Why does my mountain bike tire "wobble"? How do I correct it?

Question

I bought a new bike off the internet (unassembled), and assembled the bike on my own. I took the bike out on its maiden voyage, and everything went well with one exception: I noticed an intermittent grinding sound coming from my front disc brakes. I put the bike on a stand and spun the wheel, and noticed that the front wheel didn't seem to go straight. Instead, it had a slight wobble. I don't mean it wobbled in that it was loose or crooked. But it looked like it somewhat drifted. This travel caused a problem because it forced the disc brakes to rub against the caliper whenever the tire "drifted" to the left.

Does anyone know what causes this problem? Is it an installation error (I followed the instructions EXACTLY)? Is there any way to correct this problem?

Answer 1

There are 2 separate things going on here.

First, a disc brake wobble has nothing to do with a wheel being out-of true. The disc is attached to the hub via a machined flange. If part of the assembly required you to attach the disc then I hope you made sure that the bolts were tightened in order and at the correct torque. This is vitally important.

Staying with the disc for now, it is very to have to true the disc as a part of regular maintenance and as part of assembly. Close an adjustable wrench over the disc and coax it into place.

Regarding the wheel being out of true or out of dish, this really is something that takes practice to correct but it shouldn't make much of a difference in riding (since you have discs) unless it is more than 1/8" (4mm) out. If this really concerns you I suggest taking the wheel to a shop for truing.

Answer 2

As you spin the wheel, the *rim flanges* should spin true. If not, you may have a bent or broken axle, which is very uncommon.

I generally true my rotor by pushing on it with my thumb.

If your wheel is  wobbling - is it the rim or just the tire? If the hub is true, but rim wobbles, the spokes will need to be adjusted. Unless you know what you're doing, it's best to have a shop do it. If the rim is true but the tire isn't straight on the rim, deflate the tire and carefully go all around the bead on both sides looking for a bit of the tube stuck between the tire and the rim. Most often this happens at the valve stem. If it's at the stem, you can fix it by pushing the stem all the way into the tire, then pulling it back out or pushing it by pushing down on the tire at that spot. Then reinflate and re-check for wobble. Having a pinched tube is pretty common, and it must be fixed or eventually it will cause a blowout.

Answer 3

It sounds like the hub is not square in the drop-out. Remove wheel and reinstall making sure the hub is squarely-seated. If the disc still rubs, that means the disc is warped/bent and the disc itself will need to be straightened. There are special tools for doing this, however, I've had to straighten mine before and without the tool handy, some light taps with a rubber mallet tend to work well to get it back to true. It does not take much force to bend it back to straight.

If your wheel is out of true, that's a different subject, and since your disc is rubbing, an out of true wheel doesn't usually affect the disc. It's a good idea, however, to keep your wheels true even if you are running discs.