Likely causes of vibration when braking?

I'm dealing with a 1994 Toyota 4Runner with the 3.slow engine and about

190k miles on the chassis.

Lately and increasingly, there is a *lot* of vibration when braking from over 40mph or so. I believe that this comes from the front end because I tried applying just the parking brake at speed. In that test, it stopped smoothly.

I lifted the front end and did a good old fashioned wiggle test, but found nothing noticeably loose. I also lightly applied the brakes while rotating each wheel by hand. The grab seemed smooth, but I'm not real sure if this would be a good test for a warped rotor.

What, in your collective, esteemed opinion would be the likely causes for a rather violent shaking of the front end when braking? BTW - the brake pads are still in good shape, although I didn't measure them, but there is plenty of meat left.

Reply to
TomO
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Warped front brake rotors. Have them machined if they are thick enough or replace them if they are too thin to machine.

Reply to
Mike

It could be a warped or varying thickness rotor, but it could also be loose or worn front end parts.

If it is due to an out of spec rotor, you will usually feel a characteristic shudder. If it is a varying thickness problem, you will feel it through the brake pedal. If it is a warped rotor, you will also feel it, but it may be somewhat different.

You can measure the runout and thickness variation of rotors and know for sure how badly they are off.

I have, however, also seen cars with worn ball joints or rod ends that will start buffeting when some braking stress is put on the car. You might want to be sure that the front suspension and steering parts are in good shape.

Reply to
HLS

It may be over-torqued lug nuts/bolts, especially if you have allow rims.

Reply to
Thomas Tornblom

I'll try and get a gauge that can show me the runout/thickness first. Is there something not-too-costly that's readily available?

Reply to
TomO

You use a micrometer which has a wide enough mouth so that you can take readings at several placed if you want to measure thickness variations. Runout can be measured with a dial indicator. The hobbyist mechanic is not likely to have both these.

If you take it to a professional mechanic, you have to be careful to find someone who will check it out, not just throw money at it.

While it is likely that rotor distortion or wear is the problem, I just wanted to make a point that other things can also cause similar shuddering.

There used to be a really good article on brake rotor issues, and if I can still find it I will post it here for you. Read it until you really understand what they are saying, and you will be well armed to discuss with a professional mechanic.

Other points...never never never let anyone put your wheels back on the car with an impact wrench. EVEN if they say that they are using a Torq Stick, dont allow it.

If the mechanic doesnt true the rotor properly, or replace them with new rotors which are properly prepared, this problem can recur.

Anyway, read the article (as soon as I find it)

Reply to
HLS

Here is that link, TomO

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Reply to
HLS

One new pair of rotors later, the problem is either fixed or masked for a while. I did make sure that the surfaces where the rotor bolts to the hub were good-n-clean and that the rotors seated fully against that hub mating surface.

I replaced both rotors because the parts were cheap enough. This also gave me the opportunity to repack the wheel bearings and all. The parts I removed ( pads, rotors ) were not terribly worn, but one of them must have been severely warped.

I guess that only time will at this point.

Reply to
TomO

"Google" lining transfer.

Ed

Reply to
C. E. White

???

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Reply to
Calab

OK, well maybe try "rotor lining transfer" -

"Lining transfer is indicated by a thin layer of lining material which has become welded to the rotor braking surface. Initially the lining deposits will be spotty; however, as the problem progresses the lining deposits will be covering more of the braking surface. Lining transfer will accelerate lining wear.

"This problem is the result of extremely high operating temperatures which are usually caused by dragging brakes, continued hard stops, brake system imbalance or brake system malfunction. The rotor can be resurfaced to remove the lining deposits and restore a proper braking surface. The rotor thickness, after resurfacing, must not be below the minimum thickness stamped on the rotor."

Reply to
C. E. White

There was no noticeable discoloration or any indication that I could see on either of the rotors that I removed. But obviously, there was at least warpage in the port side rotor.

I did not check for variations in thickness around the entire surface, but I did measure one point at .989 inches. The new rotor ( also measured at one point only ) measured .999 with my el-cheapo digital dial gauge.

Reply to
TomO

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