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Old 09-05-06, 02:37 PM   #1
cyclezealot
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Does a bike mechanic have to be part detective?

Today took my bike to an older French mechanic. As I posted a couple weeks ago. My chain kept falling off of the large chainring, over onto the crank. But, the more difficult problem for a long time has been the chain getting lodged between the middle and large chain ring. Causing me to have no traction. I'd have to either down shift and up shift from the small to the largest chain ring. The difficulty always occured when upshifting.
So I did not understand all things the mechanic did today. My French is limited. He attempted to tell me. My wife will go back to the shop with me and find out exactly what happend. The best he could tell me, it appeared and sounded like one chain ring was backwards? The other problem, I saw him removing some , what looked like spacers, placed adjacent the bolts.
As Valentin did his work, he kept saying 'impossib'le and oh, la la'. All I know it worked fine going home.
So my question. Should the cluster not be bolted together properly, how does a mechanic figure out, it is assembled improperly. ( I'd only know by disassembling it and reassembling it, as the manufacturer assembled it.?) In an original inspection, he said the gap was ok.
Anyone think a spacer problem was the sole problem. Can a chain ring work, if it is backwards. I did see him remove a chain ring and it appeared it was re-assembled reverse to what it was when removed. Is this possible.
Sure rides better, when climbing it has been a pain. He said i have another 500 km left before the chain needs replacing.
thanks.
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Old 09-05-06, 03:00 PM   #2
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experience is everything for a mechanic i think.
do you live in france? either way, you're there, and so lucky. my wife and i are seriously looking into moving there, possibly the dordogne or midi-pyrenees area.
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Old 09-05-06, 03:02 PM   #3
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To answer the question of the thread title, yes, ALL mechanics must be detectives - bicycle, motorcycle, or automobile. ANYBODY can change parts - the skill (art?) is in knowing WHICH parts to change. Diagnosis is 90% of any repair, adjustment or replacement only 10%.

To answer the question in your post, yes, a chain ring can work backwards. The teeth of modern chainrings have bevels on certain teeth to "encourage" the chain to move off of or on to the ring. If the ring is mounted backwards, the bevels either don't work or prevent the chain from shifting smoothly.
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Old 09-05-06, 06:10 PM   #4
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Quote:
Originally Posted by cyclezealot
Can a chain ring work, if it is backwards. I did see him remove a chain ring and it appeared it was re-assembled reverse to what it was when removed. Is this possible.
Depending on the components, most chain rings have shift ramps that need to face the frame. It would be pretty obvious if they were installed backwards.
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Old 09-05-06, 08:38 PM   #5
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Ouch dude, limited french and living in France? Try Sheldon Brown's guide to french bike parts. print it out and practice practice practice! here's the link: http://www.sheldonbrown.com/fren-eng.html
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Old 09-05-06, 09:33 PM   #6
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Quote:
Originally Posted by cyclezealot
Anyone think a spacer problem was the sole problem.
I just read in the Zinn book that sometimes bikes leave the factory with incorrectly sized BB axles (which would cause unrecoverable shifting problems) so spacers incorrectly used in the same area of the bike is not a big stretch.
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Old 09-06-06, 12:48 AM   #7
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ALso a lot of chainrings have a bevel on them. So that the teeth are off-center from the plane of the chainring to one side. Depending upon which way the chainring was installed, you can get a variation of about +/- 2 or 3mm of lateral leeway... If you had the middle & outer chainrings facing opposite direcitions, it's very possible that the gap became big enough to let the chain drop in between...
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Old 09-06-06, 06:30 AM   #8
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I rode up in the moutains today. Valentin made it shift just fine. Can't say how many times mechanics havde agonized over tuning up my Surly derailleur's and said it's impossible.
Why did some one else not recognize this problem. My question, how does a mechanic recognize how to put a cluster back together, if it was not assembled correctly to begin with. ?Are all clusters assembled in the same exact manner?
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Old 09-06-06, 06:59 AM   #9
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Quote:
Originally Posted by cyclezealot
I rode up in the moutains today. Valentin made it shift just fine. Can't say how many times mechanics havde agonized over tuning up my Surly derailleur's and said it's impossible.
Why did some one else not recognize this problem. My question, how does a mechanic recognize how to put a cluster back together, if it was not assembled correctly to begin with. ?Are all clusters assembled in the same exact manner?
Besides being part detective, -great- bike mechanics are very observant. They'll spot something wrong just by 'eye-balling" it.

As was stated, it's very evident if a chain ring is mounted wrong-side-out due to the position of the ramps and pins found on contemporary rings. Probably, while removing the ring to switch it around, the spacer was observed and was found to be unnecessary.
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Old 09-07-06, 06:16 AM   #10
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On a slightly different tack...
How do you know how the two chainrings fit together when reassembling after a tear down? I've always marked them but is there a way to tell if they aren't marked??
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Old 09-07-06, 06:45 AM   #11
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So you did not see a cluster assembled, what do you have to do, go to manufacturers' specs.; should you be new to the wrench world.
Smiley. Thanks for the link. I think I know enough french to 'survive.' I did have basic french words from bike parts, thanks to Ulysses' Cycling in France. ' Your is far more complete. Thanks.
My favorite wrench two days ago, He kept tearing apart the cluster and reassembling it. With some added oh, la ,la's. all, i know my ride up into the mtns. yesterday, was the most enjoyable , I 've had on this bike in a very long time.
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