You know the feeling: You’re rolling along on smooth pavement, you hit cobblestones or a rutted or washboard section of road, and the effort it takes to keep your bike going at the same speed suddenly and rapidly escalates. Wind resistance and gravity did not increase. What increased was the amount of energy your tires, bike, and body were absorbing and releasing as heat.
For Paris-Roubaix, if a rider or a team can minimize this energy loss to rolling resistance, that could constitute some worthwhile marginal gains. When added up over six hours of hard pedaling and a couple of dozen sections of pavé (cobblestones), watts saved by the tires could make the difference of who can follow the critical searing attack that makes the difference at the end, and who cannot.
The question for a team wanting to see its riders reach the Paris-Roubaix podium is not only which tires, but also at what air pressure. To quantify the energy loss in racers’ tires when rolling fast over cobblestones at various pressures, we contracted with the world’s leading lab measuring rolling resistance of bicycle tires: Wheel Energy Oy, in Nastola, Finland.
For this test, we obtained as many of the actual tires that teams plan to use in the Hell of the North as we could. We tested those alongside a number of tires that represent different kinds of tire construction technologies that have been or could be used in this race. Here is why and how a tire sucks up a rider’s energy, as well as a detailed explanation of the test protocol.
Note that not all of the tires were tested to 6 bars of pressure. We began testing at 2 bar, and air pressure was only increased in each tire until its rolling resistance started going back up; then the test was stopped.
Lab results: A clincher with an inner tube was the fastest
Years ago, Specialized claimed that its Turbo Cotton handmade clincher tire is the world’s fastest tire, pointing to Tony Martin’s time trial results as evidence, and on Wheel Energy’s simulated cobblestone surface (and in our past testing there on a smooth drum), this still appears to be true.
This 28mm tire’s 14.9 watts of power loss at 5.0 bars (72.5psi) was more than two watts less than that of any other tire, and more than 15 watts less than the slowest tire in the test. And remember, you can double these savings (or losses), as each bike of course uses two tires.
This data was from tests with a latex tube. Look for our upcoming rolling resistance test comparing different inner tubes against tubeless systems.
The Turbo Cotton’s tread is made of Gripton compound, and its casing is formed by winding thin cotton thread around a long, thin spool at 320 threads per inch.
According to Specialized tire product manager Oliver Kiesel, the Bora-Hansgrohe and Deceuninck-Quick-Step teams will be choosing between this tire and the S-Works Turbo Cotton tubular for the 2021 Paris-Roubaix.
The latest testing with these two squads, which have both won this race, indicates that they will be running pressures of 4-5 bar (58-73psi) in the rear tire, and 0.2 bar (3psi) less in the front. This agrees with the 5-bar inflection point for minimal rolling resistance we found for the Turbo Cotton clincher. However, the Turbo Cotton tubular’s rolling resistance was a half-watt lower at 5.5 bars than at 5 bars. That extra 7psi in pressure will decrease chances of pinch flats and rim damage in Paris-Roubaix and, if you’re rolling faster to boot, looks like the way to go for those tubulars.
Tubeless clinchers shone
Any concern about sealant inside of tubeless tires sucking power from the rider may have been sloshed away by this test. A Continental 28mm Grand Prix 5000 TL set up with sealant and no inner tube was second fastest behind the Turbo Cotton clincher. A 25mm version of this same tire tested almost as fast as the 28mm, but that is an unrealistic size for Paris-Roubaix and is thus not included in the table.
Bahrain Victorious, Groupama-FDJ, Ineos Grenadiers, Intermarché-Wanty-Gobert Matériaux, and Movistar will ride the GP 5000.
“The teams usually go for 28 or 32mm for Roubaix; 28mm is what most teams use,” said Continental product manager Jan-Niklas Jünger, adding that the teams will generally run pressures of 3-5 bars (43-73psi).
Conti’s Black Chili compound in the GP 5000’s smooth tread, featuring cross-hatched sharkfin-shaped sections along the sidewalls, is made of synthetic rubbers mixed with natural rubber and carbon black (soot).
Aerodynamicist Jean-Paul Ballard of Swiss Side says that this tire’s GP 4000 predecessor is always the fastest tire in wind tunnel tests and that the GP 5000, whose tread pattern is similar, also tests fast in the wind tunnel. That is also a useful consideration for Paris-Roubaix that is minimally measured by this test.
The 30mm Challenge Strada Bianca Pro TLR was the only tire not branded Specialized or Continental to come in at under 20 watts of power loss. It was also the lightest tire, when you add the latex tube to the Turbo Cotton’s weight, and rotating weight is more of a concern in the Hell of the North than in most races, due to frequent heavy braking and accelerating after corners and crashes.
Like the Turbo Cotton, the Strada Bianca Pro TLR has a super-high-thread-count, spool-wound, unvulcanized casing with hand-glued tread. And at 6 watts less power cost, the TLR tubeless version of the Strada Bianca Pro tested faster than the clincher version with a latex inner tube inside. While Challenge does not sponsor a WorldTour team, it does make tires for at least one brand with a WorldTour team racing its tires.
Tread compound and puncture breakers make a difference
Specialized claims that its Gripton tread compound has exceptionally low rolling resistance and high cornering grip, and Continental says the same about its Black Chili compound. Grip was not tested here, but the two fastest-rolling tires in this test had one of these two tread compounds.
Conti developed Black Chili in-house, and German engineers who used to work at Continental came up with Gripton for Specialized. As Continental Tire is primarily focused on making motor-vehicle tires, and as automakers constantly put pressure on tire makers to reduce rolling resistance in order that their cars can hit low MPG targets, it invests heavily in developing faster tread compounds.
Even though they have almost identical construction and were both set up with a latex inner tube inside, the Specialized Turbo Cotton clincher had 10 watts less rolling resistance than the Challenge Strada Bianca Pro clincher. Since the casings are made in the same way — spool-wound with super-fine thread and not vulcanized, with the tread hand-glued on — one might conclude that it is the tread, the puncture-protection strip under it, or a combination thereof, that is making the difference. Anybody who has ridden with Mr. Tuffy puncture-proof strips between their tires and tubes knows how much energy stiff puncture belts can absorb.
Thread count can be misleading
Continental claims a thread count of 3/180 TPI on the GP 5000 TL by adding up the number of threads per inch on the three casing layers under the tread to come up with 180 TPI. Thread count is traditionally done on a single casing ply, so to compare the TPI specs the same way as the other tires in this test, you would instead call the GP 5000 a 60 TPI tire. A low thread count generally indicates that the casing is made out of thick threads that can’t pack in as tightly as thin ones, thus resulting in a stiffer, slower casing. That is clearly not the case with the GP 5000.
Conti sidewalls have a signature weave pattern with space between the threads. Those threads are thin, and the casing is light and supple. It’s understandable that Conti calls it 180 TPI rather than 60 TPI, so that people don’t assume from the number that it is a stiff, slow tire.
Furthermore, the casings of the two non-Conti tires that came in under 20 watts do not have vulcanized casings; only the tread, which is hand-glued onto the casing, is vulcanized. Vulcanization hardens the rubber and makes it durable. Leaving these spun casings in their raw state allows the casing threads to move independently and absorb bumps with less energy loss than if they were impregnated with rubber and vulcanized.
The Continental tires, however, are made in a traditional way, in which the sticky “green” tire is put into a mold and then inflated and heated by hot steam injected into a bladder inside it as well as through channels in the mold to melt the layers together while pressing them against the inside of the mold, vulcanizing the entire tire.
Tubulars did not shine
I had expected the tubulars to be fastest, not lower down with most of them clustered at the bottom. The high-thread-count, unvulcanized, handmade casings with glued-on tread that they incorporate clearly have the potential to be very fast, since they share that casing with the Specialized Turbo Cotton clincher that won this test. Why is the tubular slower?
Just barely ahead of the 30mm Maxxis Velocita tubular (to be used by Israel Start-Up Nation) in our test, the S-Works Turbo 28mm did the best of the tubulars. However, at At 21.2 watts, it was 6 watts slower than its clincher cousin.
Both Specialized tires have a latex tube inside and share the same Gripton tread and drum-wound, unvulcanized casing. The only difference is that the tubular has its casing edges lapped over and stitched around its latex tube, while the clincher’s casing edges are lapped over aramid beads that can engage a clincher rim.
Similarly, the Challenge Strada Bianca Pro clincher consumed over 4 watts less than the Challenge Strada Bianca Pro tubular, which also has the same construction type and same latex tube inside.
The AG2R Citroën, Team BikeExchange, and Trek-Segafredo teams can choose Pirelli Pzero Race TLR tubeless tires in 30mm (fifth fastest in this test) or 28mm, or the Pirelli Pzero TM Velo tubular in 30mm.
Comparing tubeless tires of this type of construction with tubulars sharing similar construction, we observed the same thing. The Challenge Strada Bianca Pro TLR tubeless clincher used 10 watts less power than the Challenge Strada Bianca Pro tubular, and the 28mm Vittoria Corsa Control TLR tubeless clincher (fourth fastest in this test) consumed 5-6 watts less than the two tubular versions of the same tire.
We’ll see which Vittoria tires Jumbo-Visma, Alpecin-Fenix, Lotto-Soudal, Astana-Premier Tech, EF Education-Nippo, UAE Team Emirates, and Team DSM will use in Paris-Roubaix.
Perhaps a tubular loses more energy than a clincher as it rolls due to flexing of the glue joint holding it onto the rim and/or energy absorbed by the thick, cotton base tape. Tubulars in this test were attached to the rim with 3M tubular tape that Challenge developed with 3M. It is part 3M VHB tape and part 3M Super Post-It adhesive, making it strong laterally but cleanly releasing from the rim. Wheel Energy compared its bonding force favorably to tubular mastic. It may or may not compare in rolling performance to mastic; sussing that out would require more testing.
Tubular base tape protects the tire’s stitches but is not a structural component. In fact, the late U.S. National Team coach Edward Borysewicz (“Eddie B.”) sometimes had team mechanics remove base tape for world championship team time trial events. When Greg Lemond, Jeff Bradley, Andy Hampsten, and Mark Frise won the bronze medal at the 1979 junior world championships in Argentina, they did it with no base tape on their tubulars. The assumption was that Eddie B. had done that to save weight, but perhaps he had insight into a rolling resistance advantage…
I had expected to find the silk-casing, super-expensive, 30mm Challenge Strada Bianca Ultra S tubular to top the list, because its silk strands are much finer than the finest strands of cotton, polyester, or poly-cotton threads found in any other tire. However, it was the slowest. The silk tire tested has gravel construction with double PPS (PPS2: 2 layers of Puncture Protection Strips), one between the tread and casing and one between the casing and inner tube; Challenge offers a more supple Strada Ultra S in 27mm and 30mm with no PPS, which we did not test. Its hand-glued tread is the same as the 30mm Challenge Strada Bianca Pro tubular and tubeless tubular, both of which tested faster and have poly-cotton casings and PPS2; the latter tires differ from each other only in that the latex inner tube is bonded to the casing in the tubeless version, and it’s free to move in the standard tubular.
Tires in this test had nominal widths between 28mm and 30mm. In each case where we had both sizes of the same tire, the wider one tested faster. Why not use wider than 30mm? If you want to minimize vertical movement of unsprung weight over cobblestones, you would run a fat tire at low pressure, so that the entirety of each bump is absorbed into the tire, and the bike doesn’t go up and down at all, right?
Thing is, there is constant braking and re-accelerating going on in Paris-Roubaix, and fat tires not only are heavier than skinny tires of the same materials and thicknesses, that extra mass to be accelerated is rotating weight at the maximum radius from the hub. Aerodynamics play a role, too, especially on the faster, smooth-asphalt sections that comprise the vast majority of the length of the race, and a fat tire is an aerodynamic liability.
The ideal pressure for Paris-Roubaix depends on the tire, with 58psi being the lowest and 80psi being the highest. In order of fastest to slowest, here are the fastest pressures for each tire.
- 28mm Specialized Turbo Cotton clincher — 5 bar (72psi)
- 28mm Continental Grand Prix 5000 TL — 4 bar (58psi)
- 30mm Strada Bianca Pro TLR — 4.5 bar (65psi)
- 28mm Vittoria Corsa Control 2.0 TLR — 4 bar (58psi)
- 30mm Pirelli Pzero Race TLR — 4 bar (58psi)
- 30mm Maxxis Velocita tubular — 4.5 bar (65psi)
The tubulars (Specialized, Pirelli, and Challenge), at 5.5 bar (80psi), had the highest pressure at which any tire had its minimum Crr (coefficient of rolling resistance).
What tires would you choose for Paris-Roubaix?
This is not as easy as simply picking the tire that won this test, since there are so many considerations to keep in mind for a race as unique and as brutal on equipment as this one. If you could be confident that the likelihood of a flat tire or a broken wheel was remote, then it would be a no-brainer to pick the fastest-testing tire: the Turbo Cotton clincher with a latex inner tube.
Tires flat and wheels break constantly in Paris-Roubaix. Neutral support teams (Mavic for 44 years, and now, Shimano) service riders from motorcycles that return to neutral-support cars for more wheels, which keep restocked from a leapfrogging box truck. After a flat, a rider will almost certainly be on a different wheel and probably a different tire from what he started on.
Even though the Specialized Turbo Cotton has a latex inner tube, which is far more resistant to pinch flats than is a butyl tube (test this yourself by hitting inner tubes with a hammer), it can still be pinch flatted by any of thousands of sharp edges of cobblestones over the course of the race. By contrast, tubeless tires, which took the next four places in the test, cannot pinch flat, since they have no inner tube to pinch. However, they have the possibility of burping air on a hard corner at low pressure, which the Turbo Cotton will not do.
Tubulars, which trail the four tubeless clinchers in Crr, are less susceptible to pinch flats than the Turbo Cotton would be, even though they also have a latex inner tube. That’s because a tubular rim lacks the tall walls sticking up to retain the tire beads that a clincher rim has. Those thin clincher walls are much more likely to cause a pinch flat than the low, rounded edges of a tubular rim. And if a tubular does go flat, it will stay on the rim, being glued on, rather than come off as a clincher can. This could allow the rider to pedal to a place where he can more easily get service (like on the paved road after the cobbled sector), or to a team staffer posted along the cobbled stretch with spare wheels.
Besides being less likely to cause pinch flats, tubular rims are also lighter by not having a clincher rim’s bead-retaining walls. Rotating weight is arguably more important in Paris-Roubaix than in any other WorldTour road race, due to frequent braking and subsequent acceleration.
Also because they lack protruding walls to grip clincher-tire beads, tubular rims are less likely to break when hitting a sharp edge than clincher rims. This is particularly a danger at the low 4.0-bar (58psi) pressures where the fastest tubeless tires have their minimum Crr (other than the Challenge Strada Bianca TLR, whose minimum Crr is at 4.5 bar / 65psi).
Even though aluminum rims would tend to bend rather than crack, riders are still likely to choose carbon wheels for this race simply to save rotating weight. Get enough cracking in a carbon clincher rim wall, and it will no longer retain the tire beads and/or will leak air from a tubeless tire. Due to their thick, blunt walls, hookless clincher rims are less susceptible to damage than crochet-style (hook-bead) rims. That point is moot, as few, if any, of the teams in this race will use hookless rims.
Consider also that the aerodynamic advantage of the second-place Continental GP 5000 tubeless tire could negate some rolling-resistance cost for a rider soloing off the front or chasing solo. Or, after umpteen times braking for corners or fallen riders or obstacles, or even after a few crashes himself, the weight advantage of the third-place Challenge Strada Bianca Pro tubeless tire could negate some rolling-resistance cost relative to faster-rolling tires.
So, the question is, do you as a rider or team manager choose to have the fastest tire and risk losing the race due to mechanical breakdown? Or do you instead opt to minimize the potential for a pinch flat with a fast tubeless tire or the potential for rim damage with a fast tubular tire? And how much do you consider weight and aerodynamic drag, if at all? It is an interesting conundrum.
In any event, the rolling resistance physics of these commercially available tires carry over to rough roads in your neighborhood, too, and don’t just apply to Paris-Roubaix. So I hope that this test data and the contextual information above are helpful to you in choosing your next set of tires.