Fat Rims and Fat Tires

A couple months ago, as my wife (Dr. G) and I were descending from Auburn, I marveled at the grip, smoothness, and confidence I felt going through turns, just as I had since I first got the bike, a 2011 Cervelo R3. Part of the ride character can be attributed to the frame, but a large part is undoubtedly attributable to the wheels as well. When I was shopping for this this bike I had a few possible frames in mind, but I had a pretty good of the finishing kit (3T bar, stem and seatpost, Specialized Romin saddle), components (SRAM Force), and, last but not least and most relevant as I swept through that turn, the Zipp 101 wheels.

My 2011 Cervelo R3, with Zipp 101 wheels.

My 2011 Cervelo R3, with Zipp 101 wheels.

The Zipp 101 was unique when it first came out in 2010. First, it has a 30mm toroidal alloy rim, which, according to Zipp, puts its aerodynamics on par with v-shaped rims as deep as 46mm. Second, the rim is 23mm wide on the outside and 16mm wide on the inside. The rim’s width, both internal and external, put the Zipp 101 in a small fraternity. At the time, most road rims were 19mm wide on the outside and 14mm wide on the inside. This started changing in 2008 when Steve Hed came out with its Ardennes, a wheel with 23mm-wide rim. The next year it incorporated the wider rim, and a new rim shape, into the Jets, its time trial clincher wheels. According to Steve Hed, this translated to superior aerodynamics (at least with a 23mm). On a standard width rim, a 23mm tire looks like a light bulb. On a the C2 rim it was much rounder. The wider rim also permitted lower tire pressure. According to Hed, with its C2 rims you can use about 10 percent less air pressure. The wider rim and altered tire shape also reduce the chances of pinch-flatting (one of a clincher tire’s biggest liabilities). These characteristics combine to result in a larger contact patch, better rolling resistance, better grip, reduced likelihood of pinch flatting, and improved ride compliance. I purchased a set of Jet 9 time trial wheels and instantly became a convert.

A front view of my "fat" Hed Jet rim at the Full Vineman in 2010.

A front view of my “fat” Hed Jet rim at the Full Vineman in 2010.

Triathletes tend to focus on the issue of whether wider rims translate into better aerodynamics. Discussions about rolling resistance also tend to become extremely academic and test protocol descriptions are rarely detailed enough to tell whether wheel tire, and air pressure variables have been properly considered. Furthermore, while a slightly wider tire is heavier than a narrower tire and would thereby increase a wheel’s rotational mass, the added rotational mass is a very, very small part of the total combined mass of the rider and bike. Plus, riders who make the switch seem to find the improved grip and cornering more than make up for any increase in rotational mass.

Bike manufacturers heavily market the ride quality and compliance of their frames, but tire pressure alone impacts ride quality more than any frame. Time trial bikes and aerodynamic road bikes are generally not known for riding smoothly over rough pavement, but using a wider rim that facilitate lower tire pressure will improve your ride quality. When shopping for a time trial bike or aerodynamic road and you are concerned about the frame’s compliance (or lack thereof), get a wheelset with wider rims and ride with less tire pressure. You could even use a 24mm or 25 mm tire (particularly in the rear) for an even more comfortable ride. Improved ride quality makes wide-rim wheels especially good for training and, since they work better with wider tires, they are also excellent for winter training. Wider rims also make wheel and tire changes much easier on time trial “superbikes” with integrated brakes, but no quick releases. If you wheel’s brake track is at least as wide as your tire, you can change your wheel or tire by simply removing it. No quick release is necessary.

Hed’s wide rim sparked a shift in wheel design paradigm, with more and more manufacturers offering wider rims. Although it may work with tires as narrow as 21mm or as wide as 35mm, the Zipp 101 works best with tires 23 to 25mm wide. It subsequently followed with its Firecrest rim design, which has a brake track nearly 25mm wide. This year, Zipp started selling a “wide rim” disc wheel, the Super-9 clincher, which it claims is its fastest wheel ever (faster even than any of their tubular wheels). Although professional cyclists had traditionally used wider tires for the spring classics, they too started shifting towards wider tires. Tony Martin, in 2010, used an uncharacteristically wide 23mm front and 25mm rear tire on his Scott Addict in the Amgen Tour of California. He similarly opted for a 23mm front tire (a clincher, no less) when he won the won the time trial world championship in 2011, a choice motivated by a desire for better grip and cornering on the wet, technical course.

Tony Martin's 25mm rear tire, match to his Hed Stinger
Tony Martin’s 25mm rear tire, matched to a Hed Stinger

Wheels with wide rims are now available in a full range of price points. The Hed Belgium and Velocity A23 are also available as rims for hand-built wheels. However, while some manufacturers include external rim width in their wheel and/or rim specifications and some even include the internal rim width, many still do not.

Regardless of what kind of wheels you have, don’t mindlessly pump your tires up to 120psi just because that is the maximum recommended pressure. Consider your body weight, tires, wheels, riding style. Check with the manufacturers of both your wheels and tires for recommended tire widths and air pressures. Many publish guidelines online (you can find Michelin here and Continental here).

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