Bicycle Mechanics - R4550 compact crank- question

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SLATS72
05-19-09, 06:58 PM
My LBS put on a Tiagra 4550 Compact crank50/34 on my Cervelo Soloist SL -- I had a Dura Ace crank 53/39 with a 12/27 cassette. The problem I first experienced with the 4550 was that I could not get into the 27 cog without the chain jumping off the big chain ring. After many trips to different bike shops it works now but I dont know exactly why nor hasnt anyone told me why .

After looking through this forum I see that the 4550 is for a nine speed. It works now but what are your thoughts on this whole thing besides having a low level used crank on such a great bike.
Would appreciate your thoughts -- the smaller crank seems to work well for me , thinking of going for a new Ultegra compact.

joejack951
05-19-09, 07:03 PM
Sounds like the front derailler needs to be positioned better. I've had a similar problem using the smallest cog when in the middle ring of a triple crank. Fixed it by moving the front derailler a little closer to the chain, but not so close that it made any noise.

The crank being 9 speed should have absolutely nothing to do with it. The only downside to using a 9 speed crank with a 10 speed chain would be slightly slower shifting. The only downside to using Tiagra in place of Dura Ace is some weight. How much do you care about a few ounces?

According to weightweenies, Tiagra standard crank is 975 grams with BB and Ultegra SL Compact FC-6650-G is 811 grams with BB. 5.7 ounce difference.
AEO
05-19-09, 07:07 PM
well, the 34T inner ring is the same thing for 9 and 10sp

only the outer ring is different between the models.

SLATS72
05-19-09, 07:53 PM
You said the 9 speed crank should have no problem handling the 10 speed cassette.That is what my worry has been after reading that it was designed for 9 speed. i have been concerned that the nine speed thing was part of my problem,. Thanks-the weight thing is not a big factor. Seems to be working ok now.
Al1943
05-19-09, 07:58 PM
Easy solution- don't use the 27 cog while on the big ring. That's cross-chaining, creating too large of an angle which increases friction, decreases efficiency, and shortens the life of the drivetrain.

Al
SLATS72
05-19-09, 08:16 PM
You are absolutely right about the cross chaining. But I still would like to be able to get into the 27 with out the chain jumping.