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Bicycle Mechanics Broken bottom bracket? Tacoed wheel? If you're having problems with your bicycle, or just need help fixing a flat, drop in here for the latest on bicycle mechanics & bicycle maintenance.

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Old 09-19-05, 12:35 PM   #1
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What could be causing this?

Ok, I picked up a single speed road bike off ebay for cheap to ride to commute on. It is a Schwinn world steel frame with a surly flip flope rear hub and an deore lx front hub, everything else older or generic parts.

So after I got it I put BMX style platforms on it, and rode probably a week before I started to get grinding/popping from the bottom bracket area. I figured I would give it some time to trouble shoot what was actually causing the noise before I went out and bought any new parts. I did the old, take the chain off and check for crank play and bottom bracket grinding test but everything felt fine. More recently I had noticed that when I really punch down on it hard to accelerate, I get a different grinding noise from the chain and the bike suddenly feels sluggish. I tried tightening and loosening the chain by moving the rear wheel back on the dropouts, but that doesnt seem to have helped.

So I am riding today and I accelerate a little bit, and this time the chain actually slips off, off the rear sprocket no less, and to the inside. I didnt crash or anything, but now I have a hole in my toe (sandals) and my front wheel is out of true.

So what could be causing these issues? I checked the chainline, and it appears to be alright, about 54 in the back and 52 at the crank. Can 2mm cause a problem? Also, the measurement method was just my hands and a ruler. The crank is the old cotterless style I think its called, with no spider, the ring is not connected to the arm. The chainring teeth engages the chain in the center of the chain on the back, but slightly toward the inside on the front. The gearing is 52/17. Also, the chainring on the crank does not appear to run 100% true.

So thats all the info I think might be related, can anyone tell me how to fix it? I think I might be flexing something too much, causing the sluggish feel. I weigh about 195, and the crank looks sketchy to me. I guess it could be the frame flexing, but its a touring frame originally, so I doubt it. The only thing I can figure is that its the crank or the chainline. If it is the crank, I guess I will have to replace the whole crank and bb set up, and go to the more modern design which will hopefully be tougher.

Thanks folks.
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Old 09-19-05, 01:07 PM   #2
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Wouldn't the magic number be a 42mm chainline? Pull the cranks off and see how smooth the bottom bracket spindle moves. Once, when my bb was seriously bound, it still spun freely because of the momentum of the cranks, but when I removed the cranks it was obvious that it wasn't working properly.

Is it possible that the lockring on the bottom bracket wasn't on as tightly as it should have been? Once the cranks are off, you might have to re-adjust the bottom bracket and re-torque the lockring.
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