F350 Super Duty vs 3500 Denali vs The Ike Pulling 30k lbs

Me Again wrote:

4x4ord wrote:

Me Again wrote:
4X4ord. 2 gear(2:1) in a Aisin and 4:10's out back is going to hold back a lot on the full exhaust brake setting. If the VGT exhaust brake makes 75HP of braking, then in 2 gear wouldn't that equal 600+ hp of braking at the rear wheels?

AT that weight, what speed would 600 hp hold.on that grade. You seem to be able to compute the numbers going up, so you should be able to computer them coming down. I know at 24,500 in 4th(1:1) and 3.42 out back, I had to step on the throttle once in a while on 6% grades to not slow down to much. Would that not be 225HP of braking at the rear wheels?

Torque is multiplied through gears not power.

So if torque is what gets a load moving would in not also be needed to slow down a load?

Sort of. It takes a force on the outer edge of the tires to hold the load back. The force on the outer circumference of the tire can be very easily approximated .... on a 6% grade the force is about 6% of the truck and trailer's weight. (actual value is 5.989) and on a 14% grade the force is about 14% of the weight of the truck/trailer. (actual value is 13.865%). (At steeper grades the error gets a little out of hand so on a 30% grade the actual calculation would be 28.73% and on a 100% grade the actual value would be 70.7%)

When the engine is holding back this force on the outer edge of the tire is producing torque on the rear axle. So the the radius of the tire is the length of the torque arm. So 6% of 24500 lbs is 1470 lbs of force. 1470 lbs x a 16 inch tire radius gives a torque of 23520 lb inches or 1960 lb ft on the rear axle. When this torque is moving backwards through the driveline gears it gets reduced. So the 3.42 axle reduces the torque to 573 lb ft of torque. The transmission could further reduce the torque. So in your case with the transmission in direct 573 lb ft would be required on the crankshaft to hold the truck from accelerating.

If you have 32" diameter tires your rear axle will be rotating 578 rpm at 55 mph. The reverse horsepower at the rear axle is 578 rpm x 1960 lbft of torque/5252 = 215 HP. Now the engine is going to be running 1977 rpm at 55 mph in 4th gear so the reverse HP at the crank is 573 lbft x 1977 rpm/5252 = 215 HP. (torque is multiplied through gears ... not power)

In actuality wind resistance and rolling resistance acting on the truck and trailer will reduce that value to something more like 115 HP. Then inefficiencies in the driveline might further reduce it to something like 100 reverse HP.