Equal Length/Torque Matched Driveshafts

[Chris Clarke]

Regardless of what you guys feel like arguing about this topic, if you don't have torque steer covered and it comes up at next year's design competition, you'll want to shoot yourself in the foot. You either say screw it, go the easy route, and don't worry about it, or you try for the extra design points and get it right. Those are the only two choices to be made. I have no idea which I'm shooting for yet, but the easy way sounds much better.

Implementing design changes based strictly on the reasoning "so and so told us to do it" will get you nowhere with the design judges.

[RacingManiac]

our drivetrain person did the calc and concluded that the torque steer effect on vehicle yaw under power is about 1 degree in our particular setup. This is the worst case scenario applies to situation like launching the car or exiting a slow corner in 1st gear, therefore we found it to be not worth while. We tell Judge Steve Fox this and his point is that even with only 1 degree of induced yaw it is something the driver have to correct for and thereby incresing his workload.

Valid point, how important you want to take it is your call.

[Chuck Maddocks]

dustin,
looking at the picture of your diff carrier, it appears that you will have a very small housing/sleeve for holding the oil, and therefore very little oil. i noticed a few other teams at competition also had this design. 1.) did you run this diff at competition? and 2.) if so, was there any issue with the tiny amount of oil (especially during endurance)?. other teams with similar design feel free to comment. We tried (unsucessfully) to move to a torsen based driveline last year, but we have a very large housing/seal so that the diff would have an oil bath that it would dip into each rotation. big moment of inertia.

as far as torque steer due to unequal shafts, search the forums, there are other posts on this topic. another way around this is to use inboard stub axles with unequal lengths/cross sections. taylor-race also has two different size tripod housings.

also, when cornering, a torque biasing diff will send unequal torque to each wheel, which can amplify or cancel out the torque steer, depending on which way you turn. therefore i feel that having equal angle of twist at the wheels is important because your torque biasing diff will amplify the torque steer in the worst case scenario. in a straight line (ie acceleration test) torque steer, no matter how small, will require some steering correction. the tires will scrub and you will not go as fast. however, if correcting this problem means axles with higher moments of inertia, you will loose horsepower everywhere. so it's a trade off. just try to get them as close as possible, with the lowest possible moment of inertia (hahah!)

[John_Burford]

Given the large slip ratios of these high power FSAE cars, torque steer as a result of shaft deflections won't be noticed. On the other hand, different angles left and right from the wheel rotional axis could cause torque steer. This can happen when you combine an offset between the diff axial and the wheel axis with a short shaft on one side and a long shaft on the other.

The bigger issue is with the strength of the shorter shaft. The shorter shaft being stiffer will result is higher impact loading and is usually the first to fail.


Dustin

Have you had problems with the diff. bolts loosening?

John Burford

[Greg 08]

our drivetrain person did the calc and concluded that the torque steer effect on vehicle yaw under power is about 1 degree in our particular setup.

RacingManiac: How did you come up with that number? What assumptions and calculation methods did you use? Do you run a torsen with unequal halfshafts? What is 1 degree measuring, the difference in yaw of the car due to the unequal rotations of the shafts? Did you account for sidewall deflection?

We figured that the minimal difference in deflection at the tire would be compensated for in sidewall deflection, but I could buy into Under the Floors comment:

If you are running a torsen with 2.6 TBR unequal drive lengths could be a problem. The half shafts act like torsional springs with, I assume a, linear spring rate. The shorter side will twist less and so the diff will bias to that side the full TBR.

I don't know how fast the tires can absorb the difference in the drive angles but I assume that the higher loaded tire will have a higher slip ratio.


Beaver Racing

Thanks Maniac, this is an interesting discussion and if Steve Fox is reading any of this, his input, line of thought, assumptions would be greatly appreciated.

Greg Ehlert
Michigan Tech

[Conor]

Originally posted by Wright D:
Equal length driveshafts are easier to do then different cross section drive shafts. You can get the driveshafts the same length by building a custom diff housing that centers the diff in the car. If you are building a custom diff case, why not also solve the driveline problems at the same time. By centering the diff there is no need for custom shafts to be made; so getting replacements, or spares is much fast/easier. Of course there are many ways to get the engine connected to the tries, this way happened to fit with our needs.

Here is a picture of what we came up with:

http://formula.engr.arizona.edu/pics...img%20(55).jpg

Sprockets on most engines are offest from the center of the engine. Even if you center the differential in the frame, the sprocket itself isn't centered if your engine is centered. How does the offset sprocket effect this? Also, thanks a million to your team. You guys really helped us out at competition and it was a pleasure sharing a paddock next to your team. Our entire team is grateful for the new starter clutch you guys so gracially provided.

[murpia]

Originally posted by Underthefloor:
If you are running a torsen with 2.6 TBR unequal drive lengths could be a problem. The half shafts act like torsional springs with, I assume a, linear spring rate. The shorter side will twist less and so the diff will bias to that side the full TBR.

This makes no sense. Diff torque bias is not influenced by the amount of shaft twist.

Regards, Ian

[RacingManiac]

Originally posted by Greg 08:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> our drivetrain person did the calc and concluded that the torque steer effect on vehicle yaw under power is about 1 degree in our particular setup.

RacingManiac: How did you come up with that number? What assumptions and calculation methods did you use? Do you run a torsen with unequal halfshafts? What is 1 degree measuring, the difference in yaw of the car due to the unequal rotations of the shafts? Did you account for sidewall deflection?

We figured that the minimal difference in deflection at the tire would be compensated for in sidewall deflection, but I could buy into Under the Floors comment:

If you are running a torsen with 2.6 TBR unequal drive lengths could be a problem. The half shafts act like torsional springs with, I assume a, linear spring rate. The shorter side will twist less and so the diff will bias to that side the full TBR.

I don't know how fast the tires can absorb the difference in the drive angles but I assume that the higher loaded tire will have a higher slip ratio.


Beaver Racing

Thanks Maniac, this is an interesting discussion and if Steve Fox is reading any of this, his input, line of thought, assumptions would be greatly appreciated.

Greg Ehlert
Michigan Tech </div></BLOCKQUOTE>

I am not the D-train man on the team, so I can't tell you for sure. Go to our website in my sig and I am sure you can find his e-mail and he can probably elaborate. We did run a Torsen in comp and I believe that was the calc based on.

[Wright D]

Originally posted by Conor:

Sprockets on most engines are offest from the center of the engine. Even if you center the differential in the frame, the sprocket itself isn't centered if your engine is centered. How does the offset sprocket effect this?

Check out the diff Taylor Race Engineering sells, it has an offset sprocket and the diff is centered. Also, if you check out more of the pictures on our site, http://formula.engr.arizona.edu, you will notice that the diff is driven from one end of the housing, allowing the sprocket to be offset.


Our ass has been saved at competition by some other nice teams help too, just pass on the favor . The more teams that do well; the better fsae competition is. The better fsae competition is; the better we all do.

[Underthefloor]

Originally posted by murpia:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Underthefloor:
If you are running a torsen with 2.6 TBR unequal drive lengths could be a problem. The half shafts act like torsional springs with, I assume a, linear spring rate. The shorter side will twist less and so the diff will bias to that side the full TBR.

This makes no sense. Diff torque bias is not influenced by the amount of shaft twist.

Regards, Ian </div></BLOCKQUOTE>

Hey Ian, I misspoke. I don't think that the shorter shaft would always get the max torque allowed by the tbr but that would be an upper limit. The difference in torque would be related to the difference in half shaft stiffness.