Chain Jump

[mace112961]

I recently purchased a used Trek 1000 equipped with Shimano 105 components, a Sachs LY 94 12-21 cassett mounted on a Dura-Ace hub.

When I shift down to the bottom two gears (12 & 13) the chain "jumps" as if it is trying to climb up the next gear, but it's not. As long as I have the bike on a stand spinning the wheel (with is no toruqe load), there's no problem. But I cannot ride the bike and shift onto these gears without the jump. I have no problems like this on the other gears.

I'm guessing it's a chain problem, but I don't know and would appreciate some help. The chain has the following markings stamped on the links:

UG
6F
VIA (I think, can't tell for sure)
Japan

Help please!

 

[ScienceIsCool]

I'm guessing that your (high) derailleur limit isn't set quite right. I would back it off by half a turn and see if the problem goes away. If that's the case, you may also have to readjust your derailleur cable tension. Let the Park Tool website be your guide - they've got great instructions.

Another guess is that it may be a stiff link. Put the bike in the stand, slowly pedal backwards and watch the rear derailleur. If there's a stiff link, it'll make the derailleur cage jump slightly and won't sit quite right on the cogs.

John Swanson

I recently purchased a used Trek 1000 equipped with Shimano 105 components, a Sachs LY 94 12-21 cassett mounted on a Dura-Ace hub.

When I shift down to the bottom two gears (12 & 13) the chain "jumps" as if it is trying to climb up the next gear, but it's not. As long as I have the bike on a stand spinning the wheel (with is no toruqe load), there's no problem. But I cannot ride the bike and shift onto these gears without the jump. I have no problems like this on the other gears.

I'm guessing it's a chain problem, but I don't know and would appreciate some help. The chain has the following markings stamped on the links:

UG
6F
VIA (I think, can't tell for sure)
Japan

Help please!

 

[Lonnie Utah]

Another guess is that it may be a stiff link. Put the bike in the stand, slowly pedal backwards and watch the rear derailleur. If there's a stiff link, it'll make the derailleur cage jump slightly and won't sit quite right on the cogs.

I've got a stiff link right now and it's driving me nuts. I've suspected as much for a few days and confirmed it last night. I found it exactly as you described. You can litterly watch it stay up before it jumps on the casette. And it's a distinct JUMP! in my 23 & 21 rear cog. Time for a tune up and maybe a new chain (hopefully I can free it up...)

 

[ScienceIsCool]

A stiff link is usually caused by too much side loading on the roller. At the stiff link, grab and bend the chain side to side with quite a bit of force. That should loosen it up and cure the stiff link. It really shouldn't need a new chain!

John Swanson

I've got a stiff link right now and it's driving me nuts. I've suspected as much for a few days and confirmed it last night. I found it exactly as you described. You can litterly watch it stay up before it jumps on the casette. It produces a distinct JUMP! in my 23 & 21 rear cog. Time for a tune up and maybe a new chain...

 

[Retro Grouch]

Another guess is that it may be a stiff link. Put the bike in the stand, slowly pedal backwards and watch the rear derailleur. If there's a stiff link, it'll make the derailleur cage jump slightly and won't sit quite right on the cogs.

I'm betting on a stiff link too. Try turning the crank backwards with your bike in the workstand. The stiff link will usually reveal itself when it comes off of the bottom derilleur pulley. I usually work over a stiff link with my chain tool to loosen it up.

 

[Lonnie Utah]

A stiff link is usually caused by too much side loading on the roller. At the stiff link, grab and bend the chain side to side with quite a bit of force. That should loosen it up and cure the stiff link. It really shouldn't need a new chain!

Yeah, at 11:30 last night I really didn't feel like messing with it. My wife doesn't like it when I come to bed with greasy hands ...

 

[Albert 50]

I'm guessing worn chain with 12/13 not providing enough contact teeth OR a new chain was put on & worn 12/13 teeth won't mesh causing it to slip

 

[2 old 2 go slow]

I'm guessing worn chain with 12/13 not providing enough contact teeth OR a new chain was put on & worn 12/13 teeth won't mesh causing it to slip

I second that. Check the chain for stretch. If you don't have the means to check it for yourself, any bike shop will be happy to check it and install a new one for you.
Chains are cheap. Always replace them before they have a chance to hurt your cogs and rings.

 

[mace112961]

I second that. Check the chain for stretch. If you don't have the means to check it for yourself, any bike shop will be happy to check it and install a new one for you.
Chains are cheap. Always replace them before they have a chance to hurt your cogs and rings.

Thanks all for the replies and input.

It's NOT a stiff link. Double checked them all. Also not the H/L/B adjustment, checked those too.

After careful study, it's definetly a mismatch of the chain linkage with the 12 & 13 gears. 2 or 3 teeth will engage the chain fine, but the next link will ride-up slightly on the 4th tooth, which leads the 5th to ride a little higher. By the time the 6th tooth engages it has the joint of the two links directly above the point of the tooth. The "jump" happens as the cog turns and releases the first 3 teeth that engaged normally and have been pulling. As these teeth release the chain they allow it to slide forward and the 4-5-6 teeth then snap down onto the gear, which starts the process over. Creating a jump/jerk every 1/2 turn of the crank.

I'll take it to my LBS to have it checked out.

Thanks all.

 

[2 old 2 go slow]

... it's definetly a mismatch of the chain linkage with the 12 & 13 gears. 2 or 3 teeth will engage the chain fine, but the next link will ride-up slightly on the 4th tooth, which leads the 5th to ride a little higher. By the time the 6th tooth engages it has the joint of the two links directly above the point of the tooth. The "jump" happens as the cog turns and releases the first 3 teeth that engaged normally and have been pulling. As these teeth release the chain they allow it to slide forward and the 4-5-6 teeth then snap down onto the gear, which starts the process over. Creating a jump/jerk every 1/2 turn of the crank...

You've just authored an excellent description of what happens with either a badly stretched chain and/or worn cog.