Join Date: Aug 2008
Location: Tucson, Arizona
Bikes: '02 Lemond Buenos Aires, '98 Fuji Touring w/ Shimano Nexus premium, '06 Jamis Nova 853 cross frame set up as commuter, '03 Fuji Roubaix Pro 853 back up training bike
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Sugino XD cranks have a 110mm bsd. That's a pretty small diameter for a 50t ring. The distance from where the ring is secured and where the chain exerts force on the ring creates room for leverage results in flex. It's physics. Same reason a longer crank arm will flex more than a shorter crank arm.
I'm pretty sure if the same make/model/length crank was 130mm bsd, using the the same bottom bracket, and using a 50t ring, you have less flex.