Close enough.
Just don't waste another question with any more "Can I ask a question?" type fluff.  Thanks.

 

Mike Halloran
Pembroke Pines, FL, USA

Ok.

I read just about everything I found on front suspension design. I'm still far from being an expert. So, here's what bothers me.

The complete weight of the vehicle should be as low as possible, within reasonable limits, it should be capable of handling rider's weight up to 150kgs.
The problem I have is the tire wear at high speed cornering. Need to reduce it as much as possible while keeping the steering/handling performance. The suspension system will be a double wishbone (two A arms) with the shock absorber (suspension element) attached between the two arms, unlike it's usual in automotive applications (lower end on the movable part, the other end fixed to the chassis)

The previous design I made was designed to accommodate use in extreme conditions (off road use) and tire wear was not such an issue. This new design is intended to be used for a comfortable ride on normal paved roads and the suspension should only make the ride comfortable and allow for predictable and reliable handling. The attached image shows the offroad version. It's a parallelogram with four rod ends at the end (holding the kingpin) and two movable pieces allowing the kingpin axis to be at desired angle - about 17 degrees from vertical to meet the tire contact patch on the road/surface.

The new design should have only two rod ends holding the kingpin (one on top, one at the bottom) meaning I need to make the upper and lower arms of unequal length. Need to get the kingpin inclination at proper angle which gives me a rough idea about the length difference between the upper and lower arm, but at the same time, need to minimize the track width changes at the lower end of the wheels as the suspension works to reduce the scrub during suspension travel. Tire scrub at cornering can be addressed with the proper setting of the Ackermann compensation and I'm aware that some will be present.

My confusion starts with choosing the roll center and reaction point (according to http://www.rqriley.com/images/fig-12.gif ) and basically I could use any of these, but I'd prefer to pick the best solution for the purpose.

I suppose I am missing some obviously better ideas and solutions, but have no doubt someone will turn my head in the right direction.

The offroad version has about 10cm/4" travel (wheel/frame), the road version will be fine with about half of that.

The trike will be pedal driven but will have an option to use an electric assist engine in the back. (rear hub/wheel)
 

• http://files.engineering.com/getfile.aspx?folder=27163354-caca-4562-9a56-08

Bike tires are not designed to scrub laterally, therefore your primary focus should be on minimising cp lateral migration in jounce and roll. and presenting the outside tire to the road at the correct angle.

 

Cheers

Greg Locock


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With the offroad version there's no significant lateral scrub problem, most of the time, the trike is used on relatively soft ground. Asphalt OTOH might be quite rough of skinnier tires. Is there a relatively simple solution to minimize differences in track width at cp with various amounts of suspension compression? A sway bar maybe?  

Have you actually built that offroad model, in hardware, in real life?

I'm curious about how you assemble the paired rod ends at the distal ends of the control arms, specifically how you swing one past the other while screwing the stems in, and how you phase the stem threads so that a bolt can go through both balls and the spindle without breaking something as you tighten the bolt.


 

Mike Halloran
Pembroke Pines, FL, USA

Mike, the (3Dmodel) image I posted is a revision model ("sort of" a new model) based on already well proven product I designed back in 2005. Since 2006 it has proved to be one of the best and most popular HPV trikes with full suspension on the HPV market. We made and sold about 550 units so far with minor changes and improvements during the past five years. The basic concept is the same for the narrow and wide track models (about 8cm difference in track width) - but I prefer not to do any improper advertising here, I'd prefer to learn more about the subject. I am completely aware that I had a lot of luck during the design/development process with the off-road model... Google images for Steintrikes Mungo/Meerkat and MadMax. I think it will provide sufficient information on solutions used... Then, if I can provide additional detailed information, I'd be glad to do so.  

That was sufficient; thanks.
 

Mike Halloran
Pembroke Pines, FL, USA

I'd do it with kinematics.

I suspect that a beam axle is actually the correct solution.

Cheers

Greg Locock


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Unfortunately, a beam axle is a hard sell against 'IFS!'.

Another possibility is parallel fore/aft arms, like in old VWs.  I would turn them around, to be leading arms, with the chassis pivots a little below the spindle pivots.  Ford's Merkur XR4Ti used something along those lines, where the lower 'arm' was actually a huge anti-roll bar.

 

Mike Halloran
Pembroke Pines, FL, USA

OP needs to decide on what what contact patch and body locations/angles he wants when cornering and straight ahead, and then work back to a mechanism.  

Cheers

Greg Locock


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Unfortunately, neither the beam axle, neither the trailing/leading arms (old WV) do the job in this specific construction. I suppose that I don't have to explain to you guys why. As mentioned, the overall weight of the whole vehicle must remain low. The design should be as simple as it's possible, but not simpler than that. The ideal case would be a tilting (leaning) trike , doable, but ends up with several problems, complex for both cost effective production and everyday use. There are some out there, but all ended up being too expensive and exotic and made in small numbers. The goal is to design a trike that would be comfortable, light in weight, reliable, with good handling, practical for use and affordable. Eleven years ago, when I started building them, someone said that I'll be the "new H.Ford of recumbent trikes" which is an overstatement, but I'm sure that since that time, the prices went down significantly, or at least, became less exotic. Most of manufacturers went to far East to cut the production cost and realized that the market will not grow if the trikes remain in the "luxury" niche...

My goal is to keep making them better and more affordable. That doesn't mean I'm a silly/crazy "treehugger", I'm just trying to do something useful in my life and doing my best to design reasonably priced vehicles for individual transportation...

  

So, do you understand enough about what you want your tires to do to define their required kinematic behaviour?

Cheers

Greg Locock


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This might be a bit daft question, but seeing that bike tyres are used wouldn't the 'leaning' concept be the way to go? I presume that bike and motorcycle tyres are not designed to deal with any significant amount of lateral loads, so from performance point of view it might be the best to use them in a way to keep normal loads (from tyre 'perspective') maximized and lateral loads to a minimum?

@Greg:

Not quite. I know what I would like, but still not enough to be sure it's really what the design needs to be well done. At this time, I'm fiddling with dimensional stuff, need to take care about disc brakes, steering and suspension linkages and stuff that all must pass each other at all times without conflict. Once I'm completely done with that, I'll have the limitations defined and I can start tinkering with the options I can have within the limitations that certain parts/assemblies require.

I'm aware that I will not get zero scrub at all times, but if I get to the point where most of the parameters are good enough most of the time, then, I think I'll be able to say I did the best to achieve the best that could be done within existing limitations. Nobody will complain if the tires get worn after 2000 miles. Everyone will complain if they start showing wear after 200 miles. That's what I'm trying to prevent. And it's possible. I just have to keep learning and working on it. Fortunately, I'm my own boss and made no promises or commitments to anyone, so, I still have enough time to do it the right way.   

@Wolf:

The leaning concept is ok, but most of the people who look at our way don't like it. There are some fantastic designs, concepts and products out there using this feature, but for some reason(s), eight of ten people on fairs don't even bother to ask for a test ride... Tripendo is a defunct German company which made some beautiful leaning trikes, but the price was too high, weight figures also, and somewhat "overengineered" which lead to their end. We base our future models and designs toward pleasing the user... Shouldn't everyone get the best? Yup, but people often don't know what is the best, they rather pay for what they think it's the best for them... And the customer is always right... Even then, when he/she is not...

When the body rolls it drives a camber change and a lateral motion at the cp.

The latter is what you want to minimise. Effectively that demands a fvic at ground height, ie a RCH of 0. The length of the fvsa will then govern the camber gain.

Another theory is that the tire wear has nothing to do with lateral motion at the cp, and everything to do with using a bike tire as a car tire. Presumably you have checked that out.

Cheers

Greg Locock


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Figured a few things (cp to stand for contact patch) but I'm lost trying to find what FVIC, FVSA and RHC are. Guess I'm right believing F would be Front, and then... Sorry, I'm not an automotive engineer and English not being my mother tongue, just one of six languages I'm occasionally using, doesn't really help, neither does Google in cases like this one. (blush)

Steinmini, you are correct about contact patch, and fvic is front view instant centre (virtual point around which the wheel is considered to rotate in bump/rebound), whlie fvsa is front view swing arm (the horizontal distance of line connecting the fvic and cp). RCH is roll centre height (distance of roll centre from the ground plane). HTH

Wolf, thanks for clarification. Now Greg's post became valuable, at least for me.
Now I'll need some time do digest it :)

This is not my area and this may be a stupid suggestion, but could a huge castor angle and minimal king pin inclination be used to create camber changes to see leaning of the wheel while cornering? Would this in fact help? I know there are other complicating issues like track change, scrub radius, ackermann and bump steer and maybe even steering input force required.

I know this does not help, but personally I see two wheels as most efficient for a HPV, with some flex in the front fork as the only suspension.

Regards
Pat
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The aero advantage of a recumbent trike is very significant, as is the cool factor. It is possible to have a recumbent two wheeler but they are not for the faint hearted. If you use proper bike wheels they are /very/ long, or the short wheelbase ones use little front wheels, and look rather Heath Robinson.

 

Cheers

Greg Locock


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OK I was a bit naughty, now back on topic. Along a similar line to Mike, but slightly simplified, would a torsion beam axle work

Regards
Pat
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After trying to get somewhere with the advices provided here, guess I'm back to square one. Need to re-think what I want to get what I need. The figures I had (regarding required clearances) simply don't add up... Funny, though, I thought that reducing expectations from a design (travel suspension and less frame/ground clearance for example) would come with somewhat easier ways to get to the desired goal...  

Ok, I'm pretty much of a "visual" type, I easily figure stuff I see, not so much things I read. Made a mock-up sketch with the basic arrangement and dimensions of the front wheel, disc brake and a sheet metal plate representing the kingpin. To get it where I need it (clearance thing) got about 18 degrees kingpin inclination from vertical which results in about 25mm/1 inch difference in distance measured horizontally from the middle of the chassis to the centers of the rod end balls which will hold the kingpin. That would be the outer limitation. The lowest end of the chassis should never get lower than about two inches from ground (suspension fully compressed) and without load, about 100mm/five inches from the ground. That places the lover arm's inner rotation point to about 120mm from the ground. The other dimensions are on the sketch. This would be some sort of basic starting set up with the possibility to change some positions on the chassis side, although not too much because of clearance issues, but I'll try whatever suggestion I get.  

• http://files.engineering.com/getfile.aspx?folder=3447123b-7da8-4fc6-84b2-a4

Can you redimension it so that each hardpoint has a horizontal and vertical dimension?

www.v8mongrel.com/_i/slarck.xls allows you to look at the geometry.

 

Cheers

Greg Locock


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Your instant center is outside the vehicle and very high on the "wrong" side of the wheel. I see a whole lot of camber change in the wrong direction with body roll and a whole lot of trouble with steering kickback on bumps (due to sideways motion of the contact patch) with that arrangement.

The normal upper-and-lower-wishbone arrangement has the lower arm almost horizontal and the upper arm shorter than the lower and its inner attachment point on the chassis lower than the attachment point at the spindle, which puts the instant center on the inside and relatively low to ground level. Explain what your rationale is for doing it differently because I don't see it.

@Brian:

There is no "rationale" for doing it the completely opposite way. The sketch defines the starting points (not fixed) in space which allow for sufficient clearances. The "hard" lines and points are something that defines the minimum, the dashed/construction lines are just to show the awful result.
I'll try to move some of the points/parts to get dimensions allowing me to get the IC to the proper location...
 

http://www.storyofstuff.org/

Does the single rear tire wear at the same rate as the fronts?

Cheers

Greg Locock


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Depends on the riding style, "normal" riding (commuting, recreational riding, etc) gives no noticeable wear on the rear tire. In average, even on well designed no-suspension trikes, the front tires always wear more than the rear tire. Most of the time, improper steering design or toe-in setting is the problem causing this. Enthusiastic riding is what wears them quickly, but people who like that sort of "rush", don't complain about it, they know that the extensive tire wear is the result of the way they ride their vehicles...

BTW, did some fresh calculation on steering, displacing the steering elements to back, now I can recalculate the required clearances on the chassis side to get the desired position of the instant center... Someone might find these free xl calc sheets to be useful, created by Peter Eland: http://www.eland.org.uk/steering.html  

I'll be back soon with a fresh sketch, just need to do some math and 3d visualization to check for clearances.

http://www.storyofstuff.org/

Rules of thumb with double wishbones. In all cases I am referring to dimensions to pivot points (i.e. between the pivot axis at the chassis side and the ball joint). The arm itself might be at a different angle but if the ball joint is bolted to it and sticking out the top, what counts is the dimensions involving the pivot axis on the chassis side and the center of the ball joint pivot. With that out of the way.

In front view, the lower control arm at nominal ride height should be horizontal give or take. (purpose: avoid excessively high roll center and tire scrub with suspension movement)

In front view, the upper control arm at nominal ride height should have the ball joint at the steering knuckle higher than the pivot axis at the chassis side. (purpose: get the instant center in a reasonable place so that body roll results in camber going in the correct direction)

In front view, the upper arm should be shorter than the lower one. (purpose: increase camber gain with suspension compression i.e. body roll)

In front view, the lower ball joint should be only slightly lower than the axle center line and the upper ball joint should be well above the axle center line. (no purpose behind this, it's just the way it normally works out if you want decent ground clearance, but it has something to do with what happens next ...)

In side view, the pivot axis of the lower control arm should be more-or-less horizontal. (In combination with the paragraphs above and below, this minimizes fore/aft movement of the wheel with suspension travel.)

In side view, the pivot axis of the upper control arm should slope downwards towards the rear. (purpose: this introduces a side-view rotation component to the steering knuckle with suspension movement, and this has the effect of producing an anti-dive effect when the brakes are applied, offsetting suspension compression due to load transfer when applying the brakes. There is a side effect that the trail and caster angle will change with suspension motion, but see below for an optional feature that can minimize this.)

In top view, the pivot axis of the lower control arm should be parallel with the vehicle centerline.

In top view, OPTIONALLY, if you make the pivot axis of the upper control arm further apart at the back than at the front, it will increase the anti-dive effect with suspension compression while allowing the change in caster angle and trail with slight deviations from nominal ride height to be minimized. Not all designs use this but several good ones do.

"By how much" you make all of these features ... depends on the situation, depends on the weight distribution, depends what your objectives are, depends how much space is available.

Those are good rules of thumb for cars, but on recumbents the CG is much lower compared with the suspension and antidive and so on become irrelevant.

Cheers

Greg Locock


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Sure, but regardless of C of G location, there's still no way that it should be designed so that the camber goes drastically the wrong way with bump travel, which would have been the case with the originally-proposed design.

Did the math on steering and some details, and this rework allows me a setup that should be a better starting point to get a decent design. I hope that this satisfies the rules of thumb :)
The drawing shows the front view, one half, no load. The lower arm is almost horizontal, about one degree off from horizontal...

http://www.storyofstuff.org/

• http://files.engineering.com/getfile.aspx?folder=c983146f-d397-4643-8af1-95

Is there a "rule of thumb" on approximate FVIC (desirable) distance from the chassis center - or from a different side, a distance ratio between pivots on the chassis side to ball joints on the outside? I can reduce the distance between the ball joints to move the FVIC farther from the chassis center...
BTW, the lowest point of the chassis is significantly lower from what the sketch would suggest - the frame doesn't follow the mounting points/pivoting points on the chassis...

http://www.storyofstuff.org/

Milliken said something like if you design a car using rules of thumb, you'll design a thumb, not a car.

Long fvsa length implies gentle curves, lower RCH, less camber recovery.

As I said before, if you are sure that your tire wear is driven by track change in roll, then SLARCK or CAD will do the job.

However, I suspect from a bit of poking around that much of the tire wear is caused by aggressive corner entry and drifting in corners.

The way I look at kinematics is to decide what the important parameters are, and then write down a set of target values, and then see if I can fiddle the geometry to get them. If not then I know I need to change the architecture, or trade off some of my targets.

So, do you have a target for camber gain? Typically on a road car it is around 30-100% (camber/roll), whereas for a race car more is often appropriate.

If you specify camber gain and track gain in roll=0 then I suspect you have pretty much defined your upper arm geometry, for a given lower arm.

 

Cheers

Greg Locock


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I think that I'm pretty much done with this setup, looks fine to me for a prototype. I agree that math could be very useful, however, with two many unknown factors, I usually build up a prototype (or a few versions) and do a real life testing. I know how it should behave when I sit on it and take it for a spin... Now I'm going back to SW and prepare the parts to be manufactured and check how it behaves. Fortunately, this is a time when I do have time to fiddle with working on new stuff. Sort of "dead season"... Not that I need to do much welding and manufacturing when it's full season, and sometimes I miss that part a bit... Thanks, I'll let you know how well this went. BTW, the trike should hit the market in 2013, so I'll have quite enough time to refine the details. Anyway, I did get what I asked for, assistance and not a completely defined design. So, back to designing a suspended trike and not a thumb :)

http://www.storyofstuff.org/

Things may have changed, but when I raced bicycles (20 + years ago)tires were basically round sectioned. You may not need to worry about compensating for body roll w/ camber like on a car.

You are absolutely correct and miss the point entirely. Well done.

Cheers

Greg Locock


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