2 way damping is a major suspension improvement. Ariel forks are only rebound damped, and the design is such that the rebound damping does not work on the initial part of the rebound stroke. The standard damping relies on oil being sucked through the small hole(s) in the stanchion above the bottom bush on the compression stroke. The oil then floods around the outside of the stanchion, between the upper and lower bushes. On the rebound stroke the leg moves downward moving the bushes closer together and forcing the oil back through the hole(s) in the stanchion into the centre of the stanchion. It is the controlled flow of oil through those orifices that provides rebound damping. However, as the design relies on suction to pull the oil into place for the rebound damping, the outer gallery never completely fills, so there is no damping on the initial part of the rebound stroke.
Fitting the Dow type dampers not only provides compression damping, it also overcomes the inadequate rebound damping, as the oil is then forced into the outer rebound gallery which is then properly filled.
The only downside of the Dow damper is that the ride will become a harsher over potholes or rough surfaces potholes. But high speed stability is greatly improved and fork dive under braking is reduced.
The Dow dampers work best with a thinner oil. I use ATF.
It is also possible to adapt cartridge emulators to the Ariel fork which provides adaptive compression damping, giving the best of both worlds, but even when fitted (a tricky job involving some machining) numerous fork strip downs are required to get the damping setting correct.
The Goff springs are ideal for the 4.