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Discussion Starter · #1 ·
I think crank flex is the issue. I had a new Force FD installed (Rival FD exploded), and it is mated to a Ritchey WCS Crankset (the CS was replaced, entirely, under the recall).

If I peddle at low force, I don't get chain rub (in the 50 ring) against the FD in the larger cogs (12-26 cassette ). As soon as I get on it, I get a rhythmic, not constant, rub of the chain against the FD.

After much trial and error whilst riding, it appears to occur as a result of non-drive side crank pressure. I'm assuming this is from crank flex. What am I to do??? Should I investigate new cranks?
 

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???

If I understand you, when you press hard on the left crank, you get the ring/chain rubbing against the FD. I don't see how crank flex could cause that, since it would be the left arm flexing, which wouldn't move the chainring sideways -- force is being transmitted from the BB spindle to the spider to the ring, without involvement of the right crank. I supppose the ring could flex a little, but most of the force on the ring is in line, not lateral.

IME, the most common reason for such rubbing is frame flex -- twisting at the BB shell. Aside from getting a different frame, the solution is fine-tuning FD position as much as possible to minimize the rub in the most-used combos.
 

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Discussion Starter · #4 ·
JCavilia said:
If I understand you, when you press hard on the left crank, you get the ring/chain rubbing against the FD. I don't see how crank flex could cause that, since it would be the left arm flexing, which wouldn't move the chainring sideways -- force is being transmitted from the BB spindle to the spider to the ring, without involvement of the right crank. I supppose the ring could flex a little, but most of the force on the ring is in line, not lateral.

IME, the most common reason for such rubbing is frame flex -- twisting at the BB shell. Aside from getting a different frame, the solution is fine-tuning FD position as much as possible to minimize the rub in the most-used combos.
You did understand me correctly. It's a ti Lightspeed Tuscany ('02). Maybe it is frame flex. I'll give adjustment a try.

Thanks.
 

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Probably BB flex. . .

LyncStar said:
If I peddle at low force, I don't get chain rub (in the 50 ring) against the FD in the larger cogs (12-26 cassette ). As soon as I get on it, I get a rhythmic, not constant, rub of the chain against the FD.
. . .and user error (mashing a big ring, larger cogs combo), a.k.a. cross chaining.
 

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That's not your crank flexing, it's your frame. I've never seen a crank flex. They're ridgid as hell, no matter what the brand. When you apply pressure to one side of the crank the force deflects the bottom bracket on your frame. You can check this by leaning the bike against a wall and putting the front wheel against another wall (so you're in a corner). Get on the bike and push down on a pedal while you're looking down at the bottom bracket area. See the frame flex? Another way to check this is to (off the bike) hold the bike by the handlebar while standing along side of it. Lean the bike away from you a bit and step on the pedal, forcing it sideways. Some bikes are certainly stiffer than others, but no bike is as stiff as a crank arm.
 
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