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Q Factor

Old 07-06-13, 09:43 PM
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UmneyDurak
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Q Factor

I have two cranks, one is Dura Ace, and another one is SRAM Force. Both compact. I run Shimano chain and rear cassete (10 speed road). Issue I am having is when I put Force crank, in Big/Small combo (big upfront small cassette) my chain drops of the crank. It does it only under load (sprinting). I adjusted FD to prevent this, but it only masks the issue. Chain still tries to fall off, but is being prevented, so it just skips. I think Dura Ace Q Factor is 147.5, and FORCE is 145. Could this be the issues, am I screwed? My bike has BB30 with BB90 adapter. So holowtech II / GXP BBs respectively.

Last edited by UmneyDurak; 07-06-13 at 09:49 PM.
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Old 07-06-13, 11:13 PM
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No Q-factor wouldn't come into play.

As you describe the situation, I assume the chain is coming off to the outside, which may be because the crank is too far inboard. You can measure chainline by measuring from the tips of the teeth on the outer ring to the down tube, add 1/2 the D/T diameter, This should be about 46mm (plus or minus 2mm is OK). If yours is more inboard than that, and you have a spacer on the left side of the spindle that you can move to the right do so.

I prefer to set all my bikes up with the chainline outboard of that 46mm figure because it improves the chain line for the gears I ride most (outer ring with outer half of the cassette), so feel free to go to the high side of the 46mm distance.

If you cannot improve the chain line there are a few more tricks available, but they involve the use of a file, and many balk at modifying hardware. Though it's fine if you know what you're doing.
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Old 07-06-13, 11:42 PM
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Originally Posted by FBinNY View Post
No Q-factor wouldn't come into play.

As you describe the situation, I assume the chain is coming off to the outside, which may be because the crank is too far inboard. You can measure chainline by measuring from the tips of the teeth on the outer ring to the down tube, add 1/2 the D/T diameter, This should be about 46mm (plus or minus 2mm is OK). If yours is more inboard than that, and you have a spacer on the left side of the spindle that you can move to the right do so.

I prefer to set all my bikes up with the chainline outboard of that 46mm figure because it improves the chain line for the gears I ride most (outer ring with outer half of the cassette), so feel free to go to the high side of the 46mm distance.

If you cannot improve the chain line there are a few more tricks available, but they involve the use of a file, and many balk at modifying hardware. Though it's fine if you know what you're doing.
Ok,will do. There is no spacer on left side.
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Old 07-07-13, 02:38 PM
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OK, so distance from DT to middle of outer ring is 35mm. Circumference is 115mm, so radius is 18.3, so total distance is 53mm. So it doesn't seem to be too far inward...
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Old 07-07-13, 03:04 PM
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Just a question, are the cranks / chain rings new or are they possibly bent / worn?
is the rear deraileur properly adjusted? B-screw keeping enough tension on the chain? are your deraileurs' pulley's clean, are the bearings spinning freely?
The alignment of the rear could also be an issue. Is the rear wheel in square, and is the rear quick release tight?
I had a loose quick release on my rear wheel, which popped out as I went to sprint away from work. I met with the top tube right away, Ouch!!
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Old 07-07-13, 04:45 PM
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Well chain and cassette are new. I don't think crank is worn. It's my backup one I use when my srm dura ace out of commission (which is the case now). Yeah rear derailure is adjusted, and everything is spinning fine. I never had this issue with my dura ace crank. This happens with a different rear wheel also, and different cassete, so not a wheel issue. I got a 1mm spacer, going to try that...
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Old 07-07-13, 10:42 PM
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Originally Posted by UmneyDurak View Post
OK, so distance from DT to middle of outer ring is 35mm. Circumference is 115mm, so radius is 18.3, so total distance is 53mm. So it doesn't seem to be too far inward...
Sorry for the delay. So chainline is OK at least at the crankset, but the chain still seems to be coming from the outside. Let's continue the process of elimination.

Find a straight edge at least 18" long. Place it against the face of the outer chainring on a secant, and extend back to the cassette. Using the 53mm chainline +2mm for half the thickness of the ring, the straight edge should hit the rear end 55mm from the center. Note which sprocket it lines up with, subtract 55mm from half the width of the rear triangle, and that should be the distance to the inside of the right dropout.

I'm guessing that you may find the distance to be more than predicted which means the entire rear end may be offset to the right (there may be another reason for the error, but it's rare these days).

Do that then come back for the next step.
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Old 07-07-13, 11:16 PM
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Will try. Also tried putting 1mm spacer in to BB, didn't improve things. Chain still drop.

From where?
subtract 55mm from half the width of the rear triangle

Last edited by UmneyDurak; 07-08-13 at 01:15 AM.
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