Historically Speaking | The Peanut: Motorcycle Fuel Tank

I guess only Harley-Davidson could name a new motorcycle after a 62-year-old fuel tank. Maybe you didn't need to read the media release to know that Forty-Eight refers to 1948, the year that the so-called peanut fuel tank first appeared on the Harley-Davidson S-125. But that tank, which became a styling touchstone on generations of both production Harley-Davidson and custom motorcycles, had its real origin in pre-war Germany.

In the '30s, the German firm DKW was one of the world's largest motorcycle manufacturers, and its lightweight RT 125 was one of its most-popular models. At the end of World War II, Allied agents scoured Germany for intellectual property to claim as war reparations. The United States government awarded the design plans for the DKW RT 125 to Harley-Davidson, which in 1947 was introduced as a 1948 model, it was a new lightweight called the S-125.

By 1952 the S-125 had a Tele-Glide fork, and ads claimed it was good for 90 mpg. (Charles Plueddeman collection)

By 1952 the S-125 had a Tele-Glide fork, and ads claimed it was good for 90 mpg. (Charles

While not an exact copy of the DKW, the little two-stroke Harley was darn close, right down to the fuel tank. According to Harley service literature, the S-125 fuel tank held 1 7/8 (1.875) gallons. Like the DKW tank, it was flat on the bottom, followed the angle of the motorcycle frame's downtube along its front edge, curved around at the steering head, and then tapered down below the front of the seat. Harley did make a slight change to the contour of the top of the DKW tank, adding a flat spot so that it could locate the fuel fill to the right and a rotary switch for the ignition on the left. The DKW tank had the fuel fill in the center. The S-125 and two related Harley lightweight models-the S-165 and the Hummer-were offered with either a battery ignition or a magneto ignition. Only models with the battery had the switch on the tank, but they all had the fuel fill to the right.

When Harley introduced its sporty K model in 1952, it had a new 4.4-gallon fuel tank. But the racing version, the KR, was fitted with the tank from the S-125, now rated at 2.1 gallons. The KR tank did not fit as low over the top tube, so it held a little more fuel and sat a little more proudly over the frame. The fuel fill was also moved to the center of the tank. The smaller tank looked right at home on a race bike. When a high-performance XLCH Sportster debuted in 1958, it also came equipped with the 2.1-gallon tank. The smaller tank thus became associated with racing and performance.

Old can always be new again at Harley-Davidson. After 62 years, the shape of this tank is essentially unchanged.

Old can always be new again at Harley-Davidson. After 62 years, the shape of this tank is

Beginning in 1972, almost all Sportster models had the 2.1-gallon tank. A 3.3-gallon tank with a similar shape came on the '81 XLS (the 2.1-gallon tank was an option), and these two tanks alternated as standard and optional on Sportster models for about a decade. In 1995 all XL 1200 models received a new 3.25-gallon tank. The XL 883 bikes continued with the 2.1-gallon tank until 1997, when the 3.25-gallon tank replaced it. And now the original 2.1-gallon "peanut" is back on the Forty-Eight, along with the "Sportster" script logo, which previously appeared in 1970-71 and in 1982. At Harley, old is always new.

Harley uses the term "peanut" to describe this tank, but always in lower case, and acknowledges that it's a street name that's probably a reference to its small capacity or compact physical size. Its clean lines and shape made it a popular chopper tank in the '60s, but it could take a Sportster rider less than 100 miles. Of course most riders were ready to get off a rigid-mounted Sportster before the "peanut" ran dry.