I was just thinking about Obree’s bike, and its very narrow bottom bracket.

He made what I thought was a very good point. Narrowing the bottom bracket reduces effective frontal area and therefore drag, and should contribute to speed. His world record was QED as far as I’m concerned.

Yet when I look at road bikes, BB width seems much as usual.

Why? Is this the dead hand of the UCI at work?

I know I don’t go fast enough to worry about aerodynamics, but there are a few roadies on here who might have the answer.

I guess it’s packaging.

You need to fit a chainstays, half a tyre, some clearance to allow for flex, a front mech and 2x chainrings on one side, and mirror it on the other.

Obree’s track bike had elevated stays, a 20mm tyre (guessing, might even be narrower) and fixed gearing.

Also probably approximately bio-mechanically optimum. Most bike fitters only tweek Q-factor by single figure millimetres and usually outwards with shims on the pedal axle.

Also again, a wider BB will be stiffer (both in bracing the chainstays and in the BB spindle). Obree’s bikes weren’t really designed for him to stand on the pedals.

The bike still has to fit the rider. Hip width. Stance width
https://books.google.co.uk/books?id=VMkEBAAAQBAJ&pg=PA66&lpg=PA66&dq=phil+burt+q+factor+stance+width&source=bl&ots=pN9U6BSKG9&sig=P8Evz_P1SIZ6Am7fyf5LBSH0WSQ&hl=en&sa=X&ved=0ahUKEwjXiJWagffRAhWGwxQKHR_GDlIQ6AEIGjAA#v=onepage&q=phil%20burt%20q%20factor%20stance%20width&f=false

http://www.slowtwitch.com/Tech/Stance_Width_2562.html

Narrowing the bottom bracket reduces effective frontal area

It does? By the frontal area lost from the BBC shell, yes. **** all.

Anyway, standards. You want to create a new one for a few mm off the shell?

I presume he meant legs not the BB. If you could tuck them in it could possibly get rid of some drag.

Aye, but you’d still only be narrower by the same amount you’d reduced the BB shell.

scotroutes – Member
Aye, but you’d still only be narrower by the same amount you’d reduced the BB shell.

That sort of difference doesn’t matter on a mtb, but for a fast roadie it may be worthwhile.

But the insistence on super stiff frames and clearance for big tyres requires wide bb shells.

But the insistence on super stiff frames and clearance for big tyres requires wide bb shells.

I did think the other day that E-stays might be a way around having to use silly wide stance cranks, once the chainring and cranks arms don’t have to clear a stay, you can narrow down the chain line and Q factor a little…

The stache heads along this line with its asymmetric stays, I reckon it could be taken a step further…

I think he who shall not be named played around with narrow q factors when he was doing the Tour de France.

If I remember he was producing less power with a narrower crank so dumped the idea.

No need really for symmetrical stays. The issue is chainring (or chainline) to tyre clearance. So on the non drive side you still have to have the same space (pedals symetrical about the centerline of the bike), may as well have a normal stiff stay in there.

aP – Member
But the insistence on super stiff frames and clearance for big tyres requires wide bb shells.

With material like carbon fibre, it must surely be possible to build a stiff narrow frame.

The limiting factor seems to be the width required by the somewhat agricultural device known as a derailleur. If the gears were contained in the hub or BB area, then the back wheel need be no wider than the front. Then the question is whether the aerodynamic gains exceed the reputedly lesser efficiency of a gear train as opposed to the derailleur.

Conjecture and idle speculation…but I’d have thought that the bike industry would move towards bmx style bottom brackets… narrow width but with honkin big bearings for strength/ longevity.

And I guess that in the future more bikes in general will be equipped to accept a frame mounted electric assist motor and thus will have bottom brackets different to what we see today.

“Take Jens Voigt for example, he looks ugly on his bike today, but he was even uglier 5yrs ago! We made significant changes to him. Here’s a good example of changes we needed to make because of sponsorship conflicts. The team was sent the new pro titanium pedal from his sponsor, which was made lighter by making the pedal axel shorter. Jens needed a wide stance because of his big bowed legs, but the pedals he was sent from his sponsors jammed him into a narrow position. We found that the sponsor actually had a less expensive product which had a longer pedal axel which significantly improved Jens’ position.”

A Chat With Andy Pruitt

Reducing stance width/q-factor doesn’t actually reduce frontal area (except for the shorter pedal axles). Unless it means you can tuck your legs in behind the head tube/down tube.
It *may* have a *very* small effect on drag (Cd) by moving your legs into the more turbulent air closer to the frame. Might make things better, might make them worse. I haven’t got numbers for that.

The only reasonable reason to play with it is for comfort/biomechanical reasons.

ghostlymachine – Member
Reducing stance width/q-factor doesn’t actually reduce frontal area (except for the shorter pedal axles). Unless it means you can tuck your legs in behind the head tube/down tube.
It *may* have a *very* small effect on drag (Cd) by moving your legs into the more turbulent air closer to the frame. Might make things better, might make them worse. I haven’t got numbers for that.

The only reasonable reason to play with it is for comfort/biomechanical reasons.

Having spent yesterday pedalling into a strong headwind I’m not so sure. Anything I did to narrow myself down seemed to help, ie elbows in, knees in, head down.

It did work for Obree. I don’t think he had any way of measuring any benefit other than from how he felt on the bike though.

Has anyone a link to actual research on this?

It seemed to work for Obree. I don’t think he had any way of measuring any benefit other than from on the bike though.

Did it though? Without measuring that one change independent of everything else there is no way to determine its effect. It’s like saying he was faster because his bike was white.

Head down and elbows in will help, as they can actually reduce frontal area. You are moving your head from sticking up into the air stream, to putting (some of it) in front of your torso, same with bringing elbows in, you bring (some) of your arms in front of your torso.

Waggling your legs around makes no difference to frontal area. You still have your entire leg exposed to the wind.

Knees in *may* have had other effects, like power delivery. Or comfort. Or in your head.

Obree may have benefited if he (for instance) has narrow hips.

I think I’d be banging my knees on the top tube if my feet were much closer together on the bike – particularly with modern carbon frames.

Q factor reduction is quite a common optimisation at the sharper end of the time trialling field.

Probably not such a big deal on road bikes as the speeds are lower and people are generally happy/unaware that they are accepting a bit more drag for stiffness.

Isn’t chainline also a factor?

Unless you reduce that then you can’t bring the chainrings in.

epicyclo – Member
Having spent yesterday pedalling into a strong headwind I’m not so sure. Anything I did to narrow myself down seemed to help, ie elbows in, knees in, head down.

paradigm confirmation bias, with added complete-lack-of-science!

I’ve always felt Q-factor is too wide on production bikes, with mtbs even worse than road. For me though it more of a biomechanics thing than aero, albeit I’m sure there would be a very slight aero benefit.

Remember reading a thread on this a while back over at TTF, one of the posters had a paper published…. (quick google later)…

Metabolic and kinematic effects of self-selected Q Factor during bike fit.

The take home message is that Q Factor is individual, but you’re likely to benefit both metabolically and kinematically if you can reduce it to around 140-145mm

The take home message is that Q Factor is individual, but you’re likely to benefit both metabolically and kinematically if you can reduce it to around 140-145mm

Road chainsets are ~145mm though? MTB is quite a bit wider though (175mm?), but have been getting narrower with 1x specific chainsets.

147.5 for DA apparently.

Considering the biomechanics things these EDCO pedals that have built in spay looked interesting. I’ve not heard much about them post launch though.

Considering the biomechanics things these EDCO pedals that have built in spay looked interesting. I’ve not heard much about them post launch though.

Didn’t TIME claim that benefit from ATAC pedals that the cleats could float in pretty much every direction including rocking?

I guess it’s like normal float, some people use fixed cleats, others floating ones. Most people probably have some sort of varus/valgus wedge built into their shoes which it the fixed version of those pedals float. Whether it’s optimal is a different matter.

cynic-al – Member
…paradigm confirmation bias, with added complete-lack-of-science!

I’m well aware of that, hence my last line Has anyone a link to actual research on this?.

mrblobby – Member
Remember reading a thread on this a while back over at TTF, one of the posters had a paper published….

Metabolic and kinematic effects of self-selected Q Factor during bike fit.

Thanks, I’ll check it out. I should have realised the TT guys would have looked at this. They are the experts at minimising drag. 🙂

EDUT: just read the forum. They are concentrating on power output, and it’s interesting to see their ideas for optimising your personal Q factor.

That sort of difference doesn’t matter on a mtb, but for a fast roadie it may be worthwhile.

especially for a slim hipped racing snake. i had a mooch around a TDF team bus park with a cycling journalist and one team (can’t remember which) had all their TT bikes running old dura-ace with narrower q factor for this reason.