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Bicycle Mechanics Broken bottom bracket? Tacoed wheel? If you're having problems with your bicycle, or just need help fixing a flat, drop in here for the latest on bicycle mechanics & bicycle maintenance.

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Old 08-21-13, 01:04 PM   #1
jawnn
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Bottom braket sizes???

What are the sizes of the BB shells on BMX and folding bikes? I want to use a Metropolis crank that needs 68mm? not to fit a MTB.
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Old 08-21-13, 01:26 PM   #2
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BB size is both shell width (side to side) and inside diameter (and threading). BMX and folding bikes probably have more variation than road/mtb bikes as to BB specs. How do you not know what the dimensions of the bike are? Can you give us a make/model or clear pic? We need info about the situation and what you want to accomplish to provide you with a full answer.

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Old 08-21-13, 01:43 PM   #3
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What sort of crank is this? BMX bikes often have one-piece cranks, and then the BB is essentially just cups.
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Old 08-23-13, 11:37 AM   #4
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this is the crank... I do not have a bike for it yet.

http://www.metropoliscomponents.com/patterson.htm

http://www.joe-bike.com/2012/03/11/r...peed-crankset/

Last edited by jawnn; 08-24-13 at 11:27 AM.
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Old 08-23-13, 12:05 PM   #5
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BMX bottom brackets are a complete smorgasbord of standards.

Most modern BMX bikes don't have one-piece cranks anymore if they're proper BMX bikes rather than BMX-shaped objects. Some of them do still use a one-piece-crank BB shell with cartridge bearings pressed in and a 3-piece crankset, known as a "USA" or "American" BB in BMX terms, some of them use "Mid" or "Spanish" BBs, which are smaller-diameter press-fit standards, and some of them use a conventional threaded BB, referred to as a "Euro" BB by BMX manufacturers.

Folding bikes tend to have conventional threaded BBs if they're at all decent (you may still find one with a one-piece crank shell though), the 68mm quoted by the OP is, I assume, the width of the BB shell required by the planetary gear system they want to use, and the good news is that most threaded BB shells are 68mm wide unless they're on an MTB.

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Old 08-23-13, 12:10 PM   #6
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Pictures: http://www.bmx-forum.com/t/243179/bottom-bracket-faq
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Old 08-23-13, 12:15 PM   #7
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Quote:
I want to use a Metropolis crank that needs 68mm? not to fit a MTB.
This One? .. http://www.metropoliscomponents.com/patterson.htm

they include an external type BB. I believe.. modified to transmit the planetary counter torque.

Perhaps you are confused just because the only picture is of an MTB install. http://pattersonbike.com/

you realize 73 is more than 68 and spacers fill space?


Have you discussed this with the seller ?

and perhaps its best to have, them, a Professional Shop install it for you.

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Old 08-23-13, 01:21 PM   #8
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You don't need a general answer - it's not important what BMX or folding bikes have but what the bikes you might consider have. Pull up the specs or check with the manufacturer/seller to find our BB width. I would guess a BMX bike would be best with the 165mm cranks, but make sure you are getting the benefit you expect from that kind of money.
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Old 08-24-13, 11:41 AM   #9
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Perhaps you forgot to read the article about the crank. After the 4th photo it states:"Compatibility: fits any standard 68 mm bottom bracket shell."



I send an inquiry to the makers of the crank.
Looks like BMX bikes are not going to work.

Last edited by jawnn; 08-26-13 at 11:45 AM.
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