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Edited Date/Time
6/20/2018 9:05pm
So I saw some photos of the new suspension and it got me wondering.
A couple of years ago Showa and Ohlins used TiN (gold mainly) coating on their top of the line suspension kits. At the same time KYB and WP used DLC.
Then Ohlins, followed by Showa made a switch to DLC coatings as well, but it seems like this year both Showa and Ohlins went back to TiN coating.
Do you think there's a performance related reasoning behind that, or purely economical?
When I asked around, TiN coating was cheaper than DLC. Could it be that top level suspension manufacturers made a switch to TiN in order to lower production costs?
A couple of years ago Showa and Ohlins used TiN (gold mainly) coating on their top of the line suspension kits. At the same time KYB and WP used DLC.
Then Ohlins, followed by Showa made a switch to DLC coatings as well, but it seems like this year both Showa and Ohlins went back to TiN coating.
Do you think there's a performance related reasoning behind that, or purely economical?
When I asked around, TiN coating was cheaper than DLC. Could it be that top level suspension manufacturers made a switch to TiN in order to lower production costs?
There was supposed to be a Vital shootout on kit suspension, I think that's where the pic was taken from. Unfortunately the whole shootout idea went to hell as soon as people realized that ML can't bottom out the a-kit suspension.
The Shop
However,
DLC coating has approximately 375% lower coefficient of friction when compared to TiN.
Does that translate to a drastic difference in suspension performance, no. Could you do a statistical analysis of the two coatings side by side on a test stand, probably, but I have yet to see any data like that. Can you feel the difference between DLC & TiN at the handlebars, no (at least not the average joe)
DLC coating is more complex than a chroming process, but less complex than TiN. This is why it is now common for DLC to be on OEM forks but not TiN. Part of what you are paying for with aftermarket suspension is the extra manufacturing time and cost resulting from the TiN process.
TiN
4-12 um thickness
2400-3200 Hardness
0.35 friction coefficient
DLC
0.5-2.5 um thickness
5000-9000 Hardness
0.1 friction coefficient
Here are some friction coefficients of interest (all against steel):
Hard Chrome: 0.39
TiN: 0.35
TiAlN: 0.35
AlTiN: 0.35
ZrN: 0.35
TiCN: 0.30
Metal based DLC: 0.1 (this is what motocross tubes use)
Amorphous Carbon DLC: 0.05-0.1 (dramatically more expensive)
Teflon: 0.04
I suspect that the difference in friction is not large enough to matter all that much between DLC and TiN for our purposes, but if you're after absolute performance then DLC is the way to go.
TiN is cheaper than DLC, at about half the cost, but I doubt that's the deciding factor here. It's most likely aesthetics more than anything.
SKF seals with fresh bushings and a good oil are the best bang for the buck.
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.
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!
" 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). "
There's a moment acting on the fork leg in those situations. So the forces on the PTFE bushings change depend on where the fork is in it's travel. The bushings will be close together (less mechanical advantage over moment) when the fork is extended, so the friction would be highest under off-axis loads at the top of the travel.
We could guess that the initial application of the front brake (especially in braking bumps at the end of a fast straight) is one of those places. Jumping down a downhill where you land on the front brake is a good example.
Slapper landings would be the opposite direction of the same kind of off-axis loading where we can feel the extra friction.
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