I am currently upgrading my mountain bike and have bought a set of wheels that accepts 8,9,10 speed cassettes but my previous wheels used a 7 speed freewheel. If i install the new wheels with an 8 speed cassette would having the standard shimano 7 speed trigger shifters and 7 speed derailleurs cause a problem or would it work fine?
The 7 and 8 speed cassettes have slightly different spacing (see http://sheldonbrown.com/cribsheet-spacing.html), so the derailleur will be slightly, almost unnoticeably, out of adjustment at largest and smallest cogs. And of course you won't be able to shift to one of the cogs.
If you can't change the shifter to 8-speed, you can install a 7-speed cassette on 8-speed hub with a spacer.
The 8-speed hub is wider than 7-speed, so you will also need to bend the rear fork slightly to install the wheel.
There is alot of information telling you that spacing between 7 and 8 speeds is different. However, if you look inside a Shimano ST-EF 7sp shifter (one that is available in 8sp versions also) you will see that there are 8 shifter positions on the ratchet, one of which is blocked off by a plastic cube. Drilling off the plastic cube opens the shifter to 8sp use. The 8sp shifter is identical internally apart from this "spacer" being omitted.
Therefore we can say that Shimano is using the same shifting lever performance with the same derailleurs for both 7 and 8 speed use and expects the operation to be correct.
The takeaway is that you can use an 8sp cassette with a 7sp shifter and expect it to index flawlessly. You can also space a 7sp cassette onto an 8sp hub by putting a 12t or 13t sprocket from an old cassette behind it, so long as the rivets don't interfere.
Tune in for more horrible hacks next week.