Where the rubber meets the road
By: Lennard Zinn
What makes a tire fast? We went to Nastola, Finland to find out. There, in the labs of Wheel Energy Oy — the best facility in the world for evaluating rolling resistance — we tested 34 tire models to separate the hype from the science.
Pushing a road bike at 40 kilometers per hour on a flat road demands power output in the range of 270–350 watts for most cyclists. Wind resistance takes up most of that. But the second largest constituent of drag is rolling resistance. For example, two tires with a rolling resistance of 50 watts each will eat up 100 watts to hold a 40kph pace. No wonder you feel so slow on thick, tough, touring tires.
What we found out: While supple and wider tires roll fast, tread compound is paramount. Like most cyclists, we assumed that a fine-thread, non-vulcanized casing (its thread is spiral-wrapped around a drum with only raw liquid latex brushed over it and no heat added) would offer the least rolling resistance. But only one of the top five tires in the test was a non-vulcanized open tubular. The other four were fully vulcanized tires (the entire tire is cooked in a mold, melting the rubber throughout all of the layers) with casings strung on a “calendar” (a big loom on which rubber is pressed into the fabric).
So if you’re looking for a fast tire, don’t be afraid to go wide, and don’t have blind faith in conventional wisdom about compounds and tubes. The fastest tire in our test — the Specialized S-Works Turbo Tubeless Light — was vulcanized and full of sealant.
It’s striking to see that four of the top five tires in our test come from Specialized. But the truth is that all five come from the same tire design team. The 25-millimeter Continental GP 4000S II — which, with its Black Chili tread compound, has been one of the world’s best-selling high-end clinchers for years—was created by the same engineers now running tire development for Specialized.
How they performed
Here, ranked in order, are the 34 tires from our test, with the power, in watts, each requires to maintain a speed of 40kph. So double it to get the figure for two tires — unless you’re trying to figure out Peter Sagan’s wheelie power.
How We Tested
Wheel Energy evaluates bike tires on a 1,200-millimeter-diameter drum with an interchangeable surface. To simulate an average asphalt road, they use a steel surface with a diamond-plate tread pattern like you might find on metal stairs
The tires ranged in width from 22 to 26 millimeters and included standard clinchers, open tubulars, and tubeless versions. We tested them at 40kph with a load of 50 kilograms, each inflated to 116psi. To simulate the way we assumed most people will run these various models, we used 70-gram Bontrager XXX latex inner tubes in higher-end clinchers and 95-gram butyl tubes in less expensive ones. Tubeless tires took 25 grams of Bontrager TLR sealant.
All standard clinchers were mounted to Mavic Ksyrium Super Light wheels, while tubeless tires went onto Fulcrum Racing 1 wheels.
Wheel Energy has a rigorous calibration protocol that the company claims delivers an accuracy of ± 0.15 watts. This involves starting up the machine 12 hours before testing begins to bring the drum and room up to a standard temperature. Technicians then take three test measurements with three known tires to check for consistency. Finally, each tire runs for 10 minutes on the drum before any measurements are taken, to warm up, settle the tire shape, and work out internal stresses. The calibration procedure means the lab can test only two tires per hour.
Though Wheel Energy has found that the difference between two tires of the same model is rarely more than 0.1 or 0.2 watts, we tested two tires for each model and averaged the results. So we spent one hour testing each model in our lineup.
Compound is paramount
Rubber compounding is the inexact science of mixing together different organic polymers and, often, inorganic compounds. Tire companies tend to hold their proprietary formulas as closely guarded secrets.
One way in which car companies have achieved reductions in average fleet fuel efficiency is by using tires with lower rolling resistance. Tire companies have begun incorporating more synthetic rubber into their compounds and adding silica to decrease rolling resistance without sacrificing traction. Silica reduces the amount of internal friction in the rubber, which is key. All rubber loses energy due to internal friction when it is deflected, since the energy required to deform rubber is greater than that required to return it to its original shape. This is why each bounce of a ball is lower than the prior one.
Consider a graph of the extension of a rubber compound versus the force stretching it. As the rubber is loaded under increasing force, the curve starts out steep then levels off, meaning the rubber initially holds its shape but then gives into increased force. When the load is released, the rubber again tries to hold its shape. It is initially slow to respond and then rapidly returns as the last bits of the load are removed.
This lag in the deflection response is called hysteresis, and it results in a graph that looks like convex lens tilted up to the right. The amount of energy loss is equal to the area in the center of this “hysteresis loop.” The faster the compound, the smaller the loop. The smaller the loop, the faster the tire.
How did compounds play out in our test? While the 24-millimeter Specialized Turbo Cotton open tubular was basically a rounding error away from being tied for first, it’s worth noting that the winner, the 26-millimeter S-Works Turbo Tubeless Light, was three watts faster than its 24-millimeter sibling. So it’s a safe bet that a 26-millimeter version of the Turbo Cotton would have been a runaway winner.
The Turbo Cotton’s performance is a combination of compound and casing. Specialized has several compounds under the Gripton moniker. The actual compound material varies depending on tire model and application. The Turbo Cotton is the only tire in the top with a width under 25 millimeters. It has Specialized’s fastest, most responsive (i.e., low-hysteresis) Gripton compound in the center tread. In the shoulders, it has a compound with higher-hysteresis polymers to maintain grip and provide a dampened, secure road feel.
The latex impregnation of the supple, 320-TPI, non-vulcanized cotton casing material ensures that it has very low hysteresis. Nylon or polyester casings with cured (vulcanized) rubber melted through the casing have higher hysteresis and, hence, higher energy loss.
So the Turbo Cotton was the fastest clincher in our test and also boasts a shoulder tread compound that provides outstanding grip. Speed is nothing without control.
The 26-millimeter S-Works Turbo Tubeless Light has a lightweight 180-TPI vulcanized nylon casing and low-rolling-resistance Gripton tread. This, plus the lack of inner-tube friction, helped it achieve the top spot.
Specialized’s first branded products were tires, and the company has invested heavily over the past few years to greatly improve its offerings in this category, notably by hiring two German engineers away from Continental to run its tire division: Wolf Vorm Walde, tire research and development director, and Wolfgang Arenz, compounding specialist.
“The tread area is where the most deflection in a tire occurs,” Vorm Walde says. “That is where the rolling resistance is created. We have a significant advantage in compounding,”
Supple casings reduce rolling resistance but cannot make up for bad compounds
All tires have multiple layers of threads holding things together. Two sheets of unidirectional threads laid across each other at right angles generally counts as one layer. Tire casings are specified in TPI (threads per inch) per sheet. The higher the number, the thinner the threads, since more of them can be crowded together.
Comparing TPIs isn’t merely a matter of comparing thread counts, however. It should be, but Continental counts things differently, which makes apples-to-apples comparisons difficult, at least when Contis are thrown into the mix. The company adds the TPI of each sheet within a layer. So a 165-TPI casing, like on the GP 4000S II, gets marketed as 330 TPI. This is misleading and renders the TPI number meaningless, because, obviously, the number gets higher by piling more layers atop each other, making the tire stiffer, rather than more supple, as a high-TPI number should indicate.
But the great performance of open tubulars in our test shows that a high TPI (correctly counted) does deliver a faster tire. The 24-millimeter Turbo Cotton tied for first. And the fastest tire not developed by one of Specialized’s Wolfs is also an open tubular — the 25-millimeter-wide Zipp Tangente Speed.
However, open tubulars from Vittoria, Veloflex, and Challenge ranked down with several vulcanized tires. What gives? We assume those results are a function of inferior tread compounds. Even within the same company, compounding can vary greatly between models. The 265-TPI polyester/cotton casing of the Challenge Elite isn’t as supple as the 320-TPI cotton casing of a Challenge Criterium, yet the former is four watts faster at the same width. Compound is the only explanation.
Tubeless road tires do not necessarily have lower rolling resistance
We had expected tubeless or tubeless-ready tires to have lower rolling resistances, since inner tubes create friction and should, thus, raise rolling resistance. Indeed, our top performer was a lightweight tubeless tire. Overall, though, our test didn’t show tubeless-ready clinchers to have lower average rolling resistance than standard clinchers except on a completely smooth drum, which is a totally unrealistic riding surface.
On rough surfaces, where imperviousness to thorn punctures and pinch flats — when used with sealant — makes tubeless tires particularly attractive, they were not always faster. A 25-millimeter Schwalbe ONE clincher with a latex inner tube, for example, had essentially the same rolling resistance as a 25-millimeter Schwalbe ONE tubeless. This is likely due to the fact that tubeless tires need a thicker layer of rubber inside the casing to hold air. Their weight is therefore comparable to a similar clincher plus inner tube.
Also, tubeless and tubeless-ready tires cannot be open tubulars; they are always vulcanized to create an airtight casing and seal along the tire bead.
The stiffer the tire, the more air pressure matters
Many riders obsess over tire pressure. But it turns out that there is less need for that with better tires. Conversely, if the casing is bad, the tire will be slow, regardless of pressure. Lower pressures may feel smoother, but dropping from, say, 109 to 73 psi in a slower, vulcanized tire, is a five-watt penalty per tire. With an open tubular, the difference is negligible.
Lower tire pressure means fewer pounds of pressure on the tire. Since the tire is supporting the same load regardless, the contact patch will be bigger at lower pressures. Wheel Energy measured a 75-millimeter-long footprint for a 700 X 23 tire inflated to 112 psi with a 50-kilogram load. At 84 psi, the patch grew to 82 millimeters.
The larger contact patch at lower pressure means tire deflection will be deeper, resulting in more internal-friction (hysteresis) losses. That would seem to argue for higher pressures. And, indeed, if you were riding on smooth glass, higher pressure would be faster. Wheel Energy did a smooth-drum test of the 22-millimeter track tubulars VeloNews’ managing editor Chris Case used for his hour record attempt and found that rolling resistance continued to drop as pressure increased from 101 psi all the way up to 203 psi.
On rough surfaces, however, a tire at lower pressure is better able to absorb bumps, rather than deflecting the entire bike and rider upward, as it would if it were pumped up harder. This is the same sprung-versus-unsprung-weight argument that explains why suspension makes a vehicle faster on rough terrain: The less weight that is sent upward with each bump, the less energy it takes to keep the bike rolling.
So what is the ideal pressure for the surface you’ll be riding on? There is a sweet spot, but it’s a big one, and it depends — of course — on the type of tire you’re running. Supple, unvulcanized, high-thread-count open-tubular tires have much more wiggle room than stiffer vulcanized tires.
In our pressure test of the Specialized Turbo Cotton versus the Hutchinson Atom, a 36-psi drop in pressure in the former had about the same penalty as just a 7-psi drop in the vulcanized Atom. One important takeaway from that: Latex inner tubes lose more air over the duration of a race than butyl tubes. But if you’re running fast tires, that won’t matter.
Wider tires are faster
Tire rolling resistance comes from internal friction within materials (energy loss due to hysteresis) and small bumps that lift the bike and rider.
When it comes to internal friction, wider tires have shorter contact patches and, thus, less deflection. If pressure is the same, the area of the contact patch must be the same to support the same load. (Since both load and pounds per square inch remain constant, the area in contact with the road will also be the same.) But a wider, shorter contact patch will have less vertical depth of deflection, So internal friction and hysteresis loss will be lower.
If a wider tire is constructed of the same materials in the same thicknesses as a narrower one, it will often roll faster on a rough surface, despite being heavier. This is due to both lower internal friction and the fact that the wider tire will better absorb imperfections in the road, thus lifting the bike and rider slightly less on each little impact.
Latex inner tubes are faster than butyl tubes.
The Challenge Elite 25mm was the only tire measured with both latex and butyl inner tubes. Power required was 1.3W lower per tire with the latex tube than with the butyl tube (latex 41.1W vs. butyl 42.4W). The difference was even greater on the smooth drum; power required was over 3W lower per tire with the latex tube than with the butyl tube (latex 27.8W vs. butyl 31.0W). The theory is that the lighter, more supple latex tube moves along with the tire casing more easily than would a stiffer tube.
The bottom line
If you want reliability and good rolling resistance, wide tubeless tires with sealant are the way to go. If punctures are not an issue, and you want pure speed and grip without having to worry too much about tire pressure, go with an open tubular with a great tread compound.