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/126287
c.o 1""""4 l-< o Z The big Thumper Testing Bengt Aberg's 500cc HallmanlYamaha By Jim Gianatsis It's hard to believe that a big 500cc four-stroke is still competitive today against high powered, lightweight, two-stroke factory machines. But Bengt Aberg proved a .lot of doubters wrong this past season when he took his 500cc HallmanlYamaha to victory , f h f h 500 In one 0 t e II!0tos,0 t e . cc World Championship, Consider- 24 ing how fast Mikkola and DeCoster have been going lately, this was quite an accomplishment, despite the fact that Aberg was World Champion on Husqvarna four-stroke s way back in 1969 and 1970. The bike is based upon the Yamaha 500 TT, but after Torsten Hallman (who just happens to be the Yamaha distributor for Sweden) got finished with it, all that remains of the stocker is the engine unit . What Torsten Hallman Racing does is provide a complete chassis kit to turn the big bore trail bike into a full -fledged Grand Prix racer. You can use some additional parts off a stock 500 TT to save on cost (like wheel assemblies , forks , shocks and handlebars) bu t to build up the ultimate GP machine you' ll want to utilize better components. Bengt uses Ceriani magnesium leading axle forks with a ir /springs that offer 270mm (10 inches) of travel on his bike . They work fine , but look a little spindly and probably the Simons fork assemblies which Hallman sells would be a better idea. Designed in Sweden for Hallman, t he frame is the brainchild of former World Champion Sten Lundin, and the production versions are manufactured in the U.S.A. The frame assembly is 4130 chromemoly steel painted black and comes complete with aluminum swingarm with needle bearings. The swingarm has two different mounting positions for shocks , one forward mount for 143/8 inch shocks providing 275mm (I I inches) of travel , and the rearward mount for 17 inch shocks to provide 250mm (10 inches) of travel. Included with the frame assembly are stee ring head bearings (Timken conical) , _ engine mounts; and aluminum upper cylinder head mounts. The upper frame and backbone of the frame serves as an oil tank and together with the engine holds 2.5 quarts of lubricant. Bengt uses Ohlin gas /oil resevoir shocks on his bik e, but the day of the test he was ha ving it fitted with Fox AirShoxs, which are mo re read ily ava ilable in Ame rica . On such a heavy machine the Fox AirShoxs should also prove to be more reliable during a season of racing. Other parts used on the bike, not available from Hallman, included parts from a Yamaha YZ 250 like the hubs and footpegs, with the seat and tank coming from a Yamaha YZ 125. Detailed and complete instructions are provided with each frame assembly kit. (Clockwise from top left) Except for.a hi virtually stock; Aberg in action at M is compression release is required to tum monster thumper underway; angled rear wheel travel. Riding an earthquake I didn't realize what I had gotten myself into, and d idn't realize how scared I really was until it came time to climb aboard and ride the big red and white thumper. I began to sweat and shake. "Maybe I stan for you," said Bengt's Swedish mechanic Lars Daniellson. I gladly stood back to let Lars kick the monster over as I recalled, with less tha n fond memories, the 441 Victors I used to ha ve to stan when I worked at a BSA shop. Those babies loved to kick your knee back up into your chin and knock out teeth. Lars tried for five minutes to get the Yamaha cranked, but not nary a pop before he collapsed beside the bike in sheer exhaustion. Bengt was standing nearby watching Lars and trying to keep from breaking up in laughter. With a few suppressed chuckles leaking ou t , Bengt walked over to the bike, played with the compression release as he located on the kickstarter the few secret degrees before top dead center, and with one easy motion rotated the big single into life on the first try. Just sitting on the bike it felt heavy, close to 250 pounds for sure, but once the clutch was out and the thumper got rolling it felt as light as any big bore production Suzuki or Yamaha. It responded as well , too , and seem ed to like to dart ba ck and forth around th e Cycle World USA Trans·AMA cou rse we were using with just a flick of throttle and twist of bars. With th e