Suzuki developed the RM125 motocross bike in 1975 as its Racing Model series to replace the Trail Model or TM series motorcycles. The RM series ceased production in late 2007. Early RM125 versions couldn't compete against the more powerful Kawasaki KX, Yamaha YZ and Honda CR series enduro bikes, and Yamaha introduced the RM125S as the answer. However, the RM125S versions are rare and the RM125 remained the standard 125 cc bike.
Suzuki produced a wide range of RM series motorcycles with engine displacements ranging from 50 cc to 400 cc. The 1975 models featured a downpipe exhaust system under the two-stroke engine, and were air-cooled. Suzuki switched the RM125 to a liquid cooling system in 1981. It also introduced the "Full Floater" swing arm monoshock system the same year. To give the RM125 a better edge against Kawasaki, Yamaha and Honda, Suzuki enlarged the engine's ports, and redesigned and enlarged the exhaust pipe. Engineers also drilled two holes in the piston skirt for better cooling. These changes increased output to 26 horsepower to compete against other Japanese bikes. The RM125S, which was only in production for four months, featured a larger Mikuni carburetor than the standard version. The only noticeable difference between the RM125 and RM125S models was the "RM125S" graphic on the fuel tank.
The liquid-cooled Suzuki RM125 suffered severe problems in the early 1980s as engine tended to seize in cold weather when not properly warmed up. Suzuki engineers discovered that differences in temperatures when the piston expanded against the cylinder liner led to the engine seizing.
Despite its inherent early engine weaknesses, rider Gaston Rahier rode the RM125 to victory in the 1975 125 cc World Motocross Grand Prix championship. Through 1984, it led its class with riders Rahier, Harry Everts, Michele Rinaldi, Akira Watanabe and Eric Geboers. One reason for its early success was its ultra-lightweight frame that gave the 1970s RM125 a dry weight of just 190 lbs. while sitting on a short 53.5-inch wheelbase. The bike measured only 80.3 inches long.
RM125 on a Diet
By 2001, Suzuki fitted the standard RM125 with slightly shorter Showa forks and improved the rear suspension system. An external water pump helped reduce the engine's weight. For the 2001 model, Suzuki changed the carburetion from a Keihin model to the lighter Mikuni TMX38S. Further weight reductions included a new aluminum exhaust pipe and three-piece aluminum/steel clutch pushrod instead of the one-piece steel version. These measures dropped the dry weight of the 2001 RM125 about two pounds from the 2000 version.
The 2005 Suzuki RM125 came with a two-stroke, single-cylinder engine with a 2.1-inch bore ad 2.1-inch stroke. The compression ratio was 8.7-to-1. A six-speed chain-driven transmission delivered the power to the wheels. The front suspension consisted of telescopic forks with 12.2 inches of travel, while the rear was a link-type, fully adjustable spring preload system with 12.6 inches of travel. The front tire was 21-inch in diameter and the rear was 19 inches. Single front and rear disc brakes provided the stopping power. It sat on a 57.1-inch wheelbase and had a total length of 84.4 inches. The seat sat high, at 37.8 inches, and the bike's ground clearance was 13.8 inches.
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