Mountain Bike Frame Sizes -- powered by eHow.com

Building and truing wheels is a very difficult thing to do, it can take years to learn, but your wheels may only be a little out, or your no where near a bike shop and you need to straighten a wheel, its good to know what to do in those circumstances.

How to Start
To build and true wheels properly you need a lot of expensive tools, such as:
1. Spoke key
2. Truing jig
3. Dishing tool
but if you only need to do a little straightening all you’ll need is the spoke key and you can do the work in the bike frame, either in a bike stand or you can turn the bike upside-down on the ground.

The next step
With your eye on the wheel where passes between the brake blocks, spin the wheel slowly, now take note of where the rim touches the blocks, if it touches the left brake block then the spoke pulling to the right needs tightening and spoke on the left heeds loosening, and so if the rim touches the block on the right then the spoke on the left needs tightening and the spoke on the right needs loosening.
Now that all sounds simple, but if you over tighten or over loosen the spokes the wheel is going to be in very bad shape, so be careful. I would say that the spokes nipple should only be turned a very small amount at a time until you can see how much difference it makes. Some rims are softer than others and that can make a difference too.

Up and down too!
If your wheel is true from left to right maybe it isn’t round and the rim goes up and down or has a “flat spot” this is the most difficult problem to sort out, it can be done, but it takes time and practice, also if it is very bad it means a new rim needs fitting.
This is how you try to fix it. If the rim dips loosen the spokes if it seems to rise then tighten them, but be careful, not too much!

And all Together
This is the tricky bit, to get the wheel straight and round at the same time, it’s a case of tighten a little here and loosen a little there until you get it right.

The other thing you have to remember is that the wheel also has to be in the middle of the axle of the hub, so that its in the middle of the bike frame, there is a special tool for measuring this, but you can do this in the frame, either buy looking down at the bottom bracket where the wheel is closest the frame and measuring the gap between frame and rim, or up at the brake you can do the same.
And Finally
There is a lot to remember, the wheel has be round, not going up and down, the wheel has to be true, not going from side to side and the rim has to be in the middle of the hub axle.
If that wasn’t enough the spokes also have to be at the correct tension, this means if the spokes are loose it makes for a sloppy, soft ride and the wheel might fall apart and if the spokes are too tight it makes for a ruff ride and the spokes might snap.
Truing wheels takes a lot of patience and practice, you may not be able to do it and in many cases it’s a job for the professionals, but if you are brave enough, give it a go, but beware you could ruine your best wheels if you are not careful, it could all end in tears.