Exact or is there a certain amount of wiggle room depending on 1X or 2X?
How important is it to adhere to the right chainline and Qfactor when sourcing a crankset with your drive train?
wb3jma
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Q factor is just personal preference and doesn't directly affect drivetrain performance. Chainline, however, can be somewhat finicky - particularly in 12 speed systems. You can probably get away with a 2-3 mm deviation from optimum chainline, but more than that may degrade shift quality in the highest or lowest gears. I know some folks get good results running non-boost cranks on boost bikes, but I think you can't guarantee this sort of thing will always work well.
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It's important.
Does deviation depend more on smaller than optimum or larger than optimum?
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Either can matter if you go too far, but I've heard of more problems shifting into the biggest cog with too large a chainline than the reverse. It also depends on chainring size since the chain angles are more extreme with small chainrings. Some manufacturers like Alugear take that into account and vary chainring offset with size.
I've found that paying attention to both the chainline and Q-factor is definitely worth it. When I set up my bike, I noticed that having the right chainline made shifting way smoother and reduced chain wear, which is a big deal if you're putting a lot of miles on it. The Q-factor also matters more than you'd think—it affects how your legs feel while pedaling, and getting that dialed in made a huge difference in comfort for me, especially on longer rides. If either one is off, you could end up with issues down the road, like funky shifts or knee strain, so I’d say it's worth making sure both are on point.
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I would stick to recommend chainline. I just tried running 55mm cranks on my 52mm Japhy and it would constantly drop the chain while back pedaling. I think I may have also had a derailleur clutch issue, but ultimately I ended up putting my derailleur into my spokes, saying "**** this", and converting it to singlespeed, where the points are made up and chainline doesn't matter
IMO, the current 55mm transmission chainline is a few mm too far to the "outside". You can see this because the chain isn't straight unless it's in the outermost/hardest gears. Supposedly solved by a twist in the lower puller; not really. In this case, you'd be fine going more inside. In other situations, you might not want to mess with it. Look at your bike to see how close to center your chainline is now and visualize what the impact of the new cranks would be. If things are shifting perfectly, I wouldn't mess with it.
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A bunch of GREAT info here! No, chainline is not absolute, modern transmission will work with 52/53/54. But as Sendits post mentioned, Q factor 'might' be absolute, ie, if the Q is not wide enough, the cranks will hit the frame. But I would bet a post covid carne asada burrito (a $15 value) that in most cases there is plenty of room between frame and cranks even with 'spec' crank. Production managers etc make sure they have some wiggle room with clearances on chainline and Qfactor.
I am constantly pushing my chainring inward on all our personal bikes to reduce the chain angle under the highest torque situations. And.. it helps keep the chain from slicing away the 'wide' teeth on the chainring as well. I also make sure I have the proper chainring sizes for our pitiful power output and cassette size, so that we spend 'most' of the ride time in the 'middlish' cogs on the cassette for all bikes, road, gravel, mtb. Drivetrains today are better than ever, but they are also more expensive than ever. Getting the CL as tight as possible and regular chain wear inspection keeps me from throwing away multi hundred dollar cassettes.
DT
I am constantly pushing my chainring inward on all our personal bikes to reduce the chain angle under the highest torque situations. And.. it helps keep the chain from slicing away the 'wide' teeth on the chainring as well. I also make sure I have the proper chainring sizes for our pitiful power output and cassette size, so that we spend 'most' of the ride time in the 'middlish' cogs on the cassette for all bikes, road, gravel, mtb. Drivetrains today are better than ever, but they are also more expensive than ever. Getting the CL as tight as possible and regular chain wear inspection keeps me from throwing away multi hundred dollar cassettes.
DT
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A LOT depends on the frame you're using. I have an older Guerrilla Gravity Pedalhead. It was one of the earlier 55mm chainlne bikes. But they made it work by offsetting the rear wheel 3mm to the drive side. So the chainline isn't as bad as 55mm on a bike with a symmetrical boost rear end. Even still, I've pushed mine inboard about as much as I can. I think I'm at something like 53.5mm. I might be able to go half a mm narrower...maybe? before I ran into problems with my chainring hitting the frame.
I would never buy a bike with a symmetrical boost rear end that called for 55mm chainline. That's just insane to me.
The spindle need be wide enough or any offset chainring will foul the frame. Newer frames do not support 2x, a pivot will be made as wide as possible. Symmetry matters to your position over the pedals. 1st gear will lose fewer watts with an inboard chainring.
Older Shimano Hollowtech II supports more fiddling. 24mm Spindle spacers are available at aliexpress.
Q factor matters for myriad reasons that are compromised by fit and chain line. Your leg width, better balance with wider stance, more aero with narrow . . .
Truth! BTW, I have always been a fan
Thanks for all the posts and explanations, just the other day Shimano finally offered up their CUES for Drop Bar bikes which has the same chainline as GRX and a difference of one Qfactor which is very much likely means I'll not have to source cranks.
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