I just picked up a WR450F and had the same questions. Most dirt bikes are under 300lbs so I figure no problem. After reading all these technical replies it's pretty obvious that 500lb tongue weight does not equal 500lbs on the hitch. I found this on the internet from a site that specializes in hitch mounted bike carriers. They have dual and even triple carriers, they also have one that mounts to the bottom of the motor/frame then pivots to within inches of the rear of the vehicle, effectively reducing the moment or the arm whichever one is the distance. No matter what rack you choose I'd avoid the giant ruts and speed bumps at anything other than a crawl.
Here is their easy to read chart that gives tongue rating on the left and a safe load on the right.
Specifications
Hitch/Vehicle Tongue-Weight Capacity (lbs)
*Use lowest figure Max. Weight Capacity of Carrier (lbs)
300 180
350 225
500 350
750 or more 400 (max.)
Copied from:
Boss Hitch Mounted Motorcycle Carrier
Since 500 tongue is close to 350 actual I went with the aluminum rack that weighs less than 40lbs not the 82lb steel rack that would put my total weight close to the limit.
I plan on using this rack a lot and will let you all know if my new dirt bike turns into street debris any time soon.