I just bought brand new Scott Sub Cross 10 mens city/crossbike. It has 30 gears and the same day after I bought it I went driving on an asphalt road and started to feel grinding/vibration in pedals at higher gears and higher speed. The front chain is on bigger ring and the cassette is on the last 4-5 smaller cogs. Also can feel it a bit if chain is on middle ring in front as well. Anyone had same problem?
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2Did you buy this bike new or is it used ? If its not working right from new, take it back to the bike shop for a tune up.– Criggie ♦Commented May 28 at 2:33
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If you have issues in larger cogs (15T and more) I would look for other issues before setteling that it's just chordal action. Perhaps talk to the store and have them take a look while it is still brand new and hopefully free of charge. My answer was just merely a suggestion.– WornChainCommented May 30 at 1:11
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3 Answers
You might be experiencing "chordal action". This is vibrations in the chain that is caused by the wrapping of the chain around a curve. This effect is increasingly pronounced with smaller cogs since the path is less smooth/greater angle per tooth.
I once read a claim that 16 teeth is a reasonable lower limit to mitigate this issue, but there is no hard cut-off. In practice I notice a fair amount of vibration in sprockets that are 14T or less, with 11T being awful.
Make sure your deraileur is indexed correctly and that your B-tension is not to blame. The chainline is hopefully correctly set up from the build of the bike. It is very helpful if you can borrow/try out some other (fancy) bike to see what can be expected even from a nice and perfectly set up bike.
It is hard to rule out other issues just based on your description, but even if everything is set up correctly you are still stuck with chordal action.
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6Just to elaborate a little if anyone's interested, chordal action is a result of the chain not being a continuously flexible belt--it can only bend at certain points. You feel vibrations because the chain tries to bend at a point that it cannot bend at, and as a result the chain lifts off the sprocket a little. Once it reaches a pivot point, it drops back down onto the sprocket. Those impacts are what cause the vibration. Commented May 27 at 17:44
Not withstanding "chordal action"² , Another contributing factor may be the frame design. The seat stay terminating into the seat tube below the top tube would change the vibration dampening of the diamond frame design. Kind of a reverse of a step through frame. Especially if the seat clamp is a smaller size internally than the seat tube and the seat post "floats" in the seat tube. A tight seat post contributes structurally a loose one does not.
² Thanks for this.
Defective bearings in the bottom bracket might also give a similar experience if you are applying high pressure / low cadence to the pedals in high gear.
