Is my hub bent or the brake rotor?

I have been going through my rm125 that I got last month, mostly just cleaning things up and replacing a few parts here and there, last week I noticed that the front disc rotor is slightly tweaked. I went ahead and ordered new pads and found a rotor on ebay for $50. I put the pads and the new/used rotor on today and it seems bent too.

If I place the rotor I got on ebay on a flat surface it appears to be straight and true, but once on the bike it appears to be bent slightly like the original, enough that the caliper has to move back and forth when the wheel is spun.

The "nubs" on the front hub where the rotor bolts on are quite short and I cant really imagine how they could have been bent. Should I drop the coin for a brand new replacement rotor, or save my pennies for a few, get a new hub and try that route?

Any suggestions appreciated.

edit:
I wanted to add that I did try loosening the bolts on the rotor enough to relive pressure from the disc and it still showed a bend in it when the front wheel was spun. The "spots" on the hub where the rotor bolts on are to "staggered" for me to check the hub with a straight-edge.

The rim is straight.

The brake rotor is what seems to be un-true/bent. Or possibly the mounting points on the hub for the rotor.

If I lift the front wheel off the ground while the bike is on the stand and spin it, you can see the caliper moving back and forth as the rotor turns through the caliper at the point where it is bent, you can also see a "wave" or bent area in the rotor itself when looking from the front when the wheel is spun. The caliper is made to move this way obviously(full floating caliper) but due to the un-true brake rotor it is having to move in and out every revolution which I am sure isnt normal with a true rotor(Im an auto mech. and disc brakes arent anything new to me).

When I spin it by hand I can get about 3 revolutions, during each you can hear the rotor come into contact with the pads and then move away from the pads as the wheel and rotor turn further through the revolution, along with the sound is the visual of the caliper moving in and out.

Its hard to guess how far the caliper is moving in and out, it is not bottoming out the floating ability of the caliper even with new pads in. I know there is a spec limit on the rotor for runout, or warpage etc, but I have no way of testing that right now.

My main concern was for the front hub, and if it could be bent, it seems to me the hubs mounting points are quite beefy and it would take alot to tweak one so I am assuming I just have two bent rotors, but placing the one I got from ebay on a flat surface and turning it remains flat on the surface as if it were perfectly true. If I had it to do over again I would have just spent the extra $20 a new replacement rotor least I would know for sure then :bang:

Anyone ever experiance a brake rotor mounting point on a hub being bent? I am sorta ruling that out just based on the strength of the hub and figuring I have two bent rotors. Ill probably pick up a new rotor sometime soon and hope its not the hub.

2-Strokes 4-ever said:

Minor damage/dips can often be straightened with a crescent wrench. Spin the wheel, find the dip, tug with the wrench clamped onto rotor........repeat until true.

Click to expand...

I may give this a try since I have two rotors to play with lol.

Bearings seem good as the front wheel doesnt have any wobble to it that I can feel by hand in any direction.

Glad to hear its most likely not the hub, those cost a bit more than a new rotor, I figure Ill probably get a replacement ebc or tusk rotor as soon as I can. My main worry about riding it this way was un-even braking ending up in front tire wash-outs when entering corners.

Thanks for the replies and as always the good advice!

2-Strokes 4-ever said:

Minor damage/dips can often be straightened with a crescent wrench. Spin the wheel, find the dip, tug with the wrench clamped onto rotor........repeat until true.

Click to expand...

I gave this a shot and had good results, its not totally straight of course, but it is allowing for more revolutions of the wheel when spun by hand now, and there is less contact with the pads audibly. Kinda hard to tell where the bends are exactly, at one point I had made it worse I think lol, but was able to tweak it back. I used a marker and that helped me to pinpoint where the bends were at.

I went ahead and pulled the new pads that I got out and put the old ones back in, and will save the new pads for a new rotor sometime in the near future.

Thanks again guys :cool: