Fork Free Piston Tech Specs (Updated 4/26/2020)
Current twin chamber (aka closed cartridge) forks are pressurized dampers similar to the rear shock of the motorcycle. KYB, Showa, & now WP twin chamber forks apply pressure against the cartridge rod (damping shaft) using a linear rate compression spring & free piston divider to account for the volume change of the oil displaced by the cartridge rod during compression. During 2007-2014 WP twin chamber forks (aka bladder fork) use a gas pressurized bladder (membrane) which has certain advantages and functional drawbacks.
The functioning difference between a 2006-2018 KYB free piston assembly design and Showa free piston is KYB pistons isolate the "pressure spring air" from the "outer chamber air". When the fork compresses the "outer chamber air spring" builds pressure, this pressure does not add to the free piston spring's pressure on the cartridge rod. In short, there are two separate air spring chambers, the free piston pressure spring air and the main spring outer chamber air. Rumor on the street is that KYB had to work around a Showa patent resulting in this design.
KYB Free Piston
Why do KYB free pistons often break, grenade, explode, crack, shatter, fracture, split,......???
- Faulty free piston seal allows piston to fill with fluid with no drain/blow-off
- Unbalanced valving
- Fast and/or heavy rider on suspension not setup for them
- Weak plastic piston from lack of fork maintenance
- Verify correct spring rates are installed for heavy riders
- Heavy riders on too soft (light) of springs bottom out and constantly generate high fork pressures
- If valving has been modified, review the setup for being unbalanced
- An overly soft basevalve and/or overly stiff midvalve
- Drill a "vent" hole in the free piston
- Install SDI Elite Free Piston
- Insurance for our faster and heavier riders
- Piston is vented (allowing for fluid to drain)
- In 2018 Yamaha introduced the updated Yamaha YZ450F that featured an updated KYB SSS/AOS fork. Inside the free piston now features two large circle vents and the plastic has been "beefed up" for strength. This update was later applied in 2019 to the Yamaha YZ250F.
- Body is made of stronger machined aluminum
- Vented air design like Showa (this is good!)
- Sold as individuals or pairs
- Strong, better, and cheaper then stock KYB free piston!
- Machined aluminum free piston body
- Free piston air chamber is part of the forks outer air chamber
- Able to drain fluid that may pass the seal
Showa Free Piston Updates
- Showa has produced a number of different/update closed cartridge forks since the 47mm CC fork. The 48mm dual cartridge dual spring fork, Showa SFF, Showa SFF TAC, and new Showa 49mm Spring fork all feature an updated now plastic free piston design. While still vented it is obvious Showa focused in increasing the area of the free piston that touches the oil.
- Pressurized nitrogen instead of spring
- Sealed off from outer chamber air spring
- Features more area of the oil to push against (aka mechanical advantage)
- Purge design to bleed off excess cartridge oil is slow compared to free piston design
- Low pressure and volume of member is sensitive to changes in temperature.
- This design has since been discounted by WP and replaced with a free piston system in the WP AER48 closed cartridge forks and all WP Cone Valve CC forks.