@james
specific engine specs might be possible to look up if you get the specific name of the engine the vehicle is equipped with.
somewhat like this. AEC reliance had a multitude of engines, and the AH590 data was here.
http://middx.net/aec/bb/viewtopic.php?f=17&t=233@neroden
converting torque to tractive force is puzzling, IMO. Basically, the tire on a truck is never expected to slip while doing a standing start, at least on dry pavement, so the torque value is more of an indication of its towing ability.
@sdog
wheels and tires come in all sorts of sizes for trucks and buses, same as cars, unfortunately.
axle load isn't as necessary as locomotives as it is on trucks. There's only one axle doing all the work, while all the rest are load bearing, or at least, I've never seen a production truck that has more than one driving axle. The only time you'll see wheel slip on trucks is when they're on mud, sand or snow.
I'm not sure if the physics model does it, but engine torque and power curves don't work in a linear fashion. For nonperformance cars and vans, the engines and gears are usually set so that they give higher torque at low rpm and higher power at mid range rpm. This gives low engine noise when cruising along the highway in the highest gear, as the engine will be at mid rpm.
For full sized trucks with large displacement engines, which can have anywhere from 8 to 16 gears, the usable power band is very narrow and gives high torque and power inside the optimum range. Even with the high power, high torque engine with lots of gears to select from, the trucks still struggle to get up mountains on extended climbs. On the heavily loaded trucks, it'll be good if they can make 60km/h up a 5% gradient.
