G`day All
There is couple of things to keep in mind with the larger trucks , generally above the 6.5 ton or 14500 lbs. It to do with how they stop. The braking systems are more complex, they are designed to control big weights. It would very expensive as mentioned they are very different to the lighter trucks. The trucks would clearly need on going parts and service support for their life cycle , this would be a significant factor to pencil in.
Changing the system to comply with federal road requirements is no small thing. The testing processes is designed to be difficult to pass ( as one would hope) and runs through various failures , meaning they intentionally make components inoperable and the vehicle still must stop under a given parameter.
What we do with FUSO and Isuzu to complete them as 4x4 vehicles goes through the same test at a Federal sanctioned third part facility as any vehicle manufacturer , be it Chev, Mercedes or EarthCruiser.
It is about testing the braking "system" , so If any component has been altered after it was testing by the vehicle manufacturer ( EarthCruiser is a vehicle manufacturer in the eyes of Government ) it is no longer in compliance and the individual or company that made that change ( say did a 4x4 convert ) they are now responsible for what ever happens as long as that vehicle drives on a public road.
Buying bits from the manufacturer and bolting them on is a great big grey area , if the vehicle manufacturers did not have 4x4 truck / car tested as a 4x4 in the USA ( sure , might have one in Aus or Europe) But IF the braking system has not been tested to the US standard and carry the certification numbers with the Federal government it MIGHT get very ugly. Only a matter of time before that one gets tested in court if it has not already, cant see why a vehicle manufacturer would agree to unauthorized modifications to their vehicles and be of support before a judge.
Steering geometry, suspension and ride quality are different subjects for another day.
Best
Lance
PS- The braking systems we use is proprietary to EarthCruiser, yes we do have well known vendors we get axles from , braking systems and certification is very different conversation .