Bicycle Mechanics - Good Upshifting, bad down shifting?
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05-22-09, 07:54 PM
I'm helping a friend get a bike on the road, and I'm almost there. This is an old sears or jcpenny bike with a suntour GT derailer and one of those ratchety suntour shifter that aren't indexed.
OK. So it upshifts greatly! Through all five gears no problem
It downshifts great from 1 (smallest cog) to 2.. but from 2 to 3 it skips to 4 (pretty much always) and then it shifts nicely from 4 to 5. There are only 5 gears on this bike (only one chainring up front). I don't even know what to do?
This derailer has 3 adjustment screws. Two limits and one that I'm not exactly sure what it does?
05-22-09, 08:17 PM
I would check to see if you have a worn chain and rear free wheel. Is it possible that the RD has a bad spring that is not able to keep tension when going in the one direction?
05-22-09, 08:19 PM
Bent derailleur? warn out pulley? bent tooth on one of the cogs?
05-22-09, 08:24 PM
Before trying to adjust a derailleur, I always say: clean it up and oil it up. Use a rag and a toothbrush to get it as clean as possible; thread the rag through any openings and use a shoe-shine motion to clean up otherwise inaccessible areas. Once it's clean, oil it up - every moving point - and there are quite a few on a rear derailleur. The 8 pivots of the parallelogram - 4 corners on the front side of the derailleur, 4 corners on the back side. The big spring. The two rotating pivots - cage and frame attachment point. And, don't forget the two jockey wheels, which should be lubricated on both sides.
When in doubt, use more oil.
When you're all done with the oiling, move the derailleur around a bit to get the oil to penetrate where it should, and wipe off the excess oil.
Then you can get on with the adjusting.
05-22-09, 09:54 PM
Will do henrick. I'll get back to you in a few days when I get this all done.
05-23-09, 07:03 AM
When in doubt, use more oil.
Oh yuck! I hate that advice. All of the good comes from the first tiniest drop of oil that you can manage to apply. After that, all the oil does is to attract dirt and gum things up.
05-24-09, 01:14 PM
Ok, so I got it working pretty ok. I think the ratcheting mechanism in the shifter is making it hard to shift into third (me thinks it pushes it past the point where it wants to be), but after a thorough cleaning and some tinkering it shifts into 3rd most of the time. I didn't notice any problems except with maybe the design of the cog itself. The chain was good the jockey pulleys were good. It seemed straight, but the rear cog has these two cogs on it that would probably be like 20 and 23 teeth if 1/2 of them weren't missing BY DESIGN. That is to save some metal there are only like 10 and 12 teeth respectively but they are way spaced out.... I would replace with a new rear freewheel cog, but meh. It works.
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