About Those Fork Seals…

While I was reading about new air forks for off-road use from KYB and Showa, I also noticed discussion of replacement of original-equipment seals with low-stiction alternatives from veteran bearing manufacturer SKF. When I was at the recent AIMExpo trade show in Orlando, Florida, I stopped by the SKF display to lay hands on a pair of fork tubes, one with original-equipment seals and the other with SKF's distinctive pale-green replacements.

It was quite noticeable that the SKF-sealed leg had less stiction (starting friction) and less sliding friction than the original. I asked about the material and was told that its properties were inherent in the rubber rather than, for example, coming from the common coating of the seal lip with brown Rulon (a heavy-duty Teflon-like material applied to engine shaft seal lips since 1969).

A bit of Internet research revealed that special low-friction/high-abrasion-resistance seals are made for the oil industry’s down-hole “mud motors” and other grim applications. Such seals are generally made of NBR (Nitrile Butadiene Styrene) base material because of its outstanding resistance to swelling and degradation in petroleum-based liquids, as well as for its lower friction. This material may be mixed with an internal lubricant, such as Teflon or moly disulfide, or other low-friction polymers may be blended with the base NBR. Some enterprising person, looking for low-stiction off-road fork seals, hit upon this family of materials.

SKF also offer split outer seals that act as an easily replaced “first line of defense” against mud and dust on the fork tube, which extends the life of the inner seal (whose replacement requires disassembly of the leg).

I thought about seal friction. Motorcyclists mostly want rubber to display maximum friction in the form of tire treads, but that is the very opposite of what is desired from fork seals. Suspension must allow supple up-and-down movement of the wheels to keep them in contact with terrain, but seal or bearing stiction directly opposes this.

We are glad to have SKF’s contribution to our sport.