front derailleur clamp size?

[Christian Schneider]

ZINN and art of Bike Maintenance doesn't tell one how to discover what size clamp to be used on seat tubes. If a bike uses a 27.2 seat post, what front derailleur clamp size would it use? How is this measured? determined?
thanks

 

[David Suro]

The seat post fits the internal diameter of the upper portion of the seat tube, while the clamp for the front derrailuer (sp?) fits the external diameter of the lower portion. Since the walls tubes can vary in thickness and some seat tubes taper as they approach the top tube, it is not a good idea to use seat post diameter as a gauge for the size of your clamp.

A set of calipers should be used to measure the seat tube where you want to install the clamp. I would take the bike to a local shop and have them measure the tube and have their best mechanic make a suggestion as to which clamp would be best. If you purchase the equipment at the shop, there should be no charge for this service.

 

[Bustedknuckle]

ZINN and art of Bike Maintenance doesn't tell one how to discover what size clamp to be used on seat tubes. If a bike uses a 27.2 seat post, what front derailleur clamp size would it use? How is this measured? determined?
thanks

Measure circumference of the tube. Divide by 3.14(pi), that's the diameter.

 

[Christian Schneider]

I measured circumference to be 104mm. When I divide by pi I get 33.12..what the heck does that mean? Clamps are 31.8; 34.9...do I go with the bigger or the smaller?

 

[David Suro]

If you have to go with one that doesn't fit exaclty, it usually best to go with the bigger one and use a shim or some electrical tape.

Do youself a favor and find the best machinc at your local shop. He/she will know exactly what to do and you will have the peace of mind knowing that the job was done correctly.

I have seen more than a few frames destroyed by people who use ill-fitting equipment and try to make it work.

 

[Bustedknuckle]

I measured circumference to be 104mm. When I divide by pi I get 33.12..what the heck does that mean? Clamps are 31.8; 34.9...do I go with the bigger or the smaller?

I would say trundle on down to a bike shop and see if a 31.8 or 34.9 fits onto that.

 

[davidg]

Measure circumference of the tube. Divide by 3.14(pi), that's the diameter.

I daren't even ask how you measured the circumference! Might explain your forum name.

Use calipers or go to the LBS.