FORK WORKS: PRO CIRCUIT YZ450F KAYABA SSS RE-VALVE

When MXA set out to build our Dean Wilson Replica Yamaha YZ450F (August 2017 issue), we wanted to duplicate Dean’s bike as closely as possible. We got off to a good start when Mitch Payton gave us Dean’s hopped-up YZ450F engine. Split Designs sent us Dean’s graphics. Dubya supplied identical Kite wheels. ARC gave us the levers that Dean used. VP supplied Dean’s fuel of choice. Cycra sent us the plastic. Moto Seat provided the saddle, and Galfer supplied the rotors. But, the sticking point was Dean’s Kayaba works suspension. The Kayaba AOSS Factory spring forks cost $4500—and we would need $900 Xtrig ROCS triple clamps to mount them to our bike. Plus, we needed the Kayaba works shock also. We were stymied by the works components, so we had Pro Circuit’s Bones Bacon re-valve the stock Yamaha Kayaba SSS forks and shocks as a bailout, just in case we couldn’t track down works suspension. But then Technical Touch, the supplier of works Kayaba parts in the USA, offered us the AOSS Factory kit forks, shock and Xtrig triple clamps to complete the project. Bone’s re-valved suspension was put in a corner and forgotten.

But, after the test was over and we returned the borrowed Kayaba works suspension, we dusted off Bone’s modified Kayaba SSS suspension and put it back on the bike. The test riders were disappointed, because the Kayaba works suspension was terrific, but they sucked it up. After all, Kayaba’s production-based SSS suspension is nothing to sniff at. It has been the highest-rated OEM suspension for the last 11 years. The Kayaba SSS spring fork far outshines all the current air-fork offerings and works for a wide range of riders, skill levels and track layouts. But, Bones promised us that he could make the SSS components as good as the Kayaba AOSS Factory spring-kit fork—minus the exotic coatings and metallurgy.

WE RACED THE KAYABA KYB FACTORY KIT SUSPENSION AND PRO CIRCUIT’S MODIFIED FORK ON THE SAME TRACK ON CONSECUTIVE WEEKENDS—AND THE TEST RIDERS SAID THAT IF YOU CLOSED YOUR EYES, YOU COULDN’T TELL THE DIFFERENCE.

The MXA test riders were in for a surprise. They were thrilled with the Pro Circuit suspension modifications. Bones swapped out the stock 0.51 kg/mm fork springs for the slightly heavier 5.2 kg/mm springs and installed Pro Circuit’s National spring tubes. The aluminum National tubes control the oil flow and replace the fragile stock plastic parts. Bones focused on the Kayaba SSS fork’s compression stack with the goal of holding the forks higher in their stroke to help alleviate any mid-stroke spike. The results were immediately noticeable. The SSS forks soaked up the big hits, tracked great and absorbed the nasty stuff without a wiggle.

The Kayaba shock received a larger shock bladder, high-volume bladder cap and stiffer 5.8 kg/mm spring (versus the stock 5.7 spring). The valving internals were modified to work hand in hand with Pro Circuit’s longer link arm, which allowed us to accelerate through rough, sharp-edged bumps without the rebound kick. For comparison, we raced the Kayaba KYB Factory Kit suspension and Pro Circuit’s modified fork on the same track on consecutive weekends—and the test riders said that if you closed your eyes, you couldn’t tell the difference.

What does it cost? The complete re-valve, including fork, shock, oil seals, dust seals, fork bushings and fluids, came to $1375. For more information contact Pro Circuit Racing at www.procircuit.com.or (951) 738-8050.