Wheel component parts!?

[AlX]

a] i need to know what is the plastic washer thing that gets installed with the wheels and stops the wheel from shaking in high speed!?

b] what is the part that sits or get attached to the centre of the wheel (dome) or somthing and where can i buy them from?

thanks

[AlX]

wheel inserts

http://www.eastsidekartcentre.com.au/pdfs/wheelhubs.pdf