# Question about warped rotors - What's the maximum acceptable runout?



## VgRt6 (Mar 8, 2002)

I've got a front end shimmy coming through the steering wheel when braking and used a dial gauge and magnetic base to check the runout of the front rotors. I measured 0.005" max on one rotor and 0.003" on the other. Could these levels produce a noticeable shimmy in the steering?
I've seen specs of 0.0005" to 0.001" max, but have no idea if these are realistic values.
The rotors are approx. 20k miles old and the lugs have been overtorqued by a wheel shop twice







. PepBoys will machine them for $8 a piece and am considering taking them there.
Thanks.










_Modified by VgRt6 at 1:45 PM 3-16-2006_


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## GT17V (Aug 7, 2001)

Behold... the power of the Haynes manual.....
Max disc runout (front and rear): 0.004in (0.1mm)
Max disc thickness variation (parallelism)
Front: 0.0004in (.01mm)
Rear: 0.0008in (0.02mm)


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## VgRt6 (Mar 8, 2002)

*Re: (GT17V)*


_Quote, originally posted by *GT17V* »_Behold... the power of the Haynes manual.....


That has to be the first time those words have ever been uttered!








Thanks for the specs. My runouts are close to the spec, but I'm pretty sure that the parallelism of my discs is WAY worse than those specs. I guess I spend the $16 to have PepBoys turn them and see what happens.


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## GT17V (Aug 7, 2001)

People don't give the Haynes manual enough credit. It does have mistakes (and pure copying from previous editions) and missing some stuff... Likewise, the Bentley manual is missing some things... for $20, it's a good supplement to the Bentley manual.
Plus, it has a color chart for spark plug deposits and diagnosis.


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## Fantomasz (Aug 15, 2001)

*Re: (GT17V)*

buy new disc set


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## VgRt6 (Mar 8, 2002)

*Re: (Fantomasz)*

Nah. I had them turned at PepBoys for $8 a piece. I measured the runout afterwards and found 0.006" and 0.012"! I complained to the service manager and he refunded my money (he then tried to sell me new rotors ... because THEY just ruined them







http://****************.com/smile/emthdown.gif ). Anyway, I put the rotors on the car and it stops smooth as silk now. http://****************.com/smile/emthup.gif


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## Pronstar (Oct 25, 2005)

*Re: (VgRt6)*

Could have been an uneven transfer layer of pad material. Turning them would remove the transfer layer entirely, curing the shimmy.


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## machschnelGTI (Jun 28, 2004)

*Re: (Pronstar)*

Brake rotor Warpage 101:
Most of the time people think that they have warped rotors when they experience a pedal vibration. Pedal vibrations are caused by thickness variations in the rotor, not runout. Excessive runout can cause a steering wheel vibration, but usually not unless its very excessive, like 0.020 or more. Very seldomely are pulsations/vibrations caused by the quality of the part, but usually by the quality of the installation. Steering vibrations are usually a result of worn front end steering/suspension components, and an excessive alignment change when the brakes are applied.
When a new rotor is installed on the car, runout needs to be measured. You must measure the installed runout. Measuring rotor runout off the car, like on a brake lathe, does nothing to help us....neither does machining a rotor on a conventional brake lathe. Every rotor is going to have a slight bit (.001 or less) of runout machined in to it due to manufacturing tolerances....this is fine. Hubs also generally will have a little runout in them. The key is to orient the rotor on the hub so that the runouts of each component cancel each other out. If the hub has rust and dirt on it, this will cause problems. When the rotor is installed, Runout must be less than .002". It also wouldn't hurt to put a coat of high temp grease or anti-seize on the hub surface so that that rust does not develop over time and jack the rotor away from the hub, causing runout. 
In order to have pedal vibration, the rotor needs to somehow cause hydraulic feedback to the master cylinder. Runout will not cause this except in some opposed piston fixed caliper applications that have a relatively small crossover orifice. On a floating caliper application (like all single piston VW applications) the caliper will simply float back and forth following the rotor as it wobbles. When you release the brakes, the rotor will still make slight contact the pads on the the high spots on either side of the rotor, which, depending on what type of pad you are using will either wear the rotor slightly or build up friction material transfer on only the high spots on either side of the rotor. Depending on the severity of the runout condition, this will usually turn into a disc thickness variation in 5 or 6K miles. A thickness variation will not only cause the caliper to float back and forth, but will also push the piston back into the caliper and allow it to return back out (the distance is the same as the thickness variation in the rotor), which makes for hydraulic feedback resulting in a pedal pulsation. 
Machining a rotor on a conventional brake lathe will accomplish only two things: Provide a flat mating surface for the brake pads and eliminate disc thickness variation. 
What it will not do in most cases is eliminate runout, unless the operator actually knows what he or she is doing and uses a dial indicator when chucking up the rotor on the lathe. To do this properly, you need to set the rotor runout to .000 on the lathe. This is very difficult to do, even with the proper tools. Even if you get the runout to 0 on the lathe, that will still not compensate for any runout in the hub. The ideal way to machine a rotor is to use an "ON CAR" lathe, which will result in 0 runout and 0 DTV while the rotor is installed on the hub. In cases where the hub has excessive runout, this may be required on even a new rotor to machine out the installed runout that was induced by the bent hub.


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## VgRt6 (Mar 8, 2002)

*Re: (machschnelGTI)*

http://****************.com/smile/emthup.gif 
Cool. That means PepBoys refunded my money for no good reason.


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## machschnelGTI (Jun 28, 2004)

*Re: (VgRt6)*

It'll work fine for a while and then start vibrating again.


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