TBR of the Torsen

[Broderick]

I've heard that you can change the TBR of the torsen by varying the thickness and friction coeffiecnts of the thrust washers in it. Seems simple enough. Does increasing the bias ratio from say 2.5 to 4 make a noticeable difference? I have a set off washers to swap in but, I have yet to take the actual torsen itself apart(long story). Can anyone share some knowledge on swapping the washers, or know where I can find some of the numbers to mathmatically calculate the TBR?

Ryan B.
RU FSAE

[Ryan Schoffer]

i know from street cars, inclreasing the TBR really only helps if you keep losing one wheel thru turns or on gravel, snow or ice

i think for FSAE such a modification would be of limited merit - but who knows?

Vehicle electronics leader

www.ucalgary.ca/fsae (http://www.ucalgary.ca/fsae)

[Disco]

Hello,
There is a really good paper that is available via the Torsen website under "General FAQ". This explain how the mathematical representation of how the TBR works. Very simply, it is a function of the friction force within the torsen. By changing the thrust force within the diff, you change the friction forces. You can either do this by changing the helix angle of the side gears (very difficult) or by changing the material of the washers, hence the friction coefficient. Just rememeber, the torsen has very small tolerences (0.06mm if I remember correctly), so be careful to meet them if your going to make new washers.

[jack]

I think Ryan is right. i cant imagine why you would need more TBR on an FSAE car--unless your suspension geometry is screwed up enough to really unwieght you inside rear. i think with stiff cars on relativley smooth surfaces with little suspension, a TBR of 2.5-3 is fine. another thing is, unless you build a test rig, you will never really know the TBR of the torsen you have anyway. the FSAE FAQ says they sart out at about 3.2 and stablize at about 2.5, so you dont really know what you had in the first place. just wondering, do you have the old diff, or the new one? anyone running a Torsen II?

jack @ WWU
http://www.etec.wwu.edu

[Denny Trimble]

Our car started to spin the inside rear at nationals (concrete surface, ~1.7g's). We run the current Torsen university special diff. Some suspension specs:
45/55 weight distribution, 55/45 roll resistance distribution, 51" frtr, 48.5" rrtr. RC's -1/+1. CG height approximately 11.5.

Another factor to consider besides inside wheelspin, is oversteer torque on corner exit. I remember reading an article about the extreme lengths F1 teams would go to ("tank steering gears" etc) to get an extremely large torque bias under power, which would prevent corner exit understeer. I still want to run the numbers on this, I think it's one of those "last few percent" things, but it's important enough to care about.

How's the WWU car coming along? We're driving again, we'll invite you guys down for another test day in a couple weeks.

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

[James Waltman]

Denny,
The car is coming along pretty well. We should have a full rolling chassis by this weekend (maybe even Friday): Wheels, Tires, A-arms, Shocks, Steering, Pedals, and some crappy brakes. We got Viking 24 and 26 in running order again (for a few more laps – cross my fingers) and we would like to drive with you guys again. I saw that Everett got snow this week, we are trying to get rain tires right now but if we have to get studded snow tires we might have to start selling off shop equipment to raise more money...

I didn't think about it until after we left last time but I think that I might have seen a UW car have a similar failure. I think that it was October of 2001 and we were down there in Everett driving Viking 24 with you guys. The UW car retired with a nasty suspension failure. I don't remember what broke but I remember how impressed everyone was with the stainless steel braided brake line. It had wrapped around the mangled A-arms several times and survived. Do you know if the failures were similar?

James Waltman
http://dot.etec.wwu.edu/fsae/
Vehicle Research Institute at
Western Washington University