Chain slipping on middle chainring I have recently replaced the chain and the cassette. Previous chain never slipped. I never had any problems with it at all. But I have made about 10 000 km on it and the wear was beyound any limits, so friends and guys in the bike service recommended replacing both chain and the cassette. I did a lot of reading and I know that chain and the cassette wear together.

So I got chain and the cassette replaced. It is a 9 speed cassette. I have asked to replace it with a little lower gear ratio. Guys in the workshop also indexed the derailleur for me.

Since I collected my bike the chain slips terribly every time I stand up on the pedals. Sometimes it also slips when I apply enough pressure even while seated. But standing up is a definite slip. I already have few bruises from falling down on the frame.

I had assumption about chain being too long, but is seems to be just about ok.

I have also figured out that the chain never slips if it is on the lowest or the highest chainring. It only happens with the middle chainring. As I have said before - it never happened before with the old chain/cassette. Now I have tried to stand up and pedal very hard in all gears on low and high chainrings. Never got a single slip in about half an hour of trying. Changed back to the middle chainring - it slips in every gear from 1 to 9 when enough torque is applied.

So the question is: can it be that chain is slipping at the chainring? Do chains ever slip at the chainring at all? I have never heard of anything like that. Is there a simple way to test it in order to be sure that the chainring is the root cause?

I really don't want to blame it on the chainring and get it replaced just to figure out that the problem never went away.
I expect that after 10,000 km your chainring also needs to be replaced (frankly, I find it astonishing that you made it that far without replacing anything else.  That's amazing.).  Am I correct in guessing that you spend most of your time in your middle chainring?  If so, it will be significantly more worn than your large or small rings, which would explain why you're only experiencing slippage in that ring.

It should be very easy to tell if it needs to be replaced--either take it to your shop and have them look at it, or look at it yourself.  Worn chainrings will show a characteristic "shark fin" on the teeth if they're worn too badly.