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Removed derailleurs but still doesn't stay in gear

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Removed derailleurs but still doesn't stay in gear

Old 10-06-10, 03:52 PM
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Removed derailleurs but still doesn't stay in gear

I just put together an old junker bicycle from the entrails of a few bikes that had serious problems, something that I can lock up in city center and not worry about.

The shifting system was hosed on this 15 speed GT MTB, so I decided to remove the front and rear derailleurs and use it as a single speed bike.

Today I had the first test. It worked well for a few miles, but soon the chain started hopping down to higher (smaller rear cog) gears. It would hop to a higher gear, hop back up to the middle gear where it should be, then hop up again. After a mile or so of this hopping, the chain came off the front ring as well.

I'm not sure whether this could be caused by:

- The old chain that has lots of flex. Is chain stiffness important even with a single speed?
- The front rings might be slightly bent out of true.
- Something else which I've not considered.

I've worked on bikes, but only as a hobby so I'm not experienced.
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Old 10-06-10, 03:57 PM
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Your chainwheel needs to be reasonably straight - ther rings are either bent or not, not might be....
Flex is not an issue, wear might be. Check Sheldon site for info on chain wear.
The cog you are using in the rear needs to line up reasonably well with the front chainring (both the same distance from the centerline of the bike) unless you leave the rear derailleur on fixed in one position to lead the chain onto the cog.
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Old 10-06-10, 06:47 PM
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How are you tensioning the chain? You either need horizontal drop-outs so you can move the axle to take up the slack or if you have vertical drop-outs you need some sort of tensioner.
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Old 10-06-10, 07:09 PM
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All that, plus cogs and chainrings made for single gear use hold onto chain much better than cogs and rings designed for shifting.

I'm surprised that GT had a 15-speed. Coulda sworn the first GT MTBs were 18-speed.
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Old 10-06-10, 08:56 PM
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You didn't shorten the chain enough. If the front and rear are fairly well in line and if the chain is slack free, but not actually tight, then it'll stay in place. But any small amount of slack and it'll autoshift like you have found.

Your best option unless the bike has semi horizontal dropouts is to just get a cheap rear derraileur and shifter and use it as a rear shift only bike.

It's not that the chain is too flexible. They are all very side to side flexible to allow for shifting when new. You just don't have the right lack of slack to lock the chain into the single gear combo.
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Old 10-08-10, 06:07 PM
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Actually the bike does have near-horizontal rear dropouts, and that's how I've been tensioning it. Now that I think about it, I wonder if the reason it started autoshifting after a few miles was that the rear wheel wasn't tightened down enough so it lost tension.

I just ordered a new chain to see if that will help. I don't want to put a derailleur back on there because the rear derailleur hanger is horribly bent up, so it probably wouldn't align well.
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Old 10-08-10, 06:09 PM
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FYI I'm using the middle ring in the front and the middle ring in back, which seem to both be close to the same distance from the center of the bike.
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Old 10-09-10, 01:29 PM
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If your rear wheel shifted a little in the dropouts then that would certainly explain it shifting around. The slight angle change at the rear would be the equivalent of moving one of the sprockets sideways by a LOT.

If you lay a long ruler or other straight edge against the chainring you'll be able to see which rear cog it lines up with for the straightest chain line. Try that combo with the axle tightened well and see if that fixes your issues.
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Old 10-09-10, 04:27 PM
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Today I went out for the second ride on this bike. It was okay for a few miles. I noticed a clicking noise while pedalling like it was hunting for another gear but mostly staying in gear. I noticed that this click corresponded to when my right foot was down, so it's probably associated with the front chainrings not the rear. The clicking got worse, there was a single bad autoshift, and then moments later the chain snapped! One of the links actually broke.

I will freely admit that I'm no pro at installing chains, and in fact my chain tool had issues when I was installing this chain, so I probably bungled the install.

I also noticed that the front chainring on the outside is very out of true, even though the inner two front chainrings seem true. I think this was causing a pinch issue where the chain was getting pinched between the middle ring that it was on, and the outer bent ring. I got out some adjustable wrenches and bent the outer front ring back close to true, and I reinstalled the chain hopefully a bit more artfully, so we'll see how the next ride goes!

Lesterofputtets wrote:
I'm surprised that GT had a 15-speed. Coulda sworn the first GT MTBs were 18-speed.
Wow, you guys are good. In fact the 5-cog rear wheel on the GT MTB was from another bike!
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