Conversation about Brakes - & Tires

[FrankBarba]

Talked with a reputable person today regarding some breaking issues. I was informed that the ECU controls the breaking according to the tires sizes. Now the tires that came orgionally on my car were 345's in the rear. New VRL tires are 335's. So from what i understand is since my ECU is still thinking i have the 345's when i run the 335's the rear wants to walk. Someone please inform me as to how i can correct this problem. Do i have to send my ECU to Evernham to be reflashed? Is this cost absorbed by dodge or I?

Why did Hoosier stop making the VRL tires in 345's?

 

[GTS-R 001]

Frank,

If the ECU does compute the tire size it is probably the circumference that it uses not the width of the tread. I also believe that there is a separate antilock breaking controller on the Comp coupe. Give Tom or Steve a calll at Viper Race ( Evernham ) and ask them about it, I haven't had any problems with changing tires on my CC, the speed sensors for the Antilock Brakes are another issue though check these little $17 suckers religiously or you may just make friends with a wall or worse.

If your problem is the rear destabilizing when braking from high speeds you may need the 13" rear brake upgrade that is in the latest catalog, the 14's on the car are a bit much sometimes.

Steve

 

[CCBrian]

Hello all, I recently took delivery of my new CC #64. The first time out I had a similar problem with the rear stepping out with deep braking. Luckily my crew chief is a former chassis engineer and quickly found that the rear need much more camber than the "book" said, and the rear end was very sensitive to tire pressure. We ended up running 2 degrees negative in the rear, and dropped tire pressures to 31 hot in the rear, 32 hot front, and what a difference at Thunderhill in CA. I was able to run 157's with traffic next time out and a 155 on a clear lap at. I didn't think too bad for second session out. That was about 5 seconds faster than with the "book" setup. Brian

 

[Janni]

Hmmm....

We "solved" the rear brake instability by adding a fuzz more rear toe - 5/32" and are running little rear camber in order to place more tire on the ground during straight line braking. Our tire temps have been within 10-15 degrees across the tire, so that tells me we're getting good, even heating. Of course we're running significantly higher tire pressures on the Hoosiers while doing this. What were your tire temps?

We've also played with front damper compression and rear rebound to help keep each tire firmly pressed against the ground under braking.

I also do believe that the Motec wants to know when you change tire sizes for it's computations, but shouldn't impact the braking "brain".

 

[CCBrian]

We had 200-210 degrees on the rears. At higher tire pressure car didn't corner as well... felt loose. Hoosier says to run 36-38 PSI but have never got that setup to work on any car I have run. Just my 2 cents worth.

 

[Stan Wilson]

Frank,

I had a similar issue with my Comp Coupe. I installed the rear adjustable toe links from Viper Race Headquarters, and the problem never occurred again. It seems the production rear toe links allow a bit of flex causing the rear tires to have dynamic toe during heavy braking. The racing rear toe links are much stronger and don't allow the flex keeping the rear end directly behind you under heavy braking.

Just my $.02.

Stan