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How much radial rotor runout is acceptable?


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Just finished up putting a new set of rotors on the car for a track event next weekend, and for kicks I decided to try out my new magnetic indicator base. Both rotors showed about 12 thou radial TIR. Lateral TIR was almost zero, barely moving the needle. In the past, I've simply bolted rotors on and run them until they were blue and canyon sized cracks with no thoughts of runout, and have not had a bit of trouble. But since I now knew about it, I had to try to correct the ? problem. Several attempts at loosening the bolts till just snug and tapping the rotor with a dead blow hammer proved one thing- it's mighty hard to move something only 0.006" with a hammer. I wasn't able to show any improvement, so I tightened the rotors down and called it good.

 

So this begs the question: How much is too much? Has anybody else checked the runout and what results did you get?

 

John

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I hope to be on route from Atlanta to Myrtle Bch. next Saturday morning and will stop at Kershaw to see that white monster fly....mayolives

 

Tom, please stop by if you're coming that way, It would be good to see you. I'll tell them at the gate you're coming, just tell them you're crewing for me.

 

Motor is doing better since bring freshened up, last month at the T1 event a corner worker said he caught me at 3 consecutive 44's. Driver still sucks!

 

John

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  • 10 years later...

.090" is waaaaaay too much. There was another thread where people talked about shimming them. That's what I did. Used 5/16 shims from www.mcmaster.com and I shimmed mine to within .002" and have no problems. I don't think you can shim out .090, you will have to turn those. The problem is that when you turn them you make the rotor thinner on one side than the other. In your case, substantially thinner. You didn't tighten the bolts around in a circle, did you? You need to do a star pattern like lug nuts. Even if you did tighten across the bolt pattern, I'd loosen them up and tighten again, and then consider trying a different rotor on that hat. .090" is a hell of a lot of runout.

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I've got .003 run-out in the lateral direction, measuring the face of the rotor. Radial run-out I believe refers to what you'd measure on the edge of the rotor. Basically, my rotor isn't centered on the hub properly. I had new rotors drilled, and the machinist didn't get it centered perfectly. I don't know if this is enough to cause some weight distribution, or balancing issues.

 

Measured like this:

post-39558-0-68289100-1459700536_thumb.jpg

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If the rotor is freshly surfaced the pads will just make a new path.  You could have the wheels balanced with the hub and rotor bolted on.  I've not seen it done, but it seems to make sense even without a runout problem.  The casting inside the vents looks pretty rough.  Can't be that well balanced from the start.  After balancing you'd want to mark the wheels to the hub to keep them indexed.  Or you could do a wheel, then bolt the hub and rotor on and re-run it to see if it even matters.  Don't forget to also have your machinist take the edges off so the wear pattern doesn't look funky.  Kidding...

 

This is kind of like the assumption made for internally balanced engines, and flywheels, and pressure plates.  Each part is assumed to be "in balance".  If you start looking a round there are all kinds of ways to be out of balance.  How many people rebalance their wheels after rotating tires?  Of course, the racers probably do, but still.  Most of us wait for funny noises.

 

http://www.brakeandfrontend.com/on-car-wheel-balancing-wheel-to-hub-indexing/

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  • 1 month later...

I've got .003 run-out in the lateral direction, measuring the face of the rotor. Radial run-out I believe refers to what you'd measure on the edge of the rotor. Basically, my rotor isn't centered on the hub properly. I had new rotors drilled, and the machinist didn't get it centered perfectly. I don't know if this is enough to cause some weight distribution, or balancing issues.

 

Measured like this:

It shouldn't matter if he didn't drill the holes accurately. The ID of the rotor should be machined to have a nice snug fit onto the hub, this is what centers it. He may have made that oversize or perhaps didnt dial the rotor in on the lathe before machining. If he didnt machine the id of the rotor then it has to be defective.

Edited by 240zdan
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