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Horsepower / Torque Calculation

Horsepower / Torque Calculation

Horsepower / Torque Calculation

I need to calculate the horsepower and torque required to move a given load at a given speed.  I've spec'd the problem as a steady-state calculation---xxx pounds at yyy mph, instead of one with acceleration from a dead stop.

I need some resource tips for (1)real world Cd for closed box long nose tractor trailers and (2)real world coefficient for rolling resistance.  My handbook gives some ballpark figures, but with all the advances in tires and aerodynamics I hope the forum can offer more realistic values.

RE: Horsepower / Torque Calculation

You want the amount of power required (I think), and torque isn't a measure of power.

Remember, whatever Cd and rolling resistance you come up with, they will be FAR worse that the values for a freight train, which is where 90+% of the stuff that trucks carry long-distance should be on.

RE: Horsepower / Torque Calculation

Trains not available in this market.  I need to calculate torque and power at a given roadspeed, and I'm aware that they are related measures.

RE: Horsepower / Torque Calculation

I have no data nor experience in the area of your specific question, so my 2 cents worth is merely about general principals.

Trains are great, when they go exactly where you needed to go, when you need to go there. So if no tripple handling and time delays are incured, trains are very efficient.

One reason for the popularity of road transport is that it is fast and point to point.

I don't know about other areas, but in Australia, road freight is heavily taxed via fuel tax and rail is subsidised, yet road freight useage is vastly more popular, except in very special circumstances such as from a mine to a port.

The initial infrustructure costs also weigh heavily against rail. I expect that there is also a heavy environmental, as well as financial cost for this infustructure.

Torque is meaningless in this calculation unless you also include overall gearing, then it is directly related to power, so why not use just power, unless you are also calculating drivetrain strength requirements.

Do you need to consider assending and decending hills, as this might be a significant variable.


RE: Horsepower / Torque Calculation

Thank you for your comments, especially about the Australian transportation system.

From my initial review of forces on a vehicle, the coefficient of rolling resistance x the normal force = the tangential force at the juncture of tire and road.  This force x the radius of the wheel + tire is the torque required at the axle.  So, it is not that I want to calculate torque, but simply that torque is what moves the vehicle.  From the torque and roadspeed and gearing and wheel + tire size, I will calculate horsepower required at the axle pinion gear.

RE: Horsepower / Torque Calculation

Crr is about .007 for a good truck tyre, but this is very dependent on speed and construction and road surface and load and inflation pressure etc etc etc.

CdA is probably best found by looking at SAE papers, I guess. 852257 and 2001 01 2072 certainly look relevant. The latter quotes a range of Cd of 0.65 to 0.9, frontal area basis. The shape of the prime mover seemed pretty irrelevant to their results, the gap between the cab and the trailer was much more important.

I suspect that skin friction drag will be more important than pressure recovery drag for this type of vehicle, by the way.

Hucho is probably a good reference book to get.


Greg Locock

RE: Horsepower / Torque Calculation

Thanks for the real world values and reference book.  I'll look it up in my company library.  I also appreciate your insight on skin friction drag, which I'll look into.

RE: Horsepower / Torque Calculation

i believe hp=torque x rpm over 5,252,  tho' i also saw it expressed hp=torque (in ft. lbs.)x distance in feet vehicle moves over time in minutes (usually 1) x 33,000 (a constant)

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