cwtoyota wrote:
I agree with that statement about the friction difference being too small to matter on a fork or shock.
We have these coatings running against PTFE bushings and rubber seals.
Since coefficient of friction is multiplied by the normal force (orthoganal or perpendicular to the surface), you can think about how little that force is compared to the force compressing the suspension.
I haven't done any calculations, but having worked on plenty of suspension over the years I think the items below add much more to static friction ("sticktion") in the suspension than the coating on the tube/shaft.
1) In a spring fork, the un-coated spring bumps into the inner surface of the steel lower fork leg and rubs on that surface.
2) Fork seals add a great deal of friction. I think switching seals makes the most difference, and likely the only difference big enough to feel.
Here's a way to prove that the coatings aren't worth the money (in terms of performance gained):
Take a set of stock forks (chromed tubes) apart, clean everything and install new OEM black rubber seals and leave out the internals (spring, damper, etc.). If you compress the fork, and hang it by the upper tube, the seal friction will hold the weight of the inner tube and the fork-lug in most OEM forks.
Replace those seals with a new set of seals like the green ones SKF makes and the friction will often be low enough that the weight of the inner tube and fork lug will overcome the friction force.
Doing the same test comparison with a set of coated tubes (DLC, TiN, etc) and you'll find the results are about the same. You get more performance from installing the SKF seals for $65.00 than you do with the coatings that cost at least ten times more.
Indeed, the best bang for your buck is in quality seals and proper maintenance schedule. But if you want the best, then coatings are still the way to go (in addition to quality seals and proper maintenance schedule).
Your points on stiction reduction are spot-on, but the real benefit to coatings is the reduction of sliding friction. While a fork compressing perfectly along its axis will have very low sliding friction, this is rarely the case in motocross. The attitude of the bike, shape of the bump, direction of travel, etc. all play a role in the direction of the force vector applied to the fork. Any deviation of that vector from the fork axis will produce frictional forces (as some component of the vector is normal to the fork axis). And those forces can be quite high since there is no compliance in any direction but the fork axis.
You can argue that high friction is beneficial (or negligible) in the case of a huge hit that's going to bottom the forks anyway, but for any other bumps you want as little friction as possible. Friction is a form of damping; but it is force related, and therefore bump intensity and direction related (how far misaligned the force vector is from the fork axis). This isn't something we want. We want to control damping in relation to fork speed and position (which is what the valving is for).
End of the day, you want as little friction as possible. Coatings help this, though there are other methods of reducing friction that help for less money. If you're on a budget, skip the coatings. But if you want the best, go for it!
