wind drag/weight conversion
I hope this doesn't stir anything up. I heard an interesting comparison idea from a friend the other day. He said with a truck pulling a trailer. For every foot the trailer extends above the roof l...
Forum Discussion
Horsepower requirements, weight vs wind, depend very much on speed and road grade.
Rolling resistance is a close to a constant, value depending on weight, tire type and pressure, road surface type, and a few other things that make it difficult to estimate. Because it is a constant, HP required is linear with speed.
Wind resistance is frontal area x a shape factor, both constant, but rises with the square of speed. Thus the hp required goes up with the cube of speed.
That's on level ground, at constant speed. To climb a grade, you need extra power, directly in proportion to weight (which is mass x acceleration of gravity). To accelerate, you need extra power in proportion to mass and rate of acceleration.
Depending on weight/mass (our culture tends to treat them as equivalent), frontal area, and the constants for shape and rolling resistance, the power required to pull the weight on wheels is at first dominant, but at some speed the power required to overcome aerodynamic drag rises to match it and from there becomes much more important.
Then there is the power required to run accessories, especially A/C and mechanical hydraulic pumps, which are load dependent, and others like cooling fans that are RPM dependent, and when you are sitting still these are dominant. Most calculators I've seen fudge these into rolling resistance, or ignore them, but a subcompact car at city traffic speeds might use more power to run the A/C than to move at 10-20 mph, but will use most accelerating in stop and go.
There are calculators out there to put these things together, and some built into spreadsheets will make pretty multi-curve graphs of the power needed but you are just guessing at some of the factors in the rolling resistance constant (which might only try to consider the tire part of that), and almost always leave you guessing at the aerodynamic drag coefficient which can range from 0.3 to more than 0.6 for typical motor vehicles and go beyond that range for exceptional ones.
Rolling resistance is a close to a constant, value depending on weight, tire type and pressure, road surface type, and a few other things that make it difficult to estimate. Because it is a constant, HP required is linear with speed.
Wind resistance is frontal area x a shape factor, both constant, but rises with the square of speed. Thus the hp required goes up with the cube of speed.
That's on level ground, at constant speed. To climb a grade, you need extra power, directly in proportion to weight (which is mass x acceleration of gravity). To accelerate, you need extra power in proportion to mass and rate of acceleration.
Depending on weight/mass (our culture tends to treat them as equivalent), frontal area, and the constants for shape and rolling resistance, the power required to pull the weight on wheels is at first dominant, but at some speed the power required to overcome aerodynamic drag rises to match it and from there becomes much more important.
Then there is the power required to run accessories, especially A/C and mechanical hydraulic pumps, which are load dependent, and others like cooling fans that are RPM dependent, and when you are sitting still these are dominant. Most calculators I've seen fudge these into rolling resistance, or ignore them, but a subcompact car at city traffic speeds might use more power to run the A/C than to move at 10-20 mph, but will use most accelerating in stop and go.
There are calculators out there to put these things together, and some built into spreadsheets will make pretty multi-curve graphs of the power needed but you are just guessing at some of the factors in the rolling resistance constant (which might only try to consider the tire part of that), and almost always leave you guessing at the aerodynamic drag coefficient which can range from 0.3 to more than 0.6 for typical motor vehicles and go beyond that range for exceptional ones.
