April 11, 2007

Brake system changes

It is clear from our last race that the Electric Imp has too much power and weight for our stock sized brake system. We can confirm this diagnoses by the simple test of getting the car up to speed and slamming on the brakes. The brakes do not have the ability to lock the race tires. We can not utilize the full grip of the tires to slow the car without a better braking system. We have tried same size racing brake pads and rotors but we have reached the conclusion that we need more pad to rotor friction area. This means bigger brake rotors and pads.
How much bigger?
All else being equal, the bigger the brakes, the quicker the car will stop (up to a point). Bigger brake systems however, mean more unsprung and rotating mass. This extra mass will require extra power to accelerate and more brake friction to decelerate. Race cars spend relatively little time braking and the rest of the time accelerating, so at some point, the benefits gained under braking could be negated by slower acceleration from the small weight (and rotating mass) increase.
Our goal is to have brakes that can quickly bring the race tires to lock-up and continue to operate without loss of performance for the duration of a 35 mile race.  
The stock rotor is solid and not very large.
stock caliper and rotor

The stock caliper uses a single hydraulic puck to push the inside brake pad into the rotor's inside surface. This pressure also pulls the outside brake pad against the outside face of the rotor.

stock caliper

We remove the caliper and rotor. We leave the ABS sensor ring for future DAQ upgrades. The white holes are where the stock calipers bolt on.

bare knuckle
We replace the stock caliper with a Performance Friction two piece floating rotor. It is substantially larger. This gives more surface area for friction and more leverage to stop rotation.
Performance Friction two piece floating rotor
The two piece rotor consists of an iron rotor and a light aluminum 'hat'. The hat and rotor are connected by pins that allows the rotor to move very slightly towards and away from the center line of the car. This should reduce drag by allowing the brake pads to push the rotor to the optimum position between the inside and outside pad, so that when the brakes are released, the pads do not drag on the rotor.
The Performance Friction Subaru Big Brake kit is larger then we think we will need and would require even larger diameter wheels. We calculate a diameter that will fit beneath a 15 inch wheel and still allow full surface area for the caliper / pad combination that we have chosen. We have the rotors turned down on a lathe to that diameter.
The larger than stock rotor requires moving the caliper outwards from the axle. We fabricate an aluminum extender bracket that bolts into the stock holes.
brake bracket
The bracket bolts to the knuckle.
extender bracket being fitted
We replace our stock calipers with Wildwood racing calipers. The Wildwood have 4 pucks. Two pressing the pad against the inside face of the rotor. Two pushing the pad against the outside face of the rotor. Two pucks help press each pad more evenly against the rotor. The caliper is aluminum and is lighter than the stock caliper.
Wildwood calipers in place
We decide on Performance Friction Race pads.
A quick road test reveals that we can now, at least lock up the street tires. Track testing will give us a more complete answer.


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April 18, 2007


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