Online tool: shows aerodynamic drag, rolling resistance, power required & est. MPG

[damunk]

Can someone explain what the aero (a) figure on the results chart means please?

i.e
1.93
7.73
17.40
30.93
48.33
69.60
94.73
123.73
156.59
193.32
233.92
278.38
326.71

The difference each time increases. What is this figure measured in. Is it just a ranking.

[MetroMPG]

Legend/explanations at the bottom of the tool output page:

eg:

(a) Aero drag formula: 0.5 * rho * V^2 * Cd * A
Resulting units = Newtons

[sheepdog 44]

Suggestions:
Multiplication factor for road gradient. This would tell me how much horsepower and mpg changes to gradient so i could better adjust my commute. Also what the minimum negative grade is that i could sustain a coast in neutral at different speeds (terminal velocity). That way i could plan routes with the longest stretches of minimum negative grade for maximum P&G. It would also be useful to calculate the minimum hp needed for climbing when building a custom vehicle.

An way to easily convert to (EV) numbers like mpge, or wh/km. Do i just set the engine efficiency at 90%? It didn't turn out quite right when i did that. I wanted to put in the numbers for my gas insight, and see what what mpge id get for a conversion, then compare that to production EV's. And just fiddle with the CD to see how much better my numbers could get.

If it is possible to implement these changes, that would be great. But if not, i'd be happy if someone could provide me with a formula and i could do the calculations myself.

[jime57]

Played around a little witht he tool. Man, just small improvements in Cd have a tremendous effect on an Insight running at 55 MPH! Gotta get back to planning that boat tail.

[sheepdog 44]

I'm with you on that one!

[ever_green]

i was wondering if the power figures are at the wheels or at the flywheel? Whenever I use this calculator the power figure estimates seem too low. For example my car uses about 40% manifold pressure at 2500rpm doing 65mph. Since my car makes 70hp at 2500rpm that would amount to ~28hp. By the way that's exactly how much the scangauge horsepower gauge displays at 65mph on flat surface cruising, nice! So i think the figures on this calculator are too conservative, either that or it was not designed for heavier 4wd vehicles.

This is for a 09 2.5i sedan and my options in the calculator were .010 rolling resistance, .78 drivetrain efficiency, 22% engine efficiency, 3240lbs weight, 22.4 sq-ft frontal area and 0.32 drag coefficient. It insists that I only need 20hp :\

[ennored]

So I've been using the tool to look at my motorhome and it's (lack of) aerodynamics.

Thinking my next step is to do some coastdowns. Thought I'd try using a GPS app for my phone to gather some data. Being the lazy kind of guy, I decided maybe someone else had already done this. Google to the rescue...

Crr Cda solver for bicyclists

So yes, it has been done.

Next up. Car version. Gotta see what kind of files come out of my GPS app...

[gone-ot]

Originally Posted by jime57

Played around a little witht he tool. Man, just small improvements in Cd have a tremendous effect on an Insight running at 55 MPH!

Drag is squared-function of velocity, but HP is a cubic-function of velocity!

[renault_megane_dci]

Noob question (and sorry about it) :
let's assume I feel there is some potential by lowering the rear only to better match the template and improve windscreen angle and that I can lower the bumper at the front to manage pressure differential underneath the car.

Is the only way to alter the car pic on the side ?

[aerohead]

Originally Posted by renault_megane_dci

Noob question (and sorry about it) :
let's assume I feel there is some potential by lowering the rear only to better match the template and improve windscreen angle and that I can lower the bumper at the front to manage pressure differential underneath the car.

Is the only way to alter the car pic on the side ?

If you lower the rear of the car you alter the inclination of the forebody's (angle-of-attack if you will).In so doing,you alter the pressure gradient and
shearing potential of the flow outside the boundary layer,limiting the amount of momentum delivered into the boundary layer which could trigger premature separation.
There's also a chance that areas of the sides of the car which were 'clean' at the factory inclination might 'catch' air,now that it is moving more obliquely.
You'd want to be very careful.
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I'm not sure about your second question as pertaining to the sides.
If you look straight down on the car from above,their is much potential for plan-view streamlining.As GM did with Impact/EV1 and Honda's 1st-gen Insight.
Again,care must be taken so as not to create adverse pressure spikes which can trigger separation of the boundary layer or transverse shear which might roll up into vorticity.
A technical drawing of the 1957 MG EX 181 is available which shows all views of that land speed record car of Cd 0.12.It's plan -view taper would be a conservative template for streamlining.Honda used such contours with their successive Dream solar racers.
I'm not sure if this answers your question.