THE TELESCOPIC FORK
FUNDAMENTALS
Technology has allowed an old concept to grow with the job
KEVIN CAMERON
Look at the bikes in today’s onor off-road competition, and you will find telescopic forks on all of them. A “tele” is structurally simple and has continued to develop new strengths.
Critics correctly note that its sliding motion generates “stiction”— initial resistance to movement—its angled legs are pro-dive, and it inefficiently transmits stress up the telescoping legs from the tire footprint to a high steering head before distributing it back down into the rest of the chassis.
Each time pivoted-link front ends have shown shorter braking distances, tele builders have cut stiction with harder, smoother forktube coatings such as TiC (gold) and DLC (black) plus slippery slider bushings. Pro-dive? Yes, but the anti-dive fad of the 1980s ended when Yamaha showed that dive, by lowering a bike’s center-of-mass height, actually shortened braking distances. When “advanced” forkless designs such as Bimota’s Tesi efficiently fed stress straight back from the front axle direct to the frame, tele builders increased tube size—a 10 percent increase in diameter boosts stiffness 46 percent-flipped the fork upside-down in 1989 to put the outer tubes at the top where the peak bending load was, and in the past two years MotoGP bikes have adopted ultrastiff carbon-fiber upper tubes.
A telescopic fork embodies both structural and suspension functions. The structure is the telescoping tubes themselves, the upper pair joined at the top by the fork crowns, and the lower pair at the bottom by the wheel axle. The suspension function is their telescoping action, resisted by internal or external springs, and damped—that is, prevented from bouncing—by using the telescoping motion to pump fluid, usually an oil of about 15W viscosity, through orifices.
The history of teles began late in the 19th century with the need to keep artillery pointed at enemy targets by letting just the barrel recoil against telescoping hydraulic dampers, rather than having the entire gun leap backward haphazardly. Almost all the concepts found in modern teleforks can be traced to artillery-recoil buffers. When BMW began work with teleforks in the early 1930s, it clearly examined recoil-system patents. Earlier teleforks had existed—Britain’s Scott two-strokes had them—but it was BMW that applied intensive research to their development. After the war, several manufacturers quietly bought BMW R75 military sidecar rigs to copy the telefork— strength, long travel, and welldesigned damping. For AMC, BMW factory rider (1937-’39) Jock West tucked one of its racing teles into his luggage on his just-in-time return to Britain. (World War II began September 1939.)
“For high performance and adaptability, the cartridge damper has emerged as the most versatile choice. ”
There has been variety and evolution in telescopic-fork dampers. The four basic schemes are: the damperrod type, the cartridge damper, the through-rod damper, and the bushing-to-bushing damping. For high performance and adaptability, the cartridge damper has emerged as the most versatile choice.
For many years, fork dampers were “atmospheric,” meaning that unpressurized damper oil partly filled each fork leg to keep the damping chamber submerged.
Such dampers usually employed fixed-orifice damping combined with a one-way valve that placed most of the damping on the less-upsetting rebound stroke. In the 1970s, gas-pressurized rearsuspension dampers came into use, and there were several attempts in the 1990s to apply them to fork dampers. The purpose is to keep damping fluid under enough pressure to prevent its being pulled apart (cavitated) as damping chambers refill. This is implemented by allowing gas pressure behind a piston or bladder to maintain the necessary pressure on the fluid.
Because fixed-orifice dampers became near-rigid at high suspension velocity—this is called “orifice limitation”—sudden pavement transitions such as Daytona’s infield-to-banking turn 5 produced serious chassis upset. Variable orifices overcame this by enlarging as damper velocity rose, keeping damping force proportional to damper-piston velocity. The mature form is the modern washer stack: a thin steel spring washer, clamped at either its OD or ID, covers a ring of holes.
As a bump or the spring pushes damping oil past such a washer, the free edge of the washer—plus washers and spacers stacked with it to modify its action—is elastically deflected, enlarging the flow path. Much of the performance of modern suspension is in the art of creating the right stack.
Separate stacks control compression-rebound movement, placing most of the damping on rebound as always. Some forks today are built with compression damping in one leg and rebound in the other.
The modern fork is externally adjustable via spring preload—part of what sets ride height along with physically moving the fork leg in the triple clamps—and the damping “clickers” for compression and rebound trim. Larger damping changes require restacking the washers.
Potential alternatives to telescopic forks come and go, but the tele lives on—perhaps because it is in harmony with the essential simplicity of the motorcycle.
"The purpose is to keep damping fluid under enough pressure to prevent its being pulled apart (cavitated) as damping chambers refill.”
