Harley Twin Shock & Softail… Explained
An Ultimate MotorCycling reader recently wrote-in: "How did Harley-Davidson’s twin-shock and Softail systems come to be?"
Answering this question can mean only one thing: more Harley-Davidson Motor Company history!
Prior to 1952, all Harley-Davidson motorcycles sported rigid frames with no rear suspension. This pounded both the rider and the bike’s components.
Spring-mounted seats were used to soften the dirt and pothole-laden roads of the time for the rider, but most offered no damping so they became less effective on bumpy roads and at higher speeds.
One unique design that continued even after the introduction of rear suspension was the single "post" spring in the frame. It attached to the underside of the seat and worked like a pogo stick.
In 1952, Harley-Davidson introduced the 750 K, a middleweight model with swinging-arm rear suspension.
Then, in the fall of 1957, the Harley-Davidson Duo-Glide was introduced, which featured a Hydra-Glide front end (introduced in 1948) joined with a swinging-arm rear suspension controlled by a hydraulically dampened shock absorber on each side.
This added 70 pounds to the motorcycle, but after a few smooth miles, riders would accept nothing less. Rigid frames quickly fell out of favor-much to the dismay of chiropractors, who began losing revenue at a rapid pace-except with custom motorcycle builders who liked the look and not the ride.
Mechanical engineer Bill Davis makes an appearance in this history lesson because he started experimenting with different suspension designs in the early 1970s that looked like a rigid frame, but actually employed a swinging arm with two shocks mounted under the seat.
Later enhancements to his design put the shocks under the transmission, freeing up more space for a horseshoe oil tank and a cleaner look. H-D then bought the design from Bill Davis, and the stories and rumors about this single transaction would likely support its own article.
Harley introduced the FXST Softail in 1984, followed by the FLST Heritage in 1986 and Bill’s beloved FLSTF Fat Boy in 1990. Until very recently, the Softail was Harley’s most popular line, and it deserves substantial credit for helping save The Motor Company from bankruptcy in the 1980s.
While the Softail and outboard shock suspensions work differently, both moderate road imperfections, and are compromises for styling sake.
The traditional twin-shock rear suspension seen on Sportster, V-Rod, Dyna, and Touring models uses a swingarm from the frame to the axle, and the bike is supported by a shock on each side of the wheel. Even a casual glance by the uninitiated at the back of a twin-shock bike will reveal how the system works.
For its suspension system, the Softail utilizes a cage that looks like an old rigid frame to replace the swingarm.
However, it still pivots in the front and supports a wheel in the rear. Instead of visible outboard shocks, Softails use a pair of shocks that operate in extension-the shocks get longer as the suspension compresses-to support and dampen the movement of the rear suspension.
In addition, springs on the Softail shock absorbers are hidden under a chrome cover, making the shock appear drastically different, even though it functions the same (although in reversed direction compared to a twin-shock system).
Sportbikes and off-road motorcycles now use single-shock rear suspensions very effectively with gains in simplicity, adjustability, and reduction of mass, but the style of twin rear shocks remains because of ease of production in inexpensive bikes, and the reality that function still follows form in the cruiser segment of the motorcycle market.