Brake Rotor Run-Out

[fbody_mike]

What have you found to be an acceptable amount of run out? For my 85 GS550E, the book calls out 0.012" as the service limit. I have been getting a noticeable pulse in the front lever so I put a dial indicator and measured 0.008 run-out on the right and 0.0015 on the left. I believe the right rotor is my problem, so I got some second hand rotors on order.

I used to think the service limits had a huge amount of safety built into them, but this time I think the service limit was way off. Personally, I think run-out is more noticeable on this bike due to the opposed piston design. The calipers are fixed, no sliders and the rotors are fixed non-floating type.

 

[bwringer]

Good question -- let us know what happens with the other set of rotors. That really doesn't seem like much runout, but I dunno. Is it perhaps the surface -- is there a spot on the surface that looks different?


I overheated a set of GS850 rotors a long time ago when my mountain riding skills were a little rough, and there was a distinct pulse in the lever. You could see a definite heat-blued portion of the rotor surface, and there would be a "thump" as the pads passed over this. No naked-eye visible runout, but I didn't have a dial indicator at the time.

 

[fbody_mike]

Well I swapped the right rotor out for the replacement. Before doing so I checked the run-out on the old rotor one more time, and it was .008", but it went from 0 to 0.008" twice during one revolution. Meaning to me the old rotor was wavy like a Ruffle's Potato Chip. The "new" rotor has 0.002" run-out but the arc of the low spot was nearly 180 degrees.

No more pulsing in the lever under hard braking. I am guessing that the amount of run-out is critical especially as it approaches the factory spec, but also the number of times the rotor flops in and out in a revolution and the length of the arc in which it does it.