What are washers?
Washers are the metal discs in your trucks. They exist in two places:
1. Between the nut on the kingpin and the roadside (RS) bushing. This is the roadside washer.
2. Between the boardside (BS) bushing and the baseplate of the truck. This is the boardside washer.
Washer shapes
Washers come in a few different shapes, which can result in different feels
Cupped
Cupped washers have a lip around the outside of the washer which adds a more restrictive feel for the bushing. This can aid in kicking out slides, and most freeride setups tend to use some cupped washers.
Flat
Flat washers are the least restrictive, as they allow the sides of the bushing to deform more under load. These allow increasing the amount of lean. Setups focused on speed/grip tend to use flat washers.
Undersized flat
These washers tend to come on Aera trucks and are a little smaller than the diameter of the bushing. These are even less restrictive than the flat washers, as they allow a bit more deformation from the bushings.
Sleeved/Top hat
These washers include a small sleeve that slides around the kingpin, and adds more pre-load on the bushing when it is centered.
I've personally never used them, however Max Dubler has an article on why exactly these suck. I wouldn't go out of my way to spend extra $ for a machined part over the washers that come with the trucks or can be bought at the hardware store.
