5th generation civic hatchback - improving aerodynamics

[AndrewJ]

And here are the mirror delete pics, used a couple of round blind-spot mirrors that I got for $1.45 at the local autoparts store for interior side-mirror duty.

So here's the deletion, I'm kinda in the middle of getting the hardware situation ironed out, need to get that shiny stuff painted black.



And the gory details of the (new) mirror and it's mounting bits

[AndrewJ]

So I actually got my lazy butt off of the couch today and went out and did some coastdown testing.
Here be the results.

Methodology:
using Iwilltrys' instructable "Measure the drag coefficient of your car"
I used a camera mounted on a tripod in the backseat to record the data points. I used my remote shutter control to take pictures of the speedometer every 10 seconds as indicated by my stopwatch.

Frontal area was measured using this technique but with photoshop's histogram tool instead of GIMP.

The Raw Data:

Vehicle Weight (with occupant) 2300lbs/1043.26kg

Frontal Area 1.768 m^2

Test run-----V1-----V2----V3----V4-----V5----V6
0 sec---------55-----55----55----55-----55----55 mph
10 sec-------49.5---51-----51----50----49.5--50 mph
20 sec-------44-----45.5---45----44----43----43 mph
30 sec-------38-----40-----40----38----37----38 mph
40 sec-------33-----35-----35----33----32----33 mph
50 sec-------29-----30------------28----27----27 mph
60 sec-------25--------------------23----22----23 mph
70 sec------------------------------------19----19.5 mph

The Results

Well, here's where it gets tricky. I used the solver function in the excel spreadsheet, but it keeps giving me really strange numbers, impossible numbers. So I tried my best to eyeball the numbers to get all of the data points to line up, here's what I got.

--------Solver-------Me eyeballing it
Cd------0.062-----------0.20
Crr-----0.0218----------0.018

Perhaps someone who has done this before can lend a hand?

[IWillTry]

Hi AndrewJ,
MetroMPG contacted me through the Instructables site and asked if I would comment here since I created the spreadsheet you're using. Unfortunately, the lower your Cd is, the less accurate the spreadsheet is since the contribution of Crr dominates the results. I punched in your numbers and the data comes out almost linear which means there isn't enough change in drag force during the test to accurately isolate the contribution of Cd from that of Crr.

The easiest way to improve results would be to determine Crr from a different test and enter that value directly. Then run solver again, changing only the Cd value to match the model to the data.

To measure Crr directly you can use a spring scale (50lb capacity should be enough). Measure the force required to pull your car at a constant speed (a walking pace) on level ground. Crr = Frr/M/g where Frr is the force in Newtons (1lb = 4.45 Newtons), M is vehicle mass in kg, and g is 9.81. Measure the force to pull the car forward in one direction, then turn the car around and measure the force pulling forward in the other direction. Use the average force to calculate Crr. Drive the car for a while first to warm up the drivetrain.

Hope that helps.

[IWillTry]

One additional comment (hopefully not too critical ) now that I've read all the other posts. You said in a prior post that the air dam would increase your CdA, but would decrease your Cd. I just wanted to clarify that CdA is what you should actually be trying to decrease (not Cd). Drag force is proportional to CdA.
Cheers and good luck.

[AndrewJ]

Thanks for clearing that up! Well, looks like I have more testing to do, and a spring-scale to locate. As for the CdA/Cd thing, I basically have no idea what I'm talking about , that's why I have all the other smart folks on here to keep me straightened out.

[AndrewJ]

Well, maybe I don't need a spring scale after all. Perhaps a humble bathroom scale will do the trick if I push the car from behind. In that case, I'll have the Crr isolated by the end of the day!

Can't believe my 2 brain cells that are awake thought that up so quick. Alternately, I can't believe it took so long to think of that.

[MetroMPG]

Originally Posted by IWillTry

You said in a prior post that the air dam would increase your CdA, but would decrease your Cd. I just wanted to clarify that CdA is what you should actually be trying to decrease (not Cd). Drag force is proportional to CdA.

Good catch, IWillTry.

And thanks for signing up to clarify the coastdown testing with your spreadsheet also.

I don't know if you have a stat counter at your Instructable on this subject, but if you do, I bet it's seen a lot of traffic in the past week.

[IWillTry]

A bathroom scale may or may not work. Some scales will only work in a horizontal orientation. They also can be difficult to balance on a car bumper (depending on the car). If you do use a bathroom scale, be sure to "zero" the scale while it is in a vertical position to calibrate out the weight of the scale platform. Good luck.

[AndrewJ]

So, using the bathroom scale trick (zeroed at vertical) I found that my car takes 35lbs of force to keep it moving on a level surface.

*a note to people who try this method*
smoothing out your stride seems to be an important part of getting an accurate measurement. Just pushing normally produces quite an oscillation of force, making the measurement "jump around"
So focus on the dial of the scale and smooth out your power delivery to keep the dial steady.

So, 35lbs = 160.2 newtons, so on an so fourth. Voila! Crr for my car is 0.0169

Plug it in to Excel, set the solver to solve for Cd (cell F41 only) and it spits out a Cd of 0.194

Hmm, still seems low to me. If the "stock" car is 0.33 and I managed to knock that down to less than 0.20 with a mirror delete, airdam/radiator block , and rear skirts? Seems unlikely.

I think I'm gonna take all the aero goodies off the car tomorrow and run another set of coastdown tests in "stock" form. That way I'll have some idea of how accurate the 0.194 actually is.

[MetroMPG]

Interesting update. *applaudsyourtenacity*