Technology: the cause of leaking fork legs
Every motocross racer has to deal with it: leaking fork seals. Of all the things that can go wrong on a dirt bike, the fork seals are the most annoying. The fork must be almost completely disassembled and you have no guarantee that they will not leak again after a few laps after installing new ones.
Fork seals have been failing since dirt bikes have existed. Despite this, they are incredibly well designed. They are self-lubricating and work in both directions. They have a special chemical composition that makes them strong and resilient. And best of all, modern fork seals last millions of cycles. Yes! However, fork seals don't leak without reason.
As motocross racers, we expect our fork seals to not leak, but we also demand that they move with minimal friction. In other words, they should not fit tightly against the fork legs. Why not? Friction is the enemy of smooth fork movements over bumps. If you reduce friction, your fork legs will glide smoothly through the uneven terrain of a motocross track, but you increase the chance of a leak in the fork legs because the seals are no longer as tight. However, we ask manufacturers to make light, durable, UV-resistant, flexible, compact seals that barely touch the fork legs. A few years ago that was impossible. Today, under the right circumstances, it is possible.
The fork seals are constantly changing. The latest seals are bi-directional with an internal metal support to create a rigid sealing structure. This design prevents pressure from pushing the edges of the seal away from the proper edge position on the fork steerer. Some seals use garter springs to maintain a good seal at low pressure, while keeping the seal in position when misaligned due to fork bend. The garter spring on the leading edge can also act as a secondary scraper to prevent dirt from entering the fork; however, garter springs reduce the fork's performance by adding extra friction.
Leaking fork seals are therefore not caused by design errors in the seals themselves. They are unintentionally caused by the chrome fork tube. Most modern fork tubes are manufactured with a special steel alloy that provides good resilience, high hardness and a low friction limit. The alloy creates a uniform structure in the tube that is just hard enough to bend before breaking. Producing high quality pipe starts with grinding the outer diameter to a high quality finish. A precision grinding operation minimizes varying diameters and creates a wide flat surface with small notches angled to the leading edge of the seal. These indentations are located at a microscopic level and are therefore not visible to the naked eye. They ensure that there is a thin lubricating film on the pipe surface. The oil layer improves the functioning of the seal and reduces friction. Contrary to popular belief, an overly smooth fork tube surface will strip oil from the surface, increasing friction and wear.
After the pipe surface has been prepared, a very thin layer of industrial hard chrome is applied to the abraded surface. The hard chrome provides a very hard and smooth surface layer that is resistant to wear. Surface treatments on the fork tube can include diamond-like carbon (DLC) and PVD (Physical Vapor Deposition) coatings. One of those popular PVD operations is applying a Ti coating or, say, Titanium Nitride coating. Although these types of coatings look very different, their general purpose is to improve the surface hardness of the outer layer and the lubrication of the pipe under high loads. The improved surface hardness maintains the ideal surface finish for a longer period of time. This has the additional advantage of extending the life of the seal and bushings. So the next time you think your seals have failed too quickly, remember that advanced seals rarely wear out with use. They wear out due to external contaminants entering the seal area.
The most common cause of seal failure can be found by running your fingers along the back of your fork leg. We bet you will find an imperfection in the chrome touching the lip of the seal. A leak will occur there quickly and by quickly we mean within a few minutes. Your fork can compress and decompress millions of times without leaking, but a small bump on the fork legs (for example due to stone chips) can cause a leak after a few laps of motocross. If you are used to buying quality seals, you do not have to pass the buck to the manufacturer afterwards. Checking your fork legs for imperfections should be a routine check for you, just like lubricating a chain or filling up with gasoline.
Photos: SKF and archive
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