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Old 07-19-11, 02:56 AM   #1
look171
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Chain rain fitment question on Sram crank.

I have a Force (2010 model if that makes a difference) crank with a 53/39 rings. Can I fit a 50 ring on there without issues? I discover that my old age can't push the young man's gear anymore. I raced with those size ring 15 years ago. I should have known better by now.
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Old 07-19-11, 03:15 AM   #2
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The sram website shows that a 50/34 set is available, but I don't know whether it uses the same BCD as the 53/39. If so, you should be able to simply replace the 53 tooth ring with a 50.
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Old 07-19-11, 10:25 AM   #3
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I expect your current crank has a 130 mm bolt circle diameter given the chainrings it came with since compact cranks that come with 50/34 rings mostly have a 110 mm BCD and I'm pretty sure yours isn't that size.

If so, you can get a 130 mm BCD 50T chainring and it should bolt right up. However, there are two considerations:

1. The ramps and other shifting enhancements on your current rings may be different and shifting won't be as good.

2. If your front derailleur is rated for a 14 tooth chainring difference (53-39=14) or anything more than an 11T difference (50-39=11) you may have to mount it higher than desired above the 50T ring to let the inner cage clear the 39T ring. That will also reduce shifting quality.

An alternative is to go all the way to a true compact (50/34) crank and have both chainrings provide lower gears. This will require a completely new crank.
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Old 07-19-11, 10:28 AM   #4
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How can I tell what bolt pattern is on there?

Last edited by look171; 07-19-11 at 06:30 PM.
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Old 07-19-11, 06:30 PM   #5
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Anyone?
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Old 07-20-11, 05:15 AM   #6
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Measure the distance across an imaginary circle formed by the bolts. 95% sure it will measure 130mm.
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Old 07-20-11, 07:10 AM   #7
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By my calculations (a bit of trigonometry) A 130mm bcd would have bolt-to-bolt spacings of just over 40mm for a 5-bolt pattern.
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Old 07-20-11, 08:21 PM   #8
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Here is Sheldon Brown's article on chainring bolt circle diameters and how to measure to determine what you have: http://www.sheldonbrown.com/gloss_bo-z.html#bcd

BTW, the center to center distance between two adjacent bolts for a 130 mm BCD is 76.4 mm.
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Old 07-21-11, 04:16 AM   #9
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Ah yes, you're right of course. That will teach me to write stuff down rather than try to do it mentally
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