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How should I build the driveline?

How should I build the driveline?

How should I build the driveline?

(OP)
Hello all,

I have a project I am building that has 8 wheels & 8 wheel steering. I have used 8 sprint control arm assemblies (strut towers and brakes and rotors.. etc) along with their CV axles to join up to four seperate differentials. I was origionally thinking that I would have an engine & transmission inline with the vehicle, droping the drive using CV axles to two 90 gear boxes, which will drive two sprockets with 80 size chain to each corresponding diff. The problem I don't like about this idea is that if I need to tension the chain (which I would) I'd need to either move both diffs side to side for each chain, or move the 90 gear box up and down. Since I wanted everything sealed in a custom box, this would make it really hard to seal.

So, I was thinking about using 3 carrier diff gears (same ones as on the current diffs) inline instead of chain to connect everything. The middle gear would connect to the 90. This way, everything can be secured down without needing future adjustments.

Anyway thoughts?

(to see some pictures of this machine for a reference of what I'm talking about got to  http://www.4wheeler.ca/index.php?showtopic=20755)

Thanks in advance,

Frazer RM Ross

RE: How should I build the driveline?

Could you drive each diff centre from a mutistart worm reduction of suitable ratio ?  

The drive to each worm would be above each crown wheel, all four worms could then be coupled together in line down the length of the vehicle.  

RE: How should I build the driveline?

(OP)
Warpspeed,

Interesting.. I like that idea. That would be great if I could do it that way! But I wanted to get at most a 2:1 reduction in speed from the output of the automatic transmission to the wheels. Is this possible with a worm gear assembly? I could of course, play with the gears on the output of the worm drive and on the diff, but my size on the diff is limited to what is on there right now (approx 6") and I not sure how much it would cost to get a bigger gear than that on the output of the worm drive.

Thanks for your response, this is definitly something I hadn't thought about yet.

Frazer RM Ross

RE: How should I build the driveline?

Yes it is possible. A worm gear can be a simple single start, or multiple start worm. When the number of "starts" becomes quite large, the drive worm begins to look more like a simple helical cut gear than a worm gear.

Two crossed helical cut gears can be meshed to have the shafts at ninety degrees (or any other desired angle), and the ratio can be anything you want to make it, even 2:1

Here is a picture of some crossed helical gears:

http://auto.howstuffworks.com/gear3.htm

RE: How should I build the driveline?

That is how I believe I would do it. The crossed helical gears could be cut to any ratio that you decided to use.

Helical gears generate considerable end thrust, so at least some of your bearings may need to be tapered roller bearings.

The whole drive system might be enclosed in a sturdy gear case that could form a box shaped backbone chassis. All the major suspension loads could be fed into it as well.

There is also the possibility of extending that long driveshaft through CV joints to power some additional wheels in a powered trailer.

RE: How should I build the driveline?

(OP)
Then I guess the final question would be, where would I be able to get gears like these?

Thanks for your help!

Frazer RM Ross

RE: How should I build the driveline?

There is a vehicle called an Argo which is similar to your idea. It uses a skid steer and chain drive. You may be able to get some ideas from it.
http://www.4x4mag.co.uk/marfeatures/magargo.html

RE: How should I build the driveline?

Rather a lot depends on where you are in the world. I am in Melbourne.

The internet or telephone book may lead you to businesses that cut custom gearsets. If you are patient, it may be possible to track down some existing gears, or something that could be readily adapted.

There is a "gear and pulley" Forum here on eng-tips, someone there near your location may be able to help better with this.

RE: How should I build the driveline?

(OP)
Aviat,

I currently own an old praire bobcat (basically an argo), which is exactly where I got the idea from. I wanted to incorporate steering so I could safely get higher speeds, and will use two brake systems (left and right) to be able to skid steering if I want. Thanks for the suggestion.

I'll be looking around for gears in the next little while.. thanks for all the help!! you guys have been great!

Frazer RM Ross

RE: How should I build the driveline?

I have never seen an argo, but have driven a thing called a wildcat.  This had four driven wheels, and chain drive with hydraulic transmission and skid steering. At speed, the skid steering was pretty frightening. Bumps would also throw it off line in a rather alarming way, as it had no suspension.

Eight wheels, properly ackerman steered, with independent suspension on each wheel sounds like a vast improvement on something crude like an argo.

RE: How should I build the driveline?

(OP)
Warpspeed,

You might have some appreciation then of why I crashed my prarie bobcat into a telephone pole at around 50kms. A little younger and wilder, I decided ditch the current rotax 503 and throw in a 993 4 stroke suzuki engine and the autotranny that went with it. After many many trail and error tests with short little drives, I finally got it working quite well. This was her madian voyage:

http://www.4wheeler.ca/index.php?act=Attach&type=post&id=20497

Hense the reason for a whole new design in the first place (glad you agree with the idea!!)

RE: How should I build the driveline?

Oh golly !!! And that thing has skid steering ?

You most certainly deserve the Eng-Tips Evil Kinevil award for that.

I often "located" a stump or a boulder in the wildcat, and it would stop from around 20Kmh to zero in about an inch, whereas I would continue on going at undiminished velocity over the dashboard.  

All jolly good fun when you are young.

RE: How should I build the driveline?

(OP)
"whereas I would continue on going at undiminished velocity over the dashboard"

You should write a book. Hehe. Yeah, that was skid steering and yes, quite crazy to think back on it. Needless to say she has a hard time floating now. This new one should alleviate the steering and rough riding issues.

RE: How should I build the driveline?

It certainly should.  

Eight independently and very softly sprung wheels should give amazing traction. Eight rigidly mounted wheels are likely to see most of them up in the air on any very uneven surface. Fully independent suspension should give a nice smooth comfortable ride too.

If you plan to drive it fairly fast, some appropriate spring damping may be well worth the trouble, or it might go into eight wheeled pogo stick mode, and jump like a spider.

I suppose only six wheels really need to be steered, and with a bit of careful design, all the correct akerman steering angles could be incorporated. That would create an amazingly agile and stable vehicle on all sorts of surfaces.

RE: How should I build the driveline?

You could look into using differentials from the M151 military quarter ton truck.

 The M151 had full independent suspension and, since the differentials were interchangeable from front to rear of the vehicle, they have power input flanges on the front and rear.

 You would not need any fancy chain or gear box for each pair of diff's. You could drive one differential directly from the other with your transmission or transfer and all 4 differentials in line

 The M151 had only a 70 hp motor so the differentials may not be able to handle a lot of torque. Also, I do not know what the gear ratio was.
 
 Have fun,
 Russ

RE: How should I build the driveline?

Wonderful amorrison4 ! I have been searching the internet for a picture exactly like that.

"Normal" direction rear wheel steering is unstable at speed because it promotes over-steer. The rearmost pair of wheels should definitely be fixed. For eight wheels, the front four or front six should ideally be steered. For very low speed vehicles, it probably does not matter.

The other point, is that for normal heavy duty road transport trucks with Bogey drive, the front diff is a "through" design with front and rear facing drive flanges.  But the way they do it is with a normal spiral bevel gear plus a drop gear from the input/output shaft.  That may end up being too heavy for your application.

I still feel that crossed helical gears may be a suitable solution for a very light weight low powered vehicle.

RE: How should I build the driveline?

Could always do this:



&



CODE


[img http://www.army-technology.com/projects/mrav/images/mrav3.jpg]
&
[img http://www.mil-plus.com/public/photo/00000087.jpg]

Best regards,

Matthew Ian Loew


Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: How should I build the driveline?

(OP)
I made a post in the "Gear and Pulley" mechanical section on using crossed helical gears and it was stated that they are not suitable for high torque applications. (Interestingly enough, I've found gear suppliers stating that they are, and some suppliers stating that they are not). I do like the elegant solution that they offer, although my limited garage tools might hinder my ability to make 4 properly working helical gear boxes.

ca2sbl - when I had initially thought of building this project, that's exactly how I envisioned the drive train to work, but only knew of semi-trucks that had the dual driveline diffs. I'll look into them more (but I imagine they will be quite pricey). Another issue is mating the current CV cups (from the GMC sprints) into this gear box.

I’m not really happy by going with chains, but it looks like it is the only solution without spending a lot of money on the project (these ideas are great though for future projects). If I did go with chains, could someone quickly scan through the calculations below to make sure I did them correctly.


As for calculations, the engine produces 58 lb/ft @ 3600 rpm. First gear reduction is 2.4:1 and the differential inside the automatic transmission is 3.16:1. That would give me a maximum torque of 58 lb/ft * (2.4 * 3.16) = 440 lb/ft at the output shaft. This shaft would be inline with the vehicle and change 90 degrees in direction via the bevel gear box, which needs to be rated at this maximum torque (440lb/ft or 5280 lb/in)..

I've found a gear box (http://catalogs.hubcityinc.com/bevel/model800.html) that is rated at 443 lb/ft @ 400 RPM.

Now given that I want a 1.45:1 reduction on the main drive axle, this would be putting 444 lb/ft * 1.45 = 643.8 lb/ft on some of the chains. The Main drive sprockets would be 22 teeth and the sprockets on the diffs would be 32. Using this ANSI Chain spec (http://www.4wheeler.ca/index.php?act=Attach&type=post&id=24693) I figured 80H Ansi size chain will work for the 22 tooth sprocket on the main shaft (can handle 446lb/ft torque) and 80H on the diffs with the 32 tooth sprockets (can handle 650 lb/ft).

Thanks for all the help! The pictures are great!

RE: How should I build the driveline?

(OP)
Warpspeed, sorry I missed your line "I still feel that crossed helical gears may be a suitable solution for a very light weight low powered vehicle." The vehicle is going to weigh around 1200-1500lbs and have the engine mentioned in the calculations above (54HP) (That might put things into perspective on it being light or heavy). I do not know what crossed helical gears can handle since I still have not been able to find any specs.

RE: How should I build the driveline?

mccheynebeeman:
 The M151 series of 1/4 ton trucks (jeep) were used by the U.S. military from 1960 until 1985. The 71 Hp gas engine put out 128 lbft of torque at 1800RPM. In first gear (5.712:1) the torque would be 731.136 lbft. The differential gear ratio is 4.86:1

 The front suspension is "A" arms, coil springs, tube shocks, open steel cross u-joints (Cardan type) with drum brakes. The whole front suspension was mounted on a subframe that was attached to the unit body with four bolts. The outside width is 64 inches.

 All of these parts are available new or used on the military surplus market. I have seen good, used differentials for $95U.S. and new units for $300U.S. There are civilian aftermarket automatic and manual locking conversions available.

 These subframes can be bolted to box section beams, with additional cross beams to make up the chassis of your machine. To connect the output from your transmission to the M151 differential, you would need M151 driveshafts cut and welded to the Sprint drive axels.

 There are several web sites for military vehicle collectors that have links to military parts dealers that   
can supply you with all of these parts, including complete assemblies.

 Keep having fun,
 Russ
 

RE: How should I build the driveline?

I really don't know either. But tooth loading is I suppose, relative to gear diameter. As the driven gear has to enclose the diff hemisphere, and the ratio is 2:1 both drive and driven gears are going to unavoidably be of a fairly large diameter.

The tooth loadings will be quite low, and even with point contact, I cannot see actual gear failure ever being an issue. For it's physical size it will be very lightly loaded.

The serious hard core power transmission engineers might look at this as being only capable of a quite low power density, and not being a particularly elegant or efficient design. But does that really matter ?

RE: How should I build the driveline?

(OP)
ca2sbl:

It would make complete sense to not re-invent the wheel and go with these diff boxes. But weight is a bit of an issue (granted a gear box capable of 731lb/ft matched to a similar angle gear reducer box is around 80-90lbs). I'll try to find some more info on these.

warpspeed:

"The serious hard core power transmission engineers might look at this as being only capable of a quite low power density, and not being a particularly elegant or efficient design. But does that really matter ?" I have always fought against what is supposed to be because it is the accepted way, compared to what I want it to be.  In that sense I would want something quite inexpensive, fairly easy for me to install (no major machining) and light (so, no.. truly it does not matter). I really do like the crossed gear set because of a possible single axle connecting all three diffs on one side and then another axle to drive the remaining diff (so the transmission can sit low in the vehicle). One problem (one I will face with a chain driven setup) is that I will need to create a custom housing for each gear set.

One thing I noticed you mentioned above about the steering is leaving the rear pair of tires fixed while the other six tires could turn and not use rear steering (ie. monster trucks). I planned on steering all 8, with the rear four steering opposite to that of the front 4. Granted of course, that this could be dangerous at high speeds, I was thinking of using a set of handlebars instead of a steering wheel. With proper gearing this would make it awkward to over steer as the user at the handle bars would comfortably only be able to articulate the system to a limited degree (for safe driving at high speeds). The benefit to all wheels steering would be that when needed, those handlebar could be completely rotated and thereby steer the vehicle at a much sharper turning radius than with only front wheel steering.

Here is a picture of my proposed setup for the steering:
http://199.126.216.230:81/pictures/random/8wheeled_steering.jpg

RE: How should I build the driveline?

I tend to agree with your line of thinking here. Four commercial military diffs, while mechanically attractive and very robust, will quite likely end up relatively heavy. And the 4.86 ratio is a bit of a worry too.  One way around that might be to consider six larger diameter wheels instead eight smaller wheels? That would simplify a lot of other things, and might also reduce vehicle weight and complexity.

Road cars that use four wheel steering, steer the rear wheels in the opposite direction to the fronts for maneuverability at parking speed only. It would be highly unstable like that at highway speeds. These systems steer the rears (very slightly) in the same direction as the fronts at high speed, which is probably not at all what you want.

That eight wheeled military vehicle above, and twin steer trucks have the right idea. All the steering should be done with the front sets of wheels, and the rear most axle should not steer at all for best stability.  The only problem I can see with that, is getting enough wheel clearance to gain sufficient steering lock at the front most axle.

I suppose the whole steering system is going to be a compromise between good low speed turning circle, and high speed stability. But if it also has the option of skid steering as well, it could turn in it's own length to negotiate obstacles at very low speed, and use the normal steering at higher speeds.

Air bag suspension with adjustable ride height would be another interesting toy to play with.

RE: How should I build the driveline?

You need to steer the rear axle if you want the tightest possible turning circle, including overhangs.

There was a WW2 UK armoured car that had 4 wheel steer, they ended up disabling the rear wheel steer as they had too many rollovers. I do not think that limiting the steer angle is sufficient, at high speed, I think you need to actively fix the rear half of the vehicle in the straight ahead configuration. Managing the transition will be complex.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: How should I build the driveline?

The chain drive on the steering may have an unacceptable amount of lash in it. I recommend using a direct mechanical linkage or gears rather than chains.

Best regards,

Matthew Ian Loew


Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: How should I build the driveline?

If I was going to steer the front and rear pairs of wheels only, one way would be to mount a conventional steering rack down the length of the vehicle and use some bell cranks at each end.  

Nothing really wrong with handlebars either. Power steering might be worth a thought on a ~1200 Lb vehicle with handlebars and a fairly direct steering ratio. A power steering rack would be a simple solution to that.

But I still believe rear wheel steering is going to require some fresh dry underwear after the first really high speed bend.

RE: How should I build the driveline?

(OP)
ca2sbl: I found the m151 differentials used for 95USD. The weight appears to be 70 pounds from what I could gather on my investigation. Granted if I had known about the m151 I would have started to build my project around these, but now heavy modifications would need to be implemented if I were to use these now.

mloew: Thank you for the advice. I was considering this as a possible issue, but now that you have stated the same I will try gears in an effort to keep the steering tight.

GregLocock: If the scenario of a driver in a vehicle turned the steering wheel 5 degrees to the left and the car was turning at X radius, was matched to a front and rear steering vehicle that when the driver turned the steering 5 degrees to the left the vehicle turned at the same X radius, would this make the vehicle anymore unstable at high speeds? Are there other factors to consider then just a sharper turning radius that might make it flip over easier givin the above scenario? hmm

Warpspeed: Power steering is a definite, as well as extra dry underwear. The skid steering (via braking one side or the other) was only going to be used where traction was an issue, since there are not going to be locking diffs I could force one side or the other to always have drive. Air bags will be thought about in the next project that will also use the m151 diffs.


RE: How should I build the driveline?

It is reasonably easy to estimate turn radius and turn centre by drawing a line through the rotational axis of every wheel.  The lines will meet (or cluster) around the turn centre. Steering both ends would approximately halve the turn radius for any given steered wheel angle. It is a bit rough and ready, but this may help visualise what is going on.

I have been thinking a bit more about the skid steering and the brakes. How about having three brake pedals ? One for the brakes on each side, and a central master pedal that operates both sides equally via a balance bar. That should feel fairly "natural" to drive.

RE: How should I build the driveline?

(OP)
Warpspeed: steering both ends would reduce the radius by half, but what if the gearing from steering wheel were to be 2:1 (ontop of whatever ratio there is now). Then the steering would be no different than that of a front steered vehicle without the 2:1 reduction (as seen by the driver), except for the fact that you can turn the steering wheel (on the all wheel steering) twice as many revolutions to get the tightest turn. If it were setup this way, I can not see it being anymore unsafe than our regular highway vehicles.

As for the skid steer effect, I'm glad that we see eye to eye in that I was going to use that descrbied setup. (If two pedals were only used, high speed braking could potentially get really dangerous)

Note: Estimated top speed is around 80kms/h, average speeds would be around 30-50kms/h

RE: How should I build the driveline?

The danger of rear wheel steering is that it will definitely cause vehicle oversteer as speed increases. Oversteer is dangerous because as the rear wheels begin to lose traction the turning radius reduces even further.

Basically the rear end slides outwards and the vehicle rapidly goes into a spin. All production cars have some built in rear understeer, even rear beam axle cars steer the rear wheels. The rear wheels are automatically steered by body roll.

Roll understeer is stable and safe.  By deliberately steering the rear wheels in the oversteering direction, high speed stability will be marginal to non existent. The vehicle is liable to very sudden and violent loss of control at higher speed.  It will either go into a sudden uncontrollable spin, or tip over.

The only road vehicles that I know of that use rear wheel steering are some really large fire engines, where a second driver at the back steers the rear wheels to get around very tight intersections.  The other application I have recently seen, was a very large mobile crane that had ten huge wheels, five down each side. The rear pair could be steered independently (by hydraulics), and did not operate directly from the steering wheel. Presumably this extra steering is used for very low speed maneuvering of the crane into position in tight situations.

You would have loved this crane, ten wheel drive ! The front four wheels were steered, then four fixed wheels, and the rear pair were locked straight ahead for road operation.

RE: How should I build the driveline?

No, at high speed there are many subtle effects to do with rear wheel steering. What you are proposing is an oversteering machine, it will have many awful steering characteristics, including a speed at which the controls will reverse, ie above that speed you will have to steer left to turn right.

Probably the best discussion of this is Dixon "Tires, Suspension and Handling" from www.sae.org, but basically you have to sit down and work it out yourself, there is not a whole lot written down that is not proprietary.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: How should I build the driveline?

(OP)

Quote (Warpspeed):

The danger of rear wheel steering is that it will definitely cause vehicle oversteer as speed increases. Oversteer is dangerous because as the rear wheels begin to lose traction the turning radius reduces even further.

Quote (Greg Locock):

What you are proposing is an oversteering machine, it will have many awful steering characteristics, including a speed at which the controls will reverse, ie above that speed you will have to steer left to turn right

hmm.. I guess I am still having a hard time fully visulizing this issue, but a solution could be that I build it with the oversteer configuration and if the vehcile is truely too unstable at proposed speeds then I can always remove the rear steering and weld up the linkages to remain fixed.

By the way, what figures of high speeds are we talking about?

RE: How should I build the driveline?

High speed is any speed where centrifugal force starts to become significant.

To try to visualise this, imagine you had a four wheeled vehicle where the front wheels were steered, and the rear wheels were castors.  You know, those little wheels fitted to furniture that swivel around and steer all by themselves.

The vehicle would have absolutely no directional stability. As soon as you try to steer the front, the back just goes wherever it wants to go. That is what Greg means by steering left to go right, and my smart comment about a change of underwear.

For directional stability, the rear wheels ABSOLUTELY MUST resist any sideways movement. What you are proposing is to steer the rear wheels in the same direction that castors would naturally try to steer, giving a similar unstable response.

Try riding an office chair down a ramp to get the feeling of having absolutely no directional stability, or control.



 

RE: How should I build the driveline?

The calculations for the critical speed  at which control reversal will occur are too complex to summarize here, but are available in every vehicle dynamics textbook that is written for student use.  The problem is that you won't know many of the numbers you'll need in those equations I think.



 

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: How should I build the driveline?

The concept of oversteer instability is difficult to express in plain easy to understand text.

But the idea is that the vehicle will have a strong tendency to try to slide sideways, or yaw uncontrollably at speed.  It will attempt to spin out, or suddenly and very quickly swap ends.

The actual point of "letting go" may be calculable, but in practice a bump, pot hole, wet patch, or sudden change in road surface, will cause a very sudden and dramatic loss of control.

As the whole machine starts to yaw, the front wheels, being attached relative to the chassis, will change direction with respect to the intended direction of travel.

This unfortunately makes things worse, which is what makes it such an unstable condition. To correct, the front wheels need to be very quickly turned in the opposite direction to the direction of the corner you are negotiating.

But because it is unstable, insufficient steering correction will not save you, too much, and it will start to spin in the opposite direction. The danger is that a sideways sliding vehicle can always hit a solid obstacle, and/or flip over.

Rear wheel steering almost guarantees these sorts of instabilities and handling problems.

RE: How should I build the driveline?

(OP)
Well, I can somewhat now see what you two are trying to explain. I have been reluctant not to change the opposed steering concept since I did not think there would be too much of a difference, but at this critical speed I can now understand the instability of the setup. I really wanted rear steering to not only have another "eye popping" visual but also greater maneuverability for the machine. But, since the terrain is not suitable for an unstable vehicle (climbing a hill side or riding through an open field with gofer holes) and I'm not about to risk a limb for looks, I'll lock down the rear 4 tires. Like you said warpspeed, I can always use the skid steering for tight maneuvers.

Thank you all for the help!

RE: How should I build the driveline?

Steering the front four is going to give you a vastly more stable vehicle. Oversteer is highly unstable, some built in rear roll understeer ADDS to high speed stability, and is built into most (if not all) production cars.

The way to do it is to have the rear wheels deliberately steer very slightly with body roll, not a lot is needed.  The correct rear steering direction is toe in on bump, and toe out on droop. As you will be using eight struts, the steering arms of the rear four struts will be attached by linkages somehow to the chassis. The location of the inner attachment point to the chassis can be selected to cause the rear wheels to steer with vertical suspension movement, or always point straight ahead.  It is only a small refinement, but it is something to consider.

RE: How should I build the driveline?

You can also use compliance steer to get a similar stabilising effect. Compliance steer is a bit faster than roll steer.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: How should I build the driveline?

Excellent point Greg, but it is a bit more tricky to judge. I would expect the cornering force levels to be quite low in this type of vehicle, particularly on off road surfaces.

Anyhow, if it has rear facing steering arms that are fairly rigidly located, and the lower suspension arm is rubber mounted (and therefore compliant), that would also rear steer in the required direction for improved stability.
 

RE: How should I build the driveline?

(OP)
Thanks guys for all the help!! I'll post pictures when I'm done!

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