Torsional deflection of your axles....

[PedalOnTheRight]

Having determined the angle the half shafts could rotate through under maximum load, I wondered "how much is too much"?

Since it is impossible to eliminate the instantaneous torsional event of the rear axles, the goal is obviously to have both half shafts deflect through the same angle - such that both tires break loose in unison. Since our differential is not centered about the rear driveline, the difference in deflection angles amounts to a little less than 4/10 of a degree.

I'm wondering if the effect of such a small difference in deflection angles, (both tires will not break loose together under max instant torque application) is enough to warrant independently sized half shafts (if the diff cannot be centered) or other considerations.

A judge last year mentioned a great loss in available power application due to torsional deflections; I'm hoping someone can compare their calculations.

Cheers!

[PedalOnTheRight]

Having determined the angle the half shafts could rotate through under maximum load, I wondered "how much is too much"?

Since it is impossible to eliminate the instantaneous torsional event of the rear axles, the goal is obviously to have both half shafts deflect through the same angle - such that both tires break loose in unison. Since our differential is not centered about the rear driveline, the difference in deflection angles amounts to a little less than 4/10 of a degree.

I'm wondering if the effect of such a small difference in deflection angles, (both tires will not break loose together under max instant torque application) is enough to warrant independently sized half shafts (if the diff cannot be centered) or other considerations.

A judge last year mentioned a great loss in available power application due to torsional deflections; I'm hoping someone can compare their calculations.

Cheers!

[murpia]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by PedalOnTheRight:
Since it is impossible to eliminate the instantaneous torsional event of the rear axles, the goal is obviously to have both half shafts deflect through the same angle - such that both tires break loose in unison. Since our differential is not centered about the rear driveline, the difference in deflection angles amounts to a little less than 4/10 of a degree. </div></BLOCKQUOTE>

Tyres don't 'break away' due to the angular position of anything in the driveline, they just operate at a slip ratio appropriate to the applied engine torque and contact patch conditions (with appropriate transient effects).

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by PedalOnTheRight:
A judge last year mentioned a great loss in available power application due to torsional deflections; I'm hoping someone can compare their calculations. </div></BLOCKQUOTE>

Power loss implies an energy loss somewhere. I doubt the internal damping in steel is worth worrying about, else everyone would be fitting cooling system to their driveshafts... I think what was meant was a delay in torque transfer to the contact patches when going on throttle. Easy enough to model if you know the stiffnesses and inertias in the system.

Regards, Ian

[PedalOnTheRight]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by murpia:


Tyres don't 'break away' due to the angular position of anything in the driveline, they just operate at a slip ratio appropriate to the applied engine torque and contact patch conditions (with appropriate transient effects).


</div></BLOCKQUOTE>

Let's say theoretically that one half shaft will deflect to an angle of 1 degree, and the other side could twist to 10 degrees during torsion. Once the shorter half shaft reaches it's limit (1deg), the energy in that shaft must now be fully dissipated at the contact patch. Meanwhile, the other shaft can still store potential energy due to its angular deflection; the power is not being transmitted equally upon instantaneous torque application.

Since the torque is being applied sooner at the wheel on the 1deg side with respect to the 10deg side at this instant, the tire will reach its maximum grip first, if the overall torque applied is great enough, causing it to slip prior to the other wheel. In the above example, this would amount to drastic torque steer.

[Biggy72]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">

drastic torque steer. </div></BLOCKQUOTE>

How much is drastic? I think that's the real question here. We have never ran unequal length driveshafts, but we will be this year. I honestly think that it would take something pretty drastic in order for this effect to be very noticeable, but I've also heard design judges don't like it much.

If you think 4/10 of a degree is too much change that axle to account for it. It probably won't hurt the car any, and the design judges won't question it.

[Composites Guy]

Perhaps you should think about the instantaneous biasing of your diff (depending on diff type) as well as break away effect that you mentioned above.

Here is a solution that might work. I assume that your outboard and inboard portions (cv, tripod etc) are the same on both sides differential so we are only dealing with a difference in axle center-section length. Why not make the shorter center-section narrower diameter than the longer center-section.... such that they have identical angles of twist. Then, put a rubber or delrin ring around the narrower diameter axle such that both axles have identical rotational inertia.

Its a heavy solution, but it should make you have zero torque steer.

[Conor]

I think you're getting a little too in-depth. Look at the entire picture. Your angular displacemnts differ by 4/10 of a degree? That's pretty damn close. Now think of the design time, machinining efforts, etc.. that will be needed for making two different diameter axles vs just one. Also, have you done the calculations on the required diameters? You might find that adjusting the diameter will produce a dimension smaller than the amount of tolerance you can actually hold. Now if you had differing angular displacement of roughly 3 or more degrees(that might even be kind of small), you'll probably want to dump the time into sizing your axles correctly. Don't qoute me on the 3 degrees, I'm obviously no expert. But the moral of the story is, weigh the amount of time you'll be putting in against the reward that you'll actually be getting out.

[Sam M]

If you're worried about both tires breaking loose at the same time, I'd say the bigger issue is the diff. If you have a diff that works, only one tire SHOULD break loose at a time.

Also, think about how quickly those axles will be loaded up. Lets say it was 5 degrees... how quickly do you think your axle rotates 5 degrees even at slow speeds? We're probably talking fractions of a second here

And as for losing energy, can't the axles be more or less be considered torsional springs? Which would mean that any energy that is "lost" (or, more accurately, stored) in the axles would be put to the road at some point?

[Composites Guy]

Sam M... I thought the same way, but at California this year I questioned our judges about torque steer. They claimed to have driven cars which have not been corrected for torque steer, and had noticeable amounts of the rear end kicking out under acceleration. Its easy to correct for... we aren't talking weeks of design time... mabey an hour, and unless you CNC lathe them making two different axles is just about as easy as making two similar axles.

[drivetrainUW-Platt]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Conor:
I think you're getting a little too in-depth. Look at the entire picture. Your angular displacemnts differ by 4/10 of a degree? That's pretty damn close. Now think of the design time, machinining efforts, etc.. that will be needed for making two different diameter axles vs just one. Also, have you done the calculations on the required diameters? You might find that adjusting the diameter will produce a dimension smaller than the amount of tolerance you can actually hold. Now if you had differing angular displacement of roughly 3 or more degrees(that might even be kind of small), you'll probably want to dump the time into sizing your axles correctly. Don't qoute me on the 3 degrees, I'm obviously no expert. But the moral of the story is, weigh the amount of time you'll be putting in against the reward that you'll actually be getting out. </div></BLOCKQUOTE>

Even if it is 3 degrees(the extreme), the weight difference from side to side in the car, the tire temperatuers and the surface available for traction will vary so who cares if one wheel moves 1/10" mroe then the other?

[Conor]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by drivetrainUW-Platt:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Conor:
I think you're getting a little too in-depth. Look at the entire picture. Your angular displacemnts differ by 4/10 of a degree? That's pretty damn close. Now think of the design time, machinining efforts, etc.. that will be needed for making two different diameter axles vs just one. Also, have you done the calculations on the required diameters? You might find that adjusting the diameter will produce a dimension smaller than the amount of tolerance you can actually hold. Now if you had differing angular displacement of roughly 3 or more degrees(that might even be kind of small), you'll probably want to dump the time into sizing your axles correctly. Don't qoute me on the 3 degrees, I'm obviously no expert. But the moral of the story is, weigh the amount of time you'll be putting in against the reward that you'll actually be getting out. </div></BLOCKQUOTE>

Even if it is 3 degrees(the extreme), the weight difference from side to side in the car, the tire temperatuers and the surface available for traction will vary so who cares if one wheel moves 1/10" mroe then the other? </div></BLOCKQUOTE>

The judges.

I'm siding with you, it is hard for me to believe. But I'm sure most of them have had for more seat time than us and understand the problem far better than we do. I'll take their word for it, even if I don't 100% agree.

[PedalOnTheRight]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Biggy72:

How much is drastic? </div></BLOCKQUOTE>

Exactly.

Some great points made here. As I mentioned earlier, the easy fix is to have the longer axle at a greater diameter, which may not be too time consuming.

Also, a good point made was that both tires will most likely slip differently due to factors other than difference in deflection. Pressure, temperature, choice of differential, etc are certainly factors.

It's obviously up to the opinion of the designer, but as many have stated, the judges do care. Thanks for all your responses!

[murpia]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by PedalOnTheRight:
Let's say theoretically that one half shaft will deflect to an angle of 1 degree, and the other side could twist to 10 degrees during torsion. Once the shorter half shaft reaches it's limit (1deg), the energy in that shaft must now be fully dissipated at the contact patch. Meanwhile, the other shaft can still store potential energy due to its angular deflection; the power is not being transmitted equally upon instantaneous torque application.

Since the torque is being applied sooner at the wheel on the 1deg side with respect to the 10deg side at this instant, the tire will reach its maximum grip first, if the overall torque applied is great enough, causing it to slip prior to the other wheel. In the above example, this would amount to drastic torque steer. </div></BLOCKQUOTE>
This makes no sense, you are assuming that the '1 degree' driveshaft has a non-linear stiffness characteristic. If both shafts are subjected to the same torque then the stiffer one will take the same time to reach peak twist as the softer one, it won't get there sooner. Also, the energy stored in the shaft twists doesn't get dissipated until the torque is removed. At that point it might get dissipated into accelerating the diff or the engine, there's no reason to assume it goes into the tyre.

I tried searching for an older post on this subject, but couldn't find it. Here's what I wrote then:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Consider the four following limit cases, all are technically impossible yet illuminate the physics.
Assume non-equal stiffness driveshafts in all cases:

1) Zero tyre slip ratio (tyres 'geared' to the road) / Unity Torque Bias Ratio (an open diff)
As torque is applied each driveshaft twists a different amount according to its stiffness. This differential twist is absorbed by the open diff and no yaw moment results. The time taken to reach a steady-state condition depends on the torque applied and the component inertias.

2) Zero tyre slip ratio (tyres 'geared' to the road) / Infinite Torque Bias Ratio (a spool)
As torque is applied the spool biases the torque in the ratio of driveshaft stiffnesses. As no slip can occur anywhere in this system this torque bias is maintained indefinitely and results in a constant yaw moment at the axle of Torque * Driveshaft Stiffness Ratio * Axle Track Width.

3) Infinite tyre slip ratio (tyres just spin and the car never moves) / Unity Torque Bias Ratio (an open diff)
As 1) except that depending on the various inertias in the system some driveshaft twist will be absorbed by the tyre slip. No yaw moment results.

4) Infinite tyre slip ratio (tyres just spin and the car never moves) / Infinite Torque Bias Ratio (a spool)
As torque is applied the spool biases the torque in the ratio of driveshaft stiffnesses. Once the driveshafts are twisted by their full amount the tyres are transmitting equal torque and are displaced rotationally by an amount equal to the difference in driveshaft twist. A transient yaw moment is applied which decays from Torque * Driveshaft Stiffness Ratio * Axle Track Width to Zero over the time taken to twist the driveshafts.

Any real car is an example of 4) where the Infinite Tyre Slip Ratio and Infinite Torque Bias Ratio are replaced by real values. To calculate the duration of the yaw moment transient requires real numbers for all the component inertias and stiffnesses, the diff TBR and the tyre slip ratio curves. </div></BLOCKQUOTE>

Regards, Ian

[PedalOnTheRight]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by murpia:

This makes no sense, you are assuming that the '1 degree' driveshaft has a non-linear stiffness characteristic. If both shafts are subjected to the same torque then the stiffer one will take the same time to reach peak twist as the softer one, it won't get there sooner. Also, the energy stored in the shaft twists doesn't get dissipated until the torque is removed. At that point it might get dissipated into accelerating the diff or the engine, there's no reason to assume it goes into the tyre.

</div></BLOCKQUOTE>

I was incorrect in stating that the energy in the twisted shafts was dissipated, I meant to explain that it is stored as potential. However, I disagree with your observation. Consider a differential mounted 20mm away from one upright, while the other axle is 100mm long. The shorter shaft is the limiting factor in the driveline; if a torque is applied at the differential (assuming all else equal) the shorter shaft will reach its maximum angle of twist well prior to the longer shaft, causing the instantaneous application of torque to be felt by the wheel connected to this short axle. It will absolutely get there sooner. That's the whole point of driveshaft induced torque steer.

[murpia]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by PedalOnTheRight:
The shorter shaft is the limiting factor in the driveline; if a torque is applied at the differential (assuming all else equal) the shorter shaft will reach its maximum angle of twist well prior to the longer shaft, causing the instantaneous application of torque to be felt by the wheel connected to this short axle. It will absolutely get there sooner. That's the whole point of driveshaft induced torque steer. </div></BLOCKQUOTE>
No, it depends on the differential and the tyres. See the above 4 cases. If you had no driveshaft on one side, and an open diff, would you get torque steer or no drive?

Regards, Ian

[PedalOnTheRight]

I completely agree with you there. I was considering the worst case scenario, which may or may not be applicable given a different type of differential. Sorry for the confusion.

[murpia]

Unfortunately this is one topic area where 'judges intuition' can sometimes bias the situation against certain design solutions. For sure, given no real evidence one way or the other, and without having conducted the necessary study personally, many experienced engineers would reject unequal length driveshafts as 'not looking right'... And I agree that if one had to decide what configuration would offer the least likelihood of problems, then symmetrical solutions will always 'look better'.

As I hope we've demonstrated here, though, there's no reason to reject unequal length driveshafts if analysed correctly and designed accordingly. That way you'll have the evidence to convince the skeptics.

Regards, Ian

[Composites Guy]

As I see it, equal length half-shafts represent an ideal low weight solution. If you are going to need unequal length half shafts, then you should make the longer shaft oversized (to balance the angular twist.) But this oversizing of the diameter is wasted weight from a strength perspective.

[Conor]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Composites Guy:
As I see it, equal length half-shafts represent an ideal low weight solution. If you are going to need unequal length half shafts, then you should make the longer shaft oversized (to balance the angular twist.) But this oversizing of the diameter is wasted weight from a strength perspective. </div></BLOCKQUOTE>

Not necessarily. There's always gun drilling and for us, using unequal length half shafts saved us from making heavy and complicated components to center our differential.

[Composites Guy]

Conor, I was thinking of the weights of the shafts alone. You make a good point though about considering the weight of the entire system. I myself am designing a car with unequal length half shafts for that reason.