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Old 11-12-07, 07:44 PM   #1
cynergy
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having trouble shifting from middle chain ring to teeny chain ring

I have an older Trek 800 mountain bike that I use as a general purpose utility bike.

I upgraded the drivetrain last year so I could have some lower gears while I tow things (mostly kids in trailers or bike seat) around the hills of New Hampshire.

It has a Bontrager mountain bike crankset (22-32-44) and a Shimano "Mega Range" 6 speed freewheel (13, 15, 18, 21, 24, 34).

I am having occasional problems when I am trying to shift from the middle front chain ring to the little chain ring. The problems usually occur when the drivetrain is under some heavier load (for example, if I'm climbing up a hill). Occasionally, one of two things happens,

1. The chain does not shift from the middle ring to the inner ring. The front derailleur just rubs against the chain but there is not enough force exherted by the derailleur to move the chain (which is under load) off of the middle ring and onto the little ring.

2. At other times, the front derailleur will move the chain off of the middle ring, and it will overshoot and the chain misses the inner ring and falls off of the chain ring completely (falls inside of the little ring and against the frame).

I've tried to adjust the high and low limits for the front derailleur. I have it so the chain just clears the derailleur in the front at the low and high gear combos. I've also tried adjusting the front derailleru cable tension. It seems to index well, but I still have problems when I shift under load from the middle to inner chain ring.

Any ideas on what I should try next?
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Old 11-12-07, 07:56 PM   #2
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Does it shift normal with no load on the pedals?
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Old 11-13-07, 05:04 AM   #3
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dr raleigh,

yes, it does seem to shift normally without load. when i put it on the bike stand to inspect it, it usually works OK. there is some slight hesitation when shifting from the middle ring to the small ring, but it does shift normally. I don't recall it not shifting.

one thing i've tried to do is to minimize the gap between the front derailleur and the chain at the Low and High limits. However, if I make these gaps too small, the chain rubs the derailleur along one spot of the chain. For example, if I make the Low limit gap really small (like the thickness of a dime) the chain clears the derailleur in most spots, except maybe one or two spots, where the chain seems to move towards the derailleur and rub the derailleur.

In general, what would cause hesitancy during shifting from the middle ring to the low ring? Is it cable tension? Is it the limit settings? The chain is pretty new (installed it this past spring) and the crankset and freewheel are also pretty new (also from this past spring). The front derailleur, however, is the original derailleur (from 1993) that came with the bike.
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Old 11-13-07, 07:51 AM   #4
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Originally Posted by cynergy View Post
Any ideas on what I should try next?
Do the procedure from the start. Then get back to us.

http://www.parktool.com/repair/readhowto.asp?id=75

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Originally Posted by cynergy View Post
In general, what would cause hesitancy during shifting from the middle ring to the low ring? Is it cable tension? Is it the limit settings? The chain is pretty new (installed it this past spring) and the crankset and freewheel are also pretty new (also from this past spring). The front derailleur, however, is the original derailleur (from 1993) that came with the bike.
Cable tension and limit screws could both screw it up. Or FD cage rotation or FD heigh, or something is bent. List goes on. There's no point trying to diagnose this. You can write a phonebook of procedure base on symptom x, do y.

Do the park tool procedure. DO NOT SKIP ANY OF THE STEPS
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Old 11-13-07, 01:51 PM   #5
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I think you may simply have to anticipate your shifts a little sooner so that you aren't shifting under heavy load. Spin faster & mash less.
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Old 11-13-07, 03:51 PM   #6
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HI Folks,

Thanks for the tips.

Operator - thanks for the links to the Park Tool website. I love their site!

I actually have downloaded the Parktool instructions already and did everything step by step to check the derailleur alignment, height, limit screws, cable tension etc.. I had to disconnect the front derailleur cable when I replaced the crankset and lowered the front derailleur. I used the Park Tool documents to adjust the FD alignment and height (i used coins as suggested in their documents as feeler gauges) and when through each step to adjust the limit screws and tension.

The bike shifts pretty well, except under heavier load (going up a moderately steep hill).

I have a question about adjusting the low and high limits. I noticed that with my bike, there is maybe one point along the chain that seems to run closer to the derailleur than the other links in the chain. For example, as I stare at the chain while I'm turning the crank slowly with my hand, I see that for most of the links, there is adequate clearance between the FD and the chain. However, as I approach one point along the chain, this clearance decreases and there is one point along the chain where the clearance is at a minimum.

I would like to decrease the clearance gaps for the L and H settings, but I have to accomodate for that minimum point on the chain. I'm not sure if I've described this well, but is it normal for a chain to have minimum clearance points for the low and high limit settings?

Do you think replacing the front derailleur to a more modern mountain bike front derailleur would help? When I switched out my free wheel from a 28 tooth freewheel to the "Mega Range" 34 tooth freewheel, I replaced my rear derailleur.

My front derailleur was originally paired with a larger crankset (I think it only had a 28 tooth inner chain ring and not the tiny 22 tooth ring).

Bill - Thanks for the suggestion - I think you are right. If I anticipate my shifts earlier and avoid putting a heavy load when I need to shift.

For the most part I haven't had any problems while riding, but I was just wondering if there was something I could do to better optimize my drivetrain. I love my Trek 800. Even though my new Trek 520 is an awesome bike, I ride the Trek 800 a lot more and use it as my towing, shopping, tote the kids around town bike.

Thanks again for the replies and suggestions. I appreciate all of your ideas.
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Old 11-13-07, 03:55 PM   #7
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I've had the same thing happen occasionally on my Cross-Check with an XT crank. I find the problem is more associated with the fact that I'm already in the lowest (big) gear on the cassette. If I'm in the second to lowest, no issue.

So to echo Bill, anticipate and shift sooner.
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Old 11-13-07, 03:58 PM   #8
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make sure it is at the right height first...

adjust the height of the derailleur, so that when it is shifted to the biggest ring,
the edge of the outer guard of the derailleur just barely clears the teeth on the ring by 2mm.

and make sure that the derailleur cage is parallel to the rings.

after that, it is making sure the thing is lubed and doesn't bind, cable has no drag or kinks, and adjustments of the limit screws.
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Old 11-13-07, 04:26 PM   #9
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Quote:
Originally Posted by cynergy View Post
Do you think replacing the front derailleur to a more modern mountain bike front derailleur would help? When I switched out my free wheel from a 28 tooth freewheel to the "Mega Range" 34 tooth freewheel, I replaced my rear derailleur.

My front derailleur was originally paired with a larger crankset (I think it only had a 28 tooth inner chain ring and not the tiny 22 tooth ring).
It might. How big was the original large ring? Ideally, you want as much curve to the FD cage as you can get and that is limited by the large chain ring clearance. With a smaller big chainring, you can probably use a more sharply curved FD cage or one shaped to optimize this shift. However I would just shift with a lighter load and call it good.

Or get an old FD that uses cable pull to shift down and spring action to shift up
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Old 11-13-07, 05:15 PM   #10
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" I have a question about adjusting the low and high limits. I noticed that with my bike, there is maybe one point along the chain that seems to run closer to the derailleur than the other links in the chain. For example, as I stare at the chain while I'm turning the crank slowly with my hand, I see that for most of the links, there is adequate clearance between the FD and the chain. However, as I approach one point along the chain, this clearance decreases and there is one point along the chain where the clearance is at a minimum."

I'd look for a bent chain ring.
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