What is causing chain to slide off cassette?
 

Hi everyone,
I recently did some major upgrades on my winter beater bike. The bike is a 2004 K2 Zed Sport with 26 inch wheels. I have installed a Shimano CS-HG50 Freehub Cassette - 12-28T with 7-Speeds on a new wheel with a free hub. The bike has 3 chain rings up front. I also put on a on a new KMC Z51 chain. I connected up the chain with SRAM power link. When I shift into the smallest 3 sprockets I get skipping and if I go to the smallest sprocket the chain comes right off the cassette and jams into the frame. What is the cause? What do people recommend as a fix? Thanks!

Clean dérailleur cables and shifters...
Then adjust and adjust - Thats just a simple start - When it makes up its mind it will behave again...

Sounds like the new parts aren't quite in the same EXACT positions as the old (assuming the old worked well...) The first thing I'd look at is whether the new wheel locates the cassette further in from the drop out then the old. if so then much/most/all of the problem might be a simple readjustment of the der's limits and cable tension. Andy.

Chain too long?

If the chain goes off the small cog the high limit screw needs to be adjusted. The skipping could have several causes, but we need more info. What does it do on the different chainwheels with the small cogs? Have you gone through the correct adjustment procedure for both limits and cable tension? Why are you replacing the drive train - was there any previous damage? Has the hanger been checked for proper alignment?

Anytime you install this many new parts in a drivetrain (especially if you did something like change the gearing), you have to go through all the adjustments to get it to work correctly - cable tensions, high/low limit screws for both derailleurs, indexing, chain tension, b-screw, etc. The fact it is derailing off the small sprocket means, at the very least, that your high limit screw is off which tells me that you probably adjusted little or nothing. So you need to go through these adjustments or if you're not qualified, find a mechanic to do so. You still may have a problem, but until these adjustments are made, all bets are off.

- Mark

I had that problem when I was rebuilding my Bianchi. In my case, the problem was not enough cable tension and a high=limit screw that was too loose.

Should we assume that the cassette is new? Post #1 doesn't say that. A new chain on an old cassette could explain the skipping part.

if the rear shifting shifts properly and accurately on one side of the cassette
and is out of adjustment on the other side of the cassette
the culprit is likely a bent derailleur hanger

bike shops have a tool to check hanger alignment
which has to be pretty close to perfect for indexed shifting to work properly

as for the chain falling off the smallest cog
the first thing you should do when setting up shifting
is set the H and L limit screws without the cable attached
to make sure the derailleur can not push the chain too far one way or the other

As a chain wears the distance between its pins increases. A chain with pins spaced more widely wears out the teeth of the cogs in the cassette and chainrings. If the teeth on the cogs of the cassette have worn down enough a new chain, with its more-closely-spaced pins, will slip off.

Check both Hi and Lo derailer settings as well as tension:

http://www.youtube.com/watch?v=SkzvfCaIbyQ

I had this same skipping chain problem before and it was caused by severely worn cogs. The gear teeth looked more like hooks when viewed from the side. It was a very old Bridgestone with 7 speed Shimano LX. The cassette was replaced and the LBS recommend that I replaced the chain as wear which showed signs of wear (stretched and deflected). However the problem persist and a week later, I had the LBS inspect the bike again. Seems like a 10-speed chain was used, and being narrower, it didn't "sit" properly on the cogs. Chain was replaced again (at no charge) and the skipping never occured again.

Since you replaced the rear wheel, I will guess that you need to adjust the limits on the rear derailleur. Even if the wheel was the same brand and model, there might be enough variation to require this adjustment. Go to the Park Tool website and follow the instructions for rear derailleur setup.

Hi everyone,
Thanks for the many responses above. I have some additional information:
-The cassette, free hub, and chain are all brand new - never been on the road.
-The chain is a KMC Z51 RB. The box states that it is compatible with 18, 21, and 24 speeds.
-The derailleur cables are a couple months old.
-I tested out the shifting. It is fairly crisp on the larger sprockets of the cassette. The chain runs fairly smoothly (apart from cross-chaining) on the larger sprockets.
-The shifting becomes a bit less crisp as I shift into the smallest 3 sprockets.
-I turned the low limiter screw on the rear derailleur as far as it would go clockwise.
-Even after adjusting the limiter screw, when I shift into the smallest sprocket, the chain immediately jumps off the cassette and wedges into the frame.
Here are my questions:
1. Based on the above what is the likely problem causing the chain to jump?
2. Someone above suggested the problem could be the chain is too long. Suppose I cut off another link. Suppose further I had in fact measured the chain correctly at the outset. Can I repair the chain by just adding an additional power link?
3. Is the mix of a SRAM power link and a KMC chain a potential problem?
4. Someone above suggested my high limit screw needs to be adjusted. If I adjust the high limit screw, won't that mess up the shifting to the larger sprockets (which is fine).
Thanks!

It is the high limit screw that keeps the chain from jumping off the smallest cog. Proper chain length is determined by looping chain over the big front and big rear without going through the derailleur.

Uh, and adding a link to the length you could make it without derailer.

Yes- thanks. Coffee hasn't fully kicked in yet...

The high limit screw is also the outer limit screw. HIGH gear is the smallest cog on the back, largest chainring on the front. If you check parktool.com/blog or sheldonbrown.com both explain which is which. It does not matter how much you screw in the limit screw - what matters is whether the bottom of the screw hits the stop on the derailleur - otherwise it's doing nothing. I'm wondering now if the wheel is compatible with the 7 speed cassette, or if the cassette needs a spacer behind it.
http://sheldonbrown.com/k7-7.shtml#stay

Chains have to be much too long to cause a problem, and then only on the small chainwheel and smaller cogs.