Minimum speed for aero advantages

[NeilBlanchard]

To check brake drag, you can jack up a wheel and give it a spin -- in neutral on the drive wheels of course.

[Fat Charlie]

Originally Posted by skyl4rk

A commuter on a bicycle slogging along at 10mph will benefit from aeromods.

Even at slower speeds, a headwind can cause aeromods to make a big difference.

I took a small sailboat out a couple of times- it doesn't take much wind to move a boat. Most of my water time is in a canoe or kayak, but either way I've got a healthy respect for the amount of force that a slight breeze has. Basically if sticking your hand out the window makes you feel pressure, you've got to think about multiplying that rearward force by your frontal area.

[gone-ot]

...an air-speed of 30 mph while driving 30 mph has the equivalent aero-drag of driving at 60 mph through still air.

...conversely, driving 60 mph with a 30 mph tail wind, has the equivalent aero-drag of driving at only 30 mph through still air...obviously a BIG difference.

[ryannoe]

My 2 cents...

Take the weight of your vehicle and multiply it times gravity times sine of any slope you may drive on.

Example:
1200 kg car (~2600 lb) x 9.81 (gravity) x sin(3 degrees) = ~620 N
If you were traveling at 30 mph (~13 m/s) power consumption = 620 N * 13 m/s = 8000 watts = ~10 hp

Civic Cd ~0.36 & frontal area ~2 m^2...
Drag = .5 x density x velocity^2 x Cd * frontal area = .5 * 1.23 * 169 * 0.36 * 2 = 74 N
Power Consumption = 74 N * 13 m/s = 962 watts = ~ 1.5 hp

This calculation can be repeated with your vehicle specs to determine when you really need to worry about aerodynamic inefficiencies. From this example of a Honda Civic, 30 mph 3 degree hill climb results in 6x the power needed to overcome drag. At these speeds, I would say internal drags are your primary source of power consumption.

[aerohead]

Originally Posted by ryannoe

My 2 cents...

Take the weight of your vehicle and multiply it times gravity times sine of any slope you may drive on.

Example:
1200 kg car (~2600 lb) x 9.81 (gravity) x sin(3 degrees) = ~620 N
If you were traveling at 30 mph (~13 m/s) power consumption = 620 N * 13 m/s = 8000 watts = ~10 hp

Civic Cd ~0.36 & frontal area ~2 m^2...
Drag = .5 x density x velocity^2 x Cd * frontal area = .5 * 1.23 * 169 * 0.36 * 2 = 74 N
Power Consumption = 74 N * 13 m/s = 962 watts = ~ 1.5 hp

This calculation can be repeated with your vehicle specs to determine when you really need to worry about aerodynamic inefficiencies. From this example of a Honda Civic, 30 mph 3 degree hill climb results in 6x the power needed to overcome drag. At these speeds, I would say internal drags are your primary source of power consumption.

This should be a thread by itself.I have some data on this and will post asap.

[Piwoslaw]

The first few times I drove around town with the kammback and rear skirts I was surprised at how much my coasting distances increased. Before I got used to it I would panic and brake because I'd be going faster than planned when rolling up to stopped cars. This was all at city speeds (~50 km/h, ~30 mph). After over a year I once took a short drive without aeromods (and tire pressure ~40psi instead of 50psi) and it felt like I was dragging a log behind the car.

[mnmarcus]

My 2 cents...
At around 35 mph (average/typical) wind drag EQUALS rolling resistance. The only way to really lower RR is to lighten your vehicle. ( I think 1/2 the weight = 1/2 the RR). You can lower your wind drag by better aerodynamics or smaller frontal area. 1/2 the cdA = 1/2 the wind drag.

If its equally easy/expensive to get X improvement in aero versus X improvement in rolling resistance do the aero mod, even if you drive 35mph 99% of the time. Because wind drag and RR are EQUAL at 35 mph. 1% of the time wind drag will grow exponentially, + any headwind will hurt mpg much more than the same tailwind would help.

Wind resistance is the bigger efficiency killer. Suppose at 35 mph you'd get 30 mpg. Say you then lightened your vehicle by 1/2, now you get 45 mpg (because your wind resistance remained constant). You could get the exact same fuel savings by instead decreasing your cdA by 1/2 (either decrease frontal area by 1/2 or drag coefficient by 1/2, or some other combination).

Now 1% of the time you drive twice as fast (or drive faster and have a headwind). Normally (no weight change or aero mods) your milage would drop to 12 mpg because when you double your speed you quadruple your wind resistance. (RR is more linear/constant).

Suppose you had lowered your RR by 1/2. Your mpg at 70 mph would be 13.33 mpg. If you had instead lowered your cdA by 1/2 you'd get a more reasonable 20 mpg.

These are all theoretical suppositions though. We all know our milage doesn't drop to 12 mpg (from 30 city) at highway speeds. This is because our cars engines are sized to give us extra horsepower for acceleration and hills and internal combustion engines are also most efficient when operating close to full load. So even though it takes 4 times the horse power to go twice as fast, the engine is operating more efficiently, so mileage doesn't drop as much as we may expect.

If you really want fuel efficiency sell your engine and buy a replacement with 1/4 the HP. I bet your milage would more than quadruple, but it would be a real pain to drive.

[FORDF250HDXLT]

this is an extreme example.but helps one get the picture.

during my dump body build,i ran out of bolts to bolt down my plywood on the front of my bed.
i bolted the top,and threw my spare 16" fully inflated truck tire behind the sheet of plywood to keep it from blowing backwards on my trip to the hardware store.
@ 15 MPH leaving town,the wind blew back the sheet of plywood,sliding my big heavy tire,which is sitting(laying down of course) on dry rough sawed lumber,like it wasn't even there.
i had to turn around,remove the plywood,so i could go to town for the bolts.
there was no wind that day.
(i didn't have the sides on yet,to hold it.)