Bicycle Mechanics - Crank/Chain Ring Flex?
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06-20-08, 08:55 AM
What's a normal amount of flex under load?
I have a Sugino XD double crank and my big ring is a 50t.
When under load (pedaling from a stop, butt off the seat) I can see a good 1/4 inch of movement (side to side) of the chain in the derailler. It's enough that the chain scrapes the inside of the derailler. Is this normal? I didn't notice this before on my Sugino XD triple with a 48t big ring. Could be that I don't have my derailler set up correctly though since I did have to remove/replace it. But I still think I would have noticed that much chain sway.
06-20-08, 10:38 AM
Loose bottom bracket bearings? Also frame-flex plays a major part of the movement you're seeing as well.
But most likely you have a bent chainring. Take the chain off and spin the crank by hand. Use the FD as a gauge and see how much the chainring moves laterally as you spin the crank by hand. Use a crescent-wrench to true it.
06-20-08, 11:03 AM
But most likely you have a bent chainring.
That was my first thought, but it's definitely more pronounced under load. During light pedaling it seems to be well within what I would think is spec for a relatively inexpensive ring. Then again, maybe it isn't.
I also tried two different phil wood bottom brackets... one of them is brand new.
Same frame as before so it shouldn't have more flex than it did before i switched cranks.
Likely frame flex. If you are a heavier and powerful rider, the amount of flex as evidenced at the gate of the front derailleur can be noticeable.
Trying moving away from a stop in a lower gear.
09-15-08, 09:56 PM
Whatever the case, never underestimate the importance of a really stiff crank ;-)
G piny parnas
09-16-08, 01:27 PM
are you cranking the bike hard side to side? can you up the wheel and cinch it closer to the
seat tube? this is a quality ride according to the pics and the bracket is bombproof--- what kind of tires are you running--- are you using spd and are they coming loose at any point?
09-16-08, 02:14 PM
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.
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