Bicycle Mechanics - Downtube shifting problems...

Whenever i'm in the largest cog in back and pedaling hard up a hill, the bike shifts itself into a smaller cog. This only happens when pedaling hard, and only when i'm in the largest cog.

Can i fix this and how?

Thanks
Brian.

are you in the smallest chainring?

It might be that the limit screw that prevents the derailleur overshifting the chain beyond the big cog needs readjustment. It may be preventing the chain from sitting squarely on the cog. It is something I would consult a bike shop about if you don't know what you are doing -- overcompensating with the limit screw could result in damaged spokes.

I doubt this has anything to do with downtube shifters or other type of shifters - it is probably mainly your rear derailler setup. Do you know if your indexing is properly adjusted with the RD's barrel adjuster? My guess is that you should turn the barrel adjuster counterclockwise a turn or two, which will mean that for each indexed stop in the shifting (assuming this is indexed-shifting on your bike, because it wouldn't make sense for you to complain about this problem otherwise) the rear derailler cage is slightly further to the left, toward the spokes.

However, since you notice this problem only on the large cog, it's possible that your indexing is lined up properly, but the limit screw on the rear derailler, the one marked "L" that limits how far the derailler cage can swing toward the spokes and larger cogs, is screwed in just a little too far. Is the shifting cable that goes to the rear derailler really tight when you're shifted into the largest cog? If so, the limit screw could well be your problem.

Is your shift cable routed under the BB shell? On older steel frames that have a bit more flex, it is possible that as you really mash the pedals the frame flexes, pulls the cable a bit and you get a phantom shift.

Here is a good bit on the issue from Sheldon Brown:
http://sheldonbrown.com/autoshift.html

I doubt this has anything to do with downtube shifters or other type of shifters - it is probably mainly your rear derailler setup. Do you know if your indexing is properly adjusted with the RD's barrel adjuster? My guess is that you should turn the barrel adjuster counterclockwise a turn or two, which will mean that for each indexed stop in the shifting (assuming this is indexed-shifting on your bike, because it wouldn't make sense for you to complain about this problem otherwise) the rear derailler cage is slightly further to the left, toward the spokes.

However, since you notice this problem only on the large cog, it's possible that your indexing is lined up properly, but the limit screw on the rear derailler, the one marked "L" that limits how far the derailler cage can swing toward the spokes and larger cogs, is screwed in just a little too far. Is the shifting cable that goes to the rear derailler really tight when you're shifted into the largest cog? If so, the limit screw could well be your problem.
+1

As Tim points out, this is almost certainly an issue of rear derailer adjustment or cable stretch. The first thing to try would be the barrel adjuster because your RD cable may have stretched just a bit. If a slight tightening of the cable doesn't do the trick, then read Sheldon Brown's excellent guide to derailer adjustment: http://www.sheldonbrown.com/derailer-adjustment.html

Is your shift cable routed under the BB shell? On older steel frames that have a bit more flex, it is possible that as you really mash the pedals the frame flexes, pulls the cable a bit and you get a phantom shift.

Here is a good bit on the issue from Sheldon Brown:
http://sheldonbrown.com/autoshift.html


bingo, that has to be it...

It's an old (80's?) steel bike. The shifters are friction shifters. It doesn't matter which chainring I'm in. I found during a ride today that it happens on at least the first 4 cogs. As I pedal hard, it automatically shifts to consecutively smaller cogs.

Adjustment screws are fine, and cable stretch would cause the opposite problem that i'm having.

bingo, that has to be it...

It's an old (80's?) steel bike. The shifters are friction shifters. It doesn't matter which chainring I'm in. I found during a ride today that it happens on at least the first 4 cogs. As I pedal hard, it automatically shifts to consecutively smaller cogs.

Adjustment screws are fine, and cable stretch would cause the opposite problem that i'm having.
Yeah, that does sound like it could be frame flex. One thing that surprises me though: you're using friction shifters, so is it possible to "overcompensate" by moving the rear derailer a bit more towards the big cogs? Or is there no amount of overcompensation that will prevent it from jumping to the next smallest cog?

By the way, cable stretch *would* cause the effect you're describing: cable stretch means a slightly longer RD cable, which means the RD moving towards the *small* cogs. So don't rule cable stretch out!

Tighten the friction adjustment on the shifter lever. When I (until recently) had DT friction shifters, I had to tighten them almost every time I rode or I would have the same problem.

friction adjustment tightened. It's an old bike so i can't expect greatness.

Mox, if left untouched, it can shift from the biggest, down to #2, then down to #3, then down to #4. No amount of adjustment or overcompensation going to save me from 3 jumps....

Oh, and on second thought, you're entirely right about cable stretch, but still this is too big of a gap, I'd think. And it doesn't make sense why it would only happen while mashing.

I just greased my cable guide on the BB and i'll hope for the best next time I'm commuting...

friction adjustment tightened. It's an old bike so i can't expect greatness.

Mox, if left untouched, it can shift from the biggest, down to #2, then down to #3, then down to #4. No amount of adjustment or overcompensation going to save me from 3 jumps....

Oh, and on second thought, you're entirely right about cable stretch, but still this is too big of a gap, I'd think. And it doesn't make sense why it would only happen while mashing.

I just greased my cable guide on the BB and i'll hope for the best next time I'm commuting...
Yeah, definitely too big of a gap to be cable stretch.

When it hops down from one cog to the next, does the derailer actually move??? I mean, if it goes from big cog to 4th cog, does the derailer move so that at the end it's lined up with the 4th cog??

You've just got to love posters who only give a quarter of the information in their original postings.

Just another question... you haven't greased the friction shifters, have you?

Tighten the friction adjustment on the shifter lever. When I (until recently) had DT friction shifters, I had to tighten them almost every time I rode or I would have the same problem.

It shouldn't be like that, there are washers and star washers that help keep the downtube shifter in harmony (not too much and not too little friction).

It might be worth it to just go down to your LBS and ask if they have an old pair of DT shifters lying around you can purchase.