I don’t want to get bogged down in arguments over motorcycle front ends. Yes, telescopic forks are pro-dive, and, yes, they have stiction. But so far, not one of the many proposed alternatives has come close to replacing the telescopic.

Sometimes it's an advantage to have worked on junk because abused equipment has pushed my face into lots of interesting problems. The fork tests I'm about to describe are much easier to perform on a racebike because it's designed for easy and rapid service access. Streetbikes require you to fight your way through layers of amenities to reach the actual moving parts.

The test is simple in concept: to feel for completely free movement. Pull the fork legs, remove their springs, and hold each one by its axle clamp in a soft-jaw vise. Now move the fork through its whole travel, feeling for any binding or roughness. If the legs are smooth and free, assemble them into the fork crowns and add the front wheel and axle with all fasteners tightened normally. Stroke the fork again. If either fork clamp or tube is even slightly bent, you will feel resistance or binding.

A surprising number of forks that have seen service do resist or bind. At this point, I would put the bare tubes on widely separated V-blocks and rotate them under the finger of a dial gauge. Any lack of straightness will immediately appear. Even straight tubes, assembled into slightly bent crowns, may not be parallel. Check for parallel by laying the tubes, assembled into the crowns with pinch bolts tightened, across a surface plate or piece of heavy plate glass.

Because racers of the 1970s bought as few new parts as possible—other than crankshafts and pistons—we undertook activities that are frowned upon today: straightening bent parts. Unless you have good confidence in your skills, it’s best to replace bent parts rather than try to straighten them. The goal remains smooth, bind-free fork operation.

It’s best not to leave anything to chance with telescopic forks. They work hard, supporting 100 percent of the weight of machine, fuel, and rider during hardest braking.

In the damper-tube, right-side-up forks of the 1970s and ’80s, it was necessary to center the damper tube, typically retained in each fork slider by a central 10mm Allen screw inserted from below. That was done by loosening the Allen screw and compressing the fork leg—with spring removed, naturally—until it bottoms, then tightening the screw. Rotate the fork tube to check for interference; the hydraulic stop fits closely in the bottom of the leg.

It’s best not to leave anything to chance with telescopic forks. They work hard, supporting 100 percent of the weight of machine, fuel, and rider during hardest braking. They work much harder if the owner performs lots of pile-driver wheelies.

Construction of so-called upside-down forks and their dampers is different, but the same concerns for free movement remain important.

Andrea Dovizioso
Supple motion is possible, but sometimes you have to work for it. Inverted Öhlins forks with carbon-fiber outer upper tubes are now the standard in MotoGP, having replaced gold-anodized aluminum units. Andrea Dovizioso demonstrates maximum braking during practice at Le Mans.Courtesy of Ducati

GMD Computrack founder Greg McDonald makes another point. Suppose you are at the throttle-up spot in a corner, but as you ease into the grip and weight begins to transfer off the front onto the rear, a slight degree of excess friction doesn’t let the front suspension extend smoothly. Your front tire, already close to its grip limit at high lean angle and now robbed of a bit of its load, goes on strike.

I learned another biggie by trying to be too clever. The Kayaba fork on the 1976 Kawasaki KR750 two-stroke racer had separate slider bushings faced with slippery Teflon-like material. I measured their clearance and found 0.005 of an inch. With the parts solvent-cleaned and dry, they actually rattled! Aha, says I, I can eliminate some of this by slipping shim stock behind the bushings. Assembled, the fork was smooth and free.

But at the track test, my rider said, “Something’s wrong. The fork is sticking down.” As quickly as I could, I pulled the lower sliders and sheepishly removed the shim stock. At next practice, fork action was normal—good. Maybe 15 years later, I had the opportunity to ask an Öhlins engineer, “What’s a good fork internal clearance?”

“We find that 0.12mm works well. Otherwise tube deflection is enough to make it bind.” Thank you. That’s 0.005 of an inch.

By the way, those slippery bushings allowed Gary Nixon on Erv Kanemoto’s KR to approach Loudon’s turn 10 holding his back wheel a smooth 4–6 inches off the deck during braking. When I asked Nixon about it, he said he’d had no idea at the time that was happening.

A year later, on a Yamaha TZ750 without such bushings the smooth rear wheel lift approaching turn 10 was replaced by a jerky up and down. That’s stiction for you.

Supple fork action does not result from a fixed series of service-book steps, but rather from thinking about and finding the sources of excess friction and eliminating them.