Hi - I just got carbon wheels but want to keep my old wheels to use on my trainer. I have Shimano 105 groupset 11 speed, about a year old. I think I need a new cassette in order to have one on both the old wheel and new wheel. What kind of cassette do I need to look for? I heard you can switch cassettes for more climbing or whatever, but I want something that's good for everything since I will be riding triathlons with the new cassette.

You will need another 11-speed cassette that works with a Shimano (or SRAM) freehub. You can go with different sized cassettes depending on the type of riding you do. The sizes are shown as the smallest and largest cog on the cassette. For example, 11-28 or 11-25 or 11-42...and lots in between. As the larger number goes up, you will find bigger jumps between each cog (and much easier pedaling well suited for very steep climbing). Many conventional road bikes with 11-speed gearing come with an 11-28 cassette from the manufacturer, which is a pretty good happy medium for both flat and hilly terrain. It's also important to note that 11-32 is about the limit for 11-speed Shimano road groupsets (the range of the rear derailleur is the rate limiting factor). Check out some of the online retailers to see the different sizes available and pricing.

Thanks! I am seeing a Shimano 105 CS 5800 11 speed cassette for sale. Any idea about the 5800 numbers? As long as I'm sticking with 11/28 or 11/32, are there types of cassettes that are "better" than others?

Check to see if your derailleur is a short, medium, long cage. If its a medium or long, you might be able to run a 11-34 cassette..

105 cassettes are fine. No need to spend a lot of money on a cassette for a trainer wheel. Rule of thumb: the more climbing you do, the bigger the spread you'll need. So my climbing cassette would be 11-30, while my crit or flat TT cassette would be 12-25.

Make sure your chain is sized close enough to cover both cassettes if you change cog sizes.