question on axle weight distribution for single axle volvo.

I have a 1995 volvo single axle tractor hooked to a 27 aluminum dump trailer with which I want to haul grain. The tractor nameplate says 12,000 lbs on the front axle and 21,000 on the rear drive axle. The tractor wheel base is 152 in or 12'8", The distance between the drive axle and the first trailer axle is 18'6" and the distance between the two trailer axles is 48" (4'). I calculated that the trailer (27') can hold about 18 tons (36,000 lbs) of grain full. I estimate the trailer to weigh less than 10,000 lbs empty. I don't know how much the tractor weighs empty but a good deal of that weigh is over the front axle.
According to the NY DOT bridge formula (W=500{(LxN/N-1)+12xN+36} I should be able to have 48,500 lbs on the rear drive axle and trailer with 34,000 lbs on the rear two axles.
Can anyone tell me whether I am in any danger of either overloading the drive axle (exceed 21,000 lbs) or violating the rules?
Thanks

 

Thanks for your reply, The problem here is that we are limited to 20,000 lb per axle or 9000 kg. I suspect that I cannot carry as much weight as the tandom trailer could hold and would like to estimate just how much I should limit the load. If I knew how much the tractor itself weighed I could estimate how much weight I could put on the axle. Of course there is the fact that some of the load is transferred to the front axle as well.