Brake shake with freshly machined rotors?

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Hi all,
got a bit of a puzzlement. Ever since I've had it, my '08 Impala has had a pronounced steering shimmy under braking - the harder you brake,
the more the wheel shakes back and forth.
I finally took it in for its first service in my care today, and asked them to balance and rotate the tires and also to turn the front rotors. I ASSumed that this would take care of the issue (the tire balance was because I was also having a shake at highway speed, not to address this problem) but it did not - it's still doing it, and almost as badly as it was before. The rotors show clear signs of having recently been machined; there's obviously marks in the iron that are not circumferential (I'm guessing they must have dressed it with a sanding disc or similar while spinning on the brake lathe after turning) so it doesn't appear to be a halfassed job on the part of the shop.
I'd suspected that the cause of this issue was because the car sat for several months unused before I started driving it, so I just ASSumed that the brake shake was due to rust buildup on the rotors (except of course where the pads had sat, causing the rotors to wear unevenly when the car was put back in service.) Of course, I suspect that also the rotors are undersized for the weight of the car, but that's typical, and if it were simply warping, wouldn't it take a while to reappear after machining?
Any ideas as to why this could be?
nate
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Maybe you have a shimmy that is unrelated to the actual braking system. Checked the tie rods? All the bushings? Get under there and start tugging on stuff and seeing if you can feel any play in anything in the front end. --scott
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Scott Dorsey wrote:

Car has only 21K miles on it (about 4K of which are mine) it'd be pretty pathetic if it had worn out suspension components already.
I didn't mention that it had a slow leak when I first got it and I'd noticed that the steering wheel was slightly off center, so when I left it to have the tire plugged I told them to check/correct the alignment. One would think that that would involve doing all that you suggest.
nate
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Nate Nagel wrote:

My guess would be a bad tire. Did they rotate them? It could also be a sticking caliper, but then you would probably feel it in the brake pedal also.
-jim

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jim wrote:

Yes, the tires were supposedly both balanced and rotated, and I know the tires are bad. They're Goodyear Integrity, the same crap tires that were on my last car. Unfortunately there's not much I can do about that unless I want to pay for new ones out of my pocket.
nate
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But don't write off tires.
Twice now I have been hjacked around because they were in warranty and they didn't want to exchange them.
One set was OEM factory on a new vehicle. Just lived with the nuances until we replaced them at 55,000 mi. or so. The new tires of a differnet brand solved a lot of problems in stearing, braking and steering. I let the dealer convince me it was "normal". Wish I hadn't, because I became quite irrate after changing the tires to find out it was tires all along.
Another time we had similar issues but also could not get a good alignment. It was pretty bad, 4 allignments, 2 brake checks later I said if you do not warranty them, I will buy a different brand elsewhere and have these sent to the safety government people. I had a new set on ASAP and the problems went away.
All too often it is the tires and they try to putz with the alignment or something else to compensate.
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Nate Nagel wrote:

Me, I would buy a generic set of Coopers for the car, keep the "badyears" for the turn in or just try to hit every piece of debris you can find and pop em to get them replaced.
IMHO, cheap Coopers kill a Goodyear integrity, Damn that is a shitty tire. Don't look to BFG for anything better and a Michelin... Only if you want 80k miles out of a shitty riding, poor handling tire.
Falken, Nokian, Coopers and Continental. Cheap tires that work quite well in standard sizes. Falken has a bad rap due to the huge (think 22") tires that are mounted on after market rims that won't balance, ever. Their standard sizes are pretty good tires. Al
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I bought Kumho Platinums last time, based on reviews on Tire Rack. So far, they are working really well. They have a high speed rating of over 160 mph, treadlife expectation of 70K miles. And are cheaper than the OEM Michelins (which had treadware number of 440).
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| Hi all, | | got a bit of a puzzlement. Ever since I've had it, my '08 Impala has | had a pronounced steering shimmy under braking - the harder you brake, | the more the wheel shakes back and forth. | | I finally took it in for its first service in my care today, and asked | them to balance and rotate the tires and also to turn the front rotors. | I ASSumed that this would take care of the issue (the tire balance was | because I was also having a shake at highway speed, not to address this | problem) but it did not - it's still doing it, and almost as badly as it | was before. The rotors show clear signs of having recently been | machined; there's obviously marks in the iron that are not | circumferential (I'm guessing they must have dressed it with a sanding | disc or similar while spinning on the brake lathe after turning) so it | doesn't appear to be a halfassed job on the part of the shop. | | I'd suspected that the cause of this issue was because the car sat for | several months unused before I started driving it, so I just ASSumed | that the brake shake was due to rust buildup on the rotors (except of | course where the pads had sat, causing the rotors to wear unevenly when | the car was put back in service.) Of course, I suspect that also the | rotors are undersized for the weight of the car, but that's typical, and | if it were simply warping, wouldn't it take a while to reappear after | machining? | | Any ideas as to why this could be? | | nate | | -- | replace "roosters" with "cox" to reply. | http://members.cox.net/njnagel
I have a 2000 Safari that has the same problem. It was okay for a short time after a complete brake job. When it came back I called my mechanic and he asked if I had had the tires rotated. Yep, just had that done. He said he was running into a lot of this problem with different brands of cars, vans, etc. He said that the rotors are warping when the wheels are torqued during the rotation process. I pointed out that the tire shop I use, uses a torque limiting air ratchet and that should not be a problem. He agreed but said that was a common denominator to all of the brake pulsating/ shimmy problems he has seen.
FWIW, If I intentionally brake heavy several times the pulsing problem gets better and sometimes disappears. The rotors were still in specs after being turned, but I think you may be right in thinking that they are under engineered.
You didn't say how many miles is on the car, but the suspension is the only thing I can think of that would cause the the highway shimmy. More specific, the front shocks or struts, which ever it has.
I had a '93 S-10 that had bad bushings in the front A frame. It caused the truck to veer to one side or the other under hard braking. There was also a loud clunking sound when braking some times. It also exibited some weird symptoms going down the highway. Sometimes, but not always, the truck would get the shimmys after hitting a pot hole or going over a bump. That all ws corrected when the bushins were replaced.
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Anyolmouse wrote:

good point. maybe I will loosen/retorque lug nuts myself and see if that helps.
nate
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It's worth a try, but I doubt it will help..
As said before, tires are a possibility, bad machine work is another, and tire/rotor reinstallation (on some models) is a third.
I had some rotors cut once and when I remounted them on the GM product, you could SEE the wobble. The machinist didnt set up the lathe correctly.
If you dont still have the Coxco (?) link, let me know and I will find it for you.
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wrote:

I have done that on my explorer and it did help.
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I pointed out that the tire shop I

Those things dont always work they way they should. I have had two sets of rotors warped because of them.
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wrote in message

It is very true that they don't always work as advertised. That said - it is important to not that the precise torque that one would achieve with a torque wrench is not necessary either. Yes - over torquing can warp a rotor, but... and it's a bit butt, you have to overtorque a lot to incur this problem. Allow wheels, or not - you have to go a long way to incur this problem. Torque is often the first thing people speak of but it is usually not the real culprit. Cheap rotors are a very common culprit. Cheap pads are another. Switching to ceramics makes a huge difference in heat dissipation, therefore the problems. So - it may be warpage, but that warpage is more likely to be component related than it is to be torque related.
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It happens, I assure you, and have seen, and been, a victim of it. None of the local tire shops here even have a torque wrench. When I have to use them, I carry mine along and torque the studs myself.
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wrote in message

What kind of vehicle? Never seen it happen. I'd like to know how. I've seen wheels with stud broken off. No problem. Now that's difference in torque.
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Ive seen it happen on lots of different types of vehicles. To name a few I have seen recently, Chevrolet truck, Chevrolet Impala, Toyota SUV, Dodge full size van, etc etc.
None of these cars had any judder until locals fitted new tires or rotated tires, and hammered on the lug nuts with an impact wrench. The judder came on days to a few weeks after the work, and were cured by machining the wobble out of them. I think heating of the discs which are unevenly torqued down induces the distortion.
Whatever happens it is real, and it is avoidable.
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wrote in message

I'm going to call you on that. Take a look at your rotor and hub. That hub is SOLID. It has to be since it houses the bearings. Now the hub goes over top followed by the wheel. Against a machined surface of the hub. Even with two opposing bolts the rotor has to run true to the hub. How can it possibly warp a rotor?
I'll keep doing all my own tire repairs and machining rotors. I've never bothered with a torque wrench. My impact would never break a stud and I have a pretty good idea what torque I'm getting if I do them by hand. I just finished correcting a poorly machined rotor on my dad's car that was done by a local shop. It was out by several thousands on the diameter. I have my own lathe. Set it up with dial gauge on inside and machined the other to match. Back on the Lumina, test drive and vibration gone.
The reason I understand that shops use a torque wrench is they don't want a broken stud and it removes the guess work out. Any jockey can do the job then and the shop isn't libel
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There are internal stresses in rotors, and rotors themselves are of different designs. The ones I've seen are hollow between the sides for cooling/weight purposes. I can imagine that uneven stress on the center can affect the outlying metal. Too many people report bad torquing causes rotor distortion for me not to believe it happens. Like you I don't bother torquing when I change tires. Never have. And the only time I had a warped rotor was right after I ran hot brakes through a deep puddle. I think some rotors aren't affected by lugnut torquing, and others are. Best not to find out which are which.. But I'll probably continue to roll the dice if the rotors are cheap.
--Vic
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