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ive always heard that it doesnt matter what drivetrain (8/9) you ride, the front and rear derailleurs are "dumb" components. i just got a new frame and need just my derailleurs now to finish it. im planning on stickin with just 8. all discriptions that ive seen for derailleurs say theyre designed for 9 speed systems. does that matter? how do i figure out my clamp size? measure circumference and figure out diameter in mm?
 

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The front derailleurs are 7/8 or 9 speed specific. The side plates on the 7/8 speed derailleurs are further apart to accomodate the wider 7/8 speed chain and the wider spacing of the chainrings on the 7/8 speed crank. If you use a 9 speed deraileur with a 8 speed drivetrain, you will have a lot of gear combinations where the chain rubs on the derailleur.

You get the clamp size by measuring the seat tube with vernier calipers.
 

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GearHead said:
The front derailleurs are 7/8 or 9 speed specific. The side plates on the 7/8 speed derailleurs are further apart to accomodate the wider 7/8 speed chain and the wider spacing of the chainrings on the 7/8 speed crank. If you use a 9 speed deraileur with a 8 speed drivetrain, you will have a lot of gear combinations where the chain rubs on the derailleur.
I took a set of calipers to a rear mech when 9 speed first came out. Unless they've gotten narrower since then the difference in the cage width was less than a millimeter.

I'm using a 9 speed front mech, and a 9 speed rear mech on three bikes with no problems.

JmZ
 

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laackund said:
ive always heard that it doesnt matter what drivetrain (8/9) you ride, the front and rear derailleurs are "dumb" components. i just got a new frame and need just my derailleurs now to finish it. im planning on stickin with just 8. all discriptions that ive seen for derailleurs say theyre designed for 9 speed systems. does that matter? how do i figure out my clamp size? measure circumference and figure out diameter in mm?
I've had problems with using 9-speed front d on a 7-speed system, but used 7-speed on 7, 8 & 9 with no problems.

You should be fine.
 

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forkboy said:
I've had problems with using 9-speed front d on a 7-speed system, but used 7-speed on 7, 8 & 9 with no problems.

You should be fine.
I believe he wants to use a 9 speed front derailleur on a 8 speed bike.

I measured my 7/8 speed front derailleur, it is 13 mm between the buldges in the side plates and 12 mm on a 9 speed front derailleur. So, the 7/8 speed is 8% wider which seems to be what is required for a chain that is 0.5 mm wider and chainrings that are also about 0.5 mm further spaced apart.
 

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GearHead said:
I believe he wants to use a 9 speed front derailleur on a 8 speed bike.

I measured my 7/8 speed front derailleur, it is 13 mm between the buldges in the side plates and 12 mm on a 9 speed front derailleur. So, the 7/8 speed is 8% wider which seems to be what is required for a chain that is 0.5 mm wider and chainrings that are also about 0.5 mm further spaced apart.
Ahh - my comprehension switch was in reverse
 

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I ran 9 speed front/rear deraileurs on an 8-speed bike for three years without any problems.

I do however, now experience front deraileur rub on my all 9-speed system. Go figure......
 
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