New Hub
 

I would like to purchase a new hub so that I can use an Ultegra cassette that I recently acquired. The bike I have now has a freewheel so I need a new hub. What will this involve? As the wheel is pretty new (only a few months of use), I figure its just a matter of swapping the new hub in, I know it will be some work, but I am willing to do it, I would just like to know what is involved before I commit on getting a new hub.

It's a fairly detail involved process. Basically you have to remove the spokes from your existing hub and replace them in the new one. Different hubs may require different length spokes but most spoke length issues are due to rim differences so you have an excellent chance of "lucking out" in that regard.

Tensioning and trueing the wheel is the big deal. The big secret is not to try to do too much at one time. Go slow and be careful with the details and you'll be OK.

After you've done all of that, you'd like for the rebuilt wheel to fit on your bike and there's a good chance that it won't. Cassette wheels for road bikes usually have 130mm dropout spacing and for mountain bikes have 135mm dropout spacing. You might be able to just force fit the new wheel into place and be OK.

Spoke count needs to be the same too. This seems to be a case of putting the cart before the horse.Unless the rim is something special, it would almost make more sense to just get a wheel with a shimano 8/9 already on it. Careful shopping can net you a whole wheel for the cost of a hub and spokes.

Nashbar is selling a tiagra hub with 36 holes(what I need), for about 20 dollars. I don't think I can get much better than that. And just for clarification, I probably won't need new spokes?

I think there's a better than 50% chance that your current spokes will work. To know for sure, look up both hubs on one of the spoke calculator programs plus or minus 1mm will work fine.

Umm. . . can't find the current hub, and used shimano rear 130 mm. Took a guess and tried suntour xc pro rear. Came out to be exactly the same spoke length.

This is related, but slightly OT; what if one had some old steel wheels and wanted to transfer over the nice old hubs to some newer alloy rims? Would one be able to use the same spokes with the newer rims, or would one have to go and replace those as well? I've been led to understand that the outside rim diameter will be significantly different between the steel and alloy rims; 'zis correct?

Most of the difference in spoke length requirements is in the rim extrusions. I mentioned in my previous post that, if you are replacing standard-looking hubs, you have a better than 50% chance of the old spokes being the right length. When you change rims you have to be pretty lucky to stumble on a rim combination that will allow you to use the same spokes.

Most of the time, for me at least, it's a moot point. It takes so much time to disassemble the old wheel and clean up the old spokes that I don't think it's worth doing even if you are willing to ignore any previous stresses on the old spokes.