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In a group I follow there was a recent post from a fellow member who complained about how his deep rims (Fulcrum Racing Speed XLR 55mm, if it matters) caused him to fall down because of a sudden wind gust and how a car behind him managed to stop in time before driving over him.

I have no experience with deep rims myself, but I have the impression that the side area of a cyclist is still larger than the rim area, thus it shouldn't make that much of a difference.

Can deep rims increase the chances of falling because of wind?

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  • Do bear in mind that the warnings given are mostly referring to track wheels with very deep rims (on the order of 15mm). May 10 at 11:39
  • @Daniel 15mm isn’t exactly a deep rim ;)
    – MaplePanda
    May 10 at 21:32
  • 1
    @MaplePanda - I meant 15cm -- about 6 inches. May 10 at 21:55
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Yes absolutely. The depth of a rim is directly related to the side-area, and a gust of wind will have more of an effect on a larger surface.

The largest wheel surface area is a solid disk, and even pros will prefer an open wheel to a disk wheel on a race day with a gusty wind forecast.

If there are a lot of sidewinds anticipated, an old-school box-section rim presents the smallest side-area to a wind, and will react the least to a sudden gust.


Consider also that a front wheel has two pivot points - the head tube and the ground contact patch. The wheel's axle is not relevant to steering. So there is more of the wheel/rim in front of a line between those two pivots, and a side wind will push the front of the wheel more than the rear of the wheel.

The result is a net turn away from the wind. If the rider is not balancing and ready to react instantly, they'll find the bike turning under them without the rider's mass following, leading to a loss of control, a wiggle/swerve, and potentially a fall. Not good in a pack!

The only correct action is to prevent the turn from starting. You have to hold firmly onto the bars, and react immediately if there's a gust. If you're watching the grass on the roadside, or other riders ahead, you can see a gust-front coming and brace yourself for it. With timing and luck, you can even turn into the wind as it hits you, resulting in a net-zero steering change.

The other good choice is to slow down in gusty areas. If your speed was halved, you'd have twice as long to react to the gust before overbalancing. (very approximately).

Additionally, don't ride too close to obstructions, the edge of the seal, other riders, or cliff edges. Having some more run-out room helps recover.

Deep rims on the back have less of an effect because they don't get involved with steering. A solid side wind can push the bike around, but its not going to steer your bike into a co-rider or a parked vehicle, or off the road completely.

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It's even worse than just the side area of the rim:

  • The cross section of a deep rim with a slim tire on it has the form of an elongated droplet. This form is very similar to that of an airplanes wings, or the blades of a wind turbine. The aerodynamics of this shape are such that you get a much larger sideward force if the wind is blowing onto the round edge than if you had any other shape. Also, wind on the round edge is much more effective than wind on the sharp edge at the same angle.

    For your front wheel, that means that the forward part acts like an airfoil in cross winds, while the rear part experiences much smaller lateral forces.

  • The steering axis is slanted, so there is more rim in front of the axis than behind, and the airfoil in front of the steering axle is also significantly farther away, increasing its lever on your steering.

This means that crosswinds significantly tug at your handlebars in an unpredictable way. And if the change in force is quick and hard enough, it may well cause you to loose control over your balance.

Note that, the deeper the rim, and the more the tire falls in line to form the droplet shape, the stronger the airfoil effect, and the more dangerous cross winds become.

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  • I can't quite visualize how the lift effect works. In an airplane wing, airflow goes from the rounded end to the tapered end, and generates lift on one side of the wing. With the rim, airflow strikes the "wing" perpendicular to its axis, effectively the same as if airflow passed over a wing from the underside to the top. Even if it did generate, lift, wouldn't that occur at the rounded or tapered ends, resulting in a lift force parallel to the radius of the wheel? The "wing" here seems to be rotated 90 degrees with respect to the airflow, compared to how a wing typically operates. May 10 at 18:18
  • @NuclearHoagie When you ride, you have the wind itself, and then you have your own head wind. These two add together to produce the effective wind speed at which the air hits your bike. Since your head wind is substantial, you get an effective wind speed that comes at an angle from the front in most cases. And that is precisely the wind directions where your airfoil (= forward part of front wheel with deep rim), generates the most lift (= lateral force that translates into a steering impulse). I hope that clears it up. May 10 at 19:05
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    I see, I hadn't considered the headwind. It's interesting though, that a pure headwind won't generate lift because the bike is laterally symmetric with respect to the headwind. The tire+rim is effectively a symmetric airfoil which generates lift at non-zero angle of attack. But I'm not sure that the teardrop shape plays as much a role as the increased surface area of the rim. Even a flat airfoil like a paper airplane generates lift, but only does so by redirecting the air, and can do that whether or not the leading or trailing edges are rounded or tapered. May 10 at 20:13
  • @NuclearHoagie Of course, the rim+tire is not a perfect airfoil, and with an angle of attack of 90° it won't behave significantly different to a plank. However, lets assume a sufficiently low angle of attack, and a cross wind coming from the left: In this case, the rounded forward edge helps to guide the air around that forward edge to the right side of the rim. As the air flows around, it is redirected towards the rear, killing much of its lateral speed. A sharp forward edge would directly induce a vortex which would reduce this redirecting effect on the passing air. May 10 at 21:01

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