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AUTO TECH: Weight reductions lead to brake pulsations

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Heavier brake rotors can absorb more heat, so they fade less and give better braking performance.

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Brake pulsation has become a common concern on many newer vehicles. The vehicle stops smoothly when new but, after a few thousand kilometres, the brake pedal starts to pulsate when stopping, the brakes feel like they're grabbing and releasing, and smooth stops are impossible.

Sometimes the problem starts after the wheels have been removed. This was seldom a concern on vehicles in the '70s and '80s, so what has changed to make it a problem today?

The answer is weight. To increase fuel efficiency of new cars and reduce manufacturing costs, the accountants ensure that the weight of every part is reduced as much as possible.

This works fine for most parts, but ask a brake-system engineer about brake rotors and the answer will be that heavier is better. Heavier brake rotors can absorb more heat, so they fade less and give better braking performance. Heavier rotors also tend to be more stable, so they seldom warp, which is one of the causes of brake pulsation.

Sometimes the brake engineers get their way; other times the bean-counters win and the brake rotors are built lighter. That's when problems may occur. Sure, they could build brake rotors that are light, powerful and stable -- carbon fibre and ceramic rotors are available and used on racing cars but the cost would be prohibitive for most passenger vehicles.

Porsche does offer a high-performance brake option on the 911 Turbo model, which stops in amazingly short distances with the stock brake system. I can't imagine how much better the $5,000 brake option would be!

So most of us are stuck with conventional cast brake rotors which, if they're not handled correctly, can warp. That's the beginning of brake pulsation.

A warped rotor wears unevenly from side to side as it rubs against the brake pads, and soon the sides of the rotor are no longer parallel to each other. Tolerances for parallelism are often less than .0005 of an inch maximum, or about one-quarter the thickness of a hair. When the rotor sides are not parallel, the pistons in the brake calipers that apply the brake pads are pushed in and out rapidly. This rapid motion is transmitted through the brake system and into the pedal. We feel it as a pulsation.

Changing a tire is one common cause of rotor warpage. If there is any dirt, rust or corrosion between the wheel and the brake rotor, it will be clamped unevenly when the wheel is installed and the rotor will warp.

Incorrect tightening of the wheel nuts also can warp a rotor. When installing a wheel, snug up the wheel nuts and then tighten them in two stages using an alternating criss-cross pattern. Using a torque wrench is critical on modern vehicles. Some shops tighten wheel nuts with air impacts. Others use "torque sticks" designed to limit the torque on the nuts. Neither is accurate enough for today's cars. Make sure they use a torque wrench

If you experience brake pulsations after changing a wheel, loosen the wheel nuts and re-torque them. If this is done as soon as possible, the rotor will usually correct itself. Leave it too long (more than 1,000 km) and it remains warped.

Another cause of brake pulsation is brake rotor run-out. Many manufacturers allow the rotor and hub to wobble up to .003 of an inch because of machining tolerances. More than that and the rotor wobbles too much - it acts like it is warped. And some cars are especially sensitive to rotor run-out -- if it's more than .001 of an inch, then brake pulsations can occur.

The answer is to correct the run-out to less than .001 inch. One way to do this is to machine the rotors in place on the car instead of using off-car brake lathes, as most shops do. This will correct for run-out in both the rotor and the hub. The trouble with this is that on-car brake lathe machines tend to be more fragile and cost three to four times as much as an off-car brake lathe, so very few repair shops own one.

Another method of correcting rotor run-out has been introduced to the market. Brake Align has designed special tapered shims that are placed between a removable rotor and its hub to compensate for rotor run-out. Run-out is measured on the rotor and one of three different-sized shims is installed. Run-out can be corrected to within .001 of an inch.

Brake Align shims have been tested for hundreds of thousands of kilometres, and research by the company shows that if rotor run-out can be kept under .001 inch, then brake pulsations are no longer a problem.

Correcting brake pulsations, or preventing them, is just a matter of details. Keep mounting surfaces clean, machine rotors so the sides are parallel, reduce rotor run-out to a minimum, and use the proper procedure to torque wheel nuts. Brake pulsations should no longer be a problem on your vehicle.

Jim Kerr is an experienced mechanic, instructor of automotive technology, freelance journalist and member of the Automobile Journalists' Association of Canada.

kerr.jim@sasktel.net

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