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I am looking to purchase a new wheelset in the not too distant future and am debating between wheels that are 58mm and 85mm deep.

Or, and this is the part I'm curious about, a hybrid of the two. (58 front, 85 rear)

In a crosswind, a deep front wheel will, I believe, have a larger negative effect on stability than a deep rear wheel.

My question is regarding this. Since I'm going to have one aero wheelset I'd like them to be able to be used in as wide a range of conditions as possible, within reason of course.

Assuming a moderate crosswind situation, which would add more instability, a 58mm front wheel, or an 85mm rear wheel?

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I believe you'd need a wind tunnel to work that out. – Daniel R Hicks Feb 19 '12 at 21:18
A wind tunnel would tell you about drag force but it wouldn't (easily) tell you how it affects handling. In particular, wind tunnels don't measure the effect of "gusts" very well since it takes some time to increase the wind speed and most wind tunnels are not designed to measure torque forces around the mount point (which is what you'd need to determine differential drag at non-zero yaw between the front and rear of the bike). – R. Chung Feb 23 '12 at 2:24
up vote 2 down vote accepted

The short answer is that a deep front rim will almost always affect handling more than a deep rear rim. The exact answer depends, as you surmised, on windspeed and yaw angle; however, it also depends on the design and geometry of the bike and the rims in question. For Tri or TT bikes where the rider is positioned maximally forward (or, under UCI regulations, the saddle tip is minimally 5 cm behind the bottom bracket), under moderate wind conditions a deeper rear rim tends to improve the bike's handling compared to a normal shallow box rim. This is not an absolute, however -- for very gusty winds at high yaw angles and very deep or disk wheels, handling can suffer. This is why rear disk wheels (though not deep rear rims) are prohibited on the windy Ironman Kona course. Wheel manufacturers have been spending a great deal of effort in recent years improving the performance of deep rim and disk wheels under non-zero yaw conditions.

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Since your front wheel directly affects your bike's handling through its direct connection to your steering, the front wheel will have a larger and more immediate effect on the stability of the bike in a crosswind.

Keep in mind, however, that that assumes equal wheel depth. In addition, the brand and design of the wheel matters a great deal. For instance, according to Zipp, the new Firecrest wheels are designed to have their best performance in a 30 degree crosswind.

It is common to see riders, especially in unusually windy, or consistently windy areas, use a lower depth front wheel compared to the rear wheel. The benefit is well known, but the amount of benefit varies depending on the wheel design.

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