Kayaba PSF forks and Showa SFF 2013

Hi all,
I know it’s just on 2013 MX bikes but i thought you might all be interested in what’s happening in the suspension world in 2013 as MX usually trickles down to offroad bikes in time.

What we are talking about here is the PSF (Pneumatic spring fork) Air fork from Kayaba (KYB) and the Showa SFF (single function fork)
KYB PSF has no fork springs, it’s air. You buy a pump and check the fork air pressure like you check your tyres. It’s light as you save 1.5kg odd in springs, it means the cartridge (dampening) can be much bigger as it doesn’t need to fit inside the spring and you can change spring rate by changing air pressure.
The Showa 47mm SFF (single function fork) came out in 2011 on the KX250F. It uses one spring in the right fork leg and dampening character in the left leg. It’s lighter, makes for only one fork to revalve and one spring to buy plus you have adjustable preload on a twin chanber fork. Cool.
KYB PSF Initially fitted to the CRF450 and KX450F in 2013, will expand to the CRF250 in 2014 and YZ450/250 in 14 or 15. Yamaha who own KYB claim the frame needs to be totally redesigned for the PSF fork so they will not use it on the current frame bikes.
Essentially it’s the current 48mm fork with the main spring and the inner chamber spring removed. This allows a bigger cartridge (32mm ID) to be fitted. This is the same cartridge size we had in 1998-2004 with KYB in the “cylinder valve” fork.

In last model of the old fork (01-04) used a 2 piece rebound piston, 8mmm bore, 32mm OD. This was an attempt to stop the mid valve shims from distorting in the earlier forks, 1998-2000. It uses a one piece piston band that is a relatively tight fit on the cartridge tube. In the PSF fork the same band is used and seals very well for the first 2-3 hours use but starts wearing and causing excessive bleed after that. The piston has a large bleed hole drilled as the tight sealing band causes no leakage new.
The mid speed shims are 11.2mm as were used in the old forks. 12mm ID shims have been trialed by pro Circuit and found to cause too much hi speed dampening so they have gone back to 11.2mm
The compression piston is 32mm OD with an 8mm bore with 28mm initial valves. The 8mm shaft was used as the nut could be torque to 9N not 5N as the 6mm post to resolve the problem of the nut coming loose when the assembly is removed.

Traditionally the reason air has not been used is initially it takes a lot of force to make the fork move. This is still the problem in Mountain bike forks to this day. This is overcome by a balance spring in the PSF, it’s essentially a big top-out spring to oppose the air spring. The PSF uses a 0.32kg balance spring. There is a 0.31,0.33 and 0.34 available. KYB have done limited testing with longer springs but the fork felt like it had a step in it. Testing that part was cancelled as they could not resolve the step issue.
The fork is designed to use between 32 and 36psi. At 35-36 psi the friction on the seals (as the pressure behind them energises the seal lips) becomes unacceptable for MX use but will be OK for SX as the valving is stiff anyway. The seals are a new design that uses the air pressure to energise the seal.

Obvious areas of weakness in the design. 2 part rebound piston. It was a patch to resolve the shims bending like a taco shell in the 98-02 forks and it’s still a compromise. KYB have designed a new piston that will be production in 2014. The current piston band wears out after 2 hours and causes excessive bleed. This problem exists with the current KYB twin chamber fork, the rebound piston band wears, the rebound gets fast and the fork starts “topping out”
The balance springs are very hard to buy from KYB and will need to be changed to the 0.34kg/mm units for using high pressure to try and keep some initial plushness. We are making springs.
There is a big scope for sub air tanks on the PSF.
There is a simple program to show the force curve of the fork springs Vs the balance spring, we are getting a copy shortly. Meanwhile http://www.vitalmx.com/videos/features/Inside-the-KYB-PSF-Fork-with-Dan-Worley,6573/GuyB,64

For Showa SFF 2013 is the first major revision. 48mm tubes (from 47mm) shorter main spring (from 743mm to 663mm). Uses same seals as KYB 48mm. The fork is now being used on the RMZ 250 and 450. There is an SFF Air fork that is a factory/pro circuit A kit fork for 2013 and rumored to be production on the RMZ’s for 2014.

We will have balance springs for the PSF by November and will start testing different rates and lengths. Getting to fork to feel supple for 100kg guys racing MX will be the biggest hurdle. We are making some internal parts as well.