What cause a Sable - Taurus to shake when stopping or slowing the car?

Answer 1

If vibration occurs while braking, then the brake rotors have a thickness variation and maybe in need of being 'turned' or "Blanchard Ground".

From StopTech:

The driver can feel a 0.0004" deposit or TV on the disc. 0.001" is annoying. More than that becomes a real pain. When deposit are present, by having isolated regions that are proud of the surface and running much hotter than their neighbors, cementite inevitably forms and the local wear characteristics change which results in ever increasing TV and roughness

An important note is that your brake rotors DO NOT WARP. They develop a thickness variation due to pad deposits on the rotor surface. This is caused by poor driving habits and improper brake application or by a poor or improper Break-in or bedding process.

Habits to avoid:

• Keeping the brakes applied after hard braking will cause pad deposition. Refrain from using the brakes after a hard application until the rotors have cooled sufficiently. (Usually a mile or two)

• "Stabbing" or quick hard brake application can cause hot spots on the rotor more likely to retain pad material.

This excess pad material (if caught in the early stages- because sometimes the above is not avoidable and vibration is starting to appear) may be easily removed with some garnet paper or with hard and even braking from 60MPH to 25MPH and allowing the brakes to cool. More stubborn deposits can possibly be removed with semi-metallic brake pads used hard (after bedding/break-in).

Allowing pad deposits to remain on the rotor surface for an extended period can cause the rotors to develop a material called "Cementite" this occurs due to the brake pad composition and the rotor composition. Every time the deposit comes in contact with the brake pad, the local area increases in temperature (much more than that of the rotor) this high heat (about 1200-1300°F; 649-704°C) transforms the cast iron under the pad deposit into an iron carbide called Cementite. Cementite is a very hard, very abrasive compound which is a poor heat conductor and causes further cementite formation. The more heat being generated (due to more pad deposits and wear from heavy brake usage) the worse the cementite gets (it grows and gets deeper into the rotor) and consequently the worse the vibrations will get. Cementite is invisible to the naked eye, as it is just simply a structural change in the metal's chemical composition.

Once cementite formation or a thickness variation has occurred, it's imperitive to "turn" (a process which the rotor is placed onto a special lathe which resurfaces the rotor's braking surface) or "Blanchard Grind" (a machine which counter rotates the rotor against a grinding stone to remove cementite) that axles rotors as soon as possible. (If only one rotor is affected, both rotors should be turned. Brakes must be done in axle pairs ONLY. The variation in thickness of the rotors, as well as the difference in stopping power can cause unpredictable handling.) Continued exposure to cementite may cause the entire rotor to have to be replaced due to the depth of the formation. Continued exposure to a thickness variation can cause premature pad failure and poor braking performance.

For do-it-yourselfers: Whenever the rotors are turned, you must first clean them with brake cleaner and a clean rag to remove excess metal shavings from the surface of the rotor. Even if the rotors look clean there are small miniscule shavings of metal that can cause damage to the pad and rotor surface, causing premature failure and result in poor break-in performance. Make sure to change the brake pads whenever you replace or turn the rotors. Remember that your rotor surface is a mirror image of the brake pad and it will cause premature braking system failure and poor braking (and break-in) performance. Ideally, if the pads do not have any thickness variation, the newly turned rotors should be bedded with the already broken-in pads. However, if the pads have thickness variations they should be replaced before bedding.

Properly break in the pads and rotors after replacement and turning.

To do so find a safe area (a long stretch of uncongested and not frequently traveled road that you will not impede traffic flow on) to accelerate to 60MPH and perform a series 10 increasingly hard brakes to approximately 10-5MPH without interruption or stopping AT ALL. During the process DO NOT STOP COMPLETELY as this will cause a pad imprint or non-uniform pad material transfer and result in a bad break in causing premature brake system failure and poor braking performance. (You will be redoing what you are trying to fix here!) Quicklyre-accelerate in between the stops to 60MPH. The object is to heat up the brakes slowly to high temperature to burn off the bonding resins in the brake pad to prevent brake fade and uneven deposits. As you stop you want to stop "with purpose." Essentially on the verge of activating the Anti-lock brake system of the car, without triggering it. As StopTech recommends (where the majority of the information in this writing comes from):

In terms of stop severity, an ABS active stop would typically be around 0.9 G's and above, depending on the vehicle. What you want to do is stop at a rate around 0.7 to 0.9 G's. That is a deceleration rate near but below lock up or ABS intervention. You should begin to smell pads at the 5th to 7th stop and the smell should diminish before the last stop. A powdery gray area will become visible on the edge of the pad (actually the edge of the friction material in contact with the disc - not the backing plate) where the paint and resins of the pad are burning off. When the gray area on the edges of the pads are about 1/8" deep, the pad is bedded.

After the 10th braking cycle you ideally do NOT want to touch the brakes at all until they have cooled down to ambient temperature. (Depending on your speed and the ambient air temperature, approximately 1-5 miles.)

The break-in process is two fold. It prepares the pads for use by removing the excess resins and paint on the surface of the pad, and "re-beds" the rotors for use.

For more information on "warped rotors" and other brake myths, visit the link in the "related links" below.