Q-factor is the width of the outside faces of the cranks where the pedals are. Obsession over Q-factor came, then left with the advent of road disc (and 12 speed) which suddenly demanded wider wheel spacing, and along with it wider chainlines and Q, and now it is no longer in vogue to talk about Q-factor. It used to be that Q didn’t vary much, cranks through the 70’s were straight, and you needed to clear the chainrings with a standard chainline, and possibly the chainstays, but since most bikes used straight tapered chainstays in lugs and it was a relatively known dimension. It used to be typical for a high end racing crankset to be maybe 135mm for a double in part just to minimize material usage and save weight.
Q-factor started increasing due to a few factors, cheap cast aluminum arms instead of steel required more aluminum, shaped FD cages requiring more clearance, mountain bikes requiring wider cranks to clear wider chainstays to clear wider tires, etc. The idea Q-factor started gaining traction in the early 90s spanning the spectrum of cycling, from Obree in designing his world record breaking bike “old faithful” in the early 90s using a banana for scale, to what I can only assume was G.P. writing articles in the Bridgestone catalog, and industry giants like Shimano. Basically, it was then that people decided this oft ignored measurement had gone too far, from something that basically no one complained about before to a problem. In the following decades, you see Q-factor starting to show on spec sheets and trying to keep it under control.
As stated previously, vintage (70’s and earlier) doubles could be around 135mm. Modern road doubles are often 145-155mm. Add 10mm to any of these for a symmetrical triple, or 5mm DS only for an asymmetrical triple. MTB cranks are often 170-180mm, sometimes less, sometimes more. Fatbikes have extra wide Q depending on tire size.
Some people are obsessive about Q-factor and find they need narrow Q-factor. There are also shorter pedal spindles for people that need even lower Q than can be provided by cranks. However, some people really did not care about Q-factor at all, pointing out that few people have problems with wide Q-factor MTB cranks, where the limiting factor is often frame clearance, not FD/chainring clearance.
Ergonomically, I share most of the same desires as low-Q obsessives, in regards to hip and knee alignment, when there is a crank I do not like, I can feel it and it results in the same sort of pain and injuries reported by low-Q obsessives. However I tend to fall into the camp that doesn’t care about Q-factor. What I found is that I do care a decent bit about U-factor. I attribute this to being somewhat duck footed with fallen arches. I imagine that low-Q obsessives are slightly pigeon toed. We are both trying to get our joints (ankles and knees) in the right place, but the contact point is at the ball of the foot or midsole. For the same knee-to-knee or ankle-to-ankle width our toe-to-toe width (correlates to Q) are much different. Furthermore, trying to maintain the same knee and ankle positions would result in my twisting my feet inwards, twisting my knee in turn, another ergonomic problem (although I can manage alright with low U and low Q cranks as long as there’s some decent Q-U delta).
A big part of this problem is caused by clipless pedals which have limited lateral adjustability. My foot position on flat pedals is not at all like my foot position on clipless. This is also why Q-U delta never bothered anyone much in vintage bike days. Short spindle pedals make the problem worse. Some people use pedal extenders. I don’t trust them and they extend pedals too far. If I go for a natural knee angle, I end up with shoe rub on the cranks, and sometimes the ankle bone smacking into the crankarm. Hip and knee pain can be exacerbated by low-Q even though it solves the same problems for low-Q obsessives, and unfortunately some have been excessively evangelical about it as a high performance cure-all, but I don’t imagine many of them get major shoe rub issues unless they have massive feet. Low-Q is a performance limitation for some cyclists, not all. It’s not clear if it’s even a majority of cyclists benefit as normal foot angle is biased slightly outwards. I imagine it’s like crank length, it really matters for people that need it, but not so much for anyone else.
I’m sure sure when the first not straight 3-piece cranks were bent (ignoring 1 piece cranks for obvious reasons), but Shimano started first just recessing the part of the crank arm at the axle in the late 70’s with 600 cranks. This didn’t do much for shoe rub because most of the crank was still straight and perpendicular to the axle, but it helped someone at Shimano to stop bashing their ankle into the crank. As cranks became more 3D and less 2D, Shimano made some effort to keep U (BB axle length) length reasonable even as Q increased. Whether it was to be stiffer, lighter or just more sculpted is up for debate, but is culminated in the DA FC-7410 crankset which was a double that utilized a super short 103mm axle, basically the shortest axle that can be fit on a 68mm shell. However, U-factors got wider when external BBs were introduced because they effectively made the BB shell wider, and the Hollowtech II design needed enough thickness for 2 M5 screws.
The now nearly extinct BB30 crank and BB30 standard tried to keep U-factor under control by using 68mm spaced internal bearings, like some BMX BBs or square taper BB. BB30 ended up having a lot of problems and frame designers liked the wider BB shells offered by other standards, but BB30 wasn’t just abut using aluminum 30mm axles (DUB is 29mm so it can have a plastic anti-galling/corrosion sleeve using the same bearings) which could have been done with BSA30/BB386 etc. An underappreciated aspect of BB30 is it tried to prevent the U-factor growth caused by external bottom brackets, but this also is what caused BB30 crank incompatibility.
Campagnolo popularized the term U-factor to as a feature of its ultratorque cranks. Although much of the press was on the superiority of the hirth joint, and many of the complaints by loyal campyphiles were about it’s inability to be adjusted for out of spec BB widths, both missed the real point of the hirth joint. It was to remove the joint, and more importantly, the extractors from the crank arms which compensated for the wider bearings. This let Campagnolo take advantage of the wider stance of external BBs, (and compatibility with wide stance press fit carbon frames like BB86). This culminated in the powertorque crank that required an external gear puller to remove that everyone hated. Performance-wise it was a good design, it used a standard spline interface like most other modern cranks instead of the expensive hirth joint but kept the low U-factor. But no one liked it because it required a $100 tool to service that no one owned, negating the cost savings and making it hard to service.
The below are all estimates and could be off by a few mm because they are based on rough measurements of a limited number of parts and imprecise measurements because I just wanted to get a ballpark estimate. I will update them when I care to actually do a good job measuring them. These are mostly based on older pre-disc cranks based on the assumption that U is dominated by BB standards, whereas Q is altered by chainline.
Square taper cranks can be estimated by taping axle length and adding 20-22mm for the extractor threads and because the axle doesn’t bottom out in the taper. This would put a vintage racing crank in the range of 135mm, a typical 80’s double crank also around 135mm, a track crank or the super low profile DA at 123mm, and a MTB crank with a shorter axle at around 135mm and a MTB crank with a longer axle at about 143mm.
If you really want to know, cottered crank Q and U are basically the same as axle length.
A Hollowtech II type design will be about 138-140mm U for road (based on NDS doubled). Even though a 73mm shell is 5mm wider than 68mm, the cups for MTB are 10mm wide each instead of 11mm, however Shimano also makes wider spindle variations of the same crank. Road triples are weird and spaced out +3mm on NDS and +6.5mm on DS making them really bad. This puts HTII crank U at a similar U as some of the worst square taper cranks currently available.
Campagnolo’s U is 128-130mm, less than many classic cranks and shy of the super low profile FC-7410.
SRAM has too many variations of cranks to keep track of, but I think they’re on par with Shimano. Instead of 2 M5 bolts, they have extractor threads like q square taper crank. I think you can assume that most external BB cranks are in this range. Newer ones might be better (or worse) though.
Rotor is a bit wider at 142mm? for road because their cranks are CNC and hollow drilled so they have to be very straight in addition to fitting wide bottom brackets.
One neat product is the Favero Duo-Shi. Most pedals don’t have options for long spindles, and when they do, it’s often just by a few mm. Pedal extenders have a minimum length of ~20mm because they must have a female socket to accept the full thread of the pedal and present safety issues. However, they are available to a limited degree. However, for people who want to use pedal based power meters, those specialty options are not viable. The Duo-Shi however takes a Shimano pedal body and adds ~10mm per side to make room for the power meter pod. This doesn’t reduce U, but it does increase Q-U delta.
Just like Q isn’t the only thing to consider, U isn’t either. The Q-U delta will get you a rough proxy of how much your feet can be splayed out, but this is also down to the shaping of the arm and pedal-foot position. Many (not all!) crank arms will be parallel with the frame from the pedal eye to the edge of the big chainring, because this is how to minimize Q for a given chainline while maintaining enough FD clearance for a modern FD. If crank rub is happening around the edge of the big chainring, then lower U probably won’t help. Moving the cleat in (and foot out) or longer pedal spindles may however. I would take a wider pedal stance over trying to force the knee with 0 float cleats. If the rub is happening between the big ring and axle low U may help, as well as arch support or varus wedges, but it really depends how bowed the crank is. Some MTB cranks are almost a straight line from the big ring to the axle, some road cranks try to go for a graceful curve. The quicker the crank moves in (like the straight line) the more clearance. If your ankle bones are smashing into the crank or your heels catch on the crank sometimes, then low U may help.
The important take-away is that low-Q isn’t always better (despite what some advocates say, some fitters have noted that wider Q has helped most of their clients), even if you have the same pains that low-Q advocates describe. These things are caused by poor leg positioning, and low-Q only solves that for some people. It’s helpful to diagnose where pedal rub occurs if it does occur, and what your natural stance on flat pedals (not quill pedals) is. Because U is a rough measure of clearance (non-contact) where as Q dictates pedal position (contact point) it’s extremely important to note that while changing Q always changes the fit, lowering U past a certain point has absolutely no effect on fit, and this point varies from person to person and on the Q of the cranks. Lower U doesn’t hurt, but it also may not help.
Kuromori 