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Rear Derailleur problems

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Old 10-14-06, 04:00 PM
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cynergy
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Rear Derailleur problems

Hi All,

I bought a fairly inexpensive "Schwinn" brand bicycle a few years ago. It's a "hybrid" cycle, sort of like a cross between a mountain bike and road bike. The bicycle is equipped with grip shifters. When you want to shift, you twist the grips on the handle bars to the indexed number of the gear that you desire.

About 2 years ago, a stout branch got caught in my rear wheel and broke the rear derailleur (it's some kind of Shimano product). The derailler cracked in a couple of pieces and the plastic gear flew out. Last year I took the frame to a local shop. They installed a Shimano derailleur on the bike. HOwever, since then, the bike has had a shifting problem. If I shift to the smallest chain ring on the crankset and the low (larger) gears on the free wheel, I notice that when I am riding up hill and putting a lot of torque on the crankset, the rear derailleur wants to shift itself. This is kinda annoying, especially when I'm riding up some steep hill in low gear. As I pedal hard (usually standing on the pedals) all of the sudden the rear derailleur makes a noise like its shifting and the resistance in the pedals gives out as the derailleur tries to shift. it usually does not succeed in shifting, but it acts as if it's going to shift to the next smaller gear.

I recently put my bike on a bike stand and adjusted the front derailleur and rear derailleur. I noticed that the rear derailleur seemed like it wanted to shift the chain from the bigger rear gear to the next smaller one (i.e., to shift "up" one gear). I adjusted the position of the derailleurs using the little adjustment screws and also adjusted the tension of the rear derailleur cable and was able to get the gears to work fine on the stand.

However, when I took the bike out for a test ride, the rear derailleur occasionally attempts to shift when I am in a low gear and putting a lot of torque on the crankshaft. I don't know for certain what's going on, but I suspect that it's trying to shift to the next smaller gear. All of the resistance in the crankshaft seems to go away momentarily when this happens.

This is a really bothersome problem, since on the repairstand (both at home and at the local shop) the problem doesn't manifest itself.

Any ideas on what the possible causes might be? How should I proceed with debugging this problem? I'm pretty new to bicycle mechanics, but I enjoy riding recreationally. I did invest in a quality bike stand, since I was tired of having to flip my bike over all of the time whenever I needed to adjust the derailleurs or brakes.

Thanks in advance!
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Old 10-15-06, 10:20 AM
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Bikewer
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provided you've adjusted the rear DR properly, a possible cause is a bent hanger which might have resulted when you caught the branch.
I would start from scratch. Remove the chain and put the bike in the stand. Remove the shifter cable. Check the angle of the DR cage with the cogs; they should be parallel. The "little screws" are the limit screws; make sure the DR idler pulleys line up exactly with the smallest cog at one end of travel, and the biggest at the other end. If they don't, you may shift "off" the cogs.
Make sure the DR moves freely from one side to the other, and stays parallel.
If so, you can put the chain back on, shift to the highest (smallest) gear and re-attach the cable.
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Old 10-15-06, 10:53 AM
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Portis
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Sounds like your cable tension may be too loose or you actually have a worn cassette. The fact that you describe it as "acting like it wants to shift" makes me wonder if the cassette is worn. Seldom do derailleurs just act like they want to shift. If cable tension is wrong they will normally go ahead and shift on their own.

I'd go to www.Parktool.com and go through the derailleur adjustment step by step. Then when you are 100% sure you have it adjusted correctly, i'd look elsewhere. Also measure your chain for wear. If it is extremely worn than your casette is probably hosed.
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Old 10-15-06, 07:36 PM
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cynergy
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Thanks for the tips folks. I suspect my problem may be tension related. The problem was originally worse - the rear DR wanted to shift when the bike was in gear 1 (the lowest gear). Yesterday I tweaked the barrel adjuster to add more tension to the cable and was able to get the bike to work properly in gear 1 and 2. It still seems to misbehave, however, for gears 3 and 4 and possibly 5. I wasn't able to add any more tension to the cable because the adjuster was at its limit.

I am going start from scratch as Bikewer suggested and double check to make sure the hanger isn't bent before I attempt to adjust the rear DR.

Thanks for the link to Park Tool. I will check it out to make sure I'm adjusting my derailleur properly. I'll post an update once I've had a chance to work on my bike. Thanks again for the replies.

Cynergy

PS This is a great web forum!
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