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sbphoto · 07-14-2011, 01:52 PM

Hello again. I have been trying to look at power requirements and the related fuel efficiency as it relates to choices of motorhome construction. Here is some interesting stuff:

Assuming a level road and correctly inflated tires, the power needed to move a vehicle (I used 115 square feet as the frontal area and 16000 pounds as the weight) increases as the cube of velocity times the drag coefficient. For a big truck the coefficient of drag is a factor we can modify with design and typically ranges between 0.77 to 1.2. Somewhat non-real world, wishful thinking could get this down to closer to 0.6. For comparison, a normal car would have drag coefficients somewhere around 0.35 to 0.45 and sports cars 0.25 to 0.35.

Anyway, here are the calculations for HP needed to move a rig at various speeds (first number) for coefficient of drag 0.77 then 1.2

45MPH: Cd 0.77=>73HP Cd 1.2=>103HP
50MPH: Cd 0.77=>95HP Cd 1.2=>136HP
55MPH: Cd 0.77=>122HP Cd 1.2=>176HP
60MPH: Cd 0.77=>153HP Cd 1.2=>223HP
65MPH: Cd 0.77=>190HP Cd 1.2=>279HP
70MPH: Cd 0.77=>233HP Cd 1.2=>344HP

Note that this does not factor in hill climbing or head winds.

So, at highway speeds we can influence our power needs (and the related fuel use) by 30 to 100HP depending on how and where we drive. I did not run the numbers on different vehicle weights but changes in weight relates more to starting, stopping, and hill climbing. From what I looked at, it is not very difficult, by design modifications, to lower the drag coefficient (aerodynamics) from 1.1 to 0.85 which are real world values that apply to rigs belonging to members of this forum.

If you prefer to think in terms of energy, like on your electric bill, 100HP translates into 75kW.

Comments?

07-14-2011, 01:52 PM	#5
sbphoto Junior Member Join Date: Jun 2011 Posts: 11	power needed for air resistance Hello again. I have been trying to look at power requirements and the related fuel efficiency as it relates to choices of motorhome construction. Here is some interesting stuff: Assuming a level road and correctly inflated tires, the power needed to move a vehicle (I used 115 square feet as the frontal area and 16000 pounds as the weight) increases as the cube of velocity times the drag coefficient. For a big truck the coefficient of drag is a factor we can modify with design and typically ranges between 0.77 to 1.2. Somewhat non-real world, wishful thinking could get this down to closer to 0.6. For comparison, a normal car would have drag coefficients somewhere around 0.35 to 0.45 and sports cars 0.25 to 0.35. Anyway, here are the calculations for HP needed to move a rig at various speeds (first number) for coefficient of drag 0.77 then 1.2 45MPH: Cd 0.77=>73HP Cd 1.2=>103HP 50MPH: Cd 0.77=>95HP Cd 1.2=>136HP 55MPH: Cd 0.77=>122HP Cd 1.2=>176HP 60MPH: Cd 0.77=>153HP Cd 1.2=>223HP 65MPH: Cd 0.77=>190HP Cd 1.2=>279HP 70MPH: Cd 0.77=>233HP Cd 1.2=>344HP Note that this does not factor in hill climbing or head winds. So, at highway speeds we can influence our power needs (and the related fuel use) by 30 to 100HP depending on how and where we drive. I did not run the numbers on different vehicle weights but changes in weight relates more to starting, stopping, and hill climbing. From what I looked at, it is not very difficult, by design modifications, to lower the drag coefficient (aerodynamics) from 1.1 to 0.85 which are real world values that apply to rigs belonging to members of this forum. If you prefer to think in terms of energy, like on your electric bill, 100HP translates into 75kW. Comments?