x514

Would this create any problems? MTB: 11-34, Road 12-25
The MTB cassette's two smallest cogs are removable, if I removed one of the cogs would it be the same as an 8 speed? Are there any real differences between road and mtb cassettes?

bradtx

x514, No, the 9S gears are closer together and you'd have problems shifting, unless you're using friction shifters, in which case you can run all nine.

Brad

CCrew

Not to mention that if you're trying to put a 34t cassette on a road rear more than likely the derailleur won't go up to 34. You'd have to switch to a MTB rear derailleur.

Al1943

If you want to upgrade to a 9-speed cassette you will need 9-speed shifters and 9-speed chain. That can be done.

JiveTurkey

As mentioned, the number of speeds of the rear shifter, cassette, and chain must match. The front derailleur should also match as the narrower cage of the 9s FD will shift the 9s chain better. Though the rear derailleur doesn't care how many speeds gears you're runing, road RDs can't handle the big cogs of a MTB cassette.

As the rear shifter is the most expensive part, you'll need to decide whether it's worth changing it and the other parts.

Assuming one of your goals is to increase the range of gearing, the cheapest option would be to just replace the cassette with an 8-speed MTB cassette. You should also replace the chain when replacing the cassette so that the worn chain doesn;t prematurely wear the new cassette.

Edit: Almost forgot.... if going to an 8s MTB cassette, you'd also need to a get a MTB RD. So, you can decide if it's worth just going ahead and getting a new rear shifter and FD since you're already changing out the cassette, chain, and RD.

x514

Thanks for the replies, I think I've got it down now.

FastJake

This isn't quite accurate. "Road" and "Mountain" cassettes are just arbitrary names. You need to look specifically at the teeth on each cassette to determine if you'd have to get a new rear derailer. For example, if you had a close ratio cassette like 11-23 and you wanted a lower "low" gear you could get something like 11-25 or 11-28. If you want to go bigger (in the 30s) you will probably need a new rear derailer. Although I'm using a 30T cog on my Ultegra short cage derailer without issue.

As mentioned, the number of speeds on the cassette is important. A 10 speed cassette must use 10 speed shifters and a 10 speed chain. 9 speed with 9 speed, etc.

HillRider

One more point, a 9-speed cassette and an 8-speed cassette are the same overall width and both fit on the same rear hub. Removing one cog from the 9-speed, won't work because, along with the reasons given above, the remaining 8 cogs would be too narrow.