Badly skipping chain

[jd123321]

OK - here's a puzzle for you all...

Bike is a 1990's SM400 (26" hardtail mountain bike, 21 speed, 3x7)
It's a little worse for wear but has much sentimental value... and I'd like to bring it back to working order..

When I'm on the smallest/highest gear in back, the chain skips on every hard pedal down push.

Short history & observations
It's had a number of bumps and bangs over the years
The rusty chain snapped, and I replaced with a 3/32 chain purportedly for a 7/8/9.
Now the chain skips horribly -- but only on the smallest ring in back.
All other gears are fine.
I've traded out rear wheels with no impact on the skipping. So, I don't think it's worn chain ring.

The skipping is not impacted by changing front chain ring.
The chain looks to be very close the the rear fork.

Could be that the derailleur is bent... or that I picked the wrong chain?

Any ideas?

Thanks much.

 

[black lightning 1987]

That's usually caused by a new chain on a worn cog. I would try replacing the cassette or freewheel with a new one.

 

[jd123321]

That's usually caused by a new chain on a worn cog. I would try replacing the cassette or freewheel with a new one.

Thanks for the suggestion... I traded out the rear wheel with no change. So, I suspect that I can rule out worn cog.

 

[MikeA]

Only on the smallest cog? Maybe the H limit screw needs adjustment.

Starts about 2:15

Also, double check the chain, 7 & 8 speed are a different chain than 9 speed.

 

[jd123321]

The plot thickens.

I put the original chain back on (less a broken link), and the problem disappeared...

So, the question is why?

Chain stretch/cog wear?
Original chain measures 12 1/8 inch for 24 links... or about 1/8 inch stretch over 24 links.
This is pretty minimal and less than the other bikes I checked. So, I eliminated stretch as a cause.

Chain size?
The new chain has inner width = 2.4mm and outer width = 7.1mm.
Per bike gremlin (link below), this is an 8 speed chain.

My original chain has inner = 2.5 mm and outer width = 7.5 m.
This doesn't match anything in the bike gremlin list... rather it's between a 6 and 7 speed chain.

The LBS told me 7 and 8 speed chains are interchangeable... but it seems not in this case for my bike.
I'll probably try a 7 speed chain next.

I'm interested to see if anyone can fill in the blanks. Did old Cannondales use a different chain dimension?


https://bike.bikegremlin.com/3555/bicycle-drive-chain-dimension-standards/

 

[MikeA]

This sounds like something that won't be solved through a forum, we don't know what information is missing without experiencing the problem in person. I feel that if you take the whole bike with you to the shop, a pair of expert eyes will spot the problem pretty quickly.

 

[black lightning 1987]

1/8" in a foot of chain is enough to cause worn sprockets, especially if a single one gets used most of the time. The small cog will be the first to cause a skipping problem if all have the same amount of use.

I am using 9 speed chains on a number of index shift 6, 7, and 8 speed bikes. They were all built up with new cassettes or freewheels and new Shimano or FSA chains. 6/7 speed chains were wider than 8 and 9 but in my experience an 8 or 9 speed chain works fine on 6/7 setups.

You can buy a new 7 speed cassette or freewheel for less than $20. Even if it turns out that it's not the problem you will have it when the time does come to replace it.