Advice

It depends on what rear derailleur your bike currently has ...

And, how it was set up.

Like the other poster said, it depends. Can you give a little more info?

Quoted post said:

Originally Posted by RonSwanson
Like the other poster said, it depends. Can you give a little more info?

It depends what kind of rear derailleur you have and how much slack was in the chain when you were running the 29t. There are no road derailleurs currently in production that are designed to get their jockey pulleys clear of a 34t cog. Then, if you somehow get your chain on the 34t cog, and if your chain is too short, you can break the chain, rip the derailleur apart, rip the derailleur hanger out of the dropout, pull the derailleur into the spokes and destroy the wheel, bend a chainring, or some combination of the above.

Post more details if you want a more helpful answer.

Quote by OBC:
"Then, if you somehow get your chain on the 34t cog, and if your chain is too short, you can break the chain, rip the derailleur apart, rip the derailleur hanger out of the dropout, pull the derailleur into the spokes and destroy the wheel, bend a chainring, or some combination of the above."

Captain Bringdown and his backup band, the Buzzkill, pretty much told you the straight dope.

You are very likely in need of another 5 links of chain and a long cage touring rear derailleur (if you do not already have one on your bike).

Quoted post said:

Originally Posted by CAMPYBOB
Quote by OBC:
"Then, if you somehow get your chain on the 34t cog, and if your chain is too short, you can break the chain, rip the derailleur apart, rip the derailleur hanger out of the dropout, pull the derailleur into the spokes and destroy the wheel, bend a chainring, or some combination of the above."

Captain Bringdown and his backup band, the Buzzkill, pretty much told you the straight dope.

You are very likely in need of another 5 links of chain and a long cage touring rear derailleur (if you do not already have one on your bike).

Captain Bringdown. I like that. I seem to have a lot of experience doing that. And it brings me no joy.

A couple weeks ago we had a guy we'd never seen before ride in on something nice and carbon, with Dura-Ace 7900, complaining that he couldn't get his rear derailleur to shift right. Sure enough, the hanger was bent, so we straightened it and readjusted the limit screws and cable tension. We also told him that whoever installed his chain had cut it too short and he was liable to break something if he accidentally shifted into big-big or something close to it, and offered to install a new chain. No, I'll be careful and take care of it later, he said, as he paid for the derailleur adjustment and rode out the door.

About 90 minutes later he was walking the bike back in with a broken hanger, bent derailleur, and broken chain. The wheel and big chainring were somehow spared.