Topic: Brake rotor warp?

[Tugboat]

As I spin the rear wheel of my '76 CB750F, I can feel it drag and can watch the pads get pushed against at some times yet not at others.. as in it drags for about 1/2 a rotation and feels more free on the other half. The rear wheel has never spun freely - even after rebuilt MC & caliper, new Henry Abe wheel and new bearings - and I'm starting to think this is why. It would also explain why my rear brake can be grabby (excess heat). Is this common? Is there a fix besides replacing it?

[flybox1]

You sure its not the chain/sprocket wear?  Remove the chain.  Spin the wheel.  Resistance still there?
Next..Remove the rotor.  Wheel back on the bike w/o the chain....Spin the wheel.  Resistance still there?

When you determine it is in fact the rotor...It's 0probably cheaper to replace it than to have it turned flat again.

[Tugboat]

New chain/sprockets in the past ~1500 mi. But I can't remember if I mounted the wheel w/o the rotor and spun it - that's something to try.

[calj737]

If it is the rotor, I'd send it to Tom @TrueDisk and him him grind it. Takes it to a beautiful finish and even dimensions.

[seanbarney41]

I can't imagine these ridiculously over-built slabs of stainless warping.  I can imagine that the aluminum center could be bent from an accident or other damage.

Tugboat, your rear is grabby because it is wayyy too big for a rear application.  F0/1 has much more braking power than the front with much less traction...not one of Honda's better ideas.

[przjohn]

If you spin the wheel and observe the rotor moving the pads off and on and not evenly the rotor may be warped. If it is not the rotor than it is either the hub is not machined right or something is binding between the rotor and the hub mounting surface. You can determine if the rotor is warped by removing it and placing it on a known flat surface like glass.

[grcamna2]

If you spin the wheel and observe the rotor moving the pads off and on and not evenly the rotor may be warped. If it is not the rotor than it is either the hub is not machined right or something is binding between the rotor and the hub mounting surface. You can determine if the rotor is warped by removing it and placing it on a known flat surface like glass.

That seems like the best way to see if it's warped,remove it and lay the rotor on a flat surface.
I vote to replace it as I was told by a machinist in the past,he said it could warp again even after it was 'faced',but he might have been referring to thin sportbike rotors.

[Jerry Rxman Griffin aka MuthaF'er]

IF it IS warped you need to determine why before repair. Perhaps one or both of the pistons are sticking? It's gotta be serious heat from some reason. I agree that these rotors rarely actually warp. You can get a rebuild kit including both pistons from SiriusConInc.com. If you do get it surfaced you might as well send front and rear and get them drilled at the same time with the same pattern.

[scottly]

If it is not the rotor than it is either the hub is not machined right

I had a 750 front rotor that had noticeable lateral run-out. The rotor itself was flat, but the surface where the rotor mounted to it's hub wasn't parallel with the surface that mounted to the wheel hub. I corrected it by mounting the rotor face against the face-plate in a lathe, and turning the rotor hub surface true. Every rotor I've checked also has a fair amount of radial run-out, which is why wheels should be balanced with the rotor attached.