Torque Steer

[Andrew Del Donno]

Has anyone run a car in the past with substantially unequal half-shaft lengths. I searched on the forum and didn't find a whole lot of information. If you have run a setup like this in the past, how bad is the torque steer?
-Andrew

[Andrew Del Donno]

Has anyone run a car in the past with substantially unequal half-shaft lengths. I searched on the forum and didn't find a whole lot of information. If you have run a setup like this in the past, how bad is the torque steer?
-Andrew

[Boston]

I'd think if your half shafts were short enough to see noticable torque steer, you would have bigger problems (extreme half shaft angles at your CV joints). Ours are about 12.5" and 15.75". Our drivers never complained about torque steer.

[Schumi_Jr]

We have always balanced the spring rate of our half-shafts. This year we will be centering our diff so it wont be an issue.

[fsae_alum]

Even though your driver may not be able to "feel" the difference, the judges will question you to assure that you have addressed the issue. This can be overcome as mentioned in the above post.

[James Morrison]

I am confused on the connection between balancing the spring rate of the half shafts and its affect on torque steer....= (

[Marshall Grice]

the torque steer comes from the half shafts twisting different amounts. So if you balance the torsional stiffness of the axles then the shafts twist the same amount, thus minimal torque steer.

[Korey Morris]

We have close to a 6" difference in shaft lengths and I've never noticed a problem. Matching the torsional stiffness seems like it would compensate well though.

Maybe our car needs more power to truly see the effects of torque steer....

[Marshall Grice]

rear track and weight also have some effect on how noticable the torque steer is. I've only really ever heard of it being a problem on front wheel drive cars (not that i know anything really)due to the forces actually overpowering the driver and turning the wheel, but i can see how if you had a ton of torque (or compliance) it could cause problems in a rwd application.

[Daves]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">We have always balanced the spring rate of our half-shafts. This year we will be centering our diff so it wont be an issue. </div></BLOCKQUOTE>

How are you (Waterloo) able to center the differential for 2006? Will you be using dual rear brakes? Are you still using the Torsen Type I differential?

[Dave Cook]

I think Marshal has the right idea. Torque steer is a noticable force felt in the steering wheel due to momentarily unbalanced forces trying to rotate the front uprights. In the rear it should only be noticed as a change of toe in or toe out under extreme circumstances. If your rear toe adjustments are stiff enough that you don't get large changes of toe in or out under braking, then the smaller unbalanced effect from the uneven twist in the halfshafts should be negligable. This would also have to be labeled something else because it won't be coming through the steering wheel.

IMHO putting the dif in the center of the car is only a mass distribution issue, unless you've got aero issues in the rear of the car. In an FSAE car, I'd move the battery or some other mass in the back to compensate for the weight shift an run unequal length halfshafts to make the packaging work better. I'm open to other thoughts.

Dave Cook

[Z]

If one rear wheel provides more forward thrust than the other, then there is a "steer" effect on the chassis, ie. the car yaws sideways due to the extra push on one side. If one driveshaft flexes more than the other during acceleration, then this can momentarily give more thrust at the stiffer driveshaft's wheel, and hence some "torque steer". But IMO this is typically so small in magnitude and short in duration that it would be hard to notice.

In comparison, if one rear wheel is on a low grip surface (sand, oil, smoother tarmac, etc.) the yawing effect can be much bigger and longer lasting, and it is noticeable. As car designer you have no control over the type of road surface so there is not much you can do about this much bigger "split-mu" effect. Well, you can have a narrow rear track and long wheelbase like a dragster...

With front-drive cars the "torque steer" comes in two different ways. Firstly, an effect similar to above - different thrusts at the two wheels due to split-mu - gives different torques about each king-pin (due to any tyre "offset" lever arm), and this steers the whole steering linkage including the steering-wheel, as well as yawing the car as above. (Edit: Front-drive cars with "negative offset steering" try to balance the chassis-steering-effect with a kingpin-steering-effect turning the wheels in the opposite direction.)

Secondly, if the driveshafts are at different angles to the kingpins they transmit different torques to the wheel assemblies. If the driveshaft is at 90 degrees to the kingpin, then no torque transmitted. If the driveshaft is (hypothetically) parallel to the kingpin, then all the driveshaft's torque acts to steer the wheel. So if a front-drive car has different length driveshafts, it will likely have different angles between driveshafts and kingpins each side, and hence have this second type of torque steer.

This second type of torque steer is felt as long as the car is accelerating - it is not momentary. (Edit: Actually this effect is felt whenever there is driveshaft torque, so the steering wheel can pull to one side even during steady cruising.) It is usually worse during cornering because the difference of driveshaft/kingpin angles gets worse due to body roll. Also, it only acts to steer the wheels so it is felt (quite strongly) at the steering-wheel, but if the steering linkage is held rigid then the effect doesn't act to steer the whole car.

So with stiff toe-links at the rear wheels this second effect shouldn't be very noticeable. But with a 2" toe-base and sloppy ball joints... http://fsae.com/groupee_common/emoticons/icon_frown.gif

Z

[murpia]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Z:
So if a front-drive car has different length driveshafts, it will likely have different angles between driveshafts and kingpins each side, and hence have this second type of torque steer.
Z </div></BLOCKQUOTE>

This makes for fun chassis dyno testing in my experience... The time-honoured solution is to wind the windows down and ratchet strap a 2"x4" to the steering wheel reacting against the sills...
Ian

[Marshall Grice]

interesting note about the half shaft angles Z. isn't learning great?

[Schumi_Jr]

Daves,

The differential housing is assymetric but the internals are centered. Both bearing blocks are to the right of the sprocket when viewed from the top. If my explanation of the layout doesn't make sense, look at pictures of Wollongong's car. They have a nice website with lots of pics. With this design we could run a single brake but aren't for various reasons.

We aren't running a Torsen and there are lots of reasons for it. Gareth, our drivetrain guy, would be glad to tell you all about his design at the FSAE competition http://fsae.com/groupee_common/emoticons/icon_wink.gif

Cheers,