Frein et répartiteurs de freinnage

Brakes

14.5.1 My front brakes seem to be doing most of the work and wearing out twice as fast as my rears, can I adjust the front/rear proportions?

The answer is, yes, you can change the front-to-rear braking bias. Unless your name is Michael Andretti, you have probably heard that many race cars have adjustable brake bias systems, which their drivers use to tune the car's front-to-rear braking.

The 924, 924S, and the eight-valve, normally aspirated 944 was not equiped with a brake proportioning valve. Because of this, the front-to-rear proportion was fixed, determined by two things:

  • the relative piston diameter of the front and rear caliper's pistons,
  • the relative bore size within the tandem brake master cylinder for the front vs. rear circuits.

The caliper piston diameters can usually only be known by taking off the caliper and measuring. The front and rear piston bores within the brake master cylinder are usually stamped or cast into the master cylinder. With the 944S, Porsche introduced a 33/5 proportioning valve. For the 944 Turbo and 944S2, Porsche originally used an 18/5 proportioning valve, then later superseded it to the same 33/5 valve used on the 944S and some 928-series models.

Why is braking proportion important? You may remember from your high school physics courses that the frictive force generated by an object moving across a surface is directly proportional to the weight of the object, that is, the force with which it pushes down on the surface. So if you double the weight on a car's tire, it can grip the road twice a strongly.

For tires, the contact patch of rubber has a total limit to the force it can generate while still rolling. This force depends on the weight on the tire: the more weight pressing a tire down onto the road surface, the more force that tire can generate along the road surface. When this force is exceeded, the rubber loses grip with the road surface and the contact patch slides along the road surface instead of interlocking with it. The car's wheel stops turning and is said to have "locked."

Another important bit of physics takes place when you use your car's brakes: weight transfer. The harder you use your brakes, the more weight is transferred to the front wheels and tires, and the less weight is borne by the rears. This affects the braking force which can be generated: the less weight on the rear tires, the less ultimate braking force they can generate before locking up.

 

So, it turns out that as you brake harder, it's best that the rear wheels do proportionally less of the braking. Since it's hard to change caliper piston size on the fly, a proportioning valve does the trick by instead reducing line pressure in the rear circuits. The chart on the right shows how this would look ideally, rear pressure increase gradually rolling off as system pressure (pedal effort) increases.


 

A proportioning valve gives you an approximation to this ideal curve, using two straight-line segments. Up to the point of the change-over pressure, both the front and rear brake circuits receive full system pressure, i.e., the pressure within the brake master cylinder. Above the change-over pressure, only a fraction of additional system pressure is shared with the rear brake circuit.

There are two ways to affect the proportioning in your Porsche. By far the simplest, and perhaps safest, is to add or change the factory-style proportioning valve, which threads into the brake master cylinder's port for the rear circuit. The 33/5 designation on the valve currently used in 944-series cars is decoded as follows. The "33" refers to a 33 bar change-over pressure. One bar is 14.5 PSI, and so this valve starts reducing incremental pressure to the rear circuit when system pressure exceeds 478.5 PSI. The "5" indicates a reduction factor of 0.46, so that each additional bar of system pressure will add only 0.46 bar to the rear circuit's line pressure.

 

A second approach to modifying braking behavior is the installation of an adjustable proportioning valve. These are commonly used in professional race cars, whose drivers will fine tune the change-over point to cope with changes in outdoor temperature, tire compound, track surface, and wet/dry condition.

Before you rush off to implement any of these features, keep a few things in mind. The first is that it is perfectly normal for you car's front brakes to be doing most of the work. For this reason, you will always go through front brake pads and rotors faster than rears. The only way to change this is to reduce front-to-rear weight transfer under braking. And the only way to change weight transfer is to alter the car's center of gravity, its weight, or its wheelbase.

Another thing to keep in mind is the effect of locking up under braking. For a pure racecar, the ideal set-up is one such that the rears and fronts lock at the same pedal pressure. For a street car, and for most track enthusiasts, biasing the set-up in a way which causes the fronts to lock a little sooner that the rears is ideal. If your car locks its rears, it becomes extremely unstable in a hurry. Think of a pendulum being held with its weight above its axis. If you lock the fronts but keep the rear turning, you may flat-spot your tires and dampen your linens, but your car will remain controllable.

If you have the 18/5 proportioning valve on your car and switch to the 33/5 valve or an even more radical valve used on late-model 911-series cars, you should be aware it may cause rear brake lock up. If your car has ABS, some folks feel that the change is worthwhile and safe, especially for track usage. However, others have expressed the fear that locking up all four tires at once would cause the ABS system to think the vehicle is stopped. The p/n for the 5/33 valve is 928.355.305.02 and lists at around $69.05.

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