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First time build question (Kinda noobish)

Singlespeed & Fixed Gear "I still feel that variable gears are only for people over forty-five. Isn't it better to triumph by the strength of your muscles than by the artifice of a derailer? We are getting soft...As for me, give me a fixed gear!"-- Henri Desgrange (31 January 1865 - 16 August 1940)

First time build question (Kinda noobish)

Old 01-12-09, 08:45 AM
  #1  
jeffsui
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First time build question (Kinda noobish)

So I'm converting an old Falcon road bike into a fixed gear (or at least trying). I've stripped off most of the parts and I'm left with the frame. I need a new bottom bracket and crank for the front. I stripped off the freewheel on the back and I was going to replace it with a track cog and a lock ring (i think thats the correct terminology).

The bottom bracket is English with 68mm width.

The spacing between the rear dropouts is 126mm.

I was looking at bottom brackets and I noticed they come in different "lengths".

Shimano UN-26 Bottom Bracket for example comes in a

122.5 mm and 127.5 mm

Do i need to take into account the rear spacing in order to select the "correct" size here? My assumption was that the length of the bottom bracket would ultimately be related to how much adjustment i need to make to the chainline.

Thanks for any help you guys can offer!
-Jeff
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Old 01-12-09, 08:49 AM
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physh
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Why do you need new cranks and BB?

What you need is a new rear wheel.
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Old 01-12-09, 08:56 AM
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jeffsui
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I had a cottered crank and when i took it off i ended up screwing up the cotters so i can't put it back on-> and the existing BB was designed for only a cotter setup. So i figured i'd replace the whole getup
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Old 01-12-09, 09:11 AM
  #4  
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The short answer is no, the spacing of the seat stays doesn't relate to bottom bracket length.

Chain line is determined by how far everything is from the center of the frame, not the outside edges. Your basic formula hub will have a 42mm chain line in a 120mm rear triangle, a 126, a 130, etc. what changes is the number of spacers you will need on the outside of the lock nut. You need a bb and crank combination that also give you 42mm (or whatever number you're using).

I would recommend you look at the Bob's Garage videos on fgg for a good tutorial on how to calculate and adjust chain line for a conversion.

Good luck and post pics when you're done.
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Old 01-12-09, 09:19 AM
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Originally Posted by jeffsui View Post
So I'm converting an old Falcon road bike into a fixed gear (or at least trying). I've stripped off most of the parts and I'm left with the frame. I need a new bottom bracket and crank for the front. I stripped off the freewheel on the back and I was going to replace it with a track cog and a lock ring (i think thats the correct terminology).

The bottom bracket is English with 68mm width.

The spacing between the rear dropouts is 126mm.

I was looking at bottom brackets and I noticed they come in different "lengths".

Shimano UN-26 Bottom Bracket for example comes in a

122.5 mm and 127.5 mm

Do i need to take into account the rear spacing in order to select the "correct" size here? My assumption was that the length of the bottom bracket would ultimately be related to how much adjustment i need to make to the chainline.

Thanks for any help you guys can offer!
-Jeff
Although I don't have the exact number the new bb should be in the 113-118 mm range depending on your crank and the chain ring position on the crank.
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Old 01-12-09, 09:20 AM
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jeffsui
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thanks guys -> i'm not 100% sure i'm clear on everything but its making a bit more sense....
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Old 01-12-09, 09:43 AM
  #7  
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If you want to run fixed gear you'll need a new rear wheel. A fixed gear hub has 2 different threading so you screw on the cog with standard threads (righty-tighty) then the lockring uses slightly smaller reversed threading to 'lock' everything in place. Threading a cog and right hand threaded lockring onto an old freewheel hub is a sketchy [read: dangerous] way of doing a conversion.

Good luck, and post some pics when you get a chance.
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Old 01-12-09, 10:58 AM
  #8  
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Originally Posted by HandsomeRyan View Post
If you want to run fixed gear you'll need a new rear wheel. A fixed gear hub has 2 different threading so you screw on the cog with standard threads (righty-tighty) then the lockring uses slightly smaller reversed threading to 'lock' everything in place. Threading a cog and right hand threaded lockring onto an old freewheel hub is a sketchy [read: dangerous] way of doing a conversion.

Good luck, and post some pics when you get a chance.

You don't need a new rear wheel, rotofix + bb lockring + loctite has held many wheels. Just make sure you run a brake.

Also, you can get new cotter pins for your crank and save a ton of money on bb/crankset. Or, put a new/used square taper spindle in (instead of buying whole new bb).

But, if you want to do it proper, new wheel, bb/crank is what you want.
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Old 01-12-09, 11:04 AM
  #9  
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+1 - for a real conversion you'll feel good about, adapt as much of what you have to the project. You can work with your present wheel. The wheels are the most expensive item, anyway. Save a little money. Try looking around at Sheldon Brown's [R.I.P., Sheldon] website; the last word on fixed gear.

http://www.sheldonbrown.com/harris/index.html
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