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There is a lot of discussion out there about spoke, rim & tyre drag, but almost nothing concerning the aerodynamics of the hubs.

What got me started on this is the fact that road hubs still have a relatively thin appearance, whereas Shimano's (and many of the rest) Mtb hubs follow their 'parallax' shape, which is much wider in diameter.

Naturally, the spokes and rim cut the air all around the hub so it can't be as important, but to what extent is the hub a factor with overall aerodynamics?

  • Mountain hubs also have to be built to be more durable; I'd guess that is the primary factor in shaping. – Batman Jul 25 '16 at 20:19
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    I seriously doubt that the difference in overall size between a thin road hub and a heftier MTB one will have measurable effect on aerodynamics. The main goal of the wimpy road hub is to reduce weight (though that likely has negligible effect as well, but it's selling point). Manufacturers likely could play games with the shape of the hub to somehow "disturb" the airflow in a beneficial fashion, but that would take more research than they're apt to do. (And again the benefits would be quite small.) – Daniel R Hicks Jul 25 '16 at 21:17
  • There are aero obsessed companies out there (Zipp comes to mind). I think if there was any benefit, someone would have already done it, or at least tried to market it. – Deleted User Jul 25 '16 at 21:56
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The key factor here is that the air at the hub is dirty - it's been disturbed by passing through the spokes and around the rim, so it's moving faster than the still air at the rim (slower relative to the bike), and it's turbulent. That makes improving aerodynamics both harder, and less important. The damage has already been done. Having a smooth outer surface is more important, since that's in cleaner air, and keeping it as small as possible since bladed spokes almost certainly have lower air resistance than the solid hub.

Mechanically a rim brake hub is a pair of flanges and a spacer. Since with bikes the goal is "everything as light as it can be", making that spacer smaller is a win. It needs to be big enough to enclose the axle, and strong enough to hold the flanges apart, but the job is not much more complex than that. So it can be very small and light.

With a disk brake the hub needs to be stronger since there's a torque between the disk rotor and the flanges. For the same reason it's rare to see a multi-part hub. That braking torque means a larger centre section allows thinner walls. Some mountain bikes also have thicker axles than road bikes, and obviously with those the minimum hub core diameter has to be larger to enclose the larger axle.

What definitely does improve the aerodynamics is making the hub much bigger, eliminating the spokes altogether. Which is why the UCI does not allow it :)

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  • Something else that reduces benefits of an 'aerodynamic hub' is that unlike spokes and rims that travel at 2x bike speed... the hub rotates pretty slow and even at the top, is not much over 1x bike speed. So, durability, weight, and strength more important for the hub. – david1024 Jul 26 '16 at 18:59

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