[QUOTE] by SinGin
Congrats!

They give out trophies for 4th place?


I think one trophy is added after every group of 5, so in a big group you might have 10+ competitors trophy. Here's the results from SD

http://www.scca.org/amateur/solo2/na...o/results.html

If you haven't, try an Auto-X it's a lot of fun to drive at or over the limit on every corner

 

I read this when it was posted, but it didn't hit me til this morning.

It sounds as if you are running a different size tire on the front and rear of the car.

This is SUCH a bad idea! The center differential and center viscous coupling in the Evo work on the basis of speed differential between the front and rear axles.

When you have the same size tire on both ends of the car, the torque split is 50/50 at all times. And when there is wheelspin, there is a difference in speed on the two axles in the viscous coupler. This difference in speed shears the fluid in the viscous coupler, and heats it up. The fluid turns to putty almost instantaneously, and locks both of the VC shafts together. This, in turn, locks the two center diff shafts together with a 50/50 torque split.

When you have different size tires on the front and rear of the car, there is ALWAYS a speed difference between the front and rear axles. This means that the viscous coupler is constantly locked and trying to force the tires at one end to turn at the same rate as the tires at the other end. This does not happen. And instead, the VC just starts to overheat. The VC is meant for quick burst of lockup operation, not continual. At some point, the fluid will break down, and you will have lost the ability to lock the front and rear axles at 50/50.

I really hope you haven't already done any damage to your VC. It can not be repaired. It can only be replaced.

On top of this, the center differential is also only meant to have to operate part of the time. With different tires on the front and rear, parts are spinning inside of it all the time that usually only spin for fractions of a second.

The moral? ALWAYS run the same size tires on the front and rear of the car! Even if you calculate it out, and the circumfrence of the tires is close, manufacturing differences between the two tires usually means that there can be a big difference between actual circumfrence and calculated circumfrence.

If you want to run race tires, use them on both ends of the car, or you will damage your vehicle!

 

Thanks for the info, would'nt want to wreck the center diff. The tires did calculate out to the same size 255/35 for the kumhos, and we stood them next to each other and they looked the same ht. Also it ran like this for 3 minutes. I will not be doing it again as it was an emergency situation. Now I'm worried about a large variation in tire pressure frt/back - I've heard that can change circumferance significantly.

 

If you really want to test it out, mark each tire with some chalk and do a roll-out with each of them. Measure the distance that each tire makes in one full revolution. If the circumfrence is the same, you should be ok.

If not, I wouldn't do it.

Tire pressure should not make that much of a difference.

 

ShapeGSX,

I am not trying to be a smart *** so take this as a honest question from someone who knows far far less than you do about these things. The Evolution is a rally car technically speaking right? Wouldn't rallying on surfaces such as dirt and gravel have the potential to create a difference in wheel speeds? If so then wouldn't it be safe to assume that this too would be bad for the VC? I am just trying to get a better understanding of how this center differential works. Of course after your previous post I already know far more than I did before.

 

Sorry, I missed your reply before.

When rallying, the thing that saves your diff is that the viscous coupler will lock up. With the same size tires and wheels on both ends of the car, and the point of slippage being the gravel under the tires, there shouldn't be any noticable accelerated wear. As well, there are times when your wheels are not slipping to give the VC a rest.

When you are on tarmac with different size tires on both ends of the car, there won't be much tire slippage. So the center differential will start to turn its spider gears, and the two shafts in the differential will spin at different speeds all the time. So the VC will constantly TRY to lock up, but it will not be successful! Instead it will just over-heat and kill the fluid inside.

 

That makes sense. Thanks for the reply. My brain just got a little bigger with good-to-know stuff.