motobecane derailer issues

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  #1  
Old 05-09-18, 07:49 PM
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motobecane derailer issues

I have a 30 year old Motobecane which I have used infrequently over the years due to poor shifting and chain coming off the sprockets. I initially brought it back to the bike shop where I purchased it but they could never get it to operate correctly. Lately the chain hangs down as if the derailleur is not keeping tension on the chain. If I push the derailleur backward with my foot I can reduce some of the sag. changing gears causes the sag to return and often the gears hop in and out of where I shifted them, at times the chain will also come off. I am assuming the derailleur is at fault. Being that my bike is not new will an after market derailleur fit?
 
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Old 05-10-18, 04:02 AM
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Something is not right because the rear derailleur should have spring action to remove the excess length of the chain as it moves between the gears on the cassette.

Will something else work, maybe, but retrofitting 30 year old equipment would take some time to sort out.

I suggest a new bike shop!
 
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Old 05-10-18, 04:48 AM
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Hi Greg,
That's some serious sag. Looks like the rear axle has more room to move back but I agree with Marq, the springs in the derailleur aren't doing their job.

Check chain length to be sure it isn't too long, should accommodate both large sprockets (front and rear with a little slack.

A new derailleur with the wide range capacity should work fine.

I'm from the repair age so I would pull the derailleur and disassemble, clean, and lube. I believe there are 2 springs, one for the lower arm and the other in the pivot at the top.

Also be sure you don't have any stiff links, relater to the skipping not the sag.

If you are not comfortable get a good bike shop.

One more, measure 12" of chain to be sure it isn't worn out. Each link should fall on a 1/2" mark with no more than 1/16" stretch over the 12". At 1/8th it id time for a new chain but I like to preserve sprockets so like the 1/16" max.

Bud
 
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