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.
