Saturday, November 22, 2008

Brake vibration

On a number of newer vehicles these days there is a concern regarding brake pulsation, there may be those of you who have experienced this problem, when new the vehicle may stop smoothly. It is later perhaps when you have driven a several thousand kilometers the brake pedal begins to pulsate during braking.

The brakes, for instance, may seem as though they are grabbing and releasing and to stop smoothly is impossible. The problem may have arisen following the removal of the wheels. This was virtually unheard of in the late 70’s and 80’s and you may ask why it happens now.

It is all to do with the weight. The idea is that by increasing the fuel efficiency in new cars, this will help to decrease manufacturing costs, so the accounts department or “bean counters” as they are sometimes referred to, tries to ensure that each part of a vehicle is reduced in weight as much as possible.

This may sound fine and for the majority of parts, it works well. But a brake system engineer may say that for brake rotors, heavier is better. The heavier the brake rotors the more heat they absorb fading less giving a better braking performance, are more stable and rarely warp as this is a cause of pulsating brakes.

Brake engineers may sometimes get their own way. Unfortunately, it is the accountants that win and brake rotors are built to weigh less. Then the problems may start. It is possible to make brake rotors that are light, stable and powerful.

On racing cars carbon fiber/ceramic rotors can be obtained and used, but for the average passenger vehicle the cost would be too high. The Porsche 911 Turbo model offers a higher performance brake option, and stops in an amazingly short distance with its stock brake system.

Many are stuck using standard cast brake rotors. But if these aren’t handled properly, they will warp and then it’s the start of pulsating brakes. When the rotor becomes warped, it wears from side to side unevenly, causing it to rub against the brake pads. Eventually, the sides of the rotor cannot remain parallel to each other any longer.

Tolerances for parallelism quite often are less than 0005 inch maximum or for example ¼ of a hair's thickness. If rotor sides aren't parallel, this will cause the pistons in the brake calipers to make the brake pads push in and out at a rapid pace. It is this fast motion being transmitted via the break system into the pedal that you feel as a pulsation.

Something as simple as changing a tire can cause rotor warpage. For instance, when changing the tire, if there is rust, dirt or corrosion in-between the wheel and the brake rotor when putting it back, it will be clamped unevenly and this can cause the rotor to warp.

When tightening wheel nuts, care has to be taken. If tightened incorrectly, this can also warp the rotor. It is far better when fitting a wheel to tighten the wheel nuts in two stages by doing an alternating in a criss-cross pattern. On modern vehicles a torque wrench is critical; there are some wheel shops that tighten the nuts using air impacts.

Torque sticks are used by many as these are designed to limit the torque on nuts. Neither one is sufficiently accurate for today’s modern cars. Try to make sure that a torque wrench is used. Any brake pulsations after the wheel has been changed may mean having to loosen the wheel nuts and re-torque. This is best done sooner rather than later and the rotor will normally correct itself, leaving it longer may result in it staying warped.

Brake rotor run-out can also be a cause of pulsing brakes. There are many manufacturers that allow a rotor and hub to wobble up to 003 inch due to machine tolerances and it will wobble too much, acting as though it has warped. There are cars that are sensitive to rotor run-out if more than 001 inch, which will cause brake pulsations will occur.

Probably it is better to correct a run-out to less than 001 inch. An answer to this is to machine rotors in their place whilse on the car rather than using the off-car brake lathes that most workshops use, this way run-out for both rotor and hub will be corrected. It is due to the cost of these machines that many repair shops unfortunately don’t have one.

It is better to take certain steps in trying to prevent or correct brake pulsations, mounting surfaces should be kept clean, if machining rotors make sure sides are parallel, rotor run-out should be kept to a minimum, to torque wheel nuts always maintain to use the proper procedure, hopefully brake pulsations will be in the past for your vehicle.

Find the best buy and sell autos, auto trader, and read the oldest to newest auto reviews at

No comments: