OPINION “Purchasers of today’s exotic but street-legal supercars: verify that the speed rating of your tires exceeds the top speed of the car. Or invest in a roll cage and drag chute” JOE WALTER S peed and distance records abound for every mode of transportation – from low-speed, human-powered aircraft to rocket-powered, hypersonic sleds on rails. Between these extremes exist many vehicle types with innumerable methods of locomotion. Unsurprisingly, automotive enthusiasts are often obsessed with the speed and performance of production and one-of-a-kind cars. Manufacturers, tuners and individual enthusiasts meet these needs by building and chasing land speed and performance records – mainly for wheel-driven, piston-powered automobiles. Consider straight-line performance from a standing start as customarily used in drag racing: vehicle acceleration is governed by the torque delivered to the traction-limited drive wheels (where EVs are better), after which a transition occurs where speed is limited by available power (where piston-powered cars are currently superior). Traction-limited acceleration is restricted by tire-road friction. In contrast, vehicle top speed (accompanied by zero acceleration) is governed by the peak power delivered to the drive axle(s) modified by velocity-dependent aero and tire drag terms – an engaging test problem for engineering students. The Bonneville Salt Flats in Utah are well known as the sanctioned venue in the US for numerous land speed records, with vehicles usually employing a flying start. Speeds are calculated by averaging each top speed achieved during one-mile runs in opposite directions. This venue was used in 1935 by Sir Malcolm Campbell when he became the first person to drive a piston-propelled car over 300mph in his unique Rolls-Royce-engined Blue Bird. Since World War II, many piston-powered, The Michelin XAS was the first (1965) production tire non-production cars have achieved speeds exceeding designed to operate with the H speed rating; it also featured Beginning in 400mph at Bonneville – but are not street-legal. tread pattern asymmetry (see the November 2014 issue, Today’s fastest production car seems to be the page 14). Tires with higher speed ratings normally feature sold-out, limited-edition Bugatti Chiron Super Sport a circumferential cap-ply from shoulder to shoulder to US and Canadian 300 fitted with a variant of Michelin Pilot Sport tires. reduce belt edge stresses, and tread rubber that delivers manufacturers agreed to Although capable of achieving speeds over 300mph, better grip at the expense of wear-out mileage. Today’s conform with European the Bugatti is capped at 273mph for highway operation. speed rating system encompasses virtually the entire For competitive reasons, tire manufacturers are notably alphabet from A to Z; the increasingly higher ratings are speed ratings reluctant to provide internal construction specifics for their W, Y and Z (with the highest ZR ratings reserved for tires supercar fitments. Design considerations for all ultra-high-capable of exceeding 186mph). speed tires might include reduced tread depth (lower weight) Then there are jet-powered cars with free-rolling, non-and beefed-up beads (increased strength) – all to handle driven wheels that have achieved supersonic speeds. In 1997, potentially large centrifugal forces. the Thrust SSC, featuring two afterburning Rolls-Royce jet Thus, the importance of speed ratings for tires. The US engines and driven by Royal Air Force Wing Commander federal government does not mandate or regulate such ratings; Andy Green, registered 763mph – with the expected sonic the rating system originated in Europe in the 1960s. The booms. This record feat was accomplished at Black Rock first ratings were designated S (standard – 112mph), H (high Desert, Nevada, 400 miles west of Bonneville. The soil there speed – 130mph) and V (very high speed ≥130mph). At that is harder than at the Salt Flats, making it better suited to the time and until the early 1990s, S-rated tires shod about 85% of non-pneumatic, solid-steel wheels fitted to the vehicle. American automobiles, while T-speed ratings (118mph) were the Final words for the purchaser of today’s exotic but street-legal norm in Western Europe. Beginning in 1991, US and Canadian supercars: verify that the speed rating of your tires exceeds the manufacturers agreed to electively mold the speed symbol on top speed of the car. Otherwise, invest in a roll cage for safety the tire sidewall to conform with European practice. and a deployable drag chute for braking. 1991 14 www.tiretechnologyinternational.com November 2024 Illustration: Phil Hackett