BRAKE

BRAKE-2

THE BRAKE PEDAL

The brake pedal should be very strong-and so should its attachment to the chassis.

Typical brake pedal has a mechanical advantage of at least 3: I and it may be as much as 8: I.

The pedal arm must be plenty stout and it must be generously gusseted at the intersection of the bias bearing tube

PEDAL GEOMETRY AND ADJUSTMENT

Adjust the fore and aft position of the brake (and clutch) pedal to suit the driver's geometry and preference.

It is vital that the swing of the pedal be properly positioned on its arc. If the pedal is allowed to go over center as it is pushed, we will have an unfortunate situation where, the harder we push on the pedal, the less braking effort we get-and confusion is a certain result.

The system's mechanical advantage is determined by the mechanical advantage of the brake pedal itself and by the mean diameter of the discs. It IS normally not possible to in- crease the diameter of the front discs since they are inside the wheels. If it is possible, do so-:-there is no disadvantage. Rear disc diameter IS usually limited by the proximity of suspension members and is also a relative function of front disc diameter. The mechanical advantage of the pedal-at least on racing cars-is somewhat limited by package dimensions and by the fact that pedal travel gets excessive as the mechanical advantage is increased. It is normally from 3: I to 5: I

The hydraulic force ratio is determined by the relative area of the master cylinder bore and the total piston area of the calipers operated by that master cylinder.

There is a definite relationship between the amount of hydraulic pressure required to decelerate a vehicle at a given rate and the brake pad compound. The softer the compound, the higher its coefficient of friction and the less force required-and, things being what they are, the lower the temperature at which brake fade will occur.

THE DYNAMICS OF FRONT TO REAR BRAKING FORCE BALANCE

If we were to develop equal braking power on all four tires, then, even under straight line braking on a smooth road, under hard braking, the rear wheels would lock due to forward load transfer. This would rob the rear tires of their cornering power and any deviation from straight line running (side gust, road irregularity, uneven load transfer or whatever) would result in a very unstable vehicle due to total oversteer-unless the driver eased off the pedal, the car would spin.

 The harder the vehicle is braked, the more load is transferred from the rear tires to the fronts and, since the tires' tractive capability is a direct function of vertical load, the less braking torque the rear tires can accept without locking. If the rear tires lock while the fronts are still rolling, we must reduce the rate of deceleration or spin.

FRONT TO REAR BRAKE FORCE ADJUSTMENT

The basic front to rear brake effort proportioning is deter- mined by the ratio of the area of the front master cylinder bore to the total front caliper piston area compared to the same factors at the rear.

To provide more relative braking force at the front we can increase the front disc diameter, pad area and/or caliper piston area or we can decrease the front master cylinder bore diameter.

THE BIAS BAR

Works by moving the pivot point of the bar towards whichever master cylinder we want to put out more pressure. This changes the mechanical advantage of the bar and proportions more of the driver's foot pressure to the cylinder closest to the pivot and less to the cylinder which is further away.

To put more effort on the front brakes you move the pivot toward the front master cylinder.

The front to rear brake bias is further complicated by a few more items-the front tire diameter is probably smaller than the rear and so is its footprint area. The tread com_ pound and carcass construction may well be different, the front wheels are being steered and, if wings are installed, we will have more download at the rear. We should be aware of these factors, but since we can't do anything about them we need not worry about them. We merely tune around them.

BRAKE PADS

The pads (or shoes) have three requirements: they must stop the car controllably and without fade; they must last long enough to do the job and it helps the budget if they don't chew up the discs.

The coefficient of friction between the pad material and II the disc is a function of operating temperature. Normally the coefficient rises pretty steeply until the threshold of the design operating temperature range is reached.