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Getting a lot of crankset flex?

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Getting a lot of crankset flex?

Old 10-25-21, 01:15 PM
  #1  
tiger1964 
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Getting a lot of crankset flex?

I just got off the bike, so I have not much investigating yet, but the Suntour Superbe crankset on my Palo Alto is perplexing me. I guess I had not noticed it before, but on a recent ride I saw the chainring wavering between the sides of the front derailleur cage. Hmm, when I got home I sent time with he usual two-Cresenct-wrenches solution to see if I could remove any runout in the spider. The results looked good to my eye -- I own a dial indicator and base but so far have not puzzled out how to set it up on a bike.

So I went on a ride just now, and a long uphill I looked down and I see it persists, and I am pretty sure it's more under load than when not. Next step? I'll recheck the crank-to-spindle bolt, of course. I cannot imagine the spindle flexing, correct? The frame? What else should i look at? Hmm, single chainring ("1x") systems' appeal are becoming more inviting by the day.

I can set the FD to clear and the 'wobble' stays within the cage for no scraping -- but barely.
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Old 10-25-21, 01:28 PM
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If you want some additional flex you need to ride some Stronglight 104s. You can make contact with both sides of the FD as you're going uphill.

Some cranks just flex more. Maybe the Superbe is one of those.
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Old 10-25-21, 01:30 PM
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Some flex seems normal with most steel bikes. I see a little in my Ironman, and more with my Univega Via Carisma with their fancypants Bi-Axial Oval tubing.

As long as it doesn't cause problems with shifting, scraping the front derailleur, etc., I've learned to ignore it.

When I first tried switching my Suntour Accushift GPX shifters from index to friction mode, that slight BB flex was enough to cause ghost shifting, especially when I stood to climb or sprint. But that turned out to be a flaw with the GPX shifters. "Friction" is just an emergency bailout mode in case the indexing system fails, something to finish a ride. To minimize shifter creep I had to crank the levers so tightly it was difficult to shift. When I tried a nice set of purpose designed friction shifters, the Shimano L-422 Light Action retrofriction, there was no ghost shifting and the lever tension was smooth and easy to use. So my original problem was mediocre shifter design, not frame/BB flex.

With square taper cranks sometimes we can minimize slight wobble by trying each of the four available orientations on the spindle.

A few years ago, after test riding my first carbon fiber bike, I noticed definite differences in stiffness. At the time I thought a stiffer BB would translate to faster climbs. And it does, a little, now that I have a couple of carbon fiber road bikes. But it's not as big a deal as I'd expected. Probably because we have no mountains or long continuous climbs here, and I'm not a sprinter. My average speed on longer segments and entire familiar routes of 20-35 miles is the same on my steel Ironman and two carbon fiber road bikes.

If anything the slight flex of the steel bike seems more comfortable without any noticeable inefficiency that I can estimate from hundreds of rides. Maybe that's why some cyclists prefer "compliance" over "flex." Sounds more like a feature than a flaw.
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Old 10-26-21, 02:10 PM
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Also, check your crankset very closely if it is developing any cracking that could cause flexing.
Some people that had their crankset break on them, usually noticed flexing, right before they gave way....
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Old 10-26-21, 02:20 PM
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Not mentioned is if the crank to spindle are designed for each other. Can make a difference.
the keen looking for cracks is good advice.
the only times I have had flexing that caused rub are when a chainring bolt was loose or one of those cranks with the 50.4 BCD attachment.

oh yeah, on a Teledyne Titan, that was the frame.
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Old 10-26-21, 04:21 PM
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Long shot but the bottom bracket may be cracked. Get off the bike and lean the bike away from you and push hard inward on the near pedal at 6 o'clock. Try both sides.
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Old 10-27-21, 11:48 PM
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Is it flex (bending under load) or is it bent (a permanent, out of plane deformation)?
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Old 10-28-21, 01:24 AM
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It's particularly noticeable when the spindle cracks, my left crankarm was making contact with the left chainstay:

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Old 10-28-21, 02:43 AM
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Is it having an effect on the bike?
If it does then it needs fixing.
If not what's the problem?
Is it just because you saw it that its a problem.
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Old 10-31-21, 06:17 PM
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The most common cause of ring wobble is bent rings. Check wobble on both rings. If both wobble, then consider the spider to be bent. If only one, usually the big ring, wobbles, the ring is the culprit.

Using an ordinary ruler or tape measure, measure from the seat tube to the tips of the ring teeth. Do this in several parts of the ring circumference. Figure out long and short spots and average of all measurements. Armed with that information, commence gently straightening the ring while it is still fitted to the spider. You can pretty accurately determine and correct ring wobble using this simple tool. If both rings are the issue, you might try bending the spider arms ever so slightly to get the rings running true. Personally, I would not attempt to bend alloy spider arms for fear of them cracking and eventually failing in a catastrophic manner.
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