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Old 05-29-06, 12:19 PM   #1
jimmuter
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Replacing drivetrain parts - frustrating

I'm getting a new bike, but want to keep my old Trek 720 hybrid as a beater. Unfortunately, the chain broke while riding it and the rear cassette and front chainrings, bottom bracket, and front derailleur really all need to be replaced. At this point, I figured it probably wouldn't hurt to replace the entire crankset and the rear derailleur too. I don't want to replace the bar end shifters if I don't need to because they seem fine. Having no experience, I decided to try and replace the parts myself. I thought I had all of the compatibility issues worked out, but didn't get far. Here are the problems I've encountered so far and questions about how to fix it if possible.

1. The old bottom bracket I'm replacing was 116 mm spindle which I don't even see available anymore. The bottom bracket and crankset combination I got does not appear to give the small chainring adequate clearance from the chainstay. Is this something I can solve by getting a different size bottom bracket? I bought a 110mm size. If I get a 118mm, will that work with the crankset or does the crankset dictate the size of the bb?

2. The front derailleur I purchased is for the wrong chainstay angle. Also, the old one was a bottom bracket mount. Other than buying one for the correct angle, what other compatibility issues do I need to look for? Do I need to get a bottom bracket mounted one again?

I'm just hoping I'm not at a complete do-over as far as buying parts at this time. I'd really like to accomplish this, but not if it's just a continual exercise in frustration.
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Old 05-29-06, 12:31 PM   #2
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1. You're right. The crankset determines the bottom bracket spindle length that you need.

2. The problem with your front derailleur probably has to do with the size of the big chainring. If you use a mountain derailleur with a road crankset, the back of the cage will rub. With a road derailleur and a mountain crankset, the front of the cage will rub.

Think of these aggrivations as tuition in the school of bicycle upgrades.
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Old 05-29-06, 01:09 PM   #3
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No point in replacing stuff that still works well, unless you're wanting to for other reasons. Although it's generally a good idea to replace the cogset when you replace the chain, the chainrings are often still fine. I'd be surprised if the derailleurs, bottom bracket & crankset all need replacing......
I'm not sure what the bike had on it, but guessing that it was a triple chainring setup on the front with a fairly wide-geared cogset on the rear...? If you can get a pretty straight chainline from the middle ring in front to the middle cog in the rear, the 118mm spindle should work fine.
The BB spindle length is usually dictated by the number of chainrings and chainring/chainstay clearance. As long as the crankarm attachment is the same (flat, splined) as on the spindle it should work fine.
I'm not familiar with the chainstay "angle" having anything to do with the front derailleur. If the derailleur capacity & cage length is sufficient for the size & number of chainrings, it should work fine if adjusted correctly (for chainring clearance as well as for shifting).
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Old 05-29-06, 02:28 PM   #4
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Quote:
Originally Posted by knjc68
No point in replacing stuff that still works well, unless you're wanting to for other reasons. Although it's generally a good idea to replace the cogset when you replace the chain, the chainrings are often still fine. I'd be surprised if the derailleurs, bottom bracket & crankset all need replacing......
I'm not sure what the bike had on it, but guessing that it was a triple chainring setup on the front with a fairly wide-geared cogset on the rear...? If you can get a pretty straight chainline from the middle ring in front to the middle cog in the rear, the 118mm spindle should work fine.
The BB spindle length is usually dictated by the number of chainrings and chainring/chainstay clearance. As long as the crankarm attachment is the same (flat, splined) as on the spindle it should work fine.
I'm not familiar with the chainstay "angle" having anything to do with the front derailleur. If the derailleur capacity & cage length is sufficient for the size & number of chainrings, it should work fine if adjusted correctly (for chainring clearance as well as for shifting).
The bottom bracket definitely needed to be replaced and the front derailleur was awfully wiggly too. I checked the chainrings and they were definitely worn too. I'm not complaining because it all lasted a long time. You are correct that the front is a triple, the cogset is 11-28 (7 speed). I'm going to exchange the bottom bracket and see if that works. The chainstay angle is stated on the front derailleurs (old and new) so I'm assuming it has importance. Thanks for the help.
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Old 05-29-06, 06:48 PM   #5
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Originally Posted by jimmuter
The chainstay angle is stated on the front derailleurs (old and new) so I'm assuming it has importance.
If it says STA that would be the seat tube angle, which may make sense. What is the angle given on the derailleur?

Al
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Old 05-29-06, 07:41 PM   #6
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If it says STA that would be the seat tube angle, which may make sense. What is the angle given on the derailleur?

Al
One range is 63 - 66 degrees (which is what my old one is). The other is 66-69 degrees (which is the one listed on the new one I purchased). It definitely says chainstay angle. It's fully spelled out. I can take a picture if you don't believe me.
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