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Design trailing arm suspension
2

Design trailing arm suspension

Design trailing arm suspension

(OP)
Hi to all; I wonder if someone could help me please ?
I'm designing / building a trailer using a trailing arm suspension system. My problem is to design the arm to accommodate the torsional stress involved.
The wheel / tire is 32" diameter, 10" wide.
Static load of 4,000 lbs per tire.
The stub axle(inc brakes) will be attached to an arm approx 20" long from axle center to pivot point.
This arm will be a "bell crank" transferring suspension loads to an air bellows suspension.
My problem is to specify the section of the arm to resist the torsion loads.
I want the arm to be as thin as practical to minimize the overall width of the system.
My thoughts are to flame cut the arm/bell crank from A36 structural plate of the appropriate thickness.
Could somebody help with the appropriate formulae so I can look at various sections of the arm,
for example: what width would I need for 1" plate, or 1.5" plate, etc.
I'm thinking I could build a table using Microsoft excel to include all the variables such as: load, g force, twisting moment, length of arm, material thickness, material width, mechanical properties of the steel, etc and play with the section of the arm to give me a good solution.
Appreciate any help.
Bob

RE: Design trailing arm suspension

How could you anticipate the structural engineer understand a mechanical/automotive setup without a sketch! Good luck.

RE: Design trailing arm suspension

Ditto to the above. Although I'm not so sure a sketch will help, either.
For the immediate issue, if this is a prismatic member loaded in torsion, refer to Roark's Formulas For Stress and Strain and you can find means to calculate the torsional stresses. Combine that with bending and shear stresses via the failure theory of choice.
The bigger issue with vehicle design is not knowing the loads in the first place. I'm not aware of any standard that tells you that you should design for 2 or 3 or N times the static weight to allow for dynamic loads. Or alternatively, that everything should be designed to hit a curb or "standard pothole" at X mph.
In a fatigue situation, using flame-cut A36 plate may not be the best approach either.
And in commercial vehicles, lighter weight translates into greater payload capacity so "when in doubt, make it stout" is not always the desired approach.

Googling the trailing arm suspension gives lots of images, one here, if that helps:

RE: Design trailing arm suspension

(OP)
Well thanks for the comments.
Here is a "sketch" to show the principle. The member I'm concerned about is shown "hatched".
The sketch shows just one side of the trailer.
Weight is not too big a concern for me in this situation.
If I can get an appropriate calculation, I can play with 2 or 3 "g" and see the effect on arm.
I'm thinking A36 plate as it is readily available and easily fabricated.

RE: Design trailing arm suspension

:) JS is correct, I am no less confused than before. I think more details/clarification are required to draw to-the-point responses. Allow me to have a try, is this what you meant?

RE: Design trailing arm suspension

(OP)
Retired,
My apologies for not being clear !
The trailing arm, which I have shown hatched, pivots about the point identified by your arrow.
This link will be a "bellcrank,transferring the load to the spring which lies in the horizontal plain.
My concern is defining the necessary section of the trailing arm, to withstand the twisting moment from the wheel / stub axle. My aim is to make the trailing arm as "thin" as practical, to keep the overall width to a minimum. The trailing arm will be a rectangular section.
There will be a crosslink between the two bellcranks to transfer the total load to the spring; "F" in your sketch.
Hopefully clear ?
Thanks
Bob

RE: Design trailing arm suspension

Does this look like and make sense? If so, you shall check bearing of the pin hole, tension on the net section, shear on the gross section, and flexural stress (δ = M*y/I ± F/A). Note F can be in reverse direction, then the horizontal arm is in compression, you have to check plate buckling stress.

Note that you shall provide fillet at the inside corner to avoid stress concentration; and shall provide a beefy safety factor as dynamic force and fatigue are involved.

RE: Design trailing arm suspension

(OP)
Retired,
You've got the concept, schematic.
I certainly will need to consider the bearing surface for the pivot, and also the filler between the vertical and horizontal arms, no problem.
However my concern is trying to establish the stress in the horizontal member "L" in your sketch, due to torque effect from the stub axle.
As stated earlier, static load about 4,000 lbs on the wheel.
Centerline wheel to centerline of trailing arm "L" = approx 7 inches.
The section of "L" would be a rectangle. I'm not understanding how I calculate the stress in this member due to twisting moment of the wheel.
Thank you.

RE: Design trailing arm suspension

Can you answer the questions below (from top down), so we might be able to be on the same page.

RE: Design trailing arm suspension

(OP)
Retired
Pivot - poly bushings of sufficient diameter and width
Name - Trailing arm
Stub axle - 3" OD welded to Trailing arm
I don't understand what you mean by "type of connection". I have not yet decided whether to butt weld stub axle to Trailing arm, or whether to bore a hole in trailing arm to pass stub axle through and weld both sides.
Thank you

RE: Design trailing arm suspension

I'm not sure why i am jumping in on this other than I am trained as an M.E.
A flat plate is not a great choice for a trailing arm. It is very inefficient at resisting torsional or lateral axle stub loads.
Maybe somewhere in Roark's there are formulas for this.
A tube or pipe section would be a better choice.

RE: Design trailing arm suspension

I agree with xr250.

A solid flat plate is terrible in torsion compared to a circular or square section tube, but they are of course a little wider than a flat plate.

The other option here is to place your flat plate thing on both sides of the wheel eliminating the torque, but of course makes it more difficult to take the tyre off...

Also is this wheel braked? Are there going to be alternating forces and torque effects on the stub axle from a brake?

Really not sure why you're literally trying to reinvent the wheel here. There are multiple off the shelf suspension designs for trailers.

does that main beam need to be directly in line with the axle stubs? what not use an axle to connect both wheels?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Design trailing arm suspension

(OP)
Thanks for the input.
I'm working on a boat trailer.
The boat weighs 24,000 lbs, 42 ft long, 13' wide and 12 feet overall height.
My design is like a "tuning fork". The frame will be two 10" sq tubes,spaced about 6 feet apart, running full length.
Eight of these suspension / axle set-ups, two each side of each each frame member for a total of eight.
Total load capacity 28,000 lbs. Yes there will be brakes on each wheel.
Detachable cross members which allow the trailer to be driven under the boat, while it sits on blocks.
The suspension will be air bags / bellows as seen on highway 53' van trailers. This gives me the ability to raise trailer to lift the boat.
The option does exist to use a solid axle that would run beneath each trailer frame, however this raises the overall height of the trailer frame. I need to keep the boat as low as possible for bridge clearances, etc.
I'm limited to 102" trailer width, ideally, to avoid need for permits when trailer is empty.
Minimizing the width or thickness of these trailing arms will maximize the space available between the tires, in a limited overall width, ie:
Tires 9.5"" wide x 4 =38"
Frame members 10" x 2 =20"
Tire clearance to trailing arm 1"x4 =4"
Trailing arm arm clearance to frame 1/2" x4 =2"
Thickness of arm say 2" x 4 =8"
Total of above =72"
Subtract from overall width of 102" gives 30" between tires. This is sufficient for the keel of the boat, but I also need to be able to change a tire if necessary. For each additional inch in section width of the trailing arms, I loose 4" of clearance between the tires.
I want to maximize this space, within the 102" limit.
I take the point that a piece of flat plate is not the best for torsion. I'm looking for the formula so I can evaluate different sections for the swing arm to minimize this thickness.
It may be that the required section of the swing arms does not allow me to stay within 102" overall width and still maintain sufficient space between the tires.
As in all design work there are compromises necessary....
Appreciate the interest and help.
Bob

RE: Design trailing arm suspension

Your constraints make sense. Makes me think of my VFR800 motorcycle which has a single side swingarm that is a pretty beefy, welded up, tube section.
How much travel does the suspension have?
Would it be possible to slot and reinforce the tube and run and axle through?
Try Roark's Formulas for Stress and Strain. Might be some info in there about flat plate torsion.

RE: Design trailing arm suspension

(OP)
Thanks XR,
Operating range on the air bags is 8". But probably more if I let most of the air out to lower the suspension for loading. Depend on the lever ratio, suspension movement >10", so that precludes a reinforced slot.
I'm beginning to realize that the thickness of the trailing arm may need to be a least 2".
If I'm forced into a thicker section and can't get sufficient clearance between the tires in the middle, I may not be able to meet my ideal of 102" overall.
This may not be the end of the world. It will remove a major constraint in the design, improve stability of the trailer and allow more room between the tires. I believe the next "sweet spot" is 120" overall width.
This would allow a box section trailing arm, of perhaps 3" section width. Also structural hollow sections indicate improved stress capacity over A36 steel.
Once again, can someone help with the required formulae for a rectangular or square box section, please ?
Bob

RE: Design trailing arm suspension

I should stop to say anything that I don't have a clear understanding. However, I guess the torsion/torque is essentially has a bending effect on the trailing arm rather than linking it to "twist", so a flat bar should be able be designed to handle the task. The reason that I asked the "connection" to the tyre is that isn't that the source of torsion, other than the horizontal force shown on the sketch? I am trying to understand the setup and source of forces, and see whether it can be solved through simple mechanics. Please ignore my questions and comments, if they are far from the picture.

RE: Design trailing arm suspension

how does your design compare to JStephen's post ?

how does your trailer differ from the millions of trailers that are out there today ?

You roll the trailer down the ramp, the boat "swims" in, the rest is history ? Beams support pivoting "paddles" that'll conform to the boat's bottom.

or possibly you lift the boat onto the trailer ?

The trailer should consist of beams running under the boat. It sounds like you're trying to have stub axles, with an offset load. This'd create bending.

another day in paradise, or is paradise one day closer ?

RE: Design trailing arm suspension

(OP)
Thanks Retired for trying to get it clear !
Think of it as a simple member, 20" long. This member lies in "X" direction.
One end is supported on a pivot, with support on both ends of the pivot.
The other end has a lever, firmly attached, that sticks out 6", in the "Y" direction.
At the end of this lever is a force, 7,000 lbs static, in the "Z" direction.
The effect of this force is to try to twist the member, resisted by the pivot.
I'm trying to calculate the torsional stress in this member, as a function of the force, distance of force from neutral axis of member, length of member, cross section of the member, be it a solid, rectangle, square or round tube. With an appropriate formula, I can consider various sections for the member, and calculate the stress.
I will have to include some dynamic loading, and also the bending load in the member. I'm starting to think tube maybe my best solution, be it round, square or rectangular section.
Thank you
Bob

RE: Design trailing arm suspension

I am with you up to this point. But confused on "One end is supported on a pivot (as shown), with support on both ends (where and how?) of the pivot". What kind of restraints the support provides? For true torsion situation, wouldn't it be T = F*e, and δ = Tr/Ip. Round bar is better fit to resist torsion, but how about the thickness constraint?

RE: Design trailing arm suspension

(OP)
For the pivot, imagine a cross tube welded to the "member", with polyurethane bushes, and brackets welded to the frame.
Regarding the thickness constraint, I need to calculate the stress in different cross sections, to see if I achieve my thickness constraint within the 102" overall trailer width. If not I have to compromise on overall width and use a larger section for the trailing arm.

RE: Design trailing arm suspension

(OP)
RB,
The "tuning fork" refers to the main structure of the trailer. Two pieces of 10" square tubing 40 feet long.
My concern is with the trailing arm, referred to as the "member" in post at time 17:01.

RE: Design trailing arm suspension

(OP)

RE: Design trailing arm suspension

(OP)
Hopefully the schematic I've just uploaded will be clear.
I'm concerned with the arm shown yellow in the sketch.
Not to scale, no bracing included etc. For the purpose of this discussion I'm just concerned with thr "yellow arm".

RE: Design trailing arm suspension

If this is correct, the short stub subjects to a bending moment of 7 kips* 7" = 49 kips-in. Then, this bending becomes torsion on the trailing (yellow) arm, assuming the brake pad is capable of restrict the rotation of the pivot. I don't think this assumption is correct though.

RE: Design trailing arm suspension

(OP)
Retired,
I don't think the rotation of the pivot or brake pads is the issue. The spring will resist the rotation of the arm around the pivot point.
The torsion in the trailing arm, yellow member is the issue.

RE: Design trailing arm suspension

The forces calculated is for the both of the bending on the 7" stub, and torsion on the 20" yellow arm.

RE: Design trailing arm suspension

At 14 tonnes weight this is no laughing matter.

One of these perhaps?

https://vanclaes.com/en/lorry-boat-trailers/a-128....

There's zero chance of this trailer being allowed onto the public road.

If it's for transport with private land then you need to go brutal engineering and not try and nuance this. IMHO.

But we'd still like to see a photo once you've built it....

like this?

https://www.nauticexpo.com/prod/roodberg/product-2...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Design trailing arm suspension

(OP)
Littleinch
This is exactly what I was looking for, thank you !!!
So after a bit of toying with ideas, I went to the formula for round hollow bar.
https://amesweb.info/Torsion/torsion-of-shaft-calc...
I put in the following variables: I'm in the US, so Imperial units
Load 7,000 lbs x 2 for shock = 14,000 @ 7" radius = 8,200 ft-lbs
Tube outer radius 1.5", inner 1" (3" tube, 2" bore).
Shaft length 20"
Mod of rigidity 11.24 (Mod elasticity 29,000 ksi, poisson ratio 0.29, 1045 med carbon steel)
Results:
3" tube, 2" bore, 1/2" wall:
Max shear stress 23,200 psi
Angle twist 1.6 degrees.
Clearly a very stout member.

3" tube, 2.5" bore, 1/4" wall:
Max shear stress 36,000 psi
Angle twist 2.4 degrees.

3.5' tube, 3.0" bore, 1/4" wall:
Max shear stress 25,000 psi
Angle twist 1.5 degrees.

3.5' tube, 2.75" bore, 3/8" wall:
Max shear stress 19,000 psi
Angle twist 1.1 degrees.

This is exactly what I was looking for - Thank you.

Clearly I will not be able to achieve 102" overall width.
If I use 3.5" tube then the measurements will look like this, with a 120" overall width:
2 trailer beams x 10" = 20"
4 tires x 9.5" = 38"
1" tire clearance = 4"
1/2" trail arm clearance x 4 = 2"
4 trailing arms x 3.5" = 14"
Total 78"
Center clearance = 120-78" = 42".
Comments ?

RE: Design trailing arm suspension

(OP)
Here are some similar designs available in the US:
22,000 lbs capacity: http://www.hostarmarine.com/road/hsta-3800.html
I believe this is also a trailing arm design. The load on each tire is about 6,000 (tire rating).

http://www.kropfindustrial.com/conolift/highway-tr...
Go to "Gallery" and ses designs with open center of the trailer.
With a dual tire option, there must be enormous torsion generated in the suspension arms.

http://www.kropfindustrial.com/conolift/highway-tr...
Once again, go to "Gallery".

https://www.youtube.com/watch?v=K80crv_U7Uo
Video of 16,000 capacity trailer

RE: Design trailing arm suspension

Every one use hydraulics.

Remind me again why you're trying to reinvent this?

Certifying a trailer for road use would hopefully be quite hard to do.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Design trailing arm suspension

(OP)
LittleInch
The hydraulics are there for the support arms, and in some cases to raise/lower trailer.
Yes I will incorporate hydraulics for support arms, but not to raise lower - I plan to use the air suspension for this feature.
Certification laws are a little different in the US. However, the design needs to be robust, etc, for all the good reasons.
Why am I doing this ? - because they are very expensive, I have access to the fabrication equipment and skills, and it is a very interesting project.
In my original thinking I was very concerned with the torsion in the trailing arm, therefore considered lots of fairly low capacity tires to reduce this. Now I'm able to calculate the torsion in the trailing arm, and after reviewing some of the photos I just posted, I'm looking at heavier capacity tires, and fewer of them.
For example 245/75R17.5 tires have a capacity of 6,000 lbs.
6 of them = 36,000 lbs, ( I don't need this much capacity, but 4 tires not enough)
So I could mount these on the outside of the trailer beams only, allowing me to reduce width to 102".
Access to the torsional stress calculations allows me to review several different design options.
Once again, Thank you.
Bob

RE: Design trailing arm suspension

Well it's certainly an interesting project alright.

Be sure and let us know how it goes or if you need anymore information, but next time you might be better off posting in the mechanical engineering forum.

This one tends to be used / populated by civil structural people ( buildings, bridges, that sort of thing) and if your post doesn't get any replies after about two days it drops off the front page....

If you post another question please just copy this post in as a reference so you don't need to go back to square one.

Good luck

LI

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Design trailing arm suspension

(OP)
Thanks LittleInch
Is there a way to make contact with folks on the forum, private message or similar ?
Bob

RE: Design trailing arm suspension

It's not normally encouraged and there isn't a button for it. Most times just post the questions for all to see is actually the best way.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Design trailing arm suspension

Don't forget dynamic/impact load factor. I wonder why you don't use solid round bar, but choosing hollow tube.

RE: Design trailing arm suspension

(OP)
Thanks Retired,
If you look at the numbers I posted above I included a factor of 2 on the load to accommodate some shock, I'm currently including a factor of 3.
Re solid bar - once again I played with the results from the calculator "LittleInch" steered me to.
I had selected a tube 3.5" OD with 0.375" wall thickness, as this gives acceptable results.
To get similar results for solid bar, I need to be about 3" dia.
Don't gain much in section width, but add a lot of weight.
It's interesting to note that 4" OD, 0.25" wall gives about the same result.
I need to add a bending component, but I don't think it will be much. If it is significant, I can "triangulate" the lever to apply force more directly to the spring.
Thanks for your interest.
Bob

RE: Design trailing arm suspension

The reason for solid bar is two fold - 1) the gain is not much, but still save space where fraction of tenth of inch counts, and 2) did you check torsional stress combined with shear, V = F*FOS = 7000^2 = 14000 #.

RE: Design trailing arm suspension

This is what I have imaged. BTW, you can draw using "OpenOffice" drawing module (free), and copy to the clip board by "Shift+window key+s", then paste to the image box (with pic of camera) of this forum by "Ctrl+v".

RE: Design trailing arm suspension

(OP)
Retired
I'm OK with an extra 1/2" section width, thanks.
No I did not check torsional stress combined with shear.

RE: Design trailing arm suspension

I will give the usual caveat here - you should engage an engineer before you build anything. IF you want this trailer to be road legal and registered, your chances are much higher if a licensed engineer has reviewed the design before you waltz on down to the DMV and have to argue about whether or not it is safe.

With that said...

If you do this with plate, the torsional loads will be immense. But you also need to take into account that stiffness is your limiting factor here.

If the trailing arms aren't stiff enough, the tires will move all over the place, and this will make the trailer very dangerous to tow. If you make them out of plate and size the arms such that they provide sufficient stiffness, they will be gigantically heavy and difficult to fabricate. Trailers, racecars, and other things that need to be strong and light weight use tube instead of plate for a good reason.

What you really need is to come up with a detail that links the pairs of wheels together across the frame member. This would cancel out your bending moment- your stub axles or thru axle will need to be substantial, and the bending moment in the trailing arm will also be substantial, but your trailing arm bushings/bearings/linkage will see zero torsion and will be much easier to make strong and reliable.

Here's a very simple arrangement. With some clever packaging you could 'hang' the frame under the wheel centerline, although that flips your bag 180 degrees and puts it underneath the frame, which has complications. Much depends on how low you need the trailer to drop, and how much time you're willing to spend designing.

The quick sketch below shows this arrangement lowered all the way down- the size of the wheel/tire and the section used for the trailing arm will have a direct impact on your lowered height. Remember that you can't lower the axle centerline unless you plan to air down the tires for loading/unloading, and even if you do that you won't gain much. So if you design all this out, member sizes have a huge effect on functionality.



A word on your stress calculations - there's zero reason you would ever use round tube for this. Square tube will be significantly stiffer and easier to fabricate.

Remember that your trailing arm does not see only torsion - it sees torsion AND a giant bending load. Calculating this combined stress state is difficult.

Also - 1+ degrees of deflection is a LOT. There's also considerable side load coming into play any time this trailer gets dragged around a corner. Side load is going to try to bend your trailing arms side-to-side, and if you don't have enough stiffness in that plane, the trailer will not behave in a predictable way.

Towing your 24,000 lb boat behind an 8,000 lb truck at 50 mph on a trailer that isn't predictable is going to be very, very dangerous.

In short, your trailing arm needs to be designed to handle the worst case loading. As a rough guess, that includes:

-Lateral loading of probably at least 2G (you'll see this at minimum if you need to do a hard lane change in an emergency, such as someone pulling out in front of you)
-Impact load in the bump direction, such as hitting a big pothole (this varies a lot due to a lot of factors including how much damping; 8-10G is in the realm of possibility without sharpening your pencil too hard. This load is huge)
-Longitudinal load of at least 1G (max braking)

Plus whatever loads exist with the trailer not moving in a parking lot (i.e. all your static loads discussed above)

All multiplied by a reasonable safety factor.

Your system has to be sized to handle all of these loads at the same time- not just one of them.

RE: Design trailing arm suspension

Where is yours?

RE: Design trailing arm suspension

This is my understanding. For properly designed towing frame, the trailing arm resists the applied force only, provides all pivot points are rotation free.

RE: Design trailing arm suspension

(OP)
Swinny
Thank you for taking the time to contribute.
I do not want to be seen to be defensive here. Just to discuss your thoughts.
My background is a retired Production / Industrial Engineer from the UK, living in the US the last 30 years; working with a major tire manufacturer.
I have built several trailers over the years, generally in 12,000 lb gross range. This is something new.
I think you will see that I've moved off the naive idea of plate, following input received from this site.
If you take a look at the images and links that I posted you will see some similar designs. One of them with dual tires rated at 6,000 lb each, using some form of trailing arm solution as I'm looking at. Due the fact they are using duals, the center of the load is further from the trailing arm, than I'm considering, but also they have twice the loads, because of the duals.
My intention is certainly not to tow this trailer behind a pickup truck, but will use a contractor who typically tows mobile homes.
I need to keep the boat as low as possible due to bridge clearance. I will use detachable cross members, maybe 4" square, not defined yet. I would like to achieve 8" ground clearance beneath the cross members. For simplicity in designing the cross members, this puts the bottom of the trailer frame at 12" from the road, in the travel state. I could then lower the trailer up to 8", depending how I attach the cross members, for loading.
It would be difficult to link the trailing arms below the frame to achieve this. Maybe they could be linked above the frame ? Tires are 32" diameter, so 16" center height.
Because of these issues and looking at the other designs above, I'm thinking of wheels on the outside of the frame only.
The challenge is the design of the trailing arm. The static loads are easy to define, the dynamic loads maybe not. In attaching the trailing arm to the frame, it should be possible to achieve stiffness in the pivot, as the pivot arrangement could be as much as 12" to 16" long. This will minimize the stability issues you cite above. As you have pointed out,there are several loads:
- torsion load from the cantilever of the stub axle
- bending load in the trailing arm due the weight, and suspension reaction
- lateral loads from maneuvers, be it in a parking lot, or lane change. You suggest 2g. 2g times what load ? The total load of the trailer ? Divided by the number of tires ?
- maybe there are others...
Thus far I've considered the torsional load, with a factor of 3 for shock loads. Maybe this is insufficient - I'm open to input based on industry standards or first hand experience, please.
I've not considered bending force from suspension loads, however I could "triangulate" trailing arm to create a more direct link to the air bag, and therefore minimize the bending load. This would also aid in adding torsional stiffness.
I not sure the lateral loads are that significant. I would imagine the other loads are more significant, particularly if the trailing arm is sized for those loads, with whatever factor is appropriate.
I have considered round tube as this would be the best solution, believe, for the torsional loads; and I do not know how to calculate torsion loads in square tube. Maybe square tube would be a better solution for the combined loads.
I have not yet had a chance to put eyes on one of these other designs - something I intend to do.
It is also my intention to get an Engineering stamp for DMV and insurance reasons. I want to present a design for consideration.
Once again, I appreciate the input, and hope I do not appear to be too defensive.
Thank you
Bob

RE: Design trailing arm suspension

If my sketch above is ever close to the central topic, for loads depict, I don't see torsion but bending - from the stand up trapezoidal plate to the trailing arm. However, for side swing (lateral) load case, torsion is possible. Again, I don't know anything about automotive design, just straight talk on simple mechanics for the given setups. Hope more automotive/mechanical engineers can join the discuss, and provide meaningful insights.

RE: Design trailing arm suspension

Torsion is generated because the load applied to the trailing arm (the lower and left most of the two forces in your diagram) is eccentric- it isn't applied along the centerline of the trailing arm. It's out of plane towards the viewer of your diagram.

Bob,
You don't sound defensive at all. I hope my post is not interpreted as an attack.

Square tube is less stiff in torsion per lb (this is a generalization but it's a reasonable rule of thumb) BUT it's MORE stiff per unit of volume. Airplanes and race cars use round tube for their space frames because they care very much about stiffness per pound. Buildings and other structures where weight does't much matter will almost always use square sections. This trailer of yours is probably going to weigh as much as the boat if you want to carry it safely. At that weight, a few hundred extra pounds in your trailing arms won't matter but the 20% stiffness you will gain will matter a LOT.

Regarding the torsional load in the arm- any suspension load applied (such as striking the edge of a pothole) adds to both the bending load and torsional loads applied. If you want to be safe, with a single tire outboard of the trailing arm, the trailing arm needs to be able to handle that impact, which is the 8-10 G number I listed as a starting point. There are people in the world who have great models of what the real loading is when a tire hits a pothole or a curb; unfortunately for the home gamer, those people all work for automotive or equipment manufacturers who REALLY don't like their models, and the secret sauce contained therein, getting out into the world.

When we're talking G in suspension, that's against the total weight of the trailer and cargo. If your fully loaded trailer weighs 100,000 lbs, has 6 wheels, and hits a pothole that puts 10 G of load into any one wheel, that wheel must be safe to a true load of ~160,000 lb. (100,000 / 6) x 10.

That sounds like a huge number (and it is) but that's the nature of building equipment to move giant objects.

2G in cornering would be 32,000 lb side load (per tire for that same 100,000 lb 1x6 trailer) ; and again, you have to size everything to provide enough stiffness that the deflection at that load doesn't send the trailer off into the ditch pulling the rig behind it.

The trailer manufacturer linked above has some fairly revealing pictures of their suspension on the site if you dig around.

Link

That trailer is listed as having a 10" x 10" frame, and the trailing arms appear to me to be very close to the same size; perhaps 10x10 or 10x8. Also take note of the thickness and length of the gussets and mounts which support the pivot pins. Everything mounted to that trailer is beefy.

RE: Design trailing arm suspension

(OP)
Swinny,
Thanks again for the input.
Can you point me to a calculation, or equations for torsion in square tube please ? I've not found anything.
Your comment regarding the swing arm being same size as the frame - in that design that arm you refer to pivots to raise the whole trailer some 24" plus for loading. The actual suspension is attached to that arm.
A photo on page 3 of their brochure shows this feature.
Thank you
Bob.
http://www.kropfindustrial.com/docs/conolift-marin...

RE: Design trailing arm suspension

Wells,

This site can help for torsion on rectangular hollow shape. Link

RE: Design trailing arm suspension

(OP)
Retired
Is the link correct ? It comes back to this post.
Bob

RE: Design trailing arm suspension

This is correct address.

Link

RE: Design trailing arm suspension

Something for you to consider. The forces on the trailing arm are quite manageable, but attention should be focused on trailing arm to tire connection, that is to take a lot of abuses.

RE: Design trailing arm suspension

(OP)
Regarding loads to be considered in this design.
In theory, the acceleration of the wheel hitting a 6" high curb at 50 mph could be calculated, but this would be quite complex as it needs to include the spring rate of the tire, suspension, and any other member that deflects under this load.
An alternative would be to see what others have designed, and back into their assumptions.
If we take a Dexter 10,000 lb capacity trailer axle,with a 5" diameter, 1/4" main tube.
The track width is 70", spring center 42", and they probably use 1026 steel or similar.
MoI of axle tube 10.55
Max stress at 1 g = 16,600 psi (5,000 x 14 x 2.5 / 10.55)
Yield stress 60,000 psi.
It seems they have considered <4g in their load calculations.

RE: Design trailing arm suspension

The view of this one is clearer.

RE: Design trailing arm suspension

(OP)
Retired,
Similar concept.

RE: Design trailing arm suspension

Another similar but much stiffer design. As the concept is getting clear, I suggest to open a new thread on the automotive forum, from which you shall get plenty of to the point advices. Good luck.

RE: Design trailing arm suspension

(OP)
Retired
I tried your link, but it won't open for me.
Not sure if there is a daily limit on how many times you can enter the site, as a guest, or whether this is in the "not free" area.
Thanks
Bob

RE: Design trailing arm suspension

check von misses stresses.

RE: Design trailing arm suspension

(OP)
Thanks Retired...

RE: Design trailing arm suspension

You are welcome. Hope things work out for you.

RE: Design trailing arm suspension

Quote (Spanish Wells)

Your comment regarding the swing arm being same size as the frame - in that design that arm you refer to pivots to raise the whole trailer some 24" plus for loading. The actual suspension is attached to that arm.

I agree, but that trailing arm sees the same torsional and suspension loads, in a similar range to what you're designing.

I'm not saying your member sizes will need to be exactly the same as that, but other designs in this load range should provide a good sanity check as to how close you are to what's already known to be durable out in the world.

The designs posted by others are instructive, and may help you work out some packaging challenges, but they are for MUCH smaller trailers with much lower load capacities.

RE: Design trailing arm suspension

(OP)
Swinny,
So, working on your suggestions, and some calculations Retired has steered me towards, here is my thinking:
6 tires, 3 each side on outside of trailer.
Capacity of 5,000 lb per tire.
Trailing arm length = 14.5"
Small bending stress, as triangulated directly to spring.
Length of stub axle = 12" - will hopefully be able to shorten.
Trailing arm square tube: 4" x 4" x 3/8"
Torsional Stress calculation = T/((2t).(a-t).(b-t1))
T = 5,000 lb-ft
t (t1) = 0.375"
a (b) = 4"
g force = 8, so T = 40,000 lb-ft
Stress = 48,700 psi.
I don't know how to calculate the twist in a square tube, but the same load applied to 4" dia x 3/8" round tube gives 2.1 degrees. ( and a stress of 67,000 psi) so I should be perhaps 1.5 degree.
This is a massive structure, but I have considered 8 g.
If I go to 5/16" wall tube, and 6.5g, then my stress stays at about 44,000 psi.
Is my logic sound, please ?

Next I need to think about the torsional stress and deflection in the side beams. 10" square by 40 ft long. Going to need lots of well attached cross-members !
Thanks
Bob


RE: Design trailing arm suspension

The sketch below is force diagrams for one of the design. There are two cases considered, 1) static weight, and 2) impact due to tire bouncing upward. For road drive, case 1 and 2 are to be superimposed. Force to trailing arm-tire connection should be calculated using the net force at point "A", with the linkage as a cantilever beam. The diagrams need to be blessed by an experienced design engineer, and you might need his/her help as the calculations are quite involved

Legend: Black Arrow - Applied Load, Red Arrow - Joint Reactions, Yellow - Cylinder Internal Force

RE: Design trailing arm suspension

Update.

RE: Design trailing arm suspension

"In theory, the acceleration of the wheel hitting a 6" high curb at 50 mph could be calculated,"

No they can't - that's my job, and I always start with actual measured data, of some form, typically a few strain gages or a load cell in the road.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Design trailing arm suspension

(OP)
Greg,
Do you have something constructive to offer ?

RE: Design trailing arm suspension

Pointing out avenues that do not work may not seem very helpful, but can stop you wasting time. You can't calculate the loads accurately, the ones you started with were hopelessly low, somebody came up with some more sensible looking ones. Since I haven't measured trailer loads I'm not going to suggest specific numbers.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Design trailing arm suspension

(OP)
Retired
Thanks for your contributions. The approach you're suggesting doesn't fit my need because to get the overall suspension movement I'm looking for I must have all the components on the out side of the beam.
I have seen the style you are looking at, particularly on extreme offroad camping trailers.
Bob

RE: Design trailing arm suspension

Spanish Wells

I've greatly enjoyed this post, but looking back through it all I'm really wondering if you've bitten off more than you can chew.

From the above you are apparently trying to create a road transporting boat trailer capable of shifting (your?) boat which weighs around 12 tonnes and is 42 feet long, 13 feet wide and 12 feet high.

This is not like building a boat trailer to tow behind your car as I think you're discovering. The forces are huge and if it goes wrong you could loose a wheel, have your boat turn over and cause a large crash. If it's your boat then fine, but the insurance won't be paying out when they find out you made it in your back yard.

The second main issue I think you really need to give some intense thought to is whether you really need as oposed to would like, this whole lift up and down business on the suspension and more to the point do you really need to lower the frame below the centre of your wheel. This is really quite hard to do and if you can keep the frame above the centre of the wheel at all times then it would get a LOT easier. If you look at the examples above I think you'll find they all work on the principle of the stub axle being below the line of the frame.

When you look at the example you posted from hostar marine which is less than your capacity - it is a beast of a trailer. There is a good reason why they will cost a lot of money. Mainly because they don't want to get sued if the trailer falls a part in use.



So I wish you luck and would love to see more information, but hope you sit back for a minute and decide if this is real or just a fantasy to build it and actually use it on the public highway. In a boat yard - No problem. On the Road - I struggle to see it happening.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Design trailing arm suspension

(OP)
LittleInch
Thank you once again for your contribution.
I accept all of your points, and it's good to have a conservative point of view, as a challenge to the thinking.
Clearly this is a massive structure. Initially I underestimated the loads involved - certainly one of the main reasons I posted on here to get better information.
I have access to a fabrication shop, and professional welders.
It is my intention to get an Engineer to review the drawings, for all the good reasons.
The main structure of the trailer is quite straightforward. Clearly all the brackets, plates, gussets etc will need to be very substantial, "over-engineered".
The suspension bellows I'm looking at are Firestone IT15L 4 bellows:
https://www.truckspring.com/Search.aspx?keyword=1t...
They have a stroke of 9.5", load capacity of 7,000 lbs at 100 psi. I don't need this much capacity, so can increase the wheel movement with appropriate lever ratios. This is how I can fairly easily raise and lower the trailer, within a limited range. See page 92 in Firestone literature.
https://www.firestoneip.com/content/dam/fsip/pdfs/...
I hope you will notice that I am very open to input, to challenge my ideas, and develop a good, robust solution.
My plan is to continue with the design process, looking for solutions to each of the challenges.
I have found appropriate axles/brakes/hubs/wheels/tires.
I feel my trailing arm design is sound, although I've not yet identified pivot bushes.
I have identified suspension bellows that give me the features I'm looking for.
I have decided on the main structure frame members.
The next step is to consider the dimensions and how to attach my cross-members so they are easily installed/removed, but the attachment gives the necessary torsional support to the main beams, and supports the load.
At the end of this process I will seek a professional review of the design.
I will also get "eyes on" other designs, so as not to reinvent every aspect of the wheel.
I may ultimately decide this is not a good idea - I'm not there yet.
Once again, thank you for your constructive contributions.
Bob

RE: Design trailing arm suspension

OK, this is making more sense and sounds like the real deal, thanks for responding.

The issue remains the amount of lift and the lowest position you want the trailer frame to get to relative to the wheel. Anything above the axle height is I think much easier to design

Then what is your minimum gap between your forks?

If you look at all the other examples and what people have drawn, everyone of the trailing leg type designs are some sort of wishbone which you need to avoid huge moments at the joint between the single trailing rod / bar/ tube and the pivot tube. This needs space.

I can't see the details on the trailer picture above, but looks like it might be a sideways wishbone to me.

Rather than this complex armand pivot arrangement I think you'd be much better off with the air spring acting directly over the stub axle connection onto the trailing arm. Removes large amounts of stress, moments, gusett plates etc and all your arm is then doing is keeping the wheel in position and transmitting braking forces.

Think you need a trip to a boat yard or three and do a bit of peering about under some trailers....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Design trailing arm suspension

(OP)
LittleInch
Once again thanks for your input.
I'm trying to avoid having any part of my swing arm arrangement under the trailer frame.
The reason for this is that the tire is 32" diameter, and stub axle tube 5"; so that would mean minimum frame height 19". If I then add suspension travel of 10", everything gets very high.
It makes the attachment of the cross-members then quite complex to keep the boat low.
You are correct about the moments generated by this approach. I am waiting for the actual dimension from Dexter for wheel hub face to back of brake drum. It seems to be in the order of 12". The wheels need to be centered over the hub, for bearing life. So this seems to force me into this 12" stub axle - creating this huge moment at the end of the trailing arm. I can reduce the effect at the pivot end by extending the pivot point in front of the tire and under the frame, and gusseting accordingly.
The bell crank arrangement allows me to lay my bellows flat with about 12" above the frame member. It allows me to choose a leverage ratio to maximize suspension travel with bellows load capacity.
If I stand the bellows up, max height is 16".
Hmmmmmmmm
I wonder if I can find a higher capacity bellows, that would not be as tall, and position it directly on the trailing arm as you have suggested.
All part of the design process.
Thank you for your input.
Bob
Dexter 10,000 lbs hub arrangement:
https://www.dexteraxle.com/docs/default-source/dex...

RE: Design trailing arm suspension

I am getting addict to this thread, and have learnt quite a bit of mechanical/automotive setups. Don't stop posting the idea you have came up with, an updated plan and cross section will be of great help. I am thinking your original plan, seems doable, but I need time to visualize it. Is the 7000# the towing force on a single arm? What is weight of the boat?

RE: Design trailing arm suspension

(OP)
Retired,
The math has evolved as different solutions appear.
The weight of the boat is 24,000 lb, supported on 6 tires.
So with the weight of the trailer itself, I'm using a design load of 5,000 lbs per tire.
That is the load supported by each tire, and therefore each suspension trailing arm.
Thank you
Bob

RE: Design trailing arm suspension

I think your trailer will end up weighing a bit more than 3 tonnes...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Design trailing arm suspension

(OP)
LittleInch,
You may be correct.
80 ft, include drawbar of 10" x 10" x 3/8 tube = 3,800 (correction, doesn't need to be full length of boat).
00 lbs.
20 ft 4' x 4" x 3/8" cross-members = 340 lbs
6 trailing arm assemblies @ 60 lbs each = 360 lbs
3 complete Dexter 10,000 lb axles @ 435 lbs ea = 1,300 lbs.
So far that's 5,800 lbs......
I might get it to under 8,000 lbs.
Tires have a capacity of 6,000 lbs each, and there are stronger axles available.
I need to check how much tongue weight the tow truck can handle. It has to be a very substantial number for stability. My Ram 2500 truck calls for 10% of trailer weight as tongue weight.
As with any design process, it's one step at a time. I had the basic concept in mind, but the issue that was really bothering me initially was the trailing arm design, so that's where I started.
Thanks
Bob

RE: Design trailing arm suspension

Quote (Spanish Wells)

Is my logic sound, please ?

I think you are moving in the right direction- but not there yet.

Before I dive in, let me say - I admire your effort and I hope it doesn't appear I'm attacking you. Merely trying to help.

1.5 degrees is a LOT of deflection. Remember that when the trailing arm absorbs load, because the primary loads are eccentric, it will deflect in multiple planes. Effectively the individual tires are going to be 'steered' in different directions by their loads. You need to control this in order for the trailer to be safe and predictable behind the truck. If I were in your shoes I would probably be targeting stub axle deflections somewhere around an order of magnitude smaller than what you're seeing, under the worst case loading condition (all loads combined simultaneously). This is why stiffness is controlling your design, not strength.

Also: 48,000 psi is a high stress level. If you were to build from A500 tube, which in the highest available grade has a yield stress of about 50,000 psi, you basically have a safety factor of 1. That's bad.

Keep in mind that all the loads we're talking about so far (even that potential 8G+ pothole) are NOT factored. Those are true loads that are easily possible in the real world.

Your approach for sizing these member seems, so far, to be this:

1) Guess what the loads are (no slight against you- you have no choice)
2) Increment up member size until what you've selected just barely meets the requirement

I would challenge you to take a 180 degree turn in your approach. If I were you, my approach for this would be:

1) Determine all functional requirements of the design: you've done this already.. you need it to lower, you need clear span on the inside, you need to mate to the stub axle arrangement you've already selected, etc
2) Determine stress limit: choose a commonly available material i.e. A500 tube, divide the yield stress by a reasonable safety factor, use that value as your yield stress limit. Important clarification: you must use yield strength for this, not ultimate tensile strength. If you exceed yield, things bend. Do it a lot, and they continue to bend until they break. Do not use UTS.
3) Package everything (decide if you're putting the frame over or under the hub centerline, etc). Determine what space you have for that trailing arm to live in
4) Select the largest sections possible based on the envelope for each component
5) Calculate stresses for the individual members. You'll need to account for all loads at once. This will be hard. Start with the thinnest wall tube available in your selected sizes.
6) If your stress calcs check out, move on. If not, repeat step 5 with the next thickest wall available in your selected size.
7) Once you meet your member stress requirements, start checking connections. If connections don't work, you may have to go up in wall thickness so that your welds are strong enough. Again, repeat until you pass.
8) As a final check, look over the design making sure it can actually be built in the real world. Engage your fabricator if they are willing (some are, some aren't)
9) Submit for review by your licensed engineer

When some things are designed, you design the physical dimensions of the part and then pick a material that works.

Steel tube is available in very limited alloys and sizes at reasonable cost, which is why you want to approach from the other direction. This approach also gives you the stiffest configuration available in the packaging space you have. And stiffness is going to control your design more than ultimate strength is. Suspension members need to be very stiff; make them stiff enough, and generally they will be strong enough.

RE: Design trailing arm suspension

(OP)
Thanks Swinny for once again taking the time to contribute.
I value your opinion and am certainly considering your input.
For discussion:
- In my post of 28 July, 13:30, I "reversed engineered" a Dexter 10,000 lb trailer axle to see if I could figure out the loads they are using. If you consider it as a beam, with 2 supports (the leaf springs) and loads at each end, using 4g gives a stress of 66,000 psi. So it seems they are considering less than 4g. Do you think my logic is sound ?
- The effect of a shock load is mitigated by the tires, and suspension components, I believe. Solid tires, and no suspension puts every bit of that load into the structure.
When I was considering 1026 DOM round tube for the trailing arm, I found a yield stress value of 60,000 psi. I didn't think to change the value in my equation for A500 steel - good catch !
- It is interesting to consider one of your earlier comments when you suggested I use square rather than round tube. Your statement was that square tube is stiffer than round tube per unit of volume. However, 1026 tube has a yield stress of 60,000 psi, whereas A500 is 50,000 psi, highest grade. So using 6g, a safety factor of 2, 60,000 in-lb torque, I could use:
5' x 5' x 5/16" sq with a SF of 1.9, or (18.5 lbs / ft). I don't know how to calc twist.
5" dia x 3/8" DOM with give an SF of 2.0. (19.1 lbs/ft). Apparently 0.9 degrees twist.
It seems that the differences in yield strength overcome some of the benefits of sq section, although I don't know whether there a significant benefit in twist.
This is very big material, and all the connections and pivots will also need to be similarly sized !
Your comment on twist or deflection is interesting, once again my only reference is Dexter, and their system appears to be liable to flexing also.
Regarding your comments on process, I fully agree.
With my overall width limitation of 102", the stub axle length, tire widths, etc and a "minimum" trailing arm thickness, the clearance between the frame members "will be what it is". I don't have a particular dimension in mind. Right now it's looking like about 40"; that's OK. I am trying to define the section of the trailing arm, perhaps the most critical component in the whole design concept.
I've just realized that increasing the section of the trailing arm actually increases the torque loads, as they are based on the centerline of the trailing arm member to the center of tire !!
Greg, I imagine you are still following this discussion.
One item not yet resolved is the g force to be considered. If I am correct in my logic, it seems Dexter is using <4g. What are your thoughts based on the measurements you have seen, please ?
Secondly, for discussion once again, the tires and suspension mitigate shock loads. The tires will mitigate the shock load into the trailing arm, and the suspension will further mitigate it into the frame. Would it be reasonable to say that the tires see the full g load, the trailing arm sees less, due to the mitigation of the tires, and the frame sees even less ?
Thank you
Bob

RE: Design trailing arm suspension

Quote:

tires and suspension mitigate shock loads. The tires will mitigate the shock load into the trailing arm, and the suspension will further mitigate it into the frame. Would it be reasonable to say that the tires see the full g load, the trailing arm sees less, due to the mitigation of the tires, and the frame sees even less ?

IMO, the tires, suspension and hydraulic cylinder will soften the below/shock, but will not reduce the force. Simply as a loaded spring, it must have a rigid base to provide the reaction that equal to the applied force.

RE: Design trailing arm suspension

(OP)
To continue my argument further, if we add a spring suspension seat into the model, clearly the driver does not feel much of the effect of a large shock load; clearly that is the point of suspension. Whereas with solid wheels, no suspension and sitting directly on the frame the driver would feel every bit of the g force directly.
Secondly, I just realized that in my comparison with the Dexter axle above, they appear to have used <4g, but also 0 safety factor, in their calculations, if my logic is sound.

RE: Design trailing arm suspension

I think it is due to effect of damping, some energy is lost. Maybe you should use conservation of energy concept to evaluate the impact, with the input from the shock/suspension supplier.

RE: Design trailing arm suspension

Quote (Spanish Wells)

- In my post of 28 July, 13:30, I "reversed engineered" a Dexter 10,000 lb trailer axle to see if I could figure out the loads they are using. If you consider it as a beam, with 2 supports (the leaf springs) and loads at each end, using 4g gives a stress of 66,000 psi. So it seems they are considering less than 4g. Do you think my logic is sound ?

The tube of a solid axle is fundamentally different than a trailing arm - it's basically a simply supported beam with two point loads, and all of the loading is in bending; torsional load is very close to zero.

I'm not sure how you calculated the stress value in your analysis of that axle; I get a much, much lower stress.

RE: Design trailing arm suspension

Agree with SwinnyGG.

RE: Design trailing arm suspension

(OP)
Swinney,
Maybe I'm wrong, it's been 45 years since I did structural stuff in college.
My point is to try to define what g and safety factor Dexter has used in their design.
In the beam axle, 5' OD, 1/4" wall,10,000 lbs load rating.
70" long, 42" between spring hangers,
Moment of Inertia for beam calculates at 10.55. (pie (D^4 - d^4))/64.
Moment of 5,000 lbs x 14" = 70,000
Stress = My/I = 70,000 x 2.5 / 10.55 = 16,600 psi
Yield stress of 1026 DOM = 60,000 psi.
So they must have used a combined g factor / safety factor of <4 ??
This is my point.
So it seems that the biggest trailer axle manufacturer is using a combined g and safety factor of <4 .
Does my logic make sense ?
Thanks
Bob

RE: Design trailing arm suspension

Quote (Spanish Wells)

Moment of 5,000 lbs x 14" = 70,000

This is where your error is.

The section of a solid axle between the flexible support (the spring) and the end (where load is applied) is NOT a simple cantilever with a fixed end.

They are fundamentally different. Analysis of a simply supported beam is more complicated.

RE: Design trailing arm suspension

(OP)
Swinney
OK. I went to Engineer's edge and found:
"Beam Stress and Deflection equations and calculation for beam supported at both ends and two equal loads"
They show the equation: Stress = Wx/Z or Wxz/I
for my example, 5" tube, 1/4" wall, 70" long, 42" between supports, equal loads of 5,000 lbs, MofI = 10.55
I get: Stress at support (spring hangar) = 5,000 x 14 x 2.5 / 10.55 = 16,500 psi.
Again my point is that with a yield stress of 60,000 psi, g + safety factor is <4.
If my calculation method is wrong, would you please give me your thoughts on the method I should use ?
Thank you
Bob

RE: Design trailing arm suspension

Here is a case of damaged suspension system for your info use, and to remind you that beware of the effect of lateral force on the connections. Link

RE: Design trailing arm suspension

I'm not saying the calculation method is wrong - I'm saying your suspension configuration is fundamentally different than a solid axle connected to a chassis with leaf springs. It is compliant against impact in ways your system is not and cannot be; compliance means lower loads during impacts. Using that system as a basis of comparison regarding the safety factor of your components is dangerous.

At the end of the day you have to decide. If you're comfortable using 4g loads, use 4g loads. We can't stop you.

RE: Design trailing arm suspension

(OP)
Swinney
Appreciate the input once again.
I believe I understand the notion of compliance. However I'm not sure that I follow how the two systems are so fundamentally different.
As I see it, the "g" force is an acceleration; "compliance" in the system slows that acceleration down so that the forces are lower. Compliance is in the suspension springs themselves, including the tires, also in the components of the mechanism.
I did not intend to use a 4g factor because it seemed too low, and also it is quite easy to include a much larger g force in the calculations, using a 4" square section.
My difficulty was bridging between an apparent g + SF of <4 according to the calculations for the beam axle, and your assertion that I should consider 8-10g + SF.
My calculations, based on the input received here, for:
- static load 5,000 lb
- stub axle length 10"
- trailing arm length 14.5"
- g force of 6g
- material section 4" x 4" x 3/8" square tube
Stress = 30,000 psi.
This gives me a safety factor of 50/30 = 1.67. or an overall factor g + SF of 10.
Do you think this is sufficient to overcome the fundamental differences in the two systems ?
Thanks
Bob
ps, if I used a rectanglar section 4" wide, 5" tall, 3/8" wall, this factor would be nearer 13.

RE: Design trailing arm suspension

The fundamental difference is that in your 'baseline' (the commercially available solid axle) the member absorbing stress is 70" long. Your cantilever is 10" long. This means that the impulse that member is required to absorb is significantly higher. Imagine two springs- one 7x longer than the other. Even if they absorb the same load, the longer one does so with less strain.

4" is also very small as far as connections. Remember that your stub axle must transmit a big moment (over 4,000 ft-lb) per your math above) into that section.

And that 4,000 lb is just a single load case.

You have to consider the worst case loading condition when you size your members- that's max braking plus maximum longitudinal impact plus max transverse impact multiplied by your chosen safety factor. All the analysis you're doing so far is valuable as a though exercise, but you won't get close to knowing your 'real' sizes until you calculate that maximum load case.

I would encourage you not to size everything based on this one load case. You'll wind up having to start over once you consider the aggregate case.

RE: Design trailing arm suspension

(OP)
Thanks Swinney

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