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Weird one. On my tripster, took it into to my friends shop for a service - all done just fine, but he pointed out the rear rotor was actually oval. He'd puzzled about it a bit, but thought it seemed to be working OK (if not 100% optimally) and as I was off on a multiday ride and hadn't spotted it before I just left it and moved on.
Fast forward 12 months, I'd replaced the rotor a few weeks later with a standard Clarkes cheapy. Just looked at it and its ovalised.. and now its bugging me for no practical reason 🙂
Another dodgy rotor? Hub (6 bolt) causing it? Maybe the mount on the frame?
Do I need to just let it go 😀
If it goes from rotor to rotor, it sounds like the hub is off centre when referencing the rotor drillings.
When you say ovalised, is there one high spot per revolution, or two?
Are the discs same diameter on the other end of the bike? If so, swap rotors and see if the 'ovalisation'
follows the disc or stays with the wheel. I suspect it's the latter...
How much run out are we talking? It may just not be centralised on the bolts
It means you get more power from smaller rotors doesn't it?
You're relying on screws to position a rotor so it's always going to be slightly out vertically, would be surprised if it was actually oval
Eccentric mounting on the hub would be my guess.
1 high spot -> most likely eccentric; multiple spots -> at least oval, maybe eccentric as well.
It usually helps to rotate the disc in the direction the brake pads will push it before tightening the bolts.
it can't be that oval otherwise it would be sawing through the caliper wouldn't it? If there is no rubbing then I wouldn't worry too much.
Both 160mm. I'm happy its not a fitting error, have plenty of bikes that don't do this.. and I'm near 100% sure it was round when I fitted it, but a slight offset in the hub drilling is possible - maybe it's slipped over time and use and exaggerated it. It is only obvious when spinning so offset mounting by fit error or hub quality issue is the most likely possibility I guess.
It means you get more power from smaller rotors doesn't it?
I see what you did there, and approve 😀
Better braking when climbing.
Are you tightening the bolts with the right pattern and to the correct torque? No grease, only anti-sieze.
For comparison, check the ovality of your chainrings. Though if they are intentionally oval, you would have to correct for that.
I've never used a torque wrench - that could be it , over tightening.. if maybe there was a thread or two extra maybe it could bend it out of shape a little that looks like the disk is oval. That said, I don't go mad and always use threadlock, but it is only this wheel so it's possible.
if it's really gone oval, which is unlikely, it'll have 2 high spots.
if it's just that the disc PCD, the hub PCD, and the disc OD don't share the same centre (they won't, nothing's perfect) to the point where you can see the run-out, then you'll only have one high spot.
normal part imperfection, and a bit of clearance on the disc PCD holes, means that simple off-centric runout is more likely than anything else...