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Front Derailleur clamp on size?

Old 08-11-15, 11:28 AM
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Front Derailleur clamp on size?

Hi, I did do a search before posting.

I'm buying a front "clamp on" FD for my carbon frame Orbea Opal; they come in 31.8mm and 34.9mm. I measured mine with a Park digital caliper several times down close to the clamp as possible without removing it and read 32.3mm.

My question is which do I buy. The guy on on-line chat at JensonUSA said 34.9 it comes with spacers. Is that accurate?

Thanks in advance,
John
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Old 08-11-15, 11:43 AM
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Generally frame tube numbers are metric equivalencies of actual Inch sizes .. 1 1/8" is steel seat tube standard,
the others are also inch, decimal - fractions.

28.6 ~ 1 1/8_1.125; 31.8 ~ 1,25" _ 34.9 ~ 1.374 [.375 is 3/8"]

32.3 mm = 1.27[165354331] in,

if he said it comes with shims for she smaller diamaters that statement must be accurate, he works there.

I'd go for the 35mm and make your own shims, since you state a non standard diameter tube measurement. .

34.9-32.3= 2.6mm halve that .. and you need a 1.3mm thick shim.

2.6 mm = 0.102[362204724] in..

0.051 " ... A wrap or 2 of tape may be sufficient.

Last edited by fietsbob; 08-11-15 at 12:54 PM.
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Old 08-11-15, 11:46 AM
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Many, but not all of the Shimano FDER's come with shims so that they can fit nominal 1-1/8, 1-1/4 & 1-3-8" seat tubes. (28.6, 31.8 & 34.9mm)

Not all the higher end "road" FDER's offer as many options...

SHIMANO Dealer's Manual / User's Manual
Download the dealers manual and look up the specific options.
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Old 08-11-15, 11:53 AM
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What clamp is on your frame now? If it is an adapter clamp for braze-on FD's, you can just buy a braze-on FD and reuse the clamp. Similarly, you can just buy a braze-on FD and buy a clamp adapter in the appropriate size separate if you don't have one.
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Old 08-11-15, 02:29 PM
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Get the larger clamp diameter, you can always shim it to fit a slightly smaller tube. You can't shim a too-small one bigger!
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Old 08-11-15, 02:41 PM
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Originally Posted by Jamminatrix
What clamp is on your frame now? If it is an adapter clamp for braze-on FD's, you can just buy a braze-on FD and reuse the clamp. Similarly, you can just buy a braze-on FD and buy a clamp adapter in the appropriate size separate if you don't have one.
The size marked on a front derailleur or a front derailleur braze-on clamp adapter is the maximum size that the clamp will accept. It's not the only size, generally the clamp will fit on tubing sizes down to the next smaller clamp size that the manufacturer offers. And as other have written, you can always shim the clamp, if it's too big to grip the tubing properly.

Get a 34.9 braze-on clamp adapter. You can get them for between under $3 shipped to $15 shipped for an Shimano clamp adapter. If you go the clamp adapter route, replacing the derailleur is easy, because you don't deal with the clamp size problem anymore, you just order a braze-on front derailleur. Bike manufacturers have been doing this for decades. My 1990 Trek 1100 has a Shimano 34.9 braze-on clamp adapter from the factory.

Recently, I bought a used Shimano RX100 braze-on front derailleur for $2 (plus $2.54 shipping), off eBay, just to get the Shimano 34.9 braze-on clamp adapter that it was attached to. I felt lucky to get a front derailleur that normally sells used for $10-$15 along with a braze-on clamp adapter that would normally sell for at least $7-$10 for a total of $4.54. I didn't need the front derailleur, but even if I threw it away (I won't because I like RX100), I'd be way ahead on the braze-on clamp adapter.

Last edited by RoadGuy; 08-11-15 at 02:50 PM.
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Old 08-11-15, 02:43 PM
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If you have a "clamp on" derailleur on the bike, I would remove it and check the size marked on the inside of the clamp. Best to avoid cheesy shims if you can.
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Old 08-11-15, 05:19 PM
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Lots of good answers I think I know which way to go now.

Thanks!
John
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