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Dura ace chain ring problem.(6 posts)

Dura ace chain ring problem.Proboscis
Sep 30, 2001 8:19 AM
I am almost finished building my new tri bike. I am having a problem with the chain rubbing against the inside of the large chain ring(while on the small chainring) when I am running on the smallest rear cog. The rear hub is centered in the wheel, the wheel is centered in the bike. The Dura ace bottom bracket does not seem to come in a longer length than 109.5mm I checked and rechecked the installation of the chainrings on the crank. I could flip the inner chain ring, but the shimano instruction sheet advises against it saying that it may cause the chain to get trapped between the chain rings during changes. What gives? did I over look something? Should I add space to the drive side of the rear axle and take off from the left side and realign the hub? Or is it simply that I am not supposed to use that gear combination. Any advise would be helpful. Thanks.
re: Dura ace chain ring problem.Birddog
Sep 30, 2001 9:52 AM
The general rule is not to X-cross your chain in the most extreme small/big, big/ small, combinations. However the situation you describe should not be occurring. Is this a true Tri bike with steep angles? If it is, then maybe you just can't get the chain line positioned because of the limited distance. This could be especially significant if this is a small frame to boot.
Good luck,
Birddog
re: Dura ace chain ring problem.Proboscis
Sep 30, 2001 12:31 PM
Thanks for the reply. Yes it is a true tri bike. The chain stays are a full two inches shorter than my road bike, even looks a little weird. I have thought of this but was not sure. If this is the cause it does present an interesting dilema. It only does it when I go into the smallest rear cog. The other eight work well. What is the use of having an eleven tooth rear cog if you can only use with the large chain ring? I wonder if anybody else out there has had or heard of this?
re: Dura ace chain ring problem.Birddog
Sep 30, 2001 12:57 PM
There is a lot of overlap between your big and small chainrings. You can calculate the gear inches and find the same or nearly the same combo in the big chainring.
re: Dura ace chain ring problem.jacques
Sep 30, 2001 6:16 PM
Re: What is the use of having an eleven tooth rear cog if you can only use with the large chain ring?

Conventional race TRAINING wisdom is to shift into to the big ring the moment you find yourself spinning out your small ring/next-to-last rear cog combination.

Conventional RACING wisdom is to use the small ring only if the terrain no longer allows you to use the big ring.

That pretty much eliminates the big ring/eleven cog combination. Nevertheless, you are right in thinking that this combo should work. New Dura-Ace 9-speed small chainrings have their teeth very slightly offset to the right to prevent chain skating. Perhaps an older small Dura Ace ring would work a bit better.
re: Dura ace chain ring problem.grzy
Oct 3, 2001 10:59 AM
Having shorter than "normal" chainstays is definitely a problem - if not the problem. 2" on something that's on the order of 20" is around 10% and from a geometric view point this is huge. If the same change were applied to a hill you'd defentely notice it. WE could brush off the calculators and run some numbers.....Nah. Just using an 8 speed chain on a nine speed setup will get you problems and there's only a few tenths of a millimeter difference in width. The tolerances applied to bikes these days are pretty tight and any deviations from normal puts you on or over the edge.

I also have some bad news for you: not only should you stay out of your small chainring/smallest sprocket you want to stay out of your large CR and largest sprocket as well. That's two "wasted" gear combinations. If you learn a little shifting awareness and discipline you'll soon avoid this problem - think of the noise as nature's way of telling you you're doing something wrong. Besides - if you wanted to shift further suddenly you'd have to do a double shift - not always the speediest way to deal with a surge.