Get a Grip
Stall also affects handling. As yaw gets closer to a stall angle, the rider is fighting a vacuum on the downwind (lee) side of the wheel that pulls sideways, and, as a result, destabilizes the bike. The rim's shape is the key to reducing that effect. And as wheels are better able to handle wider stall angles, they become less susceptible to buffeting.
Manufacturers hold different opinions on how to improve handling in these conditions. Zipp says that the center of pressure, the point between the force of the pressure vacuums at the front and rear of the wheel, is a key factor. The company says that its Firecrest shape positions the center of pressure slightly behind the front hub, or as close to neutral in the steering axis as possible, creating a wheel that's not as prone to buffeting. Hed, which shares some wheel-shape patents with Zipp, does largely the same.
Bontrager takes a different approach, claiming that center-of-pressure force is negligible, even in strong side winds, and that overall rim depth is a more important consideration, as a greater surface area creates a larger vacuum. In our tests, we found that Zipp and Hed wheels handle slightly more consistently in strong crosswinds than the Bontragers (see reviews, starting on the next page). All three, however, handle far better than traditional, V-shaped aero wheels, or even toroidal shapes in standard widths, because of their curved cross sections.
More: Do Lightweight Wheels Really Make a Difference?
Smoothing Out The Ride
Wider rims may also improve ride quality. The rims on the new wheels use bead seats (the hooks that hold clincher tires) that are 2- to 3mm wider than conventional rims, so any given tire will have a broader effective width and bigger volume.
While the tires' height from the rim edge was the same as on conventional rims, each measured about 2mm wider, roughly a 9 percent increase in overall volume. This means you can run a slightly lower pressure than you're used to, which produces a more comfortable ride. We tested the wheels with pressure as low as 85 psi, which improved cornering grip and smoothed the ride feel without any noticeable increase in rolling resistance.
More: 16 Bike Tools Every Cyclist Should Have
Conclusions
Some frames, especially on a few new multisport bikes, have tight clearances and will require small but important modifications, such as shaving some material from brake pads, to fit the wider wheels. Most conventional road brakes will work without modification, but only one we know of, TRP's R970, is designed expressly to work around wider rims. And if you swap wheelsets back and forth with a conventional rim, that will likely mean readjusting your brakes each time.
Caveats aside, wider aero wheels are here to stay and could come to dominate the market. More and more manufacturers are producing wider aero wheels, although Easton and Reynolds (excepting the RZR 92.2) are notable holdouts. While the focus is often on aerodynamics, we believe that for most cyclists, improved handling is a bigger benefit. Even a faster wheel is useless if buffeting crosswinds force you to scrub speed or, for multisport athletes, make you break your aero tuck to control the bike. So are they worth the hype, or should you save your money for other gear? Find out in When to Splurge and When to Save.
Wider rims can be found in a number of profiles, from very deep (90mm) to box sections. We tested three mid-depth clinchers, which are go-to wheels for most conditions because they improve aero performance without undue handling challenges or weight penalties. All wheels were tested with identical Michelin Pro 4 Service Course 23mm tires. Measurable differences between these wheels are pretty small. One formula suggests that in the most extreme wind conditions, there are fewer than 20 total watts of variability between them. As Steve Hed explained, "Early on, our comparison was to a box-section Mavic rim; we're not saving anyone a minute over 40km anymore. Now it's more like seconds."