Succinct... that's a vegetable, right? As long as it doesn’t mean short.
At low vehicle deceleration rates, forward weight transfer is insignificant. Rear wheel loads under these near-static conditions allow the rear brakes to provide half of the braking effort (in a hypothetical vehicle with 50% rear vehicle weight.) As brake line pressure and vehicle deceleration rates increase, weight transfer loads the front tires and unloads the rear tires. Concurrently, the tractive force of the rear wheels decreases and the rear brake calipers exert more clamping force on the brake rotor. Under extreme conditions, the rear wheels can cease to rotate and the vehicle would lose directional stability. The vehicle speed retardation of the sliding rear tires is less than that of the rolling front tires, so in addition to losing the yaw control, the rear of the vehicle will tend to travel at a greater speed than the front. Consequently, the vehicle will hit the tree rear end first.
In order to improve vehicle control, OEMs have designed, and the FMVSS regulations also require that the front tires “lock” first. Sliding front tires do not have steering capability, but a slight reduction in brake pedal application may restore it. Typical drivers can recover from temporary loss of steering (“pumping the brakes”), but are far less likely to recover from severe yaw (“spin out”) induced by rear wheel “lock up.” Also, while the loss of steering leads to hitting the tree front end first, the vast majority of safety features are designed for frontal impact (seatbelts, crush zones, air bags, collapsible steering columns, cushioned dashboard, etc.)
OEMs therefore use a “proportioning valve” which is actually a pressure-limiting valve. At low brake line pressure (gradual deceleration) all wheels are equally loaded and no pressure adjustments are made. As brake pedal application is increased, line pressure is increased and at a specific pressure (the “break point”) further pedal application does not increase rear brake line pressure at the same rate as the front. This effect limits the capability of the rear brakes, but only under heavy brake applications. The break point and the allowed rise in rear brake line pressure had been determined empirically and varied by application; a specific model vehicle for consumer use may have one “proportioning valve” and the same vehicle in taxi service may have another version that allows more rear brake line pressure. The taxi service would tend to have rear occupants, increasing rear weight, thereby allowing more rear brake work.
An adjustable proportioning valve either adjusts the break point pressure with a fixed rate of rear brake line pressure rise or may have a fixed break point and vary the rate of rear brake line pressure increase.
The Gen 1/2 Viper brake system was designed with four piston calipers in the front, single piston calipers in the rear, and the federally mandated proportioning valve. Since the clamping force on the rotor is proportional to the piston area, the front brakes have far greater capacity than the rears. Adding the proportioning valve ensured that the vehicle would safely hit the tree front end first. At the last VCA-PVO autocross challenge I asked Herb Helwig about the rear brakes, the brake bias and a 38mm or 40mm piston design and the reply was “we met the government standards.”
Opinion: Brembo redesigned the rear caliper from its previous application on a Renault Alliance, the Dodge Monaco, and Eagle Premier to accommodate a thicker rotor on the Viper. Very similar designs (using the same casting configuration of single piston and incorporated parking brake) are on the Lotus Elise and the Opel Speedster. Too bad they didn’t also consider a larger piston, and if needed, an appropriate proportioning valve strategy. If the undersized brake can be fixed up, sure would have been nice to have it that way from the factory.
Bob Woodhouse conducted brake testing that included front and rear brake line pressure measurements. His data led many to conclude that the rear brake line pressure reduction was unnecessary, since even with the proportioning valve disabled (not “removed”) the front wheels locked first. There are many posts about the procedure and precautions. Examination by several also concluded that the front and rear systems are not hydraulically linked when the proportioning valve is disabled, so that a failure in one system does not affect the other.
With the OEM brake calipers still on the car, there is no benefit to replacing the OEM proportioning valve with an adjustable valve. All valves are pressure-reducing devices, and the rear brake capacity is undersized to start. A first step by many to improve braking has been to simply remove the proportioning valve cup seal to eliminate an undesirable reduction in rear brake effectiveness.
StopTech pioneered a Viper brake system that improved the front-to-rear brake balance but reducing the size of the pistons in the front calipers. The reduced front clamping force allows higher brake pedal application overall, resulting in more rear brake work. Mathematically, the 38mm rear caliper conversion matches the front-to-rear balance by increasing the rear rather than decreasing the fronts. A subtle further difference is that the StopTech configuration, with smaller pistons, reduces total clamping force while the 38mm configuration increases clamping force given the same amount of pedal pressure. For equal braking, a StopTech car would need slightly higher pedal pressure than a 38mm car.
Empirically, even with the proportioning valve disabled, 38mm rear calipers still allow front wheel lockup. (I suspect StopTech owners, even with a disabled proportioning valve, would also experience front wheel lockup.) The rear brake caliper casting allowed for an even larger piston size of 40mm. The braking performance is further improved, and with the proportioning valve disabled, the front wheels still lock first, although now only very slightly before the rears. With the 40mm rear calipers, braking under corner entry can provide an oversteer sensation. Straight line braking under ideal conditions and sticky race tires could provide sufficient forward weight transfer that the greater rear brake capacity overpowers rear wheel loads. In this state of brake “tune,” I advise that disabling the OEM valve or using an adjustable proportioning valve is a driver preference issue. Having said that, I know many are very satisfied with a disabled proportioning valve. This level of performance improvement is exceptionally satisfying, but we should all recognize that it is several iterations away from the manufacturer’s original design.
In summary, the 40mm rear caliper conversion (with the OEM front caliper) provides near-ideal front to rear brake balance and the use or not of the OEM proportioning valve is a driver preference/tuning feature.
A rear brake larger than a 40mm rear caliper certainly will require an adjustable proportioning valve; an example is the 2001-02 ABS system that uses the same front caliper and a 43mm rear caliper. In this car, the ABS dynamically adjusts brake line pressure to the “oversized” rear brakes to prevent rear wheel lockup. Larger than a 43mm single piston or going to multiple piston calipers is greatly increasing the overcapacity. (See the thread in Performance Modifications if you have input on the benefits of large rear calipers.)
If you’re still awake, did I answer the question?
