Cycle News is a weekly magazine that covers all aspects of motorcycling including Supercross, Motocross and MotoGP as well as new motorcycles
Issue link: https://magazine.cyclenews.com/i/126842
.... v ..0 o ..... u o Front (right) and rear (left) versions of Michelin's Hi -Sport tires: these are racing tires that have been made street legal. The D un lop Sport Rad ial K700: the design is currently only offered as a rear tire. Dunlop claims the radial should run cooler, last longer, and afford , better traction than traditional bias-ply t ires. High-performancetireshootout at Willow S~rings Dunlop K700 radial vs Michelin Hi-Sport By John Ulrich ROSAMOND, CA, SEPT. 25 R adials. There's been lots of tal k a bo u t radial tires for moto rcycles. R adials transformed car tire performan ce, de liverin g better m ileage, m ore traction, better durabili ty. But whi le several co m pa n ies h a ve built radial motorcycl e . . .. tlre~ for racing a pphca~lOns, u nti l now nobody has mtro- 6 duced a true radi al for street use in the U.S. Th e new rad ia l is Dunlop 's K700 Spo rt Radi al for rear-wheel use, sho wn to th e press a t Will ow Sp rings Int ern a tio na l Racewa y and availa ble in ra ce a nd street com po u n ds in variou s si zes (120 /90- 18. 130/ 80- 18. 150/80-16, in racing R com po u n d; 130/90 -16, 150/80 -16 , ' 130/90-17, 140/ 80-17, 120/80- 18 a nd 130/ 80- 18 in str eet S co m po un d). Suggested retail p ri ce is expected to range from $127 to $168 in S com po und a nd from ~ 14 8 to $ 1 9 ?i n Rco m po u nd, depend - 109 u pon srze, .' Th e th eoreti cal ad va n tages o f rad ials for mot orcycle use a re th e same adva ntages gained wit h au tomotive rad ial s. A radia l sho u ld run coo ler, a llo wing th e use o f softer tread rubber withou t premature wear. A radi al sho u ld del iver a la rger co ntact patch th anks to increased tread deflecti on; th e bigger co n tact pat ch , com bined with the so fter tread rubber, sho u ld giv e better tracti on. A radial sho u ld be mor e resistant to flying a pa rt a t very hi gh speeds, and shou ld gi ve a smoother ride over pavem ent irregularities. Sta te-of-the -a rt non-radial street tir es are bia s-belted, with -carcasses a nd belts built up of plies made of nyl on or rayon or Kevlar or some similar synthetic cords; the p lies are imbedded in the tire.rubber. The carcass is the body of the tir e, made u p of a t least two, but co m mo n ly three or four, plies connecting the inside edges, or beads, and supporting the sidewalls and tread. Belts are made up of o ne or more p lies wra p p ing around the circu m feren ce of the tire, between the ca rcass and tread. The angle of co rds in the carcass and the belts influences the rigidity of the tire and its hand ling characteristics, as well as its durability and speed potential. A bias-belted tire has carcass plies arranged wi th the cords at the same angle (65 to 75°), alternating from left to right. The belt plies are arranged with a different cord angle (18 to 22°), a lte rna ting from left to right. The co rd a ngles in the carcass and belt p lies intersect, adding rig idi ty, increasing friction and generating heat as th e tir e flexes . Because the carcass p ly cords intersect , the tire circumference grows as speed increases. A radia l tire had carca ss p lies with co rds a rra nged so that th ey're perpendicu lar to the rim; the cords in eac h carca ss p ly are parallel to and don ' t cross the co rds in underlying or overl ying pl ies, reducing carca ss rigid ity, in tern al friction and the amount o f heat ge ne ra ted as th e tire flexes. Belts used in radial tir es have plies with cords set a t angles o f 0-22°. Becaue radial carcass p ly co rds run directl y from one bead to th e other , a radi al tire 's circu m ference does not grow as much as a bias-belted tire's circu mference as speed increases. , Th e first ca tch is that, because the co rds in th e ca rca ss plies don 't intersect, a radial tire has less sidewall stiffness a nd tends to move sideways in relation to the rim during transitions into corners ; the sidewa lls can also move back-and-forth in relationship to each other on straightaways, reducing straight-lin e stability. All other things being equal, a radial motorcycle tire shou ld make a motorcycle handle worse than it does with a bias-belted tire. All other th in gs are not equal, however. To red uce problems asso' cia ted with in creased sidewall flex , radia l tire des ig ners can change the a ngle of the ply cords in the carcass sligh tly (crea ting a close-but-notreally-radial tire, lik e Pirellis MP-7), reduce the height of th e sidewall (crea ting an u ltra-low-profile tire like Michelin and Du n lop radial racing slic ks), or do something else to make th e sidewall stiffer. For Dun lop, th e hard part was trying to ca rry th e advantages of radia l road-racing slicks in to a street-lega l radial tire compatible with existing stock street bik es. And th e immediate goa l was to match th e Hi-Sport's performance as a starting point for future development. The K700 was designed-in japan, and initia lly tested in j apan and Australia. In july, racer Ra nd y Renfrow tested th e K700 against the Hi-Sport a t Willow Springs an d liked the HiSport better. Fo llowing that tes t, ch a nges were made in th e K700's compound and con struction; the K700 was then declared ready for the market. Because th e development and sal e of th e Dun lop K700 radial is so closely tied to the ex istence o f the Michelin H i-Sport, we showed up at th e Dunlop K700 press int ro with a Michelin-equipped Yamaha FZ750 borrowed from Cycle Tune of Torrance, California. The bike was ridden to victory in the recent WERA 24-hour race at Willow, and was as equipped for that race: aft ermarket exhaust system, carb jetting changes, and a two-teeth -larger rear sprocket. Along to pi lot the bike was Cycle Tune-sponsored Doug Toland, who