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-   -   Shimano cranks q factor OFFSET (https://www.bikeforums.net/bicycle-mechanics/1255786-shimano-cranks-q-factor-offset.html)

beanbag 07-26-22 08:19 PM

Shimano cranks q factor OFFSET
 
Hello,
Does anybody know what is the typical left-right offset on Shimano road cranks? (in particular the GRX gravel cranks)
For example, the right crankarm might be a little further from the chainstay than the left one. (Assuming chainstays are symmetrical)
In my case, I have an older MTB where the triple crankset has a Q factor of 182 and seems to be offset to the drive side by 4-5mm. The left (non drive) crank is 15mm from the chainstay and the right is 24mm. (I check against the rim and the offset is similar)
I want to upgrade this to a Shimano GRX double with Q factor 151mm. IF the GRX crankset also has a 5mm offset, then this won't work as both sides would move in by 15mm and the left arm will now hit the chainstay while the right will have 8-9mm clearance. BUT if there is no offset, then both sides will have about 4-5mm clearance.

P.S. I read that there should be a spec in some Shimano technical documents about the distance from the inside edge of the crank arm to the centerline, which I guess is a more relevant number and doesn't have me guessing at the crankarm thickness, etc.

Polaris OBark 07-26-22 08:21 PM

I'm almost certain they are symmetrical. I have one I just stuck in a frame. I will go measure it.

I have 14mm clearance on both the left and right of my symmetrical chain stays, which are 96mm apart. So the crank arms are 124 mm apart (inside to inside). GRX 600 11-speed, 172.5mm.

Crank arms are 14mm thick.

Total Q measured piecewise is therefore 152mm, which agrees with the claimed Q factor within experimental error.

HTH.

beanbag 07-26-22 08:59 PM

Great. Exactly as expected.
BTW, do you know if on these road crankset / BB combos there is any room to shift the crank arms out by 1-2mm on either side via spacers? That spline clamping mechanism should have a little bit of tolerance? On my older MTB shimano cranks, there was a little piece of plastic in the gap of the crankarm clamp that slots into the spindle.

79pmooney 07-26-22 09:41 PM

My knees simply will not tolerate high Q-factors. (They consider over 145 big. And no, I don't ride bikes with bigger than 38c tires. I've known a long time that failure to listen to those knees will mean going 3rd party. And I like the ones I've got. Plus they are paid for.)

So I have been observing Q-factors and the related factors a long time. Those same knees like triple cranksets and the low gears. So I observe crank design and bottom brackets carefully. Now, I came up riding asymmetrical bottom brackets. Sugino, TA and the like. I've never noticed the asymmetry while riding. It wasn't until relatively recent years that I spent much time on Shimano cranksets with their "S" shaped cranks and symmetrical BBs. Knees started complaining! My most knee friendly bike became my fix gear with its complete lack of low gears.

So I started looking at cranks and found that Sugino, especially older Sugino, were much narrower. Also I could (yes, for a pretty price) get custom Phil Wood bottom brackets at any width and asymmetry I want. So now I ride mostly Sugino cranks with several Phil BBs that have the left crank very close the chainstay and the right inboard of the "perfect" chainline. I justify all of this under the $$s spent are probably considerably less than my out-of-pocket share of those 3rd party knees.

Good bike with triple: Q = 143
Good fix gear with velodrome standard drivetrain: 132
Peter Mooney set up fix gear with a triple crankset: 135
Winter/rain/;city Trek fix gear: 137
Raleigh Competition with triple: 150 (Farmers market bike. Very few long rides. There's room for improvement.)
Setting up a mid eighties race bike now with Campy triple and matching BB. Haven't measured it yet.

Phil Wood will cost you real money. But they can make any width and asymmetry you want. And they have the neat ability to be adjusted sise to side to dial in chainline or clearances. (Plus they are flat out gorgeous. But, being a completely hidden BB, only you will know.) And a trick - mount our crtanks on yhour Shimano. Measure how much you would like to move those cranks. Now do the math and see what that new spindle turns out to be. Place your order with Phil. (Both Shimano and Phil Wood work very close to the international standards. So close that you will barely see any difference other than the length changes you spec'd. Important because very small difference in spindle thickness make a big difference in where the cranks sit on the taper. (We are talking square taper spindles I hope. I have almost no experience with anything else and Phil does not make anything else.)

Best part about Phil, when this bike dies, it can go on your next. Might need a new spindle or this one moved a little but both are doable.

beanbag 07-27-22 01:04 AM

Sorry, should have mentioned that I am talking about the Shimano Hollowtech 2 cranks and bottom brackets which have integrated spindles.

Bike Gremlin 07-27-22 02:41 AM


Originally Posted by beanbag (Post 22588772)
Hello,
Does anybody know what is the typical left-right offset on Shimano road cranks? (in particular the GRX gravel cranks)
For example, the right crankarm might be a little further from the chainstay than the left one. (Assuming chainstays are symmetrical)
In my case, I have an older MTB where the triple crankset has a Q factor of 182 and seems to be offset to the drive side by 4-5mm. The left (non drive) crank is 15mm from the chainstay and the right is 24mm. (I check against the rim and the offset is similar)
I want to upgrade this to a Shimano GRX double with Q factor 151mm. IF the GRX crankset also has a 5mm offset, then this won't work as both sides would move in by 15mm and the left arm will now hit the chainstay while the right will have 8-9mm clearance. BUT if there is no offset, then both sides will have about 4-5mm clearance.

P.S. I read that there should be a spec in some Shimano technical documents about the distance from the inside edge of the crank arm to the centerline, which I guess is a more relevant number and doesn't have me guessing at the crankarm thickness, etc.

If it's of any help:

I'm running Sora compact cranks, Shimano Hollowtech II type, on a bike with 68mm wide BSA threaded BB shell.
I mounted Tiagra RS500 hollowtech BB cups, without the 2.5 mm spacers on either side.

This reduced the overall Q-factor by 5 mm, and it still works perfectly fine.

However, the pin used to "just in case" catch the left crank should it start sliding off needed to be removed - because the axle protruded further than Shimano designed it for (though still not too much for the plastic end cap to allow for adjusting the bearing preload).
If the pinch bolt and the plastic end-cap don't save me, and I fail to notice it's getting loose, I should have a crank arm fall off! :)

Polaris OBark 07-27-22 09:15 AM


Originally Posted by beanbag (Post 22588794)
Great. Exactly as expected.
BTW, do you know if on these road crankset / BB combos there is any room to shift the crank arms out by 1-2mm on either side via spacers? That spline clamping mechanism should have a little bit of tolerance? On my older MTB shimano cranks, there was a little piece of plastic in the gap of the crankarm clamp that slots into the spindle.

I don't know the answer to your question. Mountain bike BB usually come with some spacers, which implies that there is some ability to accommodate variance, but I don't know what the limit is. The consequence to getting it wrong kind of sucks, so best to get a definitive answer from someone who knows. cxwrench cpach


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