Equal Length Half Shafts

[Gary Norris]

Hey folks,

We were having a discussion amongst a few of us on the team, and I'd like to expand the discussion to the forums here.

What are the advantages and disadvantages to having equal length half-shafts? Those of you that have done them, what was your logic? And those of you that think it's unnecessary, why is that?

The primary issues we came up with were drive angles and weight considerations. If you guys (and girls) could open my eyes to additional considerations it would be greatly appreciated. Take Care.

Gary

[Denny Trimble]

1) Spares: make one spare that will work on either side

2) Torque Steer: if you have a long shaft on one side, of equal diameter as the short shaft on the other, the car will have torque steer. So, going to equal length halfshafts, and having the stubshaft (diff to inner CV shaft) be ~2x as stiff as the halfshaft cross section, should eliminate torque steer effects.

University of Washington Formula SAE ('98, '99, '03, '04)

[murpia]

Torque steer is more subtle at the rear of a car than the front. At the front the two wheels are obviously connected by the rack, so the torque reaction needs to be equal either side for the steering wheel not to receive a resultant torque. Therefore FWD normally requires equal length shafts.

At the rear, the wheels usually have separate toe links. So the torque reaction of a driveshaft is fed into the chassis. Therefore the torque 'steer' of the rear wheels is dependent on toe stiffness and driveshaft angularity and unequal lengths will not matter per se.

Adequate toe stiffness is an obvious requirement for good handling. So you could easily justify unequal shafts for packaging reasons if you can demonstrate that your toe stiffness is indeed adequate to cope with the greater torque reaction of the shorter shaft.

Ian

[Mechanicaldan]

Doesn't torque steer come from unequal axle twisting?

If the shafts are the same material, but different lengths, they will twist different amounts over their lengths with the same amount of torque applied to them. Do an experiment yourself. It is easier to twist a long shaft a certain number of degrees, compared to a similar short shaft.

My thought on trying to explain torque steer would be to use a left and right hand rule. Hold out both hands in front of you and curl your fingers. Your thumbs would determine the direction of torque steer.

Maybe someone could help me out here. If the left shaft is shorter and twists less, does it push the car to the right?

Theoretically, you could design the shafts using different wall thicknesses to get equal torsion.

Cyclone Racing
www.cyclone-racing.com (http://www.cyclone-racing.com)
Iowa State University
Project Director

[murpia]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Daniniowa:
Doesn't torque steer come from unequal axle twisting?<HR></BLOCKQUOTE>

No, it comes from unequal contact patch forces, either through traction forces or slip angles.

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Daniniowa:
If the left shaft is shorter and twists less, does it push the car to the right?<HR></BLOCKQUOTE>

No, the differential evens out the slight difference in angular displacements of the inboard end of the driveshafts.

[Gary Norris]

even though murpia proceeded to shoot down both of dan's statement/questions, you bring up a very valid point that was on my mind. Making equal length shafts would result in a shorter shaft on the right side of the diff, and as dan said, a shorter shaft (all else the same) will deflect less. So wouldn't this reduce the likelihood of axle wrap and just generally make life for the shafts easier? This is especially important to me, as our half-shafts are made of carbon fiber, and while resilient enough, I'd like to "make their life easier" in any way I possibly could.

thanks for all the input guys.

Gary