get a second one....even if it's a cheaper 105... then put the new one on the old wheels... really switching a cassette sucks... i did it once... never again.

I would guess 17 would be fine. All you need it for is to cover the spoke holes on the rims, so as long as it does that, you are fine. Ride with the valve extenders on. If you take them off, the valve will be open and will allow air to flow out. With the valve ext. on there, it stops the air from coming out.

I agree with kona_expat. Changing a cassette is easy with the right tools. I would also recommend using a torque wrench to tighten up the lock nut on the cassette. Many wheels have fine aluminum threads that can get messed up if you overtorque. By the same token, undertorquing can be a disaster too.

If you are going to change cassettes, do it a week or two before the race. It's never a good idea to change your equipment the day before a race.

cassette is one of the few things on a bike that I K NOW I won't put on wrong... large cog goes on first... then work your way down to small cog..

or did all the spacers go on together at the end... darnit.. now I have to go read the manual..

I always swap them. It is fairly fool proof. You should be able to buy the needed tools for $30 or so. Much cheaper than a second DA or Ultegra cassette.

If you have two sets of race wheels, you should get two cassets and two chains and keep them together?

Why do you guys change out your cassettes? Is it because they are lighter and less durable? And is the change significant?

Also, while we are on the subject. I have seen several posts on this site talking about gear ratios. But without any reference point I can't figure out what they are talking about. Are they referrring to the cassettes you bring to a race? For example, if I'm going to do a ride in the Texas hill country and will need lower gears to work with, could I swap out my cassette for this purpose?...or what about the front sprocket? For example, this link is off performance bike's website. Which one would I choose?? http://www.performancebike.com/shop/sub_cat.cfm?subcategory_id=5133

Jeremy

Couple of reasons to change out cassettes -- weight, durability, and terrain (hills) can all be factors. For your question about the hills, there are two answers. You could get a 12-27 (12 teeth on the smallest cog, 27 on the biggest) rear cassette to give you an easier cog for the hills. Or you could keep your existing cassette and get a smaller compact crank up front. This article has a long discussion about compacts that may be helpful: http://www.slowtwitch.com/mainheadings/techctr/gearing.html

The upside of a compact is that it allows you to keep a tighter & lighter range of gears (like an 11-23) on the back and maintain similar max and min speeds as using a standard crank with a more spread out 12-27 rear cassette. This means you can fine tune your shifting more while still able to climb easily in the hills.

The article doesn't really address the downsides of a compact crank--cost and ease of setup. Changing a rear cassette is relatively simple and cheaper than using a compact.