I am having knee and ankle trouble on my commuting bike (1993 Specialized mtb) and this has a 73mm BB with a spindle length I haven't measured but there is space between crankarm and frame.

I wanted to lower the Q factor and thinking that lowering the spindle length will be the easiest way - but does it actually affect the Q factor? As the cranks sit at the widest point which isn't necessarily related to the overall length.

  • The Shimano BB-UN55 is a pretty good bottom bracket line for the price, but I've removed that part of your question since its not really relevant. – Batman Feb 14 '16 at 16:03
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    I'd look elsewhere first for the source of your knee and ankle issues. I know an old mechanic who considered fat bikes the end to all the "Q Factor nonsense" as he put it. All of my knee issues (on my fat bike) have been solved with the standard seat and cleat adjustments. Have you already investigated those? – Deleted User Feb 15 '16 at 23:49
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    Have you tried raising your cadence? Knee problems can be exacerbated by low RPMs which means harder push on each stroke. – Criggie Feb 16 '16 at 0:26
  • I find the most common reason for sore knees is too tight of a knee angle. This can be opened by sliding the seat back some or sliding the cleats back some (resulting in the foot being in a more forward position relative to the pedal axle). One can accomplish the same thing with longer cranks but this is a considerably more expensive solution than a seat and/or cleat adjustment. Rick – user26154 May 22 '16 at 13:40

Yes, spindle length is a factor in determining the Q factor.

A longer spindle moves the cranks further out while a shorter one moves the cranks further in. The type of crankset will also affect the Q factor, as some will sit further out than others even with the same spindle length (I've seen cranks which on the same bottom bracket can have more than 1 cm difference in Q factor).

Whether or not you can get the chainline and Q factor you want is a different matter (and dependent on your setup).

  • This answers my question. Following on from that, how would I know what the minimum spindle length is for my bike? So if I went from a 127 to a 107mm spindle, I can safely say that the each crankarm is moving 1cm? Or is it more complicated? – lazyrabbit Feb 14 '16 at 17:02
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    Technically, there exist bottom brackets which are asymmetric (so a 20 mm reduction in spindle length would not move both sides in by 1 cm, when sticking with a similar crank+bb) and you can use spacers to control asymmetry. But assuming you're working with a similar symmetric bottom bracket, you should get 1cm inward on both sides (whether or not this is feasible depends on your frame, as Frisbee's answer shows). – Batman Feb 14 '16 at 17:48

In addition to crank arm / chainstay interference, your small chainring may get too close to the bottom bracket shell and not be able to rotate. I had that problem once when trying to install a shorter spindle.

Are you sure that your knee and ankle problems are Q factor related? Andy Pruitt's 'Medical Guide for Cyclists" has a lot of info about the causes of various knee pains and recommended remedies.

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    The reason is that I can feel the inside of my knee stretching on the down stroke and it tracks inwards. For the whole pedal stroke my leg is noticeably splayed outwards (though mainly with this bike). I am 5'9" and I am also quite narrow looking. It seemed like common sense to reduce the width. However, you are quite right that I should investigate common health issues. – lazyrabbit Feb 15 '16 at 19:18
  • @lazyrabbit If you can afford it, consider a proper bike fit. Takes a couple hours, costs around ~$250 NZ around here but will give you a list of optimal measurements for your bike. Will also show up any asymmetry in your body. – Criggie Feb 16 '16 at 0:25

If you have a triple then a double would be narrower. But as Batman pointed out you may have chain line issues.

You also need to consider clearance with the crankarm and the chainstay.

Narrow crankarms will buy a little.

Clearly another bike is going to be more expensive but a mtn frame is going to limit how much Q factor you can gain.

Article on Q factor

  • I am interested in a 'narrow' crankarm. Could you point out a particular model which is narrower? Yes, the ultimate limitation does seem to be the chainstay. - Also in your diagram, the stay seems to have been cut out a bit - I wonder if that's possible to add as a 'new' feature for an old bike. – lazyrabbit Feb 14 '16 at 17:09
  • A cut is not something you would add to a bike. Turn your bike over and have a look. No I don't have a specific model with a narrow crankarm and it may not even be an option with that BB. A used road bike with smaller BB and a double may be the most effective path. A custom type crank even it you find one is not going to be cheap and you still may have chainline problems. – paparazzo Feb 14 '16 at 17:31
  • It's not a cut, it's a dent. And it's not unknown for them to be deliberately added to a bike that wasn't designed with them later. That doesn't mean it's a good idea though. (If it's not a welded steel frame, it's almost certainly a bad idea.) bikeforums.net/framebuilders/… velocipedesalon.com/forum/f2/… – armb May 24 '16 at 17:37

All things being equal, yes the bottom bracket spindle length is related to Q factor.

I'd encourage you to rethink your problem. I've never heard of someone solving a knee problem by reducing the Q factor. Most want to reduce the Q angle which is accomplished by increasing the so-called factor.

You should do a search for pedal axle extender as I believe this might lead you to the cheapest, fastest, least complex, and widest solution.

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    It seems like they want to do the opposite, i.e. reduce Q factor, not increase it – Batman Feb 15 '16 at 5:53
  • Yup, you're right. I just re-read. – jqning Feb 15 '16 at 5:57
  • As for reducing the Q factor for your knees, it does help some people. – Batman Feb 15 '16 at 14:58
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    @Batman sure but it's not the direction I'd go first. – jqning Feb 15 '16 at 15:02
  • Voting to leave open, because its relevant. Certainly the wrong direction but still related. I'd be wary of removing any thin washers too - I've got crank arms where a specific sized washer is required to protect the alloy crank. – Criggie Feb 15 '16 at 22:16

Any bike circa 1993 is likely to have a narrower Q than a lot of newer bikes. Specialized is annoying for having very slightly different taper to their square taper BB axles, meaning you can't swap a Shimano or other brand and use the Specialized crankset. It's all or nothing. Description sounds like too low a seat, maybe? Frames and older steel like this have pretty straight chain stays, so may find interference with the crankarms hitting. Narrower BB and axles have offset for the crank to keep the chainrings out, and frames usually have a dent to miss the inner rings, but going from a 73 to 68 for example likely will cause clearance problems. If using cleats, some inward movement can be gained by adjustments but if ankle bones start hitting the crank arm you're stuck. There used to be a brand that made laterally adjustable BB spindle, but what you gain on one side you lose on the other. Not a Q factor adjustment.

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