For more than a decade, Steve Hed has been telling us that wider rims are better. Recently, other wheel makers have begun to agree, releasing deep, aero wheels with wide cross sections.
Hed, Zipp, and Bontrager now offer full lines of wide wheels at various depths, and other brands are beginning to introduce their own versions. Manufacturers claim the wheels are faster than traditional designs in real-world conditions and offer better handling in crosswinds. Of course, there are limitations: Wider rims are slightly heavier and may not fit in some frames. The aerodynamic advantages are real, but minimal. Handling, however, does noticeably improve.
Designers are creating new wheels using computational fluid dynamics (CFD) software, which allows them to analyze hundreds of shapes and quickly find the most promising ones for wind-tunnel testing. Companies also now know better how to interpret and use results, building on years of test data. The combination of improved tools and knowledge has led to faster, better wheels (to learn how to maintain a good set of wheels, read The Subtle Art of Wheel Truing).
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Original aero-wheel shapes were largely based on National Advisory Committee for Aeronautics airfoil (a wing seen in cross section) specifications. But those shapes were designed to work at aircraft speeds, and to generate and control lift. On a bicycle, because of the wheel/foil's orientation, lift is a side force, which can compromise control.
Another shift in aero research came when designers realized that cyclists almost never encounter a straight headwind. Crosswinds, or yaw angles, are far more common. So wheel makers have focused on maximizing performance in various crosswind conditions. The key element is stall angle.
In aerodynamics, "stall" is when an airfoil stops generating lift. In bicycles, it's when wind comes from the side, and instead of flowing smoothly around the rim, becomes turbulent. In the language of aerodynamics, the goal is to keep air "attached" to the foil; turbulent air is no longer attached. The goal is to push the stall point to a larger angle (more perpendicular to the rider), so that it keeps air attached over a wider range of wind angles before stall. Curved, toroidal, shapes, so far, have performed best in testing. Riding stronger and faster isn't all about your wheels, find out what it takes by reading Lessons from the Lab.
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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.
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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.
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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."
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Modeling and testing led Bontrager to design a range of wheels that are shallower than competitors' wheels. Bontrager claims this is because its wind-tunnel tests show its 35mm Aeolus 3 actually rates a class higher, offering comparable aerodynamics to a traditional 45mm wheel. But because the company won't offer the Aeolus 3 in a clincher version until summer, we tested the 50mm-deep Aeolus 5, which is closer in actual size to Zipp's 303 and Hed's Jet 4 wheels.
Unlike Zipp, Bontrager claims center of pressure has a negligible effect on handling; the primary factor, the company says, is rim depth. So designers' focus is on building shallower rims, which they claim are as fast as a deeper rim from a competitor and easier to handle. Bontrager's wheels may be faster, but the Aeolus 5 was the flightiest in crosswinds of the three we tested. That could be due to the greater depth or the shape of the cross section. But the Aeolus's handling, while a clear improvement over a traditional V-shaped rim, lacked the smoothness of the other wheels in this test.
The Aeolus 5s were snappy in climbing and solid under hard cornering, and delivered a more damped ride than some carbon wheels. Braking was smooth, but Bontrager's included cork-based pad had a spongy, indistinct feel at the lever. Modulation and feel improved when we swapped to gray Zipp/SwissStop pads.
Hed proudly sticks with tried-and-true aluminum brake tracks. Given the results, why switch? The gorgeous Jet 4 FR wheels are the same weight as the Bontragers, handle beautifully, and cost $800 less than the Zipp or Bontrager sets.
Steve Hed says that because his aluminum wheels are close to carbon in weight, he chooses to offer more-affordable wheels and focus on aerodynamics. The Jet 4 uses the same 18-front-/24-rear-spoked C2 scandium rim as the company's Ardennes model, with a carbon-fiber fairing. The rear Carbon FR Sonic hub has a grease port for easy maintenance. The carbon fairing is delicate, so take care when using a roof rack's wheel strap.
In testing, the Jet 4 proved equal to the Zipps in handling crosswinds. The gently radiused profile made them less susceptible to buffeting crosswinds than most aero wheels.
They're 100 grams heavier than the Zipps, but most of that difference is in the rear hub, so they still climb well, no surprise because the design is based on the Ardennes SL, a favorite all-around clincher. The scandium brake track is grippy, boosting our confidence on descents.
The spoke nipples anchor inside in the rim, under the fairing, which makes truing a hassle. But the longer spokes also give the wheel a more compliant ride without sacrificing lateral stiffness.
Price: $1,900 Weight: 700g, front; 910g, rear Rim Depth: 45mm brake-track Width: 23mm Widest point: 26mmThe latest addition to the Firecrest line, the all-carbon 303 clinchers (pictured on p. 26) have impressive finish quality and the same hubs found on other Zipp wheels. A 2011 redesign of the rear hub pushed the nondrive-side hub flange and cartridge bearing 7.5mm outboard, creating a stiffer wheel.
Our first test ride on the Firecrest occurred in gusty crosswinds up to 40mph. On a straight, slightly downhill section exposed to the wind, the wheels displayed only slightly more buffeting than a standard box-section rim. Later test rides bore out that initial impression: The 303s are exceptionally stable even in strong crosswinds. On local climbs, the wheels lacked the snap of a lightweight climbing wheel, but we never felt their heft was holding us back. Even on steeper sections, they responded quickly to out-of-the-saddle efforts. They were the stiffest-riding wheels in this test, but not uncomfortably so.
No carbon clincher rivals the performance of an aluminum braking surface, but these are among the best we've found. There was no pulsing under even hard, sustained braking, and the new gray pad from SwissStop that comes with the Firecrests is the best carbon-specific pad we've ever used. Paired with SRAM brakes, it was powerful without being grabby and never lost its positive feel.
Each of these three sets is ideal for different riders.
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