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I'm looking for recommendations on carbon rim depth considering my use case.

Inner width is 23mm and I'll be running 32mm tubeless. For 3 season use in the UK, mostly on (rougher) roads - some hills, not flat. I'm more of a touring cyclist; longer rides at a moderate pace and I'm about 90kg. Thinking either 44mm or 56mm, shallower is slightly lighter, no difference other than that.

(Do I need the carbon? Probably not, but there's a deal to be had and I'd like to try something different).

Thanks for your thoughts

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Per the FAQ, we don't make product recommendations. However, crosswind stability is a significant consideration. Here, you are probably better with the shallower wheel.

Ronan McLaughlin discusses the general topic of how deep your wheels should be in this Escape Collective article. The article has a graph from the wheel company Parcours showing the weighted average drag at 48 km/h of a baseline 26mm wheelset versus three of the company's offerings. The jump from a 35/39mm set of rims (front/rear) to 49/54 is decent, but the returns diminish with depth. If you compare the figures for time saved versus a set of older Mavic Open Pros for Flo Cycling's 49mm and 64mm wheels, the differences are smaller. (NB: Flo's figures are total power, including rolling resistance, at 22 mph, whereas the Parcours figures are aerodynamic drag only.)

However, the weight savings from going from a 56mm to a 44mm are also small, and probably worth less than the drag savings.

In practice, I observe that amateur/recreational roadies seem to go for 40-50mm rims, and triathletes tend to opt for the deepest they can run. Professional roadies usually seem to be going for 60mm-ish rims except on mountainous stages.

Handling considerations might tilt you towards the 44mm rim. However, the OP is heavier, and heavier riders tend to handle crosswinds better. I’m a light rider, and I did elect 60mm rims, and I have adapted to the handling in most conditions. Personal preference also can and should influence your decision; I have decided that I need every advantage I can get over heavier riders, thus I opted for deeper wheels.

I forgot that you mentioned you were going to do tours on this wheelset. You are adding weight to the system. I don't know what carbon wheels are optimized for touring purposes. I imagine they will definitely have more spokes (e.g. 28 or 32), and perhaps the rim will be heavier with a more durable layup. Given that you are 90kg, you should definitely think about how much luggage you will carry. You can check with wheel manufacturers if their rims are rated for the total anticipated load. It's possible some of them may offer heavier wheel builds, e.g. Hed has what it calls a Stallion build.


Longer discussion about crosswind stability

When you are in a crosswind, there are two phenomena that are relevant.

  1. The entire bicycle-rider system gets pushed laterally. Deeper rims add to the area exposed to wind, and they increase the lateral force.
  2. Your front wheel gets steering input. The amount depends on the distance from the wheel's aerodynamic center of pressure (like center of gravity but for aerodynamics) to the hub. Deeper rim = more steering moment, all else equal. Keep in mind that winds are usually not constant. You get intermittent gusts, and the direction may change.

The first phenomena is more manageable. When I experience this, I just lean in. The second phenomena is more unpleasant, especially in gusty winds, because you get unexpected steering input. In gusty winds, it can be a variable input, which is harder to counter. This issue might tilt a rider towards shallower rims, especially considering the marginally decreasing aero advantages of deep rims.

In general, modern aerodynamic wheels are more stable in crosswinds than older, V-shaped ones (e.g. 2010 or earlier profiles). More recently, e.g. 2020 and later, some manufacturers have focused specifically on crosswind stability, in part because there are diminishing returns to aerodynamic gains. Thus, the penalty for deeper wheels probably diminishes depending on the wheel design. Newer rims should have less of this relative penalty. The same is true for more advanced rims, i.e. from premium wheel manufacturers that have put thought into this vs. open mould manufacturers. Section 8 of this white paper by Hunt Wheels has some more technical discussion about crosswind stability.

Additionally, in a conversation with one wheel builder who had done some wind tunnel testing of stability, he believed that for tires wider than the rim, you would probably worsen the stability because you decrease the stall angle.1 That is, for low angles of attack (between the net wind direction and the wheel), drag is low, but there is a step change at a certain angle.2 This probably worsens the problem of steering input. This shouldn't affect the OP, but it may be a consideration for owners of older wheels.

Footnotes:

  1. The person in question is a principal at November Bicycles. They have a longer post on crosswind stability here.

  2. Consider this graph of drag for various wheels from a test by November Bicycles. The Zipp 303 and the AForce Al33 rims (bottom two lines) stall at around 15 degrees angle of attack. enter image description here

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