My 2001 GT has a shimmy while braking at highway speeds. It is felt in the steering wheel. This shimmy is not being caused by a warped rotor. Also... The tires are new and the pressure is perfect. I'm taking the car in to the dealer this Tuesday, but I figured I'd make a post and play "You Make The Diagnosis". I'm almost sure I know what it is, but I'll I wont say.
Anyone wanna take a guess at what it is ???
I'll post the dealer's findings when I get the car back Tuesday night.
1 of the tires. just cause there new don`t make them not be the problem. my 99 f-150 came out of the factor with a bad goodyear tire on it. around 65-70 mph the right front had a bad shake to it. come to find it had a flat spot on it. the dealer put on a new one and everything was fine. mp
Not a rotor(s) If you have warped rotors what you would feel is a hard regular knocking type of pulsation. And you'd feel it in the break pedal. Also.. The pulsation from a warped rotor tends to be noticeable at just about all speeds above say 20-30 MPH.
Now there's a missed assumption if I ever saw one... Rotors can suffer from warp or a parallelism problem... the affects can be un-noticeable at moderate speeds/braking effort and still be quite violent at higher speeds. Sometimes the affects are more noticeable in the pedal - sometimes in the steering wheel and sometimes in the feel of the car itself....
You are "almost sure" you know the concerns cause..... after almost 40 years of doing this, I have realized that "almost sure" isn't much better than "I dunno" - nothing is written in stone and vibration diagnosis with such little information is nearly impossible (and is usually coupled with a statement like "I forgot to add....)
In this case, there is NO sensation what so ever in the pedal. Not even a hint regardless of the amount of pressure applied to the pedal. Also... I did write that warped rotors "TEND" to be noticeable at above 20-30 mph. I didn't say that they all warped rotors characteristically were noticeable at speeds over 20-30 mph. I'm also saying that I don't think my shimmy problem is being caused by a warped rotor. If that turns out to be the case though, I'll surely own up to it.
No question about it. "Almost sure" of what the problem is has played itself out thousands upon thousands of times to the tune of all sorts of wasted time. Not to mention, all the perfectly good parts that were unessasrarly replaced ! I don't have 40 years of pro experience under my belt like you do. I have about four meager years pro experience. Add to that, it has been since 1991 that I repaired cars professionally. I worked my way through college working in a repair shop. Before that, I had auto shop in high school. My Jr. and Sr. years in high school, I built and raced a street rod. So.... My experience is modest and you make a good point, but I think I did give enough info so that anyone who wanted to could have fun and take a few educated guesses at what is going on.
At any rate... Do you have a likely culprit for the problem or are you refusing to take a stab at it due to the limited info I have given ??? I mean... What the hell ??? It couldent hurt to have a bit of fun...
First thing I'd do is grab the brake rotor caliper and the dial indicator.... Measure the radial runout of the rotors. Next, measure the thickness of the rotor in several places, looking for variations in both instances....
FWIW, some (of those techs that do take this sort of pride in their work) will measure in three or 4 places..... For the time it takes, I prefer to measure in line with each wheel stud both at the outer and inner perimeter of the rotor....
Other rotor concerns include poor machine finish (when discussing refinished rotors).... If the lathe is set to take too coarse of a cut, some pads can actually be forced along the resulting 'threads' in a "stick/slip" type of feel - may or may not be felt in the pedal and sometimes can be quite noisey. Some brake rotors can develop burnished areas that can give a surging feeling when stopping - also without giving a feel to the pedal...
Without the benefit of a test drive, I'm at a disadvantage but (from your initial description) rotors are the place to start.
When was the last time the car had a wheel alignment?
When i bought my 93LX 5.0 it had 3k miles on it. it braked very poorly. after i had the rotors turned, they told me that a shafts inside my diff was bent (one of them). they fixed it, but they thought it was from hitting a sidewalk. it contended it was a mfg defect.
Dropped the car off this morning and picked it up this evening. According to the service writer, they weren't sure what it was. The checked out the front end, balances and rotated the tires and re surfaced the rotors. Took the car back out for a drive and there was no shimmy. I haven't had a chance to drive the car on the highway yet, but I'll check it out tomorrow. The way the shimmy was felt in the wheel, I had the idea that there was a problem in the front end. Possibly looseness somewhere in a control arm. May be a bad busing.
Got a chance to drive the car today on the highway. No problem while braking. Given that all they did was cut all the rotors and rotate the tires, I'd have to say that it was the rotors. Though, I don't think there was a badly warped rotor. It was almost like there was one spot on the rotor that was OK, and one spot that was glazed over. That is why I felt a shimmy, but no pulsation in the brake pedal.
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