Simple math, backed up by real life experience over a span of 22 years here
Doubles at 80K, two axle tractor. Each axle handles average of 16K lbs
Triples at 105500, two axle tractor. Each axle handles just over 15K lbs
9 axles at 129K, each axle handles 14K.
In actuality, the tractor doesn't do as much braking as the trailing equipment, so as you add more trailing axles, for the comparative incremental increase in gross weight, the increased braking ability greatly increases.
Just my $.02 garnered from my 35 years of trucking.
