Chain slips from middle to small chainring (triple)

[robjh]

I have a 10 speed rear cassette, 11-28T, and triple chainring 50-39-30T. I recently noticed that when I was in the middle chainring but largest rear sprocket, the chain would slip off the middle and onto the small (inner) chainring, even if this caused it to rub and bend around the front derailleur which was still in a middle position.

So I replaced the chain (the old one was pretty worn), and also the middle chainring at the same time as this too was showing some wear - see picture of old and new chainrings further down.

But with new chain + chainring I STILL get slippage when in middle front and large rear. I have traced it to one particular tooth on the chainring, directly opposite the crank. I believe I have followed Shimano's instructions and installed the chainring in the right position on the crank.

As both chain and chainring have been replaced, there must be something else at play here. I still have the old cassette for the time being, and the front derailleur, and everything is in much the same position on the bike as before. The chain line in this combination is at quite an angle, but presumably was the same for the many months that it didn't slip.

The chains : old KMC X10-93 ; new KMC X10-L
middle chainring : old Shimano Sora 39T, new Shimano 105 39T (FC-5703)
cassette : Shimano 105 10sp RC-5700

Does anyone have any ideas?

 

[Globalti]

On most chainrings there are a few that are lower in profile to help with shifting - usually one every sixth or tenth teeth, I reckon.

Your cassette (especially the bigger cogs) will have worn to the same profile as the worn out chain so the new chain won't fit it properly. Try lifting a link off the biggest gear in the middle of the contact area - you will see then how worn the cassette has become. There's no point in fitting a new chain to worn rings or cassette, the chain will only contact the cassette at the nearest tooth to the direction of tension, not evenly all the way round, and the new chain will wear out fast.

 

[robjh]

On most chainrings there are a few that are lower in profile to help with shifting - usually one every sixth or tenth teeth, I reckon.

Your cassette (especially the bigger cogs) will have worn to the same profile as the worn out chain so the new chain won't fit it properly. Try lifting a link off the biggest gear in the middle of the contact area - you will see then how worn the cassette has become. There's no point in fitting a new chain to worn rings or cassette, the chain will only contact the cassette at the nearest tooth to the direction of tension, not evenly all the way round, and the new chain will wear out fast.

I've got a new cassette on order and will get it fitted this weekend, but I just couldn't see the link between wear at the back and chain behaviour at the front. But I live in hope.

 

[Globalti]

The big ring being of the biggest diameter, the chain will easily slip around it if the teeth are worn, especially if they are as worn as your chainring was. Don't forget that the chain will be contacting the cassette only on ONE worn out tooth. Here:

 

[Globalti]

PS: Sorry, just realised we aren't talking about the same problem here. But you shouldn't run a worn cassette with a new chain, whatever.