924/944/968 Frequently Asked Questions

Brake Pad Wear

Getting your car's brake pads to wear evenly is worthwhile. It both increases the longevity of the pad, and improves the "pedal feel:" the sense of firmness and response you get when you use your brakes.

Why pads wear unevenly

Disc brake systems operate by turning the kinetic energy of a car in motion into heat. Brake pads are pressed against iron rotors attached to the car's wheels. There is friction between the pads and the rotors, and this friction causes the car to slow as its momentum is dissipated as heat in the pads and rotors. Over time, this friction causes brake pads to wear. When enough of them has worn away, they need to be replaced.

Why would pads wear unevenly? The areas of a brake pad called upon to do more work, or which run hotter, will wear away faster. Although pressing a pad against a rotor may seem to distribute the load fairly evenly, it does not.

Uneven wear occurs in roughly three different forms:

The leading edge of a brake pad is the one that may be thought of as first touching the rotor as it turns. If your calipers are mounted behind the wheel axis, the bottom pad edges will lead. The friction of the pad against the turning rotor causes the pad itself to try to rotate about an imaginary axis behind it, with the effect that the leading edge of the pad presses harder against the rotor, doing more work and getting hotter. This uneven heating leads to longitudinal wear: uneven pad depth from leading to trailing edges.

Radial taper wear can be seen as a difference in pad depth along the radius of the rotor. A brake caliper is mounted to its strut on one edge. The forces created by applying the brake pedal cause yet another twisting moment, this time of the caliper assembly around an axis through its mounting points. There are other factors as well: a square-ish pad against a round rotor means that the radially inner part of the rotor will spend proportionally more time under the pad, and less in cooling air. Heat differences also exist because the inner edge of the rotor interfaces with the hub rather than air, limiting conduction.

Differences in wear between the inner and outer pads arise for two different reasons. The first is a car's tendency to "toe out" under breaking, that is, for the front half of a tire to move outward, the rear half inward. Because of this tendency, we compensate by giving cars a little toe-in in their alignment. This toeing places a little more load on the inner pad in a caliper mounted behind the axle.

The second reason for inner/outer wear differences has to do with floating caliper designs, such as used in the 944 and 944S. If the floating half of the caliper does not move absolutely freely, the fixed half's pad does a little more work. By the way, this is the reason 944's with a single pad wear sensor use it in the inside pad.

What happens when they do

Uneven wear from inner to outer pad will not itself have any effect on braking performance on the street -- unless you run out of inner pad altogether! Tapered wear, the other two kinds, will make it necessary for the pads to move greater distances when the brakes are used, meaning more brake fluid transfer, meaning longer pedal travel. This pedal travel by itself degrades the "feel" of the brakes, and adds to the time it takes to make them effective (response time). When combined with other sources of pedal fade, such as boiled fluid or glazed pads, you can find yourself running out of brakes.

Tapered pads increase pedal travel for two reasons. Pushing a longitudinally or radially tapered brake pad against its rotor, a brake piston will "cock," that is, move off-centre in its bore in the caliper. Cocking creates a greater volume for fluid in the caliper, which causes a spongy pedal.

The other source is "knock off." When the driver releases the brake pedal, the radial and longitudinal twisting moments disappear and the caliper falls back into nominal alignment. However, because the pad is worn at an angle, the rotor will push the pad and so its piston back farther into the caliper. Because of this, the pad will have to travel more to get to the rotor next time the brakes are used, meaning more fluid transfer, meaning more pedal travel. In fact, knock off can be a problem even with flat pads because of all the knocking about the unsprung parts of the suspension take on imperfect road surfaces. It is exacerbated by tapered pads.

How to correct for uneven wear

Uneven pad wear was taken into consideration by the original design of your car, and it should also be addressed in your maintenance. Looking first at the car's design, there is the need to apply slightly different pressure to the brake pads longitudinally and radially. The possible solutions depend on whether the car employs a fixed caliper with multiple pistons on each side (e.g., 944 Turbo, 944S2, 968), a fixed caliper with a single piston on either side (914/4), or, in the case of a floating caliper design, a single piston (924S, 944, 944S).

With multiple pistons, wear can be tuned by staggering their bores' location in the caliper. It can also be tuned by using slightly differently sized pistons. For example, the 944 Turbo and the 944S2 use fixed caliper, four-piston brakes on all four corners. The '85 944 Turbo front caliper holds a 36 mm piston on its leading end, and a larger 38 mm piston on the trailing end. The rears use 28 and 30 mm pistons.

On the other hand, the 944 and 944S use a floating caliper with a single piston. Differential pad pressure is created by removing some of the lip of the piston, so that not all the piston's circumference presses against the pad. The trailing-edge brake pistons in the '85 944 Turbo also share this "piston offset" feature.

Now let's look at what needs to be taken care of as maintenance. Foremost is basic mechanical soundness of your car. Make sure your alignment is right, and repair or replace any loose or worn ball joints, tie rods, bushings, wheel bearings, and other suspension hardware. Your brake fluid should be fresh and bled free of air.

If you have brake pistons with the offset design, you need to make sure that each piston is correctly aligned in its bore. Because of manufacturing tolerances and seal wear, pistons can tend to slowly turn over time. I recommend checking them every time you replace your pads. The correct position for the missing lip varies across models, and even from front to rear. For example, in the 930/911 Turbo, the four-piston offsets should point away from one another on one axle, and towards one another on the other axle.

The accompanying figure shows a simple tool you can make to check the alignment of the piston in a 914/4, 944, or 944S caliper. Trace it onto thin cardboard and cut it out. There should be a twenty degree angle between the caliper edge along which the top (914/4) or bottom (944, 944S) of the brake pad slides, and a line joining end-points of the missing part of the piston lip. With the pad out, you can slide the cardboard template into the caliper and against the piston to check it.

This tool is also available from suppliers such as Baum, as their tool 000-3523.

If you find that a piston has turned, you can carefully rotate it back into position. The steps are as follows:

  1. Thoroughly clean the caliper area so that you can see what you're doing. This should be normal practice when replacing pads. Using a towel moistened with rubbing alcohol, clean the piston lip and the rubber boot or seal between the piston and caliper.
  2. You can use a pair of slip joint (aka channel lock) pliers to turn the piston. By lightly depressing the brake pedal, extend the piston enough that you can get a grip on it. Be very careful not to overextend the piston, i.e., any more than it would be with minimal pad depth. A small block of wood of the right depth, inserted temporarily in place of the pad, is a good way to limit travel while you press the brake pedal.
  3. Put a strip of duct tape on each jaw of your slip joint pliers. You want to avoid scoring or bending the brake piston or damaging its boot.
  4. Gripping the end of the piston, use the pliers to gently, slowly turn it into alignment.

There is a very expensive tool available from Kukko, 126-03, which safely grips the piston from its inside. If you have that much cash burning a hole in your pocket. Their "reset tool," 126-00, also does a nice job of safely pressing the pistons back into the caliper.

When servicing floating caliper brakes, you should clean the caliper pair's sliding surfaces and lubricate them with a heavy grease or anti-seize, such as Lubriplate. This reduces work differences from side-to-side and so inner vs. outer wear differences.

One thing that affects floating caliper brakes greatly is the adjustment of the wheel bearing. On the early 924, the bearings wore quickly and gave the caliper a bell shape wear pattern because the rotor moved and pushed the caliper with it. This does not occur very often with the later 924 five-bolt disc brakes as the hub is aluminum and uses a much higher quality bearing.

I'll conclude by saying that taper wear has made a difference to me. At a track event way back when, I had more pedal travel than I was used to. I thoroughly bled all my brakes, but that didn't solve the problem. When I got home and replaced the pads, I found the fronts were tapered and two pistons had turned. With re-aligned pistons and fresh pads, my pedal feel returned to normal.

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