Of all of the forces opposing your efforts at propelling your bike down the road, those working on the wheels and tires comprise a significant percentage. Tubeless tires and rims can be part of the equation to maximize speed, as they have improved to the point that there is now no faster road tire/wheel setup. Get the combination of the tubeless tire, wheel, and pressure right, and you’ll have “free” speed.
Optimizing speed from your tubeless wheels and tires also requires hitting the tire diameter, tread compound, tread pattern, and rim shape — as well as spoke count and shape.
Aerodynamic drag increases exponentially with speed. It doesn’t take twice as much power to go twice as fast relative to the air; it takes eight or more times as much power. Since rider power is limited, going faster without producing more of it requires improving the aerodynamic shape of the object moving through it.
The 1960s solution was fewer spokes in the wheels. It took a dramatic turn in 1984, when Francesco Moser broke the world hour record riding disc wheels. Steve Hed then created the aerodynamic innovation that every pro bike racer now depends on: the deep-section rim.
Hed once again pioneered the next step, a wider rim that took the tire shape into account. This gets the rim and tire together to more closely approximate a National Advisory Committee for Aeronautics (NACA) airfoil shape for aircraft wings. The tire is wider than first-generation deep-section wheels, but the nose of a NACA airfoil is significantly narrower than the thickness of the wing further back. To approximate this shape, a bicycle rim not only needs to be wider than the tire where they meet, but it also needs to continue to get wider before it tapers toward its spoke bed.
Friction within a rolling tire slows it down, and like aerodynamic drag, friction increases with speed. Unlike aerodynamic drag, however, rolling resistance only goes up in a linear relationship to speed, so the slower the rider is going, the more important rolling resistance is relative to aerodynamic drag, and vice versa. The graph below illustrates how below about 12 mph, rolling resistance exceeds aerodynamic drag.
On a smooth surface, the tire can be hard and lose little energy when rolling, like a steel ball bearing rolling on glass. Since it rolls on surfaces much rougher than glass and requires traction for propulsion, braking, and cornering, compressed air inside cushions impacts, and rubber tread provides traction.
Friction within the tire results from chafing of the inner tube against the inside of the tire casing and of the threads in the casing against each other as the tire flexes; on top of that, internal energy losses within the rubber itself result from hysteresis — the lag between the application of a force on a material and its deflection in response. Overcoming this lag absorbs energy.
Until recently, road racers generally believed that smaller, harder tires were fastest and rode races other than Paris-Roubaix on 22 or 23mm tubulars, using yet narrower tires in time trials. Greg LeMond illustrated the cost of this in the final time trial of the 1986 Tour de France. Racing on 19mm tires pumped up high, he slid out and crashed on a sharp corner. Wider, softer tires would have not only given him more traction for that corner, but also would have rolled with less resistance. LeMond then had to change bikes and worry about losing his yellow jersey to teammate Bernard Hinault, who did win that time trial.
A shorter contact area deflects less deeply into the tire. A wider tire will have a shorter, wider contact patch on the road than will a narrower tire at the same pressure, because the surface area of the contact patch must be the same to support the same load. Less deflection will result in less internal friction/hysteresis loss in the wider tire.
With reduced pressure, the tire must support the same load with fewer pounds per square inch to do it, so the surface area of its contact patch will increase. Tire deflection will deepen with the longer contact patch, resulting in more internal friction and hysteresis loss. This makes the case that higher pressure reduces rolling resistance, but only on a smooth surface.
On a rough road, the internal friction/hysteresis losses due to the larger contact patch of the softer tire is counterbalanced by the fact that deflecting the entire bike and rider on each bump costs more energy than does absorbing the bump into the tire. Only the tiny mass of a small part of the tire moves up and down on each bump, rather than the entire bike and rider. This is the “sprung weight vs. unsprung weight” argument explaining why suspension vehicles are faster on rough roads than ones without suspension. Lower pressure is faster on rough roads, and bigger tire diameters then protect the rim.
The tubeless advantage
It’s safe to run a tubeless tire at lower pressure than a tubed tire since there’s no tube to pinch when hitting sharp bumps. This in turn reduces rolling resistance on rough roads. Eliminating the inner tube’s hysteresis and its friction against the inner tire casing further reduces a tubeless tire’s rolling resistance. First-generation tubeless road tires, which were completely airtight and designed to be used without sealant, had so much rubber coating their insides that their weight was the same as a comparable tire and tube, and the frictional rolling losses were similar. Modern, tubeless-ready tires with sealant are lighter and do have lower rolling resistance than tubed tires.
Hook-bead rims vs. hookless rims
To improve aerodynamics, Hed went to the wind tunnel and caulked the edge of the rim where it meets the tire’s sidewall so the air flowing from tire to rim meets no edges to create turbulence. Caulking tires to rims is unrealistic as well as inadmissible by UCI rules, so the key is to smooth this transition without it.
Until the advent of tubeless tires, handmade tubular tires were the rolling resistance champions. The edges of their thick base tape and the glue sticking out along the rim edge is not aerodynamic, however. Specialized lead engineer Dr. Chris Yu says, “From a speed and performance standpoint, we’ve known for a while that clinchers are superior to tubulars. They allow for better control of the shape interface between the rim and tire casing and when paired with the right inner tube, offer lower rolling resistance. This is the reason why all our pro athletes have raced TTs on clinchers for the last several years. With tubeless, if done properly, we can amplify those performance advantages. More importantly, by CT-scanning the inflated tire bead and rim interface in-house, we’re able to design a system that is secure, and we also gain the insight on how to optimize the shape transition from rim to tire.”
Like automobile rims, hookless rims lack the ‘crochet’ hooks at the inner edges of the rim walls to grasp the tire beads, so they can better optimize this shape transition from rim to tire. A standard hook-bead rim squeezes the clincher sidewall further inward than does a hookless rim, rendering the tire/rim combination less aerodynamic.
Aero carbon hookless rims have many advantages. A hook-bead rim mold requires a soft outer ring, or the hooks of the hardened rim lock to it. This single-use soft mold top is less accurate and less sustainable than the long-lasting solid-metal mold for hookless rims. The edges of hookless rims are thicker, stronger, and less easily damaged when hitting bumps than the fragile edges of hook-bead rims.
To work safely with tubeless tires, hookless rims require precise rim wall dimensions, a bead lock (a raised inner edge of the shelf the tire bead sits on), bead-shelf diameter, and central valley diameter, width and ramp slope. If you’ve ever watched installation of a car tire on a rim, you know that the compressed air blasted in through the valve must not escape from under the bead even before it’s seated; it must instead push the tire beads up the ramps of the rim valley, over the bead lock, onto the bead seat, and firmly up against the rim walls.
Specialized Tire Product Manager Wolf Vorm Walde made solid-steel rims that don’t change dimensions as tire pressure increases, in order to test tire retention. He says, “Straight-wall (hookless) rims show lower burst pressure compared to hooked rims. We do not recommend using any tires with a max pressure above 5.5bar (80psi) on straight-wall rims.”
Tire and rim width
“Tire dimension would depend on factors like road surface condition as well as how technical the course is,” says Yu, regarding the fastest tire setup. “We can quantify time lost or gained through turns based on G-loads and rider confidence, which is admittedly subjective but correlated to tire width. And rim profile would depend on course profile and wind profile.”
According to Specialized’s tire product manager Oliver Kiesel, “Typically, if we measure rolling resistance, we have three factors that vary: tire load, air pressure, and rolling speed. The graphs show how rolling resistance changes by changing one of these factors.”
Schwalbe product manager Felix Schäfermeier says, “Our general recommendation for individual competitions against the clock is a tire width of 25mm in the front and a 28mm tire on the back wheel. We recommend that the external rim width is at least 1mm wider than the (front) tire. Twenty-five millimeters in front is the best choice to reduce the aerodynamic drag on the majority of aero rims with an internal width between 19 and 22mm. Since the frame is covering the rear wheel, the wider 28mm tire doesn’t have any negative impact on aerodynamics, and it saves a few watts of rolling resistance and provides more traction.”
ENVE’s marketing manager Jake Pantone says, “The athlete should be riding a low-rolling-resistance tubeless clincher tire on a rim that is roughly 5 percent wider than the tire. As tire and rim volume increases, the need for a rim to be wider than the tire diminishes, because the increased radius of the larger tire allows for the air to attach to the rim more easily at yaw. When paired with a 28mm tire (inflated will measure around 30-31mm), an SES AR rim and tire are essentially the same width.”
Tread compound and tread pattern
Tread compound has no aerodynamic effect, but plays a big role in rolling resistance, traction, and durability. Tread compounds are generally proprietary to the manufacturers.
“We redesigned the tread pattern of the new Pro One model range to reduce the aerodynamic drag in sidewind conditions,” says Schwalbe’s Schäfermeier. “Under headwind conditions, differences between tread patterns are really minor. The impact of a road-racing tire tread design on rolling resistance is not really relevant.”
Specialized’s Kiesel says, “Slick-center road tires are the lowest in rolling resistance that we normally measure in our drum tests.” He adds that Continental’s Grand Prix 4000 tread pattern (slick with interspersed areas of cross-hatching within shark fin shapes coming up from the edges of the tread) tests “the fastest in aerodynamic wind tunnel tests. Since this was designed much earlier than comprehensive aero tunnel testing became a realistic method in the bicycle industry, this design performance was a fluke.”
The fastest setup is…
Tubeless tires now best tubulars in rolling resistance, and their clean transition with the rim, particularly with hookless rims, beats tubulars aerodynamically as well. The optimal width of the tire has grown by 2-5mm since the standards at the turn of this century, and the rim widths for optimal speed have grown by more than double that.
A 27-30mm (external width) rim mated to a 25-26mm-wide front tubeless tire creates an aerodynamically optimal setup, assuming the tire width is measured when installed on the wheel, since the tire label only gives a rough guide. On the rear, use a 25-28mm wide tubeless tire on the same rim. It’s possible to gain a bit more aero advantage with a hookless rim.
Of course, the final answer depends on X-factors like mating the tire pressure to the weight of you and the bike for the particular road surface, course profile, and much more. Dialing it in optimally requires scientific study that only the most well-funded teams could undertake. As a general rule, however, you will be in the ballpark with the above setup and tire pressure for a 150-pound rider around 70psi for a 25mm tire and 60psi for 28mm (80psi and 65psi, respectively, for a 170-pound rider).
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