new OEM front rotors out of round?

[we350z]

How likely is this? I talked to my mechanic about my steering wheel jerk/shimmy issue after changing my rotors and seems to think that the front rotors may be out of round and causing this. He says this is not uncommon and that he sees it quite a bit. According to him many of the warehouses stack the rotor boxes on stop of each other and this can cause the rotors to become out of round. From various tests in my other thread I linked to I have determined that I do not have a stuck caliper, and I just did a brake bleeding which did not help either.

I ordered the OEM rotors from Crevier a couple months ago. They look fine when I spin them on the wheel but he said I wouldn't be able to tell by eye. They probably have <200mi on them total. The indy mechanic went on to say only the fronts would cause the steering wheel to shimmy/shake.

The only other thing he thinks it could be (and everyone in my previous thread) is if one or more of the control arm/thrust arm bushings are completely shot. I did the wheel shake test with the front lifted and both wheels seem very sturdy with very little play.

I suppose i could switch the rotors side to side to see if the jerk changes direction. Is this OK to do once the rotors have been mounted?

Will I be able to get a replacement set out of Crevier if they are defective?

[we350z]

Is it worthwhile taking them to a brake shop to see if they can confirm one or both of them are bent? Is it possible/worth it to true these rotors if they are out of round? My understanding is that they are pretty thin to begin with. I would imagine if one is bad they would both need to be shaved down to the same thickness, or I suppose i could just get one replacement rotor.

What a friggin' hassle...

[JCL]

Originally Posted by we350z

How likely is this? ....

Is this OK to do once the rotors have been mounted?

Will I be able to get a replacement set out of Crevier if they are defective?

Not likely. More likely that the rotors are not seated properly on the hubs (if you didn't clean the hubs when you installed them).

Not recommended.

No, because you can't likely show them to be defective.

Originally Posted by we350z

Is it worthwhile taking them to a brake shop to see if they can confirm one or both of them are bent? Is it possible/worth it to true these rotors if they are out of round? My understanding is that they are pretty thin to begin with. I would imagine if one is bad they would both need to be shaved down to the same thickness, or I suppose i could just get one replacement rotor.

If it is runout, you can measure it with a dial gauge. If it is out of balance, you can determine it with a dynamic wheel balance.

Yes, by turning them on a brake lathe. You are taking off so little that it isn't a worry.

No need to turn both, if you can isolate the problem.

Lots of questions. Have you considered taking it to a shop as recommended in your previous post which you reference above? You can spend a lot of time and money randomly replacing parts.

[Weasel]

If it is a quick jerk/bump felt when you first hit the brakes, it is most likely the thrust rod bushings. That is a hell of alot more common than warped rotors, and warped rotors would all but shake the steering wheel out of your hand at higher speeds, and be a constant shimmy under braking.

Drive the X at about 10 mph and repeatedly give a quick application/release of the brakes, that "bump" you feel is the bushings. Another way to get an idea if it is your bushings is to feel for the tell-tale bump/clunk when you come over curbs and parking bumps. They almost always feel sturdy in a shake test, but if you peek at them with a flashlight, you will see the rubber tearing at the center when they are worn.

But from the sound of it, it is most likely the bushings in question

[we350z]

Yes. I was actually calling to schedule an appointment and this is what the mechanic told me. He said he would just inspect the thrust arm bushings and if they were fine put new rotors on... That's silly these are brand new OEM rotors and based on the tests I've done the thrust arm bushings are fine.

I know this isn't rocket science. I don't remember exactly but I am pretty darn sure I cleaned the surface underneath the rotors before putting them on. Also I would imagine that probably would have been a problem with the old rotors anyway.

As for not being seated properly, I looked very carefully at the rotors with the wheels off when turning both sides and they appear to be seated perfectly. Though I suppose it could be off by a little and imperceptable to the naked eye. Furthermore, please explain to me how 5 bolts torqued to 100 ft lb./ea would not ensure a perfect seating.

I guess I just don't get it. I don't mind taking it to the shop but i do mind taking it in the a$$, especially when i already HAVE brand new OEM parts and they just want to slap another set of new rotors on.

At least someone at brake job place can tell me if the stupid rotors are true or not. When you say dynamic wheel balance to you mean like a hunter machine you would use on a rim? I know a guy who owns a really nice wheel shop that would prob throw them on the machine for me. I suppose i could just throw the bitch back up on stands and put a brick on the gas pedal and watch for wobbling on all 4 corners too.

Last question. If the wheel always jerks to the right on the same stretch of road. Is it worthwhile to swap the rotors side to side and see if the jerk changes to the left???

Thanks for all your help again, you seem to be pretty knowledgeable about all these technical details. I am just getting antsy about getting everything fixed.

Originally Posted by JCL

Not likely. More likely that the rotors are not seated properly on the hubs (if you didn't clean the hubs when you installed them).

Not recommended.

No, because you can't likely show them to be defective.



If it is runout, you can measure it with a dial gauge. If it is out of balance, you can determine it with a dynamic wheel balance.

Yes, by turning them on a brake lathe. You are taking off so little that it isn't a worry.

No need to turn both, if you can isolate the problem.

Lots of questions. Have you considered taking it to a shop as recommended in your previous post which you reference above? You can spend a lot of time and money randomly replacing parts.

[we350z]

It is a very quick jerk then it stops abruptly only when I hit the brakes with a fair amount of pressure >10-20mph or so. Hmm... I thought if the thrust arms passed the wheel shake test they were all good. Pass side does make slight squeaking sound when i go over speed bumps and the bushing does look a little old. There is a tiny crack near the center. If I take a good pic of it would you be able to tell? I have the pass side half axle out right now I just replaced the outer CV boot and about to put the assembly back on.

Totally feels like a bump...

I suppose it's easy enough to just replace the bushings and see if that fixes the problem? Do I have to buy new thrust arms or are the bushings seperate?

Originally Posted by weasel56

If it is a quick jerk/bump felt when you first hit the brakes, it is most likely the thrust rod bushings. That is a hell of alot more common than warped rotors, and warped rotors would all but shake the steering wheel out of your hand at higher speeds, and be a constant shimmy under braking.

Drive the X at about 10 mph and repeatedly give a quick application/release of the brakes, that "bump" you feel is the bushings. Another way to get an idea if it is your bushings is to feel for the tell-tale bump/clunk when you come over curbs and parking bumps. They almost always feel sturdy in a shake test, but if you peek at them with a flashlight, you will see the rubber tearing at the center when they are worn.

But from the sound of it, it is most likely the bushings in question

[Weasel]

You can get them separate from the arms, will probably cost you $30-$60, not fully sure on the price... and can be changed with a shop press, if you know anyone who will let you use theirs... or just do it for you for a decent price.

It is a safe bet to just go ahead and change them before you throw another set of expensive rotors on it... because like I said, if it is the rotors, it would be a CONSTANT shimmy the entire time you were braking, and would be directly connected to wheel speed. You would feel the difference from lets say 30 mph to 50 mph.

[we350z]

Thanks man, this makes the most sense to me. I was leaning towards a suspension issue after all the tests i've done anyway but what your describing convincingly sounds like my issue.

I'll order the bushings, pull the arms and pay somone to seat them. The way I see it is if it fixes the problem, great. If not they prob need replacing sometime soon anyway with almost 70K on the vehicle.

I am not getting a constant shimmy, in fact when I apply the brakes softly at any speed I don't get the jerk at all. Once the jerk is done it stops. I think what you are saying sounds right.

Thanks so much for the deductive reasoning. I'll post up a pic of my 8 year old bushings when i get a chance.

Originally Posted by weasel56

You can get them separate from the arms, will probably cost you $30-$60, not fully sure on the price... and can be changed with a shop press, if you know anyone who will let you use theirs... or just do it for you for a decent price.

It is a safe bet to just go ahead and change them before you throw another set of expensive rotors on it... because like I said, if it is the rotors, it would be a CONSTANT shimmy the entire time you were braking, and would be directly connected to wheel speed. You would feel the difference from lets say 30 mph to 50 mph.

[we350z]

Oh forgot to mention anything else I should change while I am in there? Tension rod bushing? Would my air hammer do the trick in lieu of a shop press?

[Weasel]

From the sound of it, the bushings are exactly what you need. Especially when you ease on the brakes not feeling it... confident enough that I'd put money on it!

Unfortuneately the air hammer is useless in this repair, and would end up damaging the bushings in the process. But it is an easy task with the shop press. I simply line it up and use the new bushing to press the old one out, gets it all in one shot! Just make sure whoever does the pressing lines up the bushing correctly... there is a small arrow on the rubber face of the bushing that lined yp with a metal dot on the arm, should be easy to spot. And once it's pressed in, there's no turning it without pressing it back out to do so!