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Quintessentium 05-24-20 10:01 AM

Q factors of typical crankset, BB30 short spindle, squared tapered

are q factors of squared tapered cranksets or BB30 short spindle cranksets

are actually shorter than cranksets with external bearing BBs like holllowtech 2 or SRAM GXP?

HTupolev 05-24-20 12:41 PM

Square-taper cranksets often allow for lower q-factors because the 3-piece assembly gives a lot of control over how the arms are positioned: you can readily adjust the spindle length, and you can often offset the spindle if desired.

Furthermore, because friction-shift front derailleur cages were often narrower than the new stuff, less clearance was needed between the chainrings and the drive-side arm. So during the era when square-taper was dominant, many cranksets positioned the chainrings closer to the drive-side arm than modern cranksets do. For a given chainline, this results in a lower q-factor.
Similarly, the crank arms at that time were also sometimes narrower than modern crank arms. Crank arms that are ~12mm wide at the pedal eyelet are a rarity on the modern market, but were not terribly unusual in the 1970s. (Cottered steel crank arms were often even narrower.) This also reduces q-factor.

But the square-taper interface does not inherently make cranksets narrower by virtue of being a square-taper interface. If you buy a modern square-taper crankset and use it with its default recommended spindle, it'll probably have similar q-factor to competing cranks that use modern 2-piece assembly.

79pmooney 05-24-20 01:02 PM

Many of the older square taper BBs were asymmetrical. Newer tend to be like Shimanos with symmetry, putting the left crank much further out than it had to be to clear the chainstay.

One advantage of square taper if low Q is important is that you can get a Phil Wood BB with exactly the width and asymmetry you want. (It will cost real money and there might be a wait if it is not on the shelves.) Also for low Q, look for early-mid '80s Sugino or SR cranks. They were very straight.


FastJake 05-24-20 07:33 PM

In general, it is possible to get a square taper crank to a lower q-factor. With Hollowtech II, GXP, BB30, or any external bearing system you have basically zero adjustment.

Here is a list of various q-factors. I measured these without extreme care for my own reference, so take them with a grain of salt. You can surely find other lists online.

Stronglight square taper double on Peugeot, unknown bottom bracket length: 141mm
Dura Ace 7400 double, 110mm bottom bracket: 141mm
Rene Herse double, 110mm bottom bracket: 143mm
Shimano 600 6207 double, 110mm bottom bracket: 145mm
Shimano Sport LX double, 107mm bottom bracket: 148mm

Dura Ace 7800 double, Hollowtech II: 148mm
Ultegra 6600 triple, Hollowtech II: 157mm
Sora triple, Hollowtech II: 162mm

Shimano MTB triple, unknown model, Hollowtech II: 186mm

oldbobcat 05-24-20 08:41 PM

Selection of a spindle length depends, however, on the crank spec, the bottom bracket shell width, and the chain line. That is, the middle ring or the space between double rings must align with the center of the cassette or freewheel.

FastJake 05-24-20 09:29 PM

Originally Posted by oldbobcat (Post 21495299)
the middle ring or the space between double rings must align with the center of the cassette or freewheel.

These days, when possible and depending what I'm using the bike for, I like to have the big ring of a double land in in the center of the cassette. This allows full use of the cassette in the big ring without cross-chaining. The small ring is for steep hills and is only used in the first few gears of the cassette anyway. Example: 46/28 crank with a 13-26 cassette.

oldbobcat 05-24-20 09:43 PM

Close enough.

79pmooney 05-24-20 09:48 PM

Originally Posted by oldbobcat (Post 21495299)
Selection of a spindle length depends, however, on the crank spec, the bottom bracket shell width, and the chain line. That is, the middle ring or the space between double rings must should align with the center of the cassette or freewheel.

My knees do not like high Q-factors. (Chrondomalacia patellae) The doc who diagnosed me 40 years ago told me to listen to them His advice has been spot on so I do. I started noticing the bikes my knees liked best and least. Best? My fix gears. Least? My best bike, custom ti with triple Hollowtech 105, Hmm, that says something! I started on a low-Q campaign on all my bikes but the two fix gears. I went to the old straight Japanese cranks of the '80s and short spindled asymmetrical BBs. (I love and thank Phil Wood regularly.) Chainrings came inboard. OK, chain line might make a few purists cringe, The $$ cost to me of poor chainlines is far, far less than the cost of the simplest knee replacement and I would need two.

I now have all of my bikes but one under 140mm Q. (That one is 150, down from a Shimano 164) But the best bike is still a work in progress, I now cannot shift the chain onto the inner chainring. Need to figure out a way to add an 1/8" steel plate to the inside of the FD outer cage plate to assist in the push. (I use friction, A very narrow FD cage that needs a lot of adjustment is no big deal.) Ideas anyone? I could drill and tap but that's not a whole lot of metal. I tried epoxy but apparently it was too brittle to go with the FD bending and it fell off mid-ride and I did not notice it leaving so no postmortem. Anyone know of a glue with epoxy-like adhesion that allows more flex?

One challenge is that the seattube is 31.8mm,pushing the FD out that much more. I"m happy to grind the FD so the cage can come in more but that only goes so far on some FDs. I could settle for a wider Phil Wood BB (right now it is the absolute minimum that can fit that narrow crankset on this frame. Q-factor of 139 with a 50-38-24 triple. My knees so want to make this work. (I did not measure the old Shimano 105 Hollowtech Q-factor but it was big! Cranks missed chain and chainstays by a cm or more. Felt like I was riding bowlegged.

Until I get this mastered, I am stuck riding fix gear. Not all bad. Did 60 today and have a body that feels like it is buzzing all over. (Tomorrow morning getting out if bed is going to be a slow process.) Knees know they did something but feel OK.


adipe 08-24-20 06:00 PM

does anybody know the q factor of a square taper triple 105 shimano crankset?
105 FC-1057

the bb length in the specs found here is 118mm and i would need to make it (have not bought it yet) fit a mtb frame so that the cranks have clearance of the chainstays; i'd get a wider BB for that but i need to know the q factor with that reference bb of 118mm.

thanks in advance.

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