Skipping the middle ring

[RichardB]

1991 Raleigh MTB, triple Shimano Biopace, 48-38-28, Shimano 100GS shifters.

Gears shift from small to large faultlessly, exactly as intended. One click, small to middle, another click, middle to large. Going the other way, one press on the smaller lever takes the chain all the way from the largest to the smallest ring, bypassing the middle ring, and then I have to change 'up' again to get the middle. It feels like a problem with the shifter, as if the ratchet thing is slipping in one direction. However, I got a new (used but good) shifter for an unrelated reason, and despite looking unworn and almost new (and lightly lubed) it's exactly the same. It's a fairly recent issue, as in the previous 25-odd years it has always changed correctly. The fact that the problem is there even with a new shift lever suggests that the problem might be with the derailleur (or that both shifters coincidentally have the same fault, of course). However, it has a new cable, everything works smoothly and the limit stops are correct. I took everything off the frame (including front mech) when I did a light recommission/deep clean a couple of years ago, and the problem might have started then.

Any thoughts?

 

[raleighnut]

There's nothing in the derailleur to go wrong, all the movement is controlled by the shifter, it could be cable tension though, does the mech sit exactly central on the middle sprocket when it is selected ?

 

[RichardB]

Good point - if the tension is too great it could be pulling the cable through the ratchet in the shifter. Will check when I get back home. Cheers.

 

[C R]

Good point - if the tension is too great it could be pulling the cable through the ratchet in the shifter. Will check when I get back home. Cheers.

Wouldn't it be the other way around, not enough tension? Going to the small ring the cable is released for the spring in the derailleur to pull back.

 

[raleighnut]

Wouldn't it be the other way around, not enough tension? Going to the small ring the cable is released for the spring in the derailleur to pull back.

It could be either, the important thing is the mech sitting dead centre on the middle sprocket, it is the only thing that 'indexes' the middle ring on a triple

 

[nmfeb70]

Remove the inner cable from the outer and wrap it around your hand. Take up the tension and try the shifter. If it clicks three times in both directions then it's not the problem.

 

[boydj]

Has anything changed? For a front triple shifter, it's all about getting the cable tension tight, but not too tight with the chain on the small ring, and the limit screws set accurately. Once the limits are set they shouldn't need touched again.

To fix the problem, first check the shifter as indicated above, then check the limit screws by moving the mech by hand with the cable still disconnected. Then make sure the shifter is set for the small ring and re-attach the cable, making sure it is tight, but not too tight. Check the shifting and adjust the cable tension if required. This could take 5 minutes, or 30 minutes and a lot of patience.

 

[RichardB]

Thanks for these helpful replies. A quick visual check shows that the mech is dead centre over the middle ring when indexed there (one click up from the small ring). Limit stops are correct (never been moved, as far as I remember). I hadn't considered cable tension as a cause, but I can see how it might mess up the balance of springs, pawls etc in the shifter mechanism and cause it to skip. Thinking about it, I'm pretty sure the issue is in the shifter, not the mech, but it looks quite hard to dismantle to get at the guts of the thing. On a visual check of what can be seen without dismantling, it all looks clean, unworn and properly (i.e. very lightly) lubed. Will have a proper look in daylight over the weekend, Thanks all.

 

[raleighnut]

Thanks for these helpful replies. A quick visual check shows that the mech is dead centre over the middle ring when indexed there (one click up from the small ring). Limit stops are correct (never been moved, as far as I remember). I hadn't considered cable tension as a cause, but I can see how it might mess up the balance of springs, pawls etc in the shifter mechanism and cause it to skip. Thinking about it, I'm pretty sure the issue is in the shifter, not the mech, but it looks quite hard to dismantle to get at the guts of the thing. On a visual check of what can be seen without dismantling, it all looks clean, unworn and properly (i.e. very lightly) lubed. Will have a proper look in daylight over the weekend, Thanks all.

If it's central then the tension is correct.

 

[RichardB]

Issue isolated to the shifter mechanism. I ran it on the work stand without the chain, and the mech shifts correctly small-middle-large, but on the way back it the mech jumps from large to small with a big clunk. I think someone had already said that the 100GS was never a high-quality item, so the fact that the replacement shifter has the same problem is no great surprise. I can try to get another shifter, or I can replace with a friction shifter, or I can live with it. (Actually, there is a fourth possibility: I have the original shifter with a broken lever. I can take this apart for educational purposes and perhaps use that knowledge to mend the newer one. I think I might destroy the shifter by dismantling it, but I might learn enough to mend the newer one. Or I could go and have a beer, as life is short enough.)

I think I'll probably live with it, as it's not a huge problem. Thanks to everyone who responded. Really appreciate the help.