How does tire width affect aerodynamics in gravel cycling?

Question

The question that piques my interest is the role of tire width in aerodynamics for gravel cycling. Which mechanism would you suggest can possibly affect my speed with different tire sizes?

Answer 1

While conducting wind-tunnel tests, it was indeed established that tire width does influence the aerodynamics. If 35 millimeters is the reference tire size, moving to the 40 mm size was 2.3 watts slower, which translates to taking 1 minute and 42 seconds longer over a distance of roughly 200 miles. The use of 45mm tires resulted in a 5.8 watt disadvantage versus the 35mm tires, and this would take four minutes and 19 seconds longer over the same distance. Nonetheless, it is worthwhile to bear in mind that it is likely that the handling characteristics of a wider tire would help improve rolling resistance in rough surfaces which may help compensate the aerodynamic disadvantage.

Comments

Another point in favor of running larger tires is if they allow you to spend more time in the aerobars. A favorite of mine are 2.1” thunder Burt’s… they give up nothing on rolling resistance but can float over loose stuff even while in the aerobars!

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Lots of good info in this content. While it's gravel specific, there's a quite a bit of overlap that can be useful for randoneurring, where rides start at 200km. Slower speeds, more relaxed position, carrying bags and extra water, wider tires, with or without aerobars, etc. Over long distances, every watt matters. Also, I'd always opt for wider tires. The comfort, control, and puncture resistance gains far outweigh any aero penalty IMHO, especially if it allows you to use aerobars more often.

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A few suggestions:
Try transient wind tunnel. Seriously more expensive, but much closer to the real world data, because IRL the wind never attacks a vehicle from a single yaw angle for any prolonged period.

Re tyre sizes: that ideal platform in the tunnel is not the same as gravel in terms of "shakey-shakey", you know. So since bigger tyres at lower pressures are vastly easier on the rider, that would be much less fatigue accumulation over the course of the race. As it has been brilliantly illustrated by Cadex at Ironman, that will allow one to ride faster for longer. You cannot test for stuff like this in the tunnel. Or you have to at least find one of those smart treadmills - you know, those "for the ultimate Strava experience" and plot a long course with constantly adjusting grade, or something. Ideally, you'd glue assorted bits of cable (to simulate roots) and actual gravel to the treadmill belt - to feel those pesky shocks and vibrations.

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On rough gravel on a long course - comfort is a huge benefit of wider tires

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The rolling resistance vs aero vs tire pressure vs width vs any other factors of a tire in real world really interest me. Very much looking forward to your data on this. I see too much theoretical absolutes on this but like a 140psi is better on an aerodrome, but not even a smooth asphalt road, I feel like there's a lot we assume but dontnknow