Is there a rule of thumb about how many extra teeth on the biggest sprocket require a longer chain?
Just gone up 2 teeth at the back and was deciding where to cut the new chain, same as old or longer.

I’m glad you asked that – I don’t have the answer but have just gone down 2 teeth on my cassette so was wondering the same thing!

If your chain was as short as possible then you’ll need to add a full link (thick and thin bit) for two extra teeth. But if it isn’t on the limit then it may be better as it is.

If your biggest sprocket is 2 teeth bigger than before, then the ideal chain length is now 1 full link longer (i.e. two half links)

If your old chain length was a little bit generous, then you may be fine, but chain length is not something to get wrong, as it’ll be something more expensive than the chain that gives.

Is it a full link (2 teeth) as the chain only wraps half of the cassette?

Surely best to just resize it properly, using the shimano guidlines?

You can run same chain. If you change gears correctly and dont cross your gears a shorter one is OK, mind if you run a 1×10 then it depends on what tensioner etc.
Technically the best chain length is when the chain is around the largest front chain ring and largest sprocket and add 2 links [without going through the mech]

Is it a full link (2 teeth) as the chain only wraps half of the cassette?

Good point.

Surely best to just resize it properly, using the shimano guidlines?

Yep.

You can run same chain.

That’s really bad advice. If the old chain was cut just a little on the short side, you’ll be in trouble. We all know that you shouldn’t ride in big-big, but I wouldn’t want to ride a bike where the penalty for shifting into that gear was an expensive failure.

Just follow the big/big, no derailleur rule…

pdw – Member

You can run same chain.

That’s really bad advice. If the old chain was cut just a little on the short side, you’ll be in trouble. We all know that you shouldn’t ride in big-big, but I wouldn’t want to ride a bike where the penalty for shifting into that gear was an expensive failure.

So what expensive failure, you just cant get it into big and big. Too long a chain and thats when the rear mech winds up in the rear cassette. Both gears a good cyclist would never use.
Likely thing for OP is the bike has never had its chain checked, came out of a shop with too long a chain.

Is it a full link (2 teeth) as the chain only wraps half of the cassette?

You’re gonna struggle putting half a link on the chain dude….

You’re gonna struggle putting half a link on the chain dude….

Yep, but the chain may already be half a link too long.

Yep, but the chain may already be half a link too long.

Only if it’s specifically had a half link added. Unlikely on a geared bike.

re chain length, I do it by putting the gearing in big/big with the derailleur on, there should be a noticeable bend at each jockey wheel. If there derailleur is stretched straight, the chain is too short.

see http://www.parktool.com/blog/repair-help/chain-length-sizing

So what expensive failure, you just cant get it into big and big. Too long a chain and thats when the rear mech winds up in the rear cassette. Both gears a good cyclist would never use.
Likely thing for OP is the bike has never had its chain checked, came out of a shop with too long a chain.

So I stupidly had a chain that was long enough to go big to big but didn’t add sufficient extra. The expensive failure was trashing the XTR mech, snapping the mech hanger and taking a chunk out of the frame as the mech smacked into it. Cost of the mech and mech hanger replacement was well over £150. Expensive in my book.

Only if it’s specifically had a half link added. Unlikely on a geared bike.

No – it could be half a link too long because following the big-big + 1 rule didn’t give a whole number of full links, so may have needed rounding up. In which case it’ll be exactly the right length for the new cassette already.

So what expensive failure, you just cant get it into big and big.

If you’re lucky, it just won’t go. If the chain starts to shift, then it can create a huge force.

There was a thread on here a few months back with a broken chainstay as a result of too short a chain.

Love to see that thread

If you’re lucky, it just won’t go. If the chain starts to shift, then it can create a huge force.

Which is exactly what happened to me. As it went big to big the pressure on the mech was just too great. Chain didn’t give but the mech hanger did. I was lucky not to trash the chain stay – heavily marked though.

If its a full suspension bike, don’t forget about chain growth when cycling the suspension. For some suspension designs, it is minimal. For others, it could be significant enough to cause what BigSteve experienced.