16 Oct Wheel Depth – Does Size Matter?
There’s a lot happening in the world of wheels. Disk breaks are disrupting the status quo, cheap, open-mold models from Asian manufactures are ever easier to access, and brands like Zipp and US start up Princeton CarbonWorks are experimenting with non-traditional inner circumference profiles. All of this is on top of the now-routine marketing warfare where every brand claims to be the fastest, the stiffest, and the most stable. I will write about wheel marketing soon – since it’s a topic well worth covering! For now, let’s talk about rim depth and how it affects our choice of wheels.
There is a well-established belief that deep wheels are always faster – and therefor better – than their shallower brethren.
Is this true? Read on!
Before we begin, it’s useful to understand the concept of yaw angle. Stated simply, the yaw angle is the angle at which the wind strikes the bike + rider. It is the result of the wind speed, wind direction relative to the rider, and rider speed. If you’re keen on doing a little basic vector math, you can sort these out yourself. If you’d rather skip the cosines, here are the highlights.
- In pure headwind or tailwind or windless conditions, yaw angle is 0°.
- The greater the angle of the wind relative to the rider (the more side-windy the wind), the greater the yaw angle.
- The greater the wind speed, the greater the yaw angle (unless it’s a pure headwind or tailwind).
- The greater the speed of the rider, the SMALLER the yaw angle.
- 80% of conditions / rider speeds place the yaw angle at +/- 10°.
It is useful to split our conversation into what happens at the front wheel from what happens at the rear. Let’s look at front wheels first.
This plot from FLO shows a comparison of a traditional box (aka non-aero) wheel (the Mavic Open Pro) with four of their own carbon clinchers in 45mm, 60mm, and 90mm depths, as well as a full disk. It’s obvious that all four FLO wheels outperform the non-aero Open Pro by a healthy margin. Now some of that is because the tested Open Pro had 32 round spokes vs the FLOs’ 20 aero spokes, but the point stands.
A more interesting observation is how close the four FLO offerings are at low yaw angles.
- At 0° and 5 ° yaw, all FLO wheels are essentially equally aerodynamic
- At 5°, we see a delta of 16g of drag between the 60 and 45. I am ignoring the disk for reasons that will be obvious a little later. For context, 16g of drag measured at 48.3kph (30mph) is 6.1g of drag at 35kph, which requires only 0.6W to overcome. Safe to say that that difference is well within the margin of error of all testing equipment!
- It is only at 10° of yaw that the magnitude of the difference between the non-disk wheels becomes measurable. The delta between the 90 and the 45 is now 40.2g (at 48.3kph). At 35 kph, that’s 15.3g of drag force, requiring an extra 1.5W of power to overcome.
- At 15°, that delta climbs to 97.4g at test speeds, 37.1g at 35kph, requiring 3.5W of extra juice.
So we have two take-aways:
- Deeper wheels ARE more aerodynamic at high yaw angles
- Deeper wheels are essentially the same as their less-deep cousins at low yaw numbers.
Now let’s consider the ‘costs’ of going deeper. By this I mean performance costs, as most brands charge only a small (if any) premium for their deeper product offerings.
- There is a slight weight penalty. But I will argue that for steady-state applications like time trials and triathlons, that penalty is negligible.
- A deeper front wheel is more susceptible to unwanted steering torque during gusty conditions. This point bears some explaining. Any surface area (in our case front wheel rim) visible to a high yaw side wind will attempt to push the steerer of the fork – and the bars attached to that steerer. The surface area in front of the line made by the axis of the steerer and the tire ground contact patch will tend to turn the wheel in the direction of the wind. The area behind that axis will counteract this turning moment and push the wheel away from the wind flow. The trouble is that the area in front of the steerer is greater than the area behind it, so the force ends up being unbalanced. This effect is of course multiplied by the depth of the rim. That means that you the rider will have to actively steer ‘into the wind’ to compensate. And the greater the depth of the rim, the greater the input you the rider need to apply to counteract it! This may not be a big deal in steady wind conditions but is legit scary in gusts!
So if front wheels of various depths are more or less identically aerodynamic at low yaw angles AND deeper wheels are less stable at high yaw angles, then I would argue that it’s a hard sell to go deep up front.
Point 2 above applies to front wheels only. Why? Because rear wheels have no effect on steering. A side wind acting on a rear wheel – or on the frame, or on the rider – simply pushes the complete bike plus rider to the side. It does NOT tug at the bars requiring sudden adjustments in steering. The corrective action here is just a bit of a lean. To be fair, lighter riders will have to do a bit more work than heavier athletes, but a lean is a much smaller price to pay than unexpected front-end steering.
In terms of aerodynamics, a disk is clearly faster than any other wheel depth at yaw angles greater than 5°. The FLO data supports this as will any other manufacturer’s plots.
In my consultations with athletes on the subject of wheel choices, I’ve come to recommend a fairly shallow front (45-60mm) with a disk rear (whenever allowed). I believe that this is the best of both worlds. It’s a fast setup that does not compromise handling comfort.
cover image courtesy of STAC Virtual Wind Tunnel.