Quote:
Originally Posted by mohaughn  I would say stopping distance is totally dependent on the tires on the car and their current condition. Brake balance will affect it somewhat, more in feel and stability, but rotor size/shape should have no affect on the overall stopping friction the car can generate.
The brakes can almost always cause a lock up, which is dictated by tire friction, not brake friction. |
In my thread 'Why size matters' (
http://www.318ti.org/forum/showpost....51&postcount=1) I have done the following analysis:
Several test of an e36 323ti shows a stop length of 38m (see e.g. [4]), going from 100-0 km/h. An assumption is that this stop length is achieved by applying maximum braking force. Accepting this as reasonable, it is possible to determine the average coefficient of friction for the vehicle. When the average coefficient of friction is determined, the maximum deceleration is determined. Further, this also dictates the maximum braking force which can be applied before the front wheels stops rotating (or the ABS intervenes). An additional assumption is that the maximum braking force for the front tires is the same for any brake system setup for the e36 323ti (see e.g. StopTech’s “The brakes don't stop the vehicle - the tires do.”). Knowing the maximum braking force for the front wheels, it is possible to determine which brake pedal force that has to be applied in order to achieve it, given the brake system data. Doing some tests, I was amazed by how well theory and practice fitted together - at least at road conditions. Based on theory, literature studies and my own experience I think brake balance has gained too little attention.
Quote:
Originally Posted by mohaughn I'll be more interested in seeing how well you can get the rear brakes to the proper temp and hold them at temp, than how much faster the car can stop. You'll have to carry a lot more heat in the rears with the larger and vented rotors. I know I have a hard time getting my standard/non-vented rotors to a high enough heat to run track pads in cooler weather. |
My experience is a bit different. I have never had any problems getting the rear rotors up to a temperature where the rear pads work as intended.
The temperature change (rate of change) in a rotor can be described as
dT/dt = 1/(m*Cp)*(Qin-Qout) [deg C/s]
Qout can be seen as the energy leaving the rotor;
Qin can be seen as the energy put into the rotor.
m is the mass of the rotor
Cp is the heat capacity.
A simple model for the energy put into the rotor is
Qin = n*Ff*vf
where:
n is the amount of friction energy loss that is transferred to heat.
Ff is the friction force (or braking force) and
vf is the slip velocity
Further,
Ff=u*Fn
where
u is the pad friction coefficient and
Fn is the clamping force
This makes
Qin=n*u*Fn*vf
To maintain the stopping distance again and again the following is important:
The brakes system must be able to deliver a sufficient friction force and the friction between the tires and the surface must be sufficient.
At the track some of us has discovered that very often both the tires and the pads suffer by temperature issues. I will leave the tire-issue here, but discuss some of the brake related issues.
As is stated above Ff=u*Fn.
Fn is given by the brake system design. The friction coefficient is pad design specific and varies with temperature.
Here are some examples (can be found here:
http://www.apracing.com/info/index.a...Materials_2858 ):
Figure 1: Pad temp char 1 (as for PFC01(?)...)
Figure 2: Pad temp char 2
Figure 3: Pad temp char 3
My thinking is that this shows the importance of having the right brake bias and to select pads that has the intended temperature characteristics. Further, the brake system design must also be such that it can cope with the friction energy in a way to keep the temperature of the brake pads within the operating temperature envelope - which is where dT/dt comes into account.
For the interested reader a discussion of how to model the temperature in a rotor can be found here:
http://www.comsol.com/showroom/gallery/102/ 
5/96 318TI Sport BIG TURBO 
Linear Mode
