In the process of developing the Wilhelm Raceworks big brake kit, I have performed numerous calculations of the brake bias resulting from various combinations of rotors, calipers, proportioning valves, pad friction values, etc. In the interest of better explaining why I make certain recommendations, the results of some of those calculations are outlined below. All of the calculations on this page were done assuming a 3000lb MR2 with 42/58 weight distribution and a cg height of 18". Unless otherwise stated, .4µ (coefficient of friction) brake pads are assumed. This is a common friction level for high performance street pads. OEM / OEM replacement pads are usually about .3µ, and "race" pads are often .5µ and up.
The graph below shows the theoretically optimal brake bias for a MKII MR2 at various deceleration rates, along with the bias for an OEM 93+ Turbo brake system. Points above the "optimal" line indicate bias that is too far forward, points below it are too far rearward. The OEM bias curves are based .3µ (coefficient of friction) pads I also plotted the effect of switching to higher friction .4µ brake pads. Since they generate more brake torque for a given brake system pressure, the proportioning valve "engages" later, causing the brake bias to shift rearward.
This graph shows the bias curve for my kit using the Wilwood FSL calipers with 1.38" front and 1.25" rear piston sizes and .4µ pads and the 12.19" rotors. Note that with the NA proportioning valve the bias is very good, reaching optimal at about 1g deceleration while not being excessively forward at lower deceleration rates. However, the turbo proportioning valve with it's much higher knee point causes the bias to be much further rearward, about 5% to far rearward at 1g.
For cars with the OEM turbo proportioning valve it becomes necessary to change the calipers to move the bias more forward. Switching the rear calipers to the 1.12" piston model produces optimal bias at 1g deceleration, however as the graph below shows the bias is further forward than optimal at lower deceleration rates. Swapping to the NA proportioning valve and using the 1.38/1.25 calipers will provide better braking under most conditions, especially low grip situations such as wet roads.
Bottom line, there is no "one size fits all" brake setup for the MR2. Due to the differences between the NA and Turbo proportioning valves, different calipers are optimal for each.
Street cars on street tires are unlikely to significantly exceed 1g under braking. Sticky race tires are another story, and so it becomes necessary to optimize brake bias for deceleration rates above 1g. Also, most race pads have much higher friction levels than street pads, delaying the effects of the proportioning valve and shifting the bias rearward. Additionally, weight reduction will cause the optimal bias to shift forward slightly.
The graph below shows the effect on brake bias of switching from a .4µ street pad to a .55µ race pad, for the 1.38/1.25 calipers with the NA proportioning valve.
As you can see, this setup has the potential to cause rear lock-up issues under hard braking. To address this you could adjust caliper piston sizes. The graph below shows the combination of a 1.38" front / 1.12" rear piston.
As you can see this will sacrifice braking at lower decel rates in favor of optimizing it at a higher rate. For those looking for a hassle free "safe" setup this combination will work. However, the better solution is an adjustable proportioning valve with a steeper slope than the OEM valves. The graph below shows the effect of replacing the NA valve with Wilwood adjustable valve with a .37 slope and the knee point set to 400psi. This produces a brake bias that is nearly optimal from .6g to 1.2g.
Another thing to keep in mind is the effects of temperature on pad friction. Nearly all pads have friction coefficients that vary with temperature (some more than others), and it is unlikely that the front and rear brakes will operate at exactly the same temperature. This can have significant effects on brake bias.
That said, I run the 1.38/1.25 piston calipers on my car with Porterfield R4 pads (advertised friction level of .55µ). So far I have had zero issues with rear brake lockup, at deceleration rates at and slightly over 1g, even though the data above would suggest that this setup has about 3% too much rear bias. This leads me to believe that the optimal braking curves presented here are somewhat conservative in nature.
If you wish to further explore the topic or to calculate brake bias for alternate setups download the spreadsheet below. The input data is pre-filled with values typical of a modified MR2 with my brake kit.
For reference, data on various proportioning valves. OEM data is from the MR2 factory service manual. Data on the Wilwood adjustable proportioning valve is from my testing described in my Brake Proportioning Valve blog post. Please note that this data is based on testing ONE valve, so there is no way of knowing how consistent the Wilwood valves are. But, it should be a good starting point.
|Stock Proportioning Valves|
|Valve||Slope||Knee Point (psi)|
|Wilwood Proportioning Valve|
|# Turns||Knee Point||Slope|