Reader Comments: More on Racing Tires

Admin · August 26, 2022

Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

Readers continue to express puzzlement at MotoGP’s biggest limiting factor: tires. Most racers and observers know that “cold tires” (tires that are not up to temperature) can cause crashes in the first three laps.

A more recent understanding is that when in a drafting pack, running in slipstreams that have been heated by the radiator airflow and exhaust of other bikes can cause the quickest riders’ tires to mysteriously underperform. Tires can also cool off enough between corners to lack initial grip during turn-in. All strange occurrences to those of us who ride only on public streets and highways.

Reader Green Rocket speculates that tracks may be too smooth, and that “bumps like a typical road” might “improve the racing.”

James Spry asks, “How much of this tire [properties] loss is compound, [and] how much is construction?”

Nigel Rue suggests organizers “give ‘em all sets of classic Avon Speedmaster MKIIs, a 3.25-19F and 4.00-18R” as a possible way to restore consistent performance.

The purposes of street tires and racing tires are quite different. Street riders make moderate demands on their tires, but many ride year-round in all kinds of weather. That dictates that their tires must give reliable grip across a wide temperature range, wet or dry.

Tread compound engineers discovered long ago that maximum grip from rubber occurs in a limited temperature range beginning not far above the material’s glass transition temperature (Tg), sometimes called its “glass point.” At Tg, the rubber becomes a rigid solid and loses all ability to conform to pavement texture—the very definition of losing its grip. In the narrow temperature range just above Tg, the rubber is flexible but sluggish rather than “snappy.” Too high above Tg the rubber loses grip by becoming too flexible; too far below by not being flexible enough.

Related: Explaining The Motorcycle Racing Tire Problem

Because street tires must function in winter or summer, the rubber’s glass point has to be below the lowest temperature of use.

As with so many physical phenomena, the harder you work to achieve maximum effect, the narrower its range becomes. Rubber for street use absolutely must function safely across the temperature range nature throws at us. Rubber for racing, on the other hand, can be (and is!) optimized for grip at the predicted operating temperature of the tread, on a particular track, for the style of a particular rider, in the weather on that day. This is why MotoGP tires offer a range of choices (for simplicity referred to as soft, medium, or hard), made available for each event, with a range of wet tires in case of rain.

The process of pushing up grip to extremely high values within a narrow operating temperature range has been extremely successful, both in terms of success on the track and in terms of discovering new techniques that can be of use in production tires.

Grip and Lean Angle

On those occasions when Dorna video coverage has presented a MotoGP rider’s lean angle, we have seen incredible values, as much as 63 degrees from the vertical, implying that the tires are laterally generating twice the acceleration of gravity to turn the bike. Watch films from earlier times and you’ll see nothing like this.

Indeed today’s actual MotoGP lateral acceleration may be greater than this, suggesting that the actual lean angle of the machine/rider combination may surpass the lean angle of the bike alone—because the rider is so offset to the inside.

When riders shift their weight to the inside during turning, they are doing all they can to push the bike toward upright. There are two reasons they do this.

Tire flex is greater on the edges than in the center. (MotoGP/)

The first is to preserve the vulnerable tire edges. These are less supported by the air-filled carcass and therefore flex more than the rest of the tire, making them more likely to lose grip from excess temperature.

Second, although manufacturers narrowed engines and mounted them higher, both measures aimed at increasing cornering clearance, grounding remains possible.

Tires and Braking

Now consider braking. The front brake on Barry Sheene’s championship-winning 1976 Suzuki two-stroke was a pair of 290mm steel discs, capable of reliable operation up to 500–550 degrees Celsius. His front rim was 2.5 inches wide. In MotoGP today Brembo offers discs in three diameters and a variety of weights (because more heat must be stored on certain tracks). Those diameters are 320, 340, and (for tracks like Motegi or Phillip Island) 355mm. Their carbon-carbon material operates reliably up to 800 degrees Celsius and exceptionally to 1,000 Celsius. Front rim widths are 3.5 or 4.0 inches.

The tires Sheene used were capable of handling the braking force from his Suzuki’s modest-sized discs over a speed range topping at about 180 mph. Today’s much more capable fronts must handle braking from speeds up to 220 mph. Since kinetic energy varies as the square of speed, the brakes on today’s 357-pound MotoGP bike must deal with 49 percent more energy from its higher top speed and maybe 13 percent more energy as a result of the increased all-up weight.

Related: Guide To Understanding Everything About Your Tires & Tread

The forces applied to MotoGP tires are far beyond what modern production street tires are expected and capable of handling. (MotoGP/)

Now for acceleration. At 114 hp, Sheene’s Suzuki was making 37 percent more power than the 1,000cc production four-strokes of its time. Moreover, four years earlier at Daytona, the new 750 two-strokes from Suzuki and Kawasaki had destroyed every tire that was tried on them through practice—and destroyed them so dramatically that Don Emde won the 1972 200-mile event on a little 350 two-stroke Yamaha twin running on traditional all-weather (patterned tread) racing tires.

Engine output has more than doubled since then, with present-day MotoGP engines likely giving about 280 hp.

Such performance is out of reach of wide-range production tires—especially those designed decades ago and made in narrow sizes! Therefore I must interpret Mr. Rue’s Avon Speedmaster proposal as either tongue-in-cheek, which I appreciate, or as a variant of the suggestions sometimes made seriously: that current MotoGP spec-tire supplier Michelin be asked to reduce performance. It should supply, for example, tires that skid off turns at 1 G rather than 2, or tires that turn to grease, blister, or chunk when called upon to transmit 2.6 times more power than that of a 1972 Suzuki.

Track Roughness and Tire Temperature

I assure “Green Rocket” that real racetracks are not smooth. This is precisely why the corner-speed MotoGP bikes (Yamaha, Suzuki), with their laterally flexy frames, hook up well in long fast corners, where the stiffer chassis needed by point-and-shoot riders (Marc Márquez and company) tend to skip from peak to peak on the pavement profile and so generate less grip there. Read the race reports and discover that riders complain constantly about rough pavement, pavement coming apart in hot sun, and wavy pavement. In particular, read Casey Stoner’s frank evaluation of the very bumpy Indianapolis circuit when MotoGP first ran there in 2008. The sports editor of the Indianapolis Star didn’t like it one bit!

And as to the loss of tire properties Mr. Spry refers to, that is the direct consequence of narrowing the range of operating temperature in order to push peak properties up to levels that can handle MotoGP’s extraordinary demands. If the actual tread temperature for any reason creeps out of that range—the day was hotter/colder than the forecast, the new pavement was more/less abrasive than anticipated, and so on—expect substantial changes in tire properties.

Yes, operating temperature can be raised or lowered by making changes to either tire construction (Stiffer? More supple?) or tread compound (Greater heat-generating internal friction? Less?). Tires run cooler for slower riders, hotter for quicker ones. Different riding styles place different demands on tires.

Related: How To Pick The Best Motorcycle Tires For Your Bike

In general terms the direction of tire development is determined by the expressed needs of riders. When Kenny Roberts introduced throttle steering to 500 GP racing in 1978-80, the development emphasis was on abuse tolerance. But when more riders began to arrive in the big class through 125 and 250 where corner-speed is the dominant style (because their smaller engines lack the acceleration needed for point-and-shoot), the tire makers focused more on raising peak grip. When a few years ago I asked the manager of Pirelli’s World Superbike tire program how he served the two conflicting masters (corner-speed and point-and-shoot) he replied, “I try to adjust things so that both groups complain about equally.”

MotoGP’s New “Sprint Race”

Today comes news that may reassure readers who feel that tire technology has too large a say in race results—news that in 2023 Dorna will introduce an additional “sprint race” on Saturday, half as long as Sunday’s GP and counting half points Some riders welcome this, others regard it as copying World Superbike. You can be sure that tire manufacturing and tire strategy will evolve to deal with this change, bringing fresh complexity.

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