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Old 10-23-08, 04:11 PM   #1
bustinxjustin
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Question About Bottom Bracket Fitting

Hi.

I'm trying to put together an Iro Rob Roy frame as a single speed and I'm having some problems figuring out what size Shimano bottom bracket I should get.

The BB shell width is just about 68mm so that means I should a 68 right? I'm kind of confused about which size spindle length I should get though.

I was looking at a RR product page, and it said the BB is IRO Cro-Mo 68X110.

Will a 68 x 109.5 BB fit or will this be too small?

Thanks in advance for your time and help.
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Old 10-23-08, 07:57 PM   #2
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68 is the width in millimeters of your bottom bracket shell. Any 68mm bottom bracket will thread into your frame properly. The spindle length will determine your chainline. For a single speed, you want as perfect of a chainline as possible, meaning that the chainring is perfectly in line with the rear cog. Assuming the crank you are using has the same chainline as the crank of the standard RR, a 109.5mm BB should fit just fine. The effect on the chainline should be only half the difference in BB spindle length, or 0.25mm, a negligible difference.
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Old 10-23-08, 08:08 PM   #3
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68 is the width in millimeters of your bottom bracket shell. Any 68mm bottom bracket will thread into your frame properly. The spindle length will determine your chainline. For a single speed, you want as perfect of a chainline as possible, meaning that the chainring is perfectly in line with the rear cog. Assuming the crank you are using has the same chainline as the crank of the standard RR, a 109.5mm BB should fit just fine. The effect on the chainline should be only half the difference in BB spindle length, or 0.25mm, a negligible difference.
I'm not exactly sure how you're answering his post without knowing what crank/hub/cog he's planning on using.

OP: The crank what length bb gives what chainline.

So all you need to tell us is what crank you plan on using and what chainline in the rear is going to be.
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Old 10-23-08, 08:11 PM   #4
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I'll be using a Shimano 105 Crank and a Shimano B5500 BB.
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Old 10-23-08, 08:16 PM   #5
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I'll be using a Shimano 105 Crank and a Shimano B5500 BB.
Which shimano 105 crank?
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Old 10-23-08, 08:59 PM   #6
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I'm not exactly sure how you're answering his post without knowing what crank/hub/cog he's planning on using.
My comments on BB length assumed he was using a crank with the same chainline as the standard RR. I made that same comment in my post.

I did make an undisclosed assumption that he would be using a single speed hub given his comments about setting up the bike as a single speed. Perhaps I'm wrong in assuming that single speed hubs generally yield the same chainline though.
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Old 10-24-08, 07:59 AM   #7
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Shimano FC-5502 with the B5500 Bottom Bracket, but I will be taking off the smaller chainring on the crank.

So the crank will be using the 53 tooth chainring (not sure if that matters).

For the back wheel I'll be using a 135mm 9spd Shimano freewheel hub with a spacer kit and a Shimano BMX freewheel cog.

I hope this helps you guys.

Thanks for everything so far.

Last edited by bustinxjustin; 10-24-08 at 08:06 AM.
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Old 10-24-08, 07:11 PM   #8
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Anybody have any more help?

Thanks in advance!
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Old 10-24-08, 07:21 PM   #9
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If you are using a regular freewheel hub with a spacer kit and a single cog, you have a lot of liberty with your crank and bb since the cog can be positioned to match the crank's chainline by arranging the cog and spacers where you want to.

You are going to use a 9-speed 105 double crank and the recommended Octalink bb so your choices of spindle length are limited but you should be fine.
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Old 10-24-08, 08:28 PM   #10
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9speed double shimano crank that uses octalink requires a 109.5 spindle.
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