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  1. #1
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    Shortened crank 9 speed vs 10 speed issue

    Here are the specifics: Current bike is a Litespeed Sirius, 9 speed double, with a Shimano 105 (FC-5502) Octalink crank that has been professionally shortened to 148mm. New bike coming in soon is a 09 Look 566, 10sp double, with Ultegra SL crank and components. Assuming that the Look bottom bracket will accept the Octalink components, I want to replace the 10sp Ultegra crank with the shortened 105 crank. Shortening the Ultegra 10sp crank or any other 10sp crank is currently not viable and is not an option. The shop that shortened the crank said that I can purchase 9/10 sp compatible chainrings from FSA and they will make my FC-5502 10sp compatible. My LBS said that if I try that terrible things will happen like my frame will crack and the rear derailleur will buckle. Referencing Sheldon Brown's website, many cyclists have just changed to a 10sp chain and have not had any problems. Now it is my understanding that the inner ring is closer to the outer ring on a 10sp crank vs a 9sp crank. If that is really the case, why can't I just take my FC-5502 and mill the inner ring mounts to match the spacing on the 10sp Ultegra? I'd appreciate opinions on the FSA chainring option, is my LBS correct and is milling the proper path forward? Any other ideas?

  2. #2
    Certified Bike Brat Burton's Avatar
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    Sure sounds like a lot of trouble to go through to move from 9 to 10 speeds!

    Quote Originally Posted by Fleet59 View Post
    Here are the specifics: Current bike is a Litespeed Sirius, 9 speed double, with a Shimano 105 (FC-5502) Octalink crank that has been professionally shortened to 148mm. New bike coming in soon is a 09 Look 566, 10sp double, with Ultegra SL crank and components. Assuming that the Look bottom bracket will accept the Octalink components, I want to replace the 10sp Ultegra crank with the shortened 105 crank. Shortening the Ultegra 10sp crank or any other 10sp crank is currently not viable and is not an option. The shop that shortened the crank said that I can purchase 9/10 sp compatible chainrings from FSA and they will make my FC-5502 10sp compatible. My LBS said that if I try that terrible things will happen like my frame will crack and the rear derailleur will buckle. Referencing Sheldon Brown's website, many cyclists have just changed to a 10sp chain and have not had any problems. Now it is my understanding that the inner ring is closer to the outer ring on a 10sp crank vs a 9sp crank. If that is really the case, why can't I just take my FC-5502 and mill the inner ring mounts to match the spacing on the 10sp Ultegra? I'd appreciate opinions on the FSA chainring option, is my LBS correct and is milling the proper path forward? Any other ideas?
    Hi - read the posting a couple times to make sure I grasped what you want to do. Looks to me like you had a set of 9 speed crank arms shortened and want to use that with a 10 speed setup. The bike it would go on is pretty incidental.

    So yup, there is apparently a difference in the front chainring spacing between the 9 speed and 10 speed cranksets. The chainrings are slightly closer together on the 10 speed setup.

    Would that make your frame crack or other terrible things? It would give you some shifting issues with the front derailleur but unless the chain comes off and gets jammed in the rear wheel while you`re going downhill at speeds that should get you arrested - I personally think any danger is being a little exaggerated.

    But would suggest you skip mixing in the FSA stuff and stick with a drive train that comes from ONE manufacturer. You can put Shimano 10 speed chainrings on that 105 crank/spider assembly and thats what I`d suggest.

    Unless you plan on changing the entire drive chain over to another brand, which is possible since you`ll be using the 10speed cassette and chain from that other bike anyway.

    Shouldn`t have to machine anything, but double check me. I`ve been known to be overly optimistic.

  3. #3
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    I've done some similar drive train mixing recently, and had no real problems. Mixing an 8 speed (shimano) crank with a 10-speed (sram) cassette almost worked perfectly -- there were some skating issues when shifting from the large to small crank ring if I was in one of the smallest two sprockets in the back (a shift I shouldn't be doing anyway). Replacing the inner chainring to a 10-speed (shimano) one fixed the problem, so I can't imagine you'd have any problems doing the same.

    Assuming the BCD is the same, it would probably just be easiest to move the chainrings from your new Shimano SL crank to your older 105 crank.

  4. #4
    pmt
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    You'll have to move both the crankset and the BB, but it should shift and work fine. Don't overthink it.

  5. #5
    Senior Member CACycling's Avatar
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    You'll need to swap the BB as well. The new crankset will use an external bearing BB and your crankset is octalink. Other than that, the only issues may be skating as mbusha noted so try it and see how it shifts. If it is an issue, swap out the chain rings.

  6. #6
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    Assuming that you have the right BB for good chainline and that your cranks are in good shape, don't sewat moving the cranks over. The only material difference between 9s and 10s cranks and chainrings is the separation of the chainrings which is narrower for 10s matching the narrower chain.

    Often 10s chains run fine on 9s cranksets, but just as often problems occur with the chain hanging between the top of the inner and inner face of the outer ring on down shifts.

    If that's a problem you need to narrow the gap. Some companies do this by milling a bit off the inner face of the outer ring where it bolts on so it moves inboard a bit, others narrow the section of the spider between the rings for the same purpose. Check with various companies to see if they offer rings with the 9s/10s conversion built in. If not you might try thinning the tab, but I warn you it's a slow process and difficult to do well without decent hand filing skill.
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  7. #7
    Senior Member liquefied's Avatar
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    I use 9sp DA cranks with my 10sp Ultegra 6700 group. It works perfectly.

  8. #8
    Senior Member TLCFORBIKES's Avatar
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    What are the chainring tooth count on the old 105 crank?

  9. #9
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    The old 105 is 53/39 and it appears that the new Ultegra is 50/34.

  10. #10
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    Sorry, I think I took the specs on 2010 Look. I believe the 2009 uses a Shimano FC-6601 with 53/39.

  11. #11
    Senior Member TLCFORBIKES's Avatar
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    The chainrings from the FC-6601 (53/39) will work fine.

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