Crank Q-factor, bike fit and foot pain

[wafter]

While I love the numbers, this is one particular rabbithole I've not been keen on venturing down..

This became necessity recently when playing with the cranks on the Fuji; and I've come to recognise that some cranksets are considerably wider than others - with potential repercussions on rider comfort.

For those who don't know, crank Q-factor is the distance across the bike between the outside faces of both crank arms at the pedals (actually might be pedal register face where it screws in, which is typically counterbored slightly).

Road groupsets are apparently narrower; partially because they have less need for wide spacings to allow clearance with at tyres and give appropriate chainlines with wide rear axles / dropout spacing.

I plan to do some tickling with the measuring stick later, but for now published values suggest Q-factors of 146mm for R7000 105 (road), 151mm for 810 GRX (gravel) and 172mm for M8100 Deore XT (MTB). That's 26mm or pushing 20% difference between the road and MTB cranksets; so not insignificant.

I have a queer, fairly wide stance with a lot of toe-out and tend to concentrate load on the outsides of my feet. I've got wedges in my cycling shoes to try to counteract this, but having got out on the Genesis last night (105 crankset) I noticed quite a lot of aching in the outsides of my feet and wonder if this is partially due to the crank's narrow Q-factor - since the closer together my feet become the less flat their soles are to the ground. I don't seem to get this issue so much with the Fuji, which runs an older M590 Deore crankset which I believe is around 170-175mm Q-factor (granted it's also running flats so not a direct comparison).

I'm tempted to sling some pedal spacers on the Genesis; while I've oft-been tempted by a new crankset as the 50/34 gearing's a bit high for its intended purpose and I'd prefer shorter cranks. As a bit of an aside, I also wonder how optimised the 105 crank's chainline is with the MTB-derived 142mm axle spacing on the rear, when road stuff is typically 130mm IIRC. I've been looking at GRX cranks but wonder if something MTB-focussed would be more appropriate - ratios and fitment permitting...

As always I'm interested to hear any thoughts anyone might have on Q-factor, fit / comfort or anything to do with the thoughts above

 

[cougie uk]

I would think it might be more to do with the shoes than the Q factor.

 

[fossyant]

I don't get any issues with Q factor, other than riding the road bikes feels like my legs are very close together for the first 10 minutes. The shoes are also closer to the cranks than MTB SPD. My commuting is on 175mm MTB. The CX bike feels closer as thats on SPD's and the shoe sits filurther out despite the more road orientated crankset.

 

[wafter]

I would think it might be more to do with the shoes than the Q factor.

Thanks - certainly a possibility. If I can pick up some cheap pedal spacers I definitely think it'd be worth seeing if a wider stance makes any difference though.

 

[Bollo]

I remember discussions about Q-factor flaring up around the time that pedal-based power meters (either integrated or fitted after-market) started to appear that increased the effective Q factor. It’s also a reason that the crank-arm definition alone for Q isn’t always entirely representative. In particular people seemed to get het up about the Q increase for the infamous Limits power meter, when really they should have been more concerned about their own gullibility.

Anyways, the anecdotal general consensus I've picked up over the years is that it doesn’t matter too much over the typical ranges for Q, although some people appear to be sensitive.

I couldn‘t find much research out there, although this guy did a PhD on it (and also has a great name!).

https://etheses.bham.ac.uk/id/eprint/5523/1/Disley14PhD.pdf

TL;DR; He reckons that most cyclists would pick a smaller Q than the 150mm ish standard and that there are advantages to be had.

 

[wafter]

I don't get any issues with Q factor, other than riding the road bikes feels like my legs are very close together for the first 10 minutes. The shoes are also closer to the cranks than MTB SPD. My commuting is on 175mm MTB. The CX bike feels closer as thats on SPD's and the shoe sits filurther out despite the more road orientated crankset.

Thanks - I don't neccessarily get on the bike and instantly feel that one crank is different to the next. However I'm prone to foot, knee and to a lesser extent hip pain and given my wonky feet it would fit that the narrower the spacing the more it's contorting my extremities away from where they want to be.

In this regard I've perhaps found the Fuji the most comfortable bike I own; however it's not a level playing field as the pedals and footwear are also different to that used on my road / gravel bikes (flats on the Fuji and A600 SPDs and Shimano gravel shoes of some description on the other two).

I'd expect Q-factor to be less of an issue for those with less wonky legs and feet..

I remember discussions about Q-factor flaring up around the time that pedal-based power meters (either integrated or fitted after-market) started to appear that increased the effective Q factor. It’s also a reason that the crank-arm definition alone for Q isn’t always entirely representative. In particular people seemed to get het up about the Q increase for the infamous Limits power meter, when really they should have been more concerned about their own gullibility.

Anyways, the anecdotal general consensus I've picked up over the years is that it doesn’t matter too much over the typical ranges for Q, although some people appear to be sensitive.

I couldn‘t find much research out there, although this guy did a PhD on it (and also has a great name!).

https://etheses.bham.ac.uk/id/eprint/5523/1/Disley14PhD.pdf

TL;DR; He reckons that most cyclists would pick a smaller Q than the 150mm ish standard and that there are advantages to be had.

Thanks - that all looks like a lot of interesting stuff that I'll take a look at when I get a bit more time. I'd concur that it's probably a bit like crank length - tolerable within reasonable limits for most, but potentially more critical for some. Will be interesting to see what Mr. far-too-many-quad-barreled-moniker has to say about it in his paper


This afternoon I've bothered the most pertinent / accessible bikes with the vernier, with eye-opening results.

Measured across the crank outer faces at the pedal (not the pedal counterbore) the Q-factors of the road 105 R7000 crankset on the Genesis and MTB Deore M590 on the Fuji came in at around 147 and 175mm respectively; so the crank spacing on the Fuji is 28mm or nearly 20% wider than that on the Genesis.

More startlingly, when factoring in the dims of the pedals (skinny SPD PD-A600s on the Genesis, fat MTB-esque flats on the Fuji) the distance between pedal centrelines comes in at around 251mm for the Genesis and 303mm for the Fuji; 52mm or a little over 20% more on the Fuji..

Having had a bit of a look at pedal extenders the consensus reflects the "common sense" conclusions I'd reached; that my specific set of physiological perculiarities can benefit from wider pedal spacing. I'm very happy to chuck a few quid at some cheap pedal spacers to see if they help; however my SPD pedals are hex-drive only, so won't work with these.

Ultimately I've been toying with the idea of replacing the crankset on the Genesis anyway so this is just another compelling reason to do so... problem is that the GRX cranks I'd earmarked are only fractionally wider and I don't think an MTB crankset would allow large enough rings.

Bugger.

 

[wafter]

Having trawled through myriad technical documents it seems that - as with most things I desire on life - the ideal solution for my predicament doesn't exist.

Chainring-wise I like the 46/30 GRX600 for pseudo-gravel jazz, but that crankset is only slightly wider than most road offerings at Q=151mm. All the MTB offerings are great from a Q perspective, however the largest chainrings I can find are on the 7000-series SLX at 38/28; which is both a bit small at the top end and stingy in terms of overall gearing range. There doesn't appear to be an easy option for bodging gravel / road chainrings onto MTB cranksets. I think Hollowtech axles are cast into the DS crank arm assy, so any swappery there is probably a non-starter.

So, moving on from OEM-facing lashups we move down the heirachy to less ideal solutions. As above pedal spacers / extensions are available; however all the spacers from the reputable suppliers are solid - meaning there's no access to the 8mm hex-socket my pedals require. Moving to ebay and there are many apparently hollow extensions that would apparently "work"; however they also have much, much less meat on them than the solid items.

The core of the standard 9/16" UNF threads used on pedals is around half an inch or 12.7mm; while clearance on an 8mm A/F hex key is, at its very mininum 9.2mm. This would mean an absolute max. wall thickness of of 1.75mm; not sure I'd trust that to react my bulk at full chat. Will do some stress calcs in a bit if viable after my 3rd G&T...