Boxster/Cayman brake bias
10-17-2017, 03:50 PM
#46
Rennlist Member
Originally Posted by rbahr
Couple of thoughts:
The following will affect your braking:
What was the original problem you were trying to solve?
Ray
The following will affect your braking:
- You went with a MC assembly designed for a car with more rear weigh bias
- You do not have a LSD: breaking loads will not be distributed in the rear
- Your suspension is likely a bit wonkey - you have no idea where your bump is currently at - AND you should do front and rear. What you are adjusting for is essentially no camber change over the region of interest
What was the original problem you were trying to solve?
Ray
Since it's designed for more rear weight bias, does that mean its applying more pressure in the rear than stock, therefore less aggressive pad in rear to compensate?
Measure bump steer front and rear?
I'm also on stock shocks with over 50K miles in them. Maybe they're past it.
I have a set of PSS9 with swift springs ready to go on next season that I bought used from another R'lister.
10-17-2017, 10:36 PM
#48
There is so much misinformation/misunderstanding in this thread that it really deserves some clarification.
Firstly, brake "power" does not cause the car to dive pitch or roll. The whole idea of "big powerful brakes" is misleading. Small single piston brakes with discs half the size of most modern cars' stock rotors are "powerful" enough to lock up any street tire on the track. The only benefit to going with a bigger rotor and caliper/more pistons is to allow for greater cooling, easier brake modulation and to handle the grip that slick racing tires have. Any diving, rolling or pitch of the car is suspension related.
Secondly, the GT3 master cylinder does nothing with brake "power". It simply pushes more fluid per distance of pedal depression than the stock MC. Shorter throw, that's it.
Increasing rear brake bias or adding more aggressive bite with pads will cause the rear to lock up sooner and cause even more instability under braking. If you haven't addressed your suspension issues first, this will be treacherous. Unless you're locking up your front wheels before you're able to slow the car adequately, then you really don't need more rear bias. Adjusting the attitude of the car under braking with suspension (ie less dive) will provide better weight over the rear wheels and allow them to retard the car more effectively.
Caymans have a significant amount of rubber in their stock rear suspension and it moves A LOT under load. I know exactly what the OP is talking about as I've felt my cayman dart about under heavy braking. It's a combination of front and rear bump steer and the flexing through the rubber mounts. I have been able to cure most of this phenomena by carefully adjusting ride height, appropriate bump steer correction, minimizing dive during braking (increasing rear rebound damping really helps), adjusting rear toe (I'm running a total of 3mm in) and swapping out much of the rubber bushings with spherical metal bushings where possible.
I still have the stock open diff and I can feel when the rear is unsettled because of the uneven braking. A locking diff would certainly help this but at the cost of significantly more heat in the transmission (that I already have difficulty cooling) and more difficulty in rotating the car in the slower corners. Also, I am not spinning up the inner wheel under acceleration, so I am leaving well enough alone for the time being. Perhaps if I get a proper transmission cooler, I will give it a go...
There. Now I feel better. Carry on.
Firstly, brake "power" does not cause the car to dive pitch or roll. The whole idea of "big powerful brakes" is misleading. Small single piston brakes with discs half the size of most modern cars' stock rotors are "powerful" enough to lock up any street tire on the track. The only benefit to going with a bigger rotor and caliper/more pistons is to allow for greater cooling, easier brake modulation and to handle the grip that slick racing tires have. Any diving, rolling or pitch of the car is suspension related.
Secondly, the GT3 master cylinder does nothing with brake "power". It simply pushes more fluid per distance of pedal depression than the stock MC. Shorter throw, that's it.
Increasing rear brake bias or adding more aggressive bite with pads will cause the rear to lock up sooner and cause even more instability under braking. If you haven't addressed your suspension issues first, this will be treacherous. Unless you're locking up your front wheels before you're able to slow the car adequately, then you really don't need more rear bias. Adjusting the attitude of the car under braking with suspension (ie less dive) will provide better weight over the rear wheels and allow them to retard the car more effectively.
Caymans have a significant amount of rubber in their stock rear suspension and it moves A LOT under load. I know exactly what the OP is talking about as I've felt my cayman dart about under heavy braking. It's a combination of front and rear bump steer and the flexing through the rubber mounts. I have been able to cure most of this phenomena by carefully adjusting ride height, appropriate bump steer correction, minimizing dive during braking (increasing rear rebound damping really helps), adjusting rear toe (I'm running a total of 3mm in) and swapping out much of the rubber bushings with spherical metal bushings where possible.
I still have the stock open diff and I can feel when the rear is unsettled because of the uneven braking. A locking diff would certainly help this but at the cost of significantly more heat in the transmission (that I already have difficulty cooling) and more difficulty in rotating the car in the slower corners. Also, I am not spinning up the inner wheel under acceleration, so I am leaving well enough alone for the time being. Perhaps if I get a proper transmission cooler, I will give it a go...
There. Now I feel better. Carry on.
10-17-2017, 11:03 PM
#49
^^^
I agree with a lot of what you say, especially regarding the MC and large brakes.
Changing to a higher friction brake pad on one end does not necessarily lead to locking up that end before the other. Yes, locking up the rear would be something to avoid, but adding an incremental bit of friction does not lead to instability... All cars stock should lock the fronts first, that is how they are designed and that is safe. But that doesn't mean there may not be some rear brake bias still available to gain. Not always, not in all cars but sometimes there is.
I agree with a lot of what you say, especially regarding the MC and large brakes.
Changing to a higher friction brake pad on one end does not necessarily lead to locking up that end before the other. Yes, locking up the rear would be something to avoid, but adding an incremental bit of friction does not lead to instability... All cars stock should lock the fronts first, that is how they are designed and that is safe. But that doesn't mean there may not be some rear brake bias still available to gain. Not always, not in all cars but sometimes there is.
10-18-2017, 08:55 AM
#50
Rennlist Member
P = w/t
Note my previous comments about time/velocity curves.
Lots of good info here for the OP. Fundamental solutions to the problem were already covered in post #11.
In a track car, setup begins with understanding what the diff does. You have to compensate for extremes i.e. spool vs. open diff.
Note my previous comments about time/velocity curves.
Lots of good info here for the OP. Fundamental solutions to the problem were already covered in post #11.
In a track car, setup begins with understanding what the diff does. You have to compensate for extremes i.e. spool vs. open diff.
10-18-2017, 02:09 PM
#51
Rennlist Member
Thanks everyone for the discussion. The variables that can be adjusted at this time are
Rear toe
Bump Steer
Rear Pad compound.
I'm going to start with the first 2 and see what happens.
I'll keep this thread updated.
Rear toe
Bump Steer
Rear Pad compound.
I'm going to start with the first 2 and see what happens.
I'll keep this thread updated.