MXA TECH SPEC: SOLVING FORK-SEAL CONUNDRUMS

Every hardcore motocross racer hates his fork seals. Of all the things that can go wrong on a motocross bike, fork seals are the most devastating—not just because fixing them is a giant pain, but because after you put brand-new seals in your forks, there is no guarantee that they won’t blow 15 minutes into your next ride.

Fork seals have failed for as long as forks have been on race bikes. So, why doesn’t some enterprising company make one that works? Surprise! The fork seals in your bike are incredible. They are self-lubricating. They work in both directions. They have specialized chemistry that makes them tough and resilient. Best of all, modern fork seals are rated for millions of cycles. Millions! Fork seals don’t leak without a reason.

As racers, we expect our fork seals not to leak, yet we also require them to move with minimal “stiction.” In other words, they can’t fit tightly against the fork legs. Why not? Stiction is the enemy of fluid movement of the forks in bumps. Reduce stiction and your forks will glide through the clatter of a racetrack’s terrain; however, reducing stiction increases the potential for a fork-seal leak, because the seals aren’t as tight. Paradoxically, we are asking the fork-seal manufacturers to build lightweight, long-lasting, UV-light-resistant, tear-proof, flexible, compact fork seals that barely touch the fork legs. A few years ago, it couldn’t be done. Today, under the right circumstances, it can be done.

Fork-seal designs are constantly changing. Most current seals are bidirectional with an internal metal backing to create a firm seal structure. This design prevents the pressure from extruding the edges of the seal away from proper edge position on the fork tube. Some seals use garter springs to keep a good seal at low pressures while maintaining seal position when misaligned due to fork flex. The garter spring on the leading edge can also act as a secondary scraper to prevent contamination from entering the fork; however, garter springs reduce fork performance by adding friction.

Fork oil running down the left fork leg is not only bad for your forks, but also for your brakes.

In defense of the rubber rings, the majority of fork-seal leaks are not caused by design flaws in the fork seals; they are caused, inadvertently, by the fork tube’s chrome plating. Most modern fork tubes are manufactured using a special steel alloy that creates good resilience, high hardness and ultimate yield strength. The alloy creates a uniform grain structure in the tube that is hardened just enough to bend before it breaks. Producing a high-quality tube starts with grinding the outer diameter to a high-quality finish. A precise grind minimizes wavy diameters and, at a microscopic level, creates a broad, flat surface with small troughs or valleys that lay at an angle to the seal’s leading edge. These valleys allow for a thin film of lubrication to be present on the tube’s surface. That layer of oil improves seal function and reduces friction. Contrary to popular opinion, if a fork tube’s surface is too smooth, the seal will remove the oil from the surface, increasing friction and wear. 

After the tube surface is prepared, a very thin flash of industrial hard chrome is applied to the ground-down surface. Chrome, when applied very thinly, will mirror the prepared surface under it. The hard chrome produces a very hard and slick surface coating that will resist wear. Surface treatments on the fork tube can include diamond-like carbon (DLC) and physical vapor deposition (PVD) coatings. While these types of coatings vary greatly in appearance, their general aim is to improve the surface hardness of the outer layer and the lubricity of the tube under high loads. The improved surface hardness maintains the ideal surface finish for a longer period of time. This comes with the added benefit of improving the longevity of the seal and bushings.

So, the next time you think your seals failed too soon, remember that state-of-the-art seals rarely wear out from use. They wear out from outside contaminants egressing the seal’s territory. And, the most common cause of seal failure can be found by running your fingers up the back of your fork leg. We bet you’ll find that ding in the chrome that nicked the seal’s lip.

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