New chain slipping on Trek 4300
 

Trek 4300

Snapped a chain in half on my last ride. Replaced chain. Everything went to hell. I've readjusted everything to get it shifting smoothly again. I also removed 4 links from a 116 link chain to get the proper length.

I would venture to say there is not 1000 miles on the cassette and chainrings. They don't appear worn. The cassette in particular still looks good.

When I stand and apply moderate heavy load to the cranks, chain slips and jumps like crazy on almost every pedal stroke that I push hard.

Chain slips on every front chainring and is worst on cogs 5-8 of 8 (smallest cogs).

I'm reasonably savvy with these types of issues, but I cannot figure out what the hell is going on.

Sounds to me like worn cogs (and probably cassette). The old chain and cogs/cassette had worn together -- the new chain upset the "happy family".

I can't imagine a chain slipping on the rings after only 1000 miles, cogs maybe. Too hard to diagnose online; best bet is to take it into your preferred LBS.

Could also be the new chain isn't properly measured. Did you remove any links?

He said he removed four.

same issue w/ an XT mid-cage
 

I've got the same problem. Just changed to a mid-cage rear derailleur since I replaced the large chain ring in the front (SC Nomad) w/ a bashguard. Chain is new, cassette is new. I pulled a few MORE links off last night and tested it in the drive way. It didn't skip under heavy pedaling so it could just be a chain length issue. My derailleur / dropout LOOKs straight but It's taken some hits so that could be a contributer as well. Good luck....

I had a chain skipping problem caused by a stiff chain link, does it only happen on your smaller cogs? If so the chain might not be flexing correctly...

No knowing how to adjust chain length seems to be an every day issue on this forum.

The shifting should be adjusted as well as possible on the workstand, then taken out on the road. Don't mix up a shifting problem with chain skip, which is a failure of the chain to mesh with worn cogs. One has nothing to do with the other.

The chain length being incorrect will not affect shifting to substantially or the ability of the chain to mesh with the cogs. An overly long chain would just hang loose in the little ring and smallest cog(s), but it would still shift and drive the bike. A too short chain, shifted into the big/big can be a disaster.

I took the bike to the shop. The mechanic took one look at the crank and said "You stand up on the pedals and push hard, don't you?"

I still couldn't see the wear, but he threw a length of test chain around the crank and showed me how it wasn't meshing properly.

He then tested the rear cogs. Those were borderline as well.

I knew the SR Suntour crankset on the 4300 was bad when I got it, but gee-wiz... less than a 1000 miles and I need a new drivetrain... because EVERY front chainring was worn out? I ride hard up hills, but I don't think I'm so hard on my components that they should have worn out so prematurely.

Another nugget... the BB that Trek used on that bike wasn't properly sealed. The mechanic popped it out and showed me... it wasn't a single cartridge, but 2 cartridges held together by a rubber seal... Grrr...

In any case, problem solved and now my bike has an upgraded drivetrain.

I replaced my primary chainring and that fixed it (my cassette only had around 40 miles so I left it alone). May want to throw yet another new chain on with the new chain ring(s)

1000 miles off-road, with wet and dirt, is likely to wear drivetrain parts faster, unless you are meticulous about maintenance.