Q: I have two sets of wheels for my bike, but every time I swap them, I have to adjust my derailleur to shift properly on the cassette. Is there a way to make them the same?
A: Wheels and hub spacing can be a bit different between brands and even with different models of the same brand. To start, if you are looking for a second set of wheels, I would always recommend trying to pick some that have the same hub if possible. This increases your chances of them having the same spacing, but even better, it will essentially double your small parts selection in case something like a hub body fails.
If you already have both wheels, and the difference is enough that you need to make adjustments to your limit screws and tension, then there is an option. First, you need to ensure the issue is the hub and not the cassette. For example, one wheelset will have a Shimano Ultegra cassette, while the other has a 105. Check and see if swapping the cassette fixes the problem. If it does, just buy a matching cassette and save the other as a backup.
If that doesn’t fix the problem, then install each wheel and measure the distance between the smallest cog and a fixed point on the frame and compare. Make sure the wheel is tensioned into place as if you are going to ride it before taking the measurement. The one that sits the widest or has the smallest distance is going to be the one you want to match. Calculate the difference between the two; it should be under 2mm. Next, find a spacer that is about the thickness of the difference, and install it between the hub body and cassette to move the cassette away from the center point and match the other wheel. You don’t want to go too far, because you won’t be able to get the smallest cog to seat, but normally it’s a small-enough difference that things will still work. If you can’t find a spacer or shim in the exact size, it’s also okay to combine very thin shims together. This is normally better because you can fine-tune by adding and subtracting the thin shims until things are matching.
This same technique can be used on trainers as well as disc brake rotors. It works on center-lock and six-bolt versions. If you have a six-bolt rotor that you need to space out, make sure that each shim on the six bolts is the same; otherwise, it will make the rotor seem warped and result in a godawful noise, making the problem worse.