i have the 4 pot calipers on my car.( mazda calipers and vented disk ). they work well
.i have a extra set of 4 pot calipers. thinking about putting them on the rear.
..brake balance..hum, that the problem..
. my thought.. lets say the stock front brake pad is 48sq. cm. and the rears are 75% of that...i am thinking about putting the 4 pots on the rear..and trimming the friction part of the pad to 75% of org. sizr. leaving the metal plate the same size..
. is this a workable idea?
I would not do it if I where you.
One of the many design factors that goes into the development of a base braking system in OEM cars is the "bias" or "balance." It's a pretty simple concept to grasp: for vehicle stability under braking, it is required that the rear brakes do NOT lock before the front brakes. Simple, right? Most of you probably knew that already.
So what governs the 'lock up' point of the rear brakes? :
1. tire tractive capability (friction)
2. tire normal force (weight on the tire)
This can be proven from looking that the fundamental relationship for maximum sustainable tire force: F=µN, where:
F = the lock up point, or peak force
µ = tire-road coefficient of friction
N = normal load sitting on the tire
When the OEM is designing a brake system, they size the system components (calipers, master cylinder, rotor OD, etc.) to generate the proper amount of torque at both ends of the vehicle so that the front brake force ('F' above) exceeds its peak traction first. At this point, the front brakes lock and the car slides in a nice, stable straight line.
Fortunately (from a safety standpoint anyway), when most big-brake suppliers adapt a ginormous rotor and caliper package to a vehicle, they end up actually increasing the FRONT bias. How? By increasing the effective caliper piston area and the rotor effective radius, these two factors work together to increase the 'mechanical gain' of the front brakes, building more torque for the same pressure, everything else being equal. So, from a bias perspective we are not pushing the vehicle toward instability, but rather just the opposite - we are underbraking the rear axle! The obvious impact would be an increase in stopping distance - probably the one thing the new owner was actually hoping to reduce. Ironic. So, say you chose to install these big brakes on the front axle but want to maintain the OEM bias. What's the answer? Well, one way would be to invest in big rear brakes too which increase the rear mechanical gain to the point that the system is balanced once again.
Well, let's look at why we upgraded the front brakes in the first place. Contrary to popular belief, the real reason sports- and racing cars use big
brakes is to deal with heat. Period. There has been a bunch of stuff published which will disclaim this, but when you look at the braking system from a design standpoint, making them 'bigger' doesn't fundamentally do anything for stopping distance. It's all about the heat. So, you upgraded the front brakes because of thermal concerns but as a hidden surprise got a shift in brake bias. As a band-aid to this condition, you now spend thousands more on a rear brake upgrade because the front system was not sized correctly in the first place. Sure, it looks great, but there is another option.
When upgrading your front brakes, it is possible to size the caliper pistons and rotor effective radius to maintain the original brake system's pressure-torque relationship. Yea, it takes more engineering know-how and you can't sell the same part to everyone anymore, but you are not altering the base brake balance from what the OEM intended. Now, if you sized the front brakes correctly, why would you need to change the rear brakes? Good question. If there are no thermal concerns with the rear brakes (and on a front-engine street car there rarely are) then by installing a rear big-brake kit all you are doing is
(a) spending money and (b) adding unsprung weight. This is not usually viewed as favorable, unless you like driving a heavy car that won't perform anymore.
Under an OEM bias condition, the rear brakes only contribute only about 15-20% of all the braking force the vehicle generates, and when you install sticky tires you actually
DECREASE the amount of work they need to do. Why? Because at the higher deceleration levels afforded by race tires, there is more weight transfer taking place, reducing the normal force on the rear tires and increasing it on the front (remember F=µN from above?). If anything, we now want to decrease the rear effectiveness. Ironic once again.
Of course, if you decide to upsize your rear brake system components you can also impact the front-rear torque relationship, and consequently you can "bias" the "balance" more toward the rear. Go too far, and the rear brakes could lock before the fronts even with brake proportion valves. Again, not the end result you were expecting, right?
So I suggest that you don't bother with them. Spider's have pretty good braking out of the box. Buy better tires and pads to increase your braking capabilities. I understand your using the same rotor diameter but the facts still hold using a larger caliper.
I think the vented rotor in front and caliper should be plenty braking enough for that car. Especially for normal street driving.
I think your going over board with the rear.
Jason
