Gas pressure shock absorber with external reservoir.
This technology keeps the shock absorber oil and gas always separate, so the oil cannot foam and the damping remains stable. The reservoir in the external expansion tank ensures perfect thermal stability.
Specially developed for the Vespa PX
Very tight but not hard road holding due to the neatly tuned load capacity of the spring.
The rebound and compression stages of the damping are perfectly tuned for sporty driving while at the same time providing good comfort
Steplessly adjustable spring preload
Pair with matching front here https://pistonported.com/shock-absorber-front-yss-pro-x-vespa-p-px/