Rear Derailleur help?

[jeinan3]

I had a chain break on my hoo koo e koo. I bought another 9spd chain that I made sure was compatable with all my gear. I have a SRAM x-7 rear derailleur. Things look and shift well when I'm on the largest front cog, but when I change it to the smallest one in front, and the smallest one in the rear the chain looks like it's really too long. Also when it's on the largest one in front and in back it looks too tight. The manual for the chain said the length needed to be "by bypassing the rear derailleur, wrap the chain around the largest gears and add two links" any ideas on what's going on?

[tryandgetme]

well one thought: you shouldn't be using those gear combinations anyway. if you're in the little ring in the front, only use the largest 4 in the rear, if in the middle ring in the front, you can use any in the rear, if in the largest in the front only use the smallest 4 in the rear. it'll help keep the amount of wear on your chains and chainrings down to a minimum as well.

as for why it does it now and not before, the broken chain going through the derailleur could have bent something. but if it's shifting good, I'd just follow the rule of thumb above and ignore it.

[dirtworshipper]

Yep, your bike might be able to put up with you running little/little or big/big gear combinations, but I wouldn't expect it to run well (or look good) like that. It sounds to me like everything is pretty much as it should be.

[OTBSkinloss]

I'd just follow the rule of thumb

I thought the rule of thumb was that you couldn't beat your wife with any stick bigger around than your thumb. :D

We now return you to your regularly scheduled thread, already in progress.

[tryandgetme]

I thought the rule of thumb was that you couldn't beat your wife with any stick bigger around than your thumb. :D

We now return you to your regularly scheduled thread, already in progress.

there's a rule of thigh for beating mountainbikers that think they're funny ;)

[davkatreb]

Singleator. Done and done.

[Zinger]

It sounds like you've got it about right. As Tryandgetme said, don't ride on those weird crossovers (big/big, little/little), but you already knew not to do that (right?) and know that this is just for measuring the chain.

I'll add this: put it on big/big and see if you can downshift off of that. If you can't, it's too tight, so add a link. This happened to Ned Overend in a race, so yes it can happen to YOU. If you think you can take out a link and still have it shift (I don't know, eyeball the thing), do so and test again.

But my favorite method: cut the new one to the same length as the old one, *then* discard. (Actually, I cut the old ones up into bracelets to leave in geocaches, but that's another thread altogether. I used to save them to hand off to a friend who was building a chain-drive for his wife's observatory dome, but they've got enough chains plus an LX crankset now. You always get cosmic bonus points if you can find a way to reuse or recycle the thing, and the more creative the better.)

[nogearshere]

STOLEN FROM SHELDON (http://sheldonbrown.com/derailer-adjustment.html#chain)

Chain Length
If you replace your chain or sprockets, you should check your chain length. New chains come longer than they need to be for the vast majority of bicycles. You will almost certainly need to shorten a new chain before installing it on your bicycle. If your large sprocket sizes are anywhere near the maximum your rear derailer can handle, the chain length can be quite critical.

If the chain is too short, it will be at risk for jamming and possibly ruining the rear derailer if you accidentally shift into the large-large combination. Never run with a chain that is too short, except in an emergency.

If the chain is too long, it will hang slack in the small-small combinations. You should never use those combinations anyway, so this is not a serious problem. If you exceed the recommended gear range for a particular rear derailer, you may have to accept droop in these gears.

The best technique for setting chain length is to thread the chain onto the large/large combination, without running it through the rear derailer. Mesh the two ends on to the large chainwheel so that they could be connected (outer link meets inner link), then make the chain one complete link (one inch) longer than that. In almost all cases, this will give the optimum length.