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Shimmy when braking - Not warped rotors
I've been dealing with this shimmy for some time and have done lots of work on the brake system to try and resolve. I'm convinced it is not warped rotors because I've gotten new rotors and have measured the run-out several times and confirmed the run-out on the front rotors are within spec. The steering wheel shakes like crazy when braking at high and medium speed -doesn't shake as much at low speed braking. I've checked everything in the suspension area to confirm nothing is loose.
Any ideas of what else I could check? My only other ideas would be to replace the pads although I'm not sure how they could cause this issue. Thanks |
#2
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Do you feel the shimmy at the steering wheel only or you feel a pedal vibration as well? if you do feel it at the pedal, then you should check the rear brakes too, otherwise you may have bad wheel bearings.
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#3
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Any little bit of anything on the wheels or hubs can cause runout. Wheel torque must be perfect and must be done in star pattern. Remember the reason the use 'on car brake lathes' is that there may be runout in the hub, bearing and rotor. Sometimes with a runout gauge you miss a problem by not testing at multiple points across the rotor face. Overall, what tends to happen is ANY runout at all makes the brake friction material deposit unevenly on the rotors. Uneven deposits are so thin that you can sometimes see them (color), but you cannot even spot them with a runout gauge. But to the pad it's a big friction difference which feels like a shimmy. See attached photo: that rotor gave MASSIVE shimmy when hot, but the runout gauge shows it's not warped. It's the friction material deposits that cause the shimmy I have a theory on these cars, and that is the huge wheel/tire package is hard on the wheel bearings, and that the brakes run hot, which also does not help the bearings any. The other issue with lots of cars is that a brake imbalance (remember the system is split diagonally) can cause one or the other front (or rear) brakes to overheat, and then you glaze a pad, and it's all downhill from there. Getting the brakes 'just right' on the Maxima is a real art. I've tried several different rotor/pad combos and have spent lots of time with the runout gauge, micrometer, digital temp gauge, etc. I am running motul 5.1 fluid with Earl's SoloBleeds and this weekend am installing goodrich stainless lines to go with my Akebono ProAct ceramic pads I did last month (with my new Centric rotors). I plan to rebuild the front calipers when I swap brake lines. Anybody wants a free set of cooked and warped EBC UltiMax Slotted Rotors, they're yours. I'll throw in the glazed EBC greenstuff kevlar pads as well. Noisiest brakes I ever had. Last edited by Nissan2007; 04-12-2012 at 05:41 PM. |
#4
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Assuming that the vibration is in the front, it could be some worn front end part(s) eg: lower ball joint, tie end rod. If the bearings are worn, you should hear the noise all the time.
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#5
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Thanks |
#6
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Good inputs. sounds like you've had lots of history of issues and have worked even more than I have to resolve. I've spent a lot of time on the front brakes trying to get them perfect but like you say - it must be a real art form. I've never had another vehicle seemingly this sensitive on the brakes. I've tested runout across the rotor (front and back) and the hub itself. I've indexed the rotors and cleaned all mating surfaces multiple times. I've checked suspension parts for looseness, etc., etc., etc. I'm going with the theory that it is either pads, uneven friction material, rear brakes or wheel bearings. I'll probably attack it in that order as time permits. Thanks for the inputs. Last edited by Longhorn101; 04-14-2012 at 05:51 PM. |
#7
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i had an hour to check some stuff yesterday so I checked the runout on the rear brakes. It was virtually perfect. No play in the rear bearings either so I'm crossing off the rear brakes.
I jacked up of the front and checked both bearings for play again and found none. So that pretty much leaves just front pads or uneven friction build-up or some other loose suspension component. I've checked most or all suspension points for play and found none. Pretty much back to were i've been the entire time with these brakes.... |
#8
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It's funny, I expected to see all sorts of runout issues on my maxima, and everything is very true, like within a thousandth... If I had to bet on how to get rid of your shimmy, I would put the car on an on-car brake lathe and give all four rotors a light turning. Never assume that a new rotor is perfect; sometimes I've heard that they need to give a light turn even to a new rotor. My understanding is that normally, if the rotor is true, then the friction material SHOULD deposit evenly on the rotor, and then no shimmy. The brake bed-in process does two things, cooks the glue-vapors from the pads and puts an even layer of friction material on the rotors. If you block-sand (or flex hone) the rotor faces, you essentially 'un-bed in' your brakes, since you scrub off the friction material. Also, if the pads are glazed, you can block sand them to de-glaze them, as long as they are not really cooked. Glazed pads are not too common, but if you feel increased stopping distance after a panic stop, thats glazed. For a non-directional finish, after turning a shop should use a brake rotor hone. This tool mounts in a drill and is pressed against the surface of the rotor while it is turning on the brake lathe. It is designed to smooth out the peaks and valley. You can do the same thing with a sanding blocks with 180-200 grit paper. (that's what I do). You can also buy your own flex-hone to chuck into a drill, but you would have to have a lathe to get it even. There is a brake supply place called 'GoodTool' that markets some amazing stuff, there is this stuff called PasteLub that is a hi-temp (2400 F) grease for pad backs and caliper sliders, and they make a 'break in spray' called 'the silencer' that you paint your rotor faces with; it helps the pads to bed-in faster. PasteLub Link: GOODSON : Brake Work : BPL-2400 Pastelub™ Silencer Spray Link: Engine Building Tools, Engine Builders Supply | GOODSON Tools & Supplies Tips on brake service http://www.autoshop101.com/forms/brake07.pdf Last edited by Nissan2007; 04-16-2012 at 06:54 PM. |
#9
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Great info. It will be a while before I can work on it again but hopefully next time i'll find the smoking bullet.
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#10
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Warped rotors. It's like a patient going to the dr. and saying, doc I have this pain and tightness in my chest but I know it's not a heart attack because I never had one before.
Really? |
#11
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To be specific, in this case I am guessing the rotors have thickness variation issues, but may not appear to be warped if measured for runout.
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#12
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Solved: Front end shimmy while braking: not warped rotors
I had a bad shimmy and replaced the rotors. Solved the problem for a while but the shimmy slowly returned. I removed the lower control arms and the rear pivot bushings were shot. I replaced the lower control arms and viola - NO MORE SHIMMY!!!
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Tags |
braking, rotors, shimmy, warped |
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