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(Just up front: (1) All comparisons made keeping all other factors equal! (2) I know this is a really hard comparison, but there seems to be virtually nothing canonical on here or the rest of google, so I figured it was worth a shot to ask.)

Currently I own a beautiful 80s Gazelle, full chrome, steel frame, on which I occasionally do trips of various distances. I'm struggling to cycle faster than 36 kph avg on it (no aerobars) over say 100k. I have an 8 speed, and I believe my highest gear is 12/54.

Because of this I'm thinking of making an upgrade to a proper aero bike, I think it might make a difference. But how much of an impact can I really expect? I'm looking at the Canyon Aeroad CF SLX 8.0, which I might fit with ENVE 7.8 racing wheels - I'd say that's about as aero as it would get. What kind of (rough) speed improvement can I expect?

  • Pro tip: you can find out the gearing by counting the teeth in front and rear. – ojs Jun 25 '17 at 14:44
  • @ojs Thanks! When I get home I'll fix my post – 1010011010 Jun 25 '17 at 14:55
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    There isn't really an a satisfactory answer for just the frame. There are dramatic differences between modern non-aero road bikes and aerobikes (when isolated to the frame). However, when you include the rider, in a decent amount fo cases, that can be teh dominant factor – Batman Jun 25 '17 at 18:30
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    In brief, the bike's aero features pale in comparison to the rider's aero effect. The big wins come from bike design allowing/forcing the rider into a more aero position for more of the time. So your aerobars force a more streamlined position of the rider. Turning all that into a speed improvement number is really more guesswork than science. – Criggie Jun 25 '17 at 20:29
  • I believe one of the GBN presenters owns one youtube.com/watch?v=ZH9FMHR6lcU and youtube.com/watch?v=oQ8RjFfc6xQ – Criggie Jun 25 '17 at 20:31
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The bike's proportion of total air drag is roughly 20% (for a road bike, more for a TT bike as the rider assumes a more aero position), with the rider accounting for the balance.

So assuming exactly the same bike fit and rider position, then even if the new bike halved the drag coefficient of the old bike, it would mean a drop of only ~10% in your total coefficient of drag.

A 10% reduction in CdA, while a decent improvement, would result in ~ 3.4% increase in speed on flat terrain.

So a bike with 50% the drag coefficient of another results in ~ 3.4% speed gain.

If the aero differences of the bikes are less, then of course the potential speed differences will be quite small. What is significant of course depends on how much a given speed gain matters to you.

And just to set an upper limit to speed gain from changing bikes (same bike position), if the bike had zero drag (impossible of course) then you'll still only attain a ~ 7-8% increase in speed. That's the approximate theoretical upper limit.

If the new bike fit is aerodynamically worse than on your current bike then you may even go slower!

So while a nice aero road bike, assuming same bike position, will result in some speed gain, the bigger bang for buck is attaining a more aerodynamic position on the bike. As others have commented, this is often done with the use of aerobars fitted to the front of the bike to enable the rider to attain a much more aerodynamic bike position.

Of course you can do both and have fun trying!

  • Thanks for your answer. One thing I'm missing in this picture are Watts: I was under the impression that this was really the meaningful statistic. In particular, a better bike would proportionally improve speed averages more. Example: the speed difference between 100W and 130W is very different than the speed difference between 300W and 330W... – 1010011010 Jun 26 '17 at 6:18
  • "A 10% reduction in CdA ... would result in ~3.4% increase in speed on flat terrain." Even less than that much increase if your route isn't flat, of course. – R. Chung Jun 26 '17 at 6:41
  • @1010011010 Watts are measuring your power output - the bike doesn't change that unless one of them is a terrible fit. – Useless Jun 26 '17 at 14:33
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    @1010011010 It makes more sense to report in terms of a CdA difference because this will remain essentially the same no matter the power applied or speed you are travelling at (save for some special cases). That enables you to make your own wattage comparisons using any one of a number of useful website calculators such as www.analyticcycling.com. – alexsimmons Jun 27 '17 at 7:44
  • In case anyone was wondering, in my case I decreased my CdA on the bike from approximately 0.48 m2 to 0.31 m2, with some spacers left underneath my handlebar (so theoretically I could become even more aerodynamic). Of course your mileage may vary. Happy cycling! – 1010011010 Apr 28 '18 at 15:21

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