neilfein

Sorry if this has been covered before. Didn't find anything in the threads, but I could have missed it.

I've been having gears slipping when shifting on any significant grade. Usually happens in the lower (hill-climbing) gears, and more often when the pedal chainring is in the granny gears. Any thoughts how I can start looking to find the problem? My bike is an 18-speed MTB with Shimano SIS twist shifters.

Thanks!

BeckyW

My chain slips if I wait too long to downshift, when climbing a hill, and end up pushing too hard while shifting. Turns out you're not supposed to be pedaling hard when shifting. Sorry if you're not a beginner - this was news to me, not too long ago. If that sounds like your problem, try shifting sooner, or easing up on the pedals while you're shifting.

frankiee

I would suggest getting some chain lube from bike store and turning bike upside down and slowly pedle the bike while applying the lube and going slowly thur the gears.
Watch the chain and gears and listen to them. They should be smooth.
If not then go from there.

neilfein

I hope you're wrong, but I have a feeling you're not.

I lube the bike ~once a week or after a serious cleaning (e.g., when covered in mud, etc), but I usually just put a few drops on the chain and casette, and just ride it to work the lube in. I'll thy this and see if it helps.

Steev

The most likely problem is that your chain has worn (stretched) until it can no longer mesh properly with the cogs. Do this. Take a ruler and place it along the chain. Set the zero mark at the middle of one rivet, look at the 12" mark. There should be a rivet right there. If the rivet is more than 1/16" away from the 12" mark, your chain is worn. You'll probably have to replace the rear cogs as well. If the stretch is 1/8" or more, your chain-rings may well be toast as well.

neilfein

Thanks for all the advice, everyone. I've puzzled out a few more things about this. (Checking out the chain and gearing is on the horizon. I'll be changing the tires soon, so I'll take a closer look then. Thanks, Steev.)

I've since noticed that when this problem happens, I can fix it immediately by sliding a little know on the side of the friction shifter.



(The shifter stays in the gear you shift it into when the knob is against the side, and when the know is at the bottom (where it is in the photo, I'm holding it upside-down), it goes to the highest (hardest) gear for the in the cassette. Pushing the knob to "lock" solves the problem - until the next time.

I read elsewhere on BF that this can be fixed by simple cleaning out the plates inside the shifter. Seems like it couldn't hurt, so I went to give it a shot. I had the shifter off the handlebars, and couldn't figure out how to open the thing! I can't imagine it'd be a sealed unit, you'd need to get in there if you're changing the cables. Here's what it looks like:




Anyone worked on one of these lately? Thanks again.

Old School

You better know how to put that back together again, Mister!!

neilfein

I never figured out how to do this. Anyone have any idea?

cny-bikeman

A couple of notes:

The problem is not due to lubrication or chain wear. The OP noted it happens only when shifting. Poor shifting to a higher gear in the rear (or lower in the front) can be due to friction or binding in cables or the derailleur, but not the chain. Skipping in the same gear under pressure is due to chain or cog wear.

It is indeed true that it is best to have low pedal pressure when shifting, esp to a lower gear in back.

Measuring a chain must be done at the portion of the chain that goes over the top of the chainring and rear cog, and must be done under tension (with pressure on the pedals) to get a correct reading.

DMF 

What exactly do you mean by "slipping"? Do you mean that it under or over-shifts (skips past the desired gear)? Or that you lose resistance at the pedals?

neilfein

I understand what's happening a little better than when I made the OP:

The thumb shifter works fine in friction (non-indexed) mode, but when in index mode, it tends to spontaneously shift into higher (harder) gears on the casette. I thought this was a friction issue, but since it's only happening in index mode (there's a toggle on the shifter), I'm thinking it's eithe cable tension or the rear deraileur, neither of which I know well mechanically.

DMF 

It's a simple adjustment. Check the Park Tool site.

cny-bikeman

OK, completely different problem than it first appeared. Do a full adjust from scratch, and be sure to first make sure all cables move smoothly. In friction mode one tends to move the lever until the derailleur shifts, but indexing moves the lever a specific amount. If the cable is binding then it won't always move a corresponding amount right away. Bouncing or the stress of hard pedalling may allow the cable to move the rest of the way, thereby shifting to a higher gear. Probably would not happen in the stand.

neilfein

Finally, I've tracked down the problem. And it happened by accident.

I've taken to making holders out of clear plastic with a rubber band taped to the back to hold a cue sheet or map, and wrapping the rubber band around my handlebars. Well, last weekend, I went for a long ride and noticed that the slipping problem wasn't happening. The rubber band had slipped between the base of the thumb shifter and the lever that switches in and out of index mode, pushing it up maybe half a millimeter. So the problem is in the shifter -- which is a sealed unit. A not-so-L LBS grees, and I'm replacingt the shifter.

Fixed by a rubber band... sheesh.

cyccommute 

He doesn't need to start from scratch. The derailer isn't lining up with the cogs due to a stretched cable. As he applies torque while riding uphill, the derailer is trying to find a happy place which happens to be between the rear cogs, hence the slippage. On a stand the gears will clatter a little. Tighten the cable in 1/4 turns until it gets quiet. Should fix the problem.