FG MTB drivetrain problem
11-03-13 | 01:24 PM
#1
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From: Kansas City
FG MTB drivetrain problem
Hi all, I was trying to figure out a front-end click in my drivetrain of my fixed-gear MTB when I noticed that the chain tension varies pretty dramatically during the pedal stroke. Something must be bent or out of round?
My plan is to fix that something, repack the BB, and see if the click goes away. The problem is that I don't know exactly what to do next. Possible sources of the problem:
1) the chainring (steel 32t, a bit worn) could be out of round
2) (the crankarm/spider on the Truvative "power spline" cranks could be bent
3) the BB could seated funny or messed up in ways I don't know about
4) ???
Does anyone have advice about how to diagnose and resolve the problem? I prefer to do all my own wrenching and I'm on a budget. My instinct is to buy a new chainring, but they are expensive and I would hate to replace the chainring only to find that the chain tension continued to vary. Replacing BB is even dicier as I don't think the "power spline" BB is still in production, and my cranks only mate with that spindle.
Here is a mostly irrelevant picture of the bike just so you don't leave the thread empty handed.
My plan is to fix that something, repack the BB, and see if the click goes away. The problem is that I don't know exactly what to do next. Possible sources of the problem:
1) the chainring (steel 32t, a bit worn) could be out of round
2) (the crankarm/spider on the Truvative "power spline" cranks could be bent
3) the BB could seated funny or messed up in ways I don't know about
4) ???
Does anyone have advice about how to diagnose and resolve the problem? I prefer to do all my own wrenching and I'm on a budget. My instinct is to buy a new chainring, but they are expensive and I would hate to replace the chainring only to find that the chain tension continued to vary. Replacing BB is even dicier as I don't think the "power spline" BB is still in production, and my cranks only mate with that spindle.
Here is a mostly irrelevant picture of the bike just so you don't leave the thread empty handed.
11-03-13 | 02:01 PM
#2
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Joined: Apr 2009
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From: New Rochelle, NY
Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter
The variation on chain tension is normal (within limits) and caused - as you believe - by eccentric chainrings and to a much lesser extent, rear sprockets.
This is one reason it's so important not to pull all the slack out of the chain, and to make sure there's some slack at the tightest point.
Many 5-pin chainring mounting systems have some float, especially those where the ring doesn't have a turned inner diameter that fits snugly on shoulders on the crank arms. With these you can sometimes improve things by loosening the chainring bolts to where they hold fairly lightly, rotating the crank to find the point where the chain is tightest, and tapping the ring toward the back of the bike at 3 o'clock. Do this trial and error until it's as good as it gets, then tighten the bolts, and adjust chain slack accordingly (vestigial slack at tightest position).
This is one reason it's so important not to pull all the slack out of the chain, and to make sure there's some slack at the tightest point.
Many 5-pin chainring mounting systems have some float, especially those where the ring doesn't have a turned inner diameter that fits snugly on shoulders on the crank arms. With these you can sometimes improve things by loosening the chainring bolts to where they hold fairly lightly, rotating the crank to find the point where the chain is tightest, and tapping the ring toward the back of the bike at 3 o'clock. Do this trial and error until it's as good as it gets, then tighten the bolts, and adjust chain slack accordingly (vestigial slack at tightest position).
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FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
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FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.
11-03-13 | 08:05 PM
#3
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Joined: Aug 2011
Posts: 22,676
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From: CID
Bikes: 1991 Bianchi Eros, 1964 Armstrong, 1988 Diamondback Ascent, 1988 Bianchi Premio, 1987 Bianchi Sport SX, 1980s Raleigh mixte (hers), All-City Space Horse (hers)
Speaking of slack, from this angle, it doesn't appear that the brake cable has any...
11-03-13 | 08:16 PM
#4
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Joined: Sep 2008
Posts: 426
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From: Kansas City
The variation on chain tension is normal (within limits) and caused - as you believe - by eccentric chainrings and to a much lesser extent, rear sprockets.
This is one reason it's so important not to pull all the slack out of the chain, and to make sure there's some slack at the tightest point.
Many 5-pin chainring mounting systems have some float, especially those where the ring doesn't have a turned inner diameter that fits snugly on shoulders on the crank arms. With these you can sometimes improve things by loosening the chainring bolts to where they hold fairly lightly, rotating the crank to find the point where the chain is tightest, and tapping the ring toward the back of the bike at 3 o'clock. Do this trial and error until it's as good as it gets, then tighten the bolts, and adjust chain slack accordingly (vestigial slack at tightest position).
This is one reason it's so important not to pull all the slack out of the chain, and to make sure there's some slack at the tightest point.
Many 5-pin chainring mounting systems have some float, especially those where the ring doesn't have a turned inner diameter that fits snugly on shoulders on the crank arms. With these you can sometimes improve things by loosening the chainring bolts to where they hold fairly lightly, rotating the crank to find the point where the chain is tightest, and tapping the ring toward the back of the bike at 3 o'clock. Do this trial and error until it's as good as it gets, then tighten the bolts, and adjust chain slack accordingly (vestigial slack at tightest position).
11-03-13 | 08:54 PM
#6
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Joined: Aug 2011
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From: CID
Bikes: 1991 Bianchi Eros, 1964 Armstrong, 1988 Diamondback Ascent, 1988 Bianchi Premio, 1987 Bianchi Sport SX, 1980s Raleigh mixte (hers), All-City Space Horse (hers)
Whew! 
One thing that may help, if loosening and shifting the chainring doesn't even things out enough, is to take it off and rotate it on the crank spider, then starting the process again. Ultimately, you may have to live with a little extra slack at some points in the crank rotation, and it's hard to avoid with parts intended for multi-speed use. It generally only bothers me when I have the bike on the stand -- it rides great otherwise.
One thing that may help, if loosening and shifting the chainring doesn't even things out enough, is to take it off and rotate it on the crank spider, then starting the process again. Ultimately, you may have to live with a little extra slack at some points in the crank rotation, and it's hard to avoid with parts intended for multi-speed use. It generally only bothers me when I have the bike on the stand -- it rides great otherwise.
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