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Bottom bracket/crankset question

Old 12-01-10, 07:24 AM
  #1  
RVachon
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Bottom bracket/crankset question

Hi everyone;

I know cars, not bikes when it comes to working on them, but I'm slowly learning. I have a question regarding bottom brackets/cranksets & a possible conversion.

I have a 2003 Rocky Mountain Metro which was born as a hybrid, but at this point I've somewhat converted it to my road trainer/CX bike. The current BB is an internal square taper 73x113, with a cheap, heavy Shimano Nexave T300 triple crankset.

Now, my recent purchase of a Jamis Comet tri bike, it came with an FSA Gossamer EXO crank setup, but I will be changing it to a Shimano R700 compact setup. I would like to use the FSA crank setup on my RM Metro. Is that conversion even possible? If so, which BB would I use and is there anything else I would need?

Thanks for any knowledge you can pass along.

Last edited by RVachon; 12-01-10 at 07:32 AM.
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Old 12-01-10, 10:09 AM
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Possibly. THere are a few bottom bracket standards that you might encounter. A mid-end hybrid the the RM probably has an English threaded 68mm wide bottom bracket shell. Almost all road bikes made between the mid 90s and mid 00s also have this setup. THe swap would simply be to remove both c ranks from their bottom brakets, unscrew the bottom brackets from the frame, and screw the bottom brakcet fromt he Jamis into the rocky mountain and re-attach the cranks... then adjust the front derailleur to match the new cranksset. There are a few specialized tools required for this and you can damage your frame if your are careless and cross-thread the BB when you screw it in.

However, in the past few years manufacturers have gone ot oversized cartrige bearings in some high-end bikes... systems like BB30 have become common. If the Jamis has a BB30 bottom bracket or similar then the two bikes are not compatible.
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Old 12-01-10, 10:18 AM
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Thanks, but like I mentioned, the shell on the Rocky is a 73mm, english, and 113m wide spindle internal square taper BB. The Jamis has a standard 68mm MegaExo external cup BB. But I see on the FSA website that they make a 73mm MegaExo BB, but I'm wondering about the spindle/shaft length on the FSA Gossamer that's on the Jamis. It's probably narrower than 113mm, so how would that work? or wouldn't it?
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Old 12-01-10, 10:37 AM
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Ignore the "113mm" bb spindle length you currently have; these vary with the crankset used.

Your FSA crank is a "two-piece" design, with the bb spindle integrated with the drive side crank. FSA notes that this crank is a road crank and is compatible with 68mm bb shells only:

see: webstore.fullspeedahead.com/downloadfly.aspx?download...MegaExo.pdf
Are MegaExo Cranks compatible with 73mm shell width frames?

Only FSA MTB and Gravity MegaExo cranks are compatible with 73mm shell frames. All road MegaExo
cranks come with a bottom bracket compatible with 68mm shell width frames only. 70mm Italian threaded
frames require a different bottom bracket, which is available aftermarket.
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Old 12-01-10, 10:41 AM
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Originally Posted by desertdork View Post
Ignore the "113mm" bb spindle length you currently have; these vary with the crankset used.

Your FSA crank is a "two-piece" design, with the bb spindle integrated with the drive side crank. FSA notes that this crank is a road crank and is compatible with 68mm bb shells only:

see: webstore.fullspeedahead.com/downloadfly.aspx?download...MegaExo.pdf
Thanks, that's all I needed to know.

For Sale: FSA Gossamer crankset, 175mm arms, with MegaExo BB - New pull.
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Old 12-01-10, 10:48 AM
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you won't be able to use an R700 compact on a frame that has a 73mm shell unless you do some modifications to the frame.
Mega EXO uses a 2-piece system that has a fixed length axle, just like shimano's R700 compact. Some cranksets have a long axle, mainly road triple and the entire MTB range, but others, like shimano's road double regular and compact have a shorter axle. When you apply the fixed axle length to the BB, shorter axles will not work on frames that require the longer length, but the longer can work on the shorter with the use of spacers.

if 68mm is the road size and 73mm is the MTB size, then the frame's BB will need to be brought down to 68mm if you want to use a road compact on it. Even then, it's not a guarantee that it will work, because there are certain chainring size limits to frames.
All in all, if you have a shop bring down the BB size to 68mm, then the R700 should work on that frame, as I see it has a 28-38-48 triple, which means a 34-50 compact will fit 99% of the time.
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Old 12-01-10, 11:07 AM
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Originally Posted by AEO View Post
you won't be able to use an R700 compact on a frame that has a 73mm shell unless you do some modifications to the frame.
The R700 is the replacement on the Jamis
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Old 12-01-10, 11:21 AM
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Originally Posted by desertdork View Post
The R700 is the replacement on the Jamis
R700 is designed for 68mm and 70mm BB shells.
if you used a road double on a 73mm BB shell frame, it wouldn't fit properly. The axle will be 5mm too short.
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Old 12-01-10, 11:24 AM
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Originally Posted by AEO View Post
R700 is designed for 68mm and 70mm BB shells.
if you used a road double on a 73mm BB shell frame, it wouldn't fit properly. The axle will be 5mm too short.
The R700 is going to be installed on the Jamis that has a 68mm shell, not on the Rocky with the 73mm.
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