12strings

New bike, Sora equipped, with FSA triple crank, sealed bottom bracket, steel frame.

Symptoms only occur in big ring, and then only in highest (smallest) 3 cogs, maybe a bit in the 4th smallest...

When pushing these gears, I feel a vibration/grinding (not sure how else to describe it)...mostly on the power part of the pedal stroke. I can even reproduce it in the shop just spinning the pedals with my hand...same thing...I don't really hear anything...front derrailler is not rubbing...I just feel it.

I have already taken off my spd pedals and put back on the stock flat pedals that came with the bike, and there is no difference...

Any ideas?

cny-bikeman

Look up info on chordal action, may be part of what you are experiencing. What are your rpm's when you are experiencing the vibration?

dsbrantjr

+1 2.2.1 Chordal Action
Cozy Beehive: 52x12 vs 52x11 Gearing : A Look At Chordal Action

12strings

Rpm and load don't seem to matter...it's the same sprinting, or peddalling freely on a long downhill...or turning with my hand in the shop, basically goes away at cog #4

2manybikes

Go to the link in post #3 .

ThermionicScott 

+1 on chordal action. A minor but contributing factor for me in switching to cassettes that start out with a 13T.

12strings

So, since I only use those gears on down hills, it doesn't happen that often, but is there nothing to do about it?

Most people don't describe it happening on a 13 tooth cog? ( I think the smallest cogs are 11-12-13)

ThermionicScott 

Right, the smaller the cog, the more pronounced it is, so if you can use a bigger chainring/cog combo (like a 53/12 instead of 50/11) that'll lessen the effect somewhat. Relubing the chain with something thick can mask it a little. Otherwise, there isn't much you can do but learn to deal with it. It's harmless.

trailflow1

Check the chainring bolts are tight.

12strings

Rode to work today, This "Chordal action" from what i've read, shouldn't be showing up at all in the 15T cog should it? It definetly is more pronounced in the 11-13 cogs, but I felt it even on the 15 tooth cog...and I'm using a 52 tooth big Ring.

ThermionicScott 

When I first started noticing the effect, it was on a triple with a 38T middle ring and cassette with 12-14-16-18-etc cogs, and I was in the small end of the cassette a lot. I could feel it very clearly on the 12T, less so on the 14T, a little on the 16T, but things were pretty smooth on the 18T. Now that I've switched to that 13-X cassette and try to spend more time in the big ring, I don't use cogs smaller than 15T very often.

It's one of those things that will annoy you the more you pay attention to it, sort of like tinnitus. The good news is that you can train yourself to worry about it less and focus on other things while riding.

cny-bikeman

Frame flex can also contribute to the problem - not every frame is amenable to higher pedal forces and out-of-the saddle riding. Part of the problem is the poor choice of a cassette starting with 11 teeth paired with a 52 ring.

12strings

SCOTT: Interesting...I also use my 38T middle ring most of the time...I really never use the Big ring unless I'm going down a hill, while also trying to keep up with the group. But I suppose if bigger cogs were in line with the 52 tooth ring, I could use it more, perhaps with less vibration. Sounds like an option...In a few years when I need a new cassette.

TRAILFLOW: I did tighten my ring bolts, and found several that seemed pretty l loose, we'll see if that makes a difference on my way home tonight.

CNY-BIKEMAN: The bike is a Jamis Quest Comp (Steel...Reynolds 520 tubing). I'm just curious that I don't remember having this problem on my $200 Walmart Schwinn flat-bar road bike, which had a Triple Chainring paired with 7 speed Tourney stuff...I was riding the same group rides, which include one long downhill section where everybody tops out their gears.

trailangel

When and what are you using for chain lube?

12strings

WD 40.... Once a week or so.






...Yeah, I know. :-)

fietsbob

Not many teeth actually engaged when there is only 11 total .. barely 5.

mercator

You mean this:

Not this:

I hope.

ThermionicScott 

Learning to put up with your current parts is always the cheapest option... once you're comfortable swapping parts, it's hard to stop buying extra parts and tinkering.

Aha... that Schwinn's freewheel would have started with a 13T or 14T.

And if you're using regular WD-40, the thin residue left inside your chain isn't doing you any favors wrt chordal action.

12strings

Yep, Regular WD 40. Get over it.

ThermionicScott 

That's the spirit!