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Dura Ace FC-7900 chainring width offset?
Posted: Mon Jul 16, 2012 2:55 am
by hjb1000
I'm sure I've read the figure on here somewhere but I couldn't find it. Apologies if already been asked.
Basically I've got a set of 7900 crank arms with an ultegra 6700 39T chainring on the inner, and a generic 53T chainring on the outer. When in the the little ring, the chain rubs on the inside of the the large chainring when in the last couple of sprockets.
Therefore I need to use some spacers to move the larger chainring further out. I need to know the default offset figure so I can order the correct width spacers. Thanks.
Re: Dura Ace FC-7900 chainring width offset?
Posted: Mon Jul 16, 2012 2:59 am
by thisisatest
i would not suggest that. the 7900 ring is more than just a chainring with spacers. it has its ramping occupying the space you intend to fill with spacers. you'll likely jam your chain between the rings.
youre much better off living with the chain rubbing the outer ring in the three hardest cogs. use it as a reminder to shift to the big ring.
Re: Dura Ace FC-7900 chainring width offset?
Posted: Mon Jul 16, 2012 2:59 am
by Weenie
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Re: Dura Ace FC-7900 chainring width offset?
Posted: Mon Jul 16, 2012 3:24 am
by hjb1000
Thanks for the reply. Yes, good point didn't think of that.
This was meant to be a bit of a temporary work-around but it's starting to sound like a PITA. I think I'll just save up for a proper new set of 7900 cranks!
Re: Dura Ace FC-7900 chainring width offset?
Posted: Mon Jul 16, 2012 6:21 am
by ultimobici
hjb1000 wrote:[b]When in the the little ring, the chain rubs on the inside of the the large chainring when in the last couple of sprockets.]/b]
Therefore I need to use some spacers to move the larger chainring further out. I need to know the default offset figure so I can order the correct width spacers. Thanks.
This is rarely a function of chainring offset/spacing and normally a function of chain stay length & the fact that you shouldn't be in that combination.
Re: Dura Ace FC-7900 chainring width offset?
Posted: Mon Jul 16, 2012 7:01 am
by pawnii
Every bike I've ever had rubs the large chainring when i'm in 39/11 or 39/12. (small frame size with short chain stays)
Technically you shouldn't be in that gear. Best to change to the big ring before you get to the 11 or 12 cog when in the small ring.