# Measuring/estimating your drag coefficient,



## major (Apr 4, 2008)

Rational said:


> Cd. How do you do it?
> TIA.


Stick in a wind tunnel. Or do a google search. I think you can find almost all the Cd for production cars published somewhere. You can also perform coast down tests and come up with some estimates for your particular vehicle.


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## Rational (Nov 26, 2011)

major said:


> Stick in a wind tunnel. Or do a google search. I think you can find almost all the Cd for production cars published somewhere. You can also perform *coast down tests* and come up with some estimates for your particular vehicle.


That's what I had in mind, taking into account rolling resistance. You have a formula?


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## notailpipe (May 25, 2010)

Yes. Essentially you measure how long it takes drag to slow you down and do a curve fit to the data, knowing your frontal area and also knowing at the high speeds the drag coefficient will dominate (it's a cubic term).

It's a little inaccurate because you are guessing at the component that is rolling resistance, but like I said, that washes out at higher speeds. These guys made it easy with a spreadsheet. I haven't done it yet.

Let us know how it works out...


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## Rational (Nov 26, 2011)

notailpipe said:


> Yes. Essentially you measure how long it takes drag to slow you down and do a curve fit to the data, knowing your frontal area and also knowing at the high speeds the drag coefficient will dominate (it's a cubic term).
> 
> It's a little inaccurate because you are guessing at the component that is rolling resistance, but like I said, that washes out at higher speeds. These guys made it easy with a spreadsheet. I haven't done it yet.
> 
> Let us know how it works out...


Thanks for the link.
The Crr can be measured by towing the car very slowly in both directions on the same road with a strong spring in the tow rope and measuring the spring extension.
With the drag, going back and forth over the same road on a windless day several times should give you a good idea of the true value. That'll probably take the whole morning and if the numbers are widely scattered there's probably some confounding factor.


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## notailpipe (May 25, 2010)

Rational said:


> That'll probably take the whole morning and if the numbers are widely scattered there's probably some confounding factor.


Whelp, that's how you'd do it though. 

Weather.com?


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## dtbaker (Jan 5, 2008)

once again... lost in the trees.

The most accurately measured cd and crr in the world for every model car out there won't help you accurately predict a range (or energy required) in any case but steady state speed.

since we all drive different speeds, and different styles, and different freaking air densities depending on elevation..... gathering cd data is useless to come up with a generalize range predictor.

the best idea I've seen is put forth in a different thread; to correlate the mpg obtained by given original vehicle on gas versus observed post-conversion consumption mile/kWhr with the same vehicle, same driver, and same type of use over a 'average week' or 'average 1000 miles'.

The goal would be to come up with that single multiplier... with all the hard-to-measure factors washing out on both sides of the equation and not needed to develop the correlation. In fact a pretty good starting place for that value has already been kicked around as being pretty close to .10 for DC motor conversions and .15 for AC. I'd suggest that one added qualifier might be the age/condition of the ICE. The mpg for a given 'new' vehicle is probably significantly better than a much older vehicle with improvements in efficiencies like fuel injection, variable timing, etc. So if you REALLY wanted to fine tune multipliers you could group ICE data by major engine/drive-train efficiency types.... but that probably falls within error tolerance for other factors.

i.e. with a 'modern' ICE vehicle getting 40mpg, a starting point for estimated consumption would be 4.0 miles/kWhr for a DC motor conversion.

gathering vehicle data like cd, crr, frontal area, etc all become irrelevant.

see the forest now?


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## fb_bf (Jul 6, 2011)

I've been working on measuring the drag on my car for some time now. I've used the approach listed on the site in this thread, and then I found I could get accurate speed versus time by using a GPS running watch. It records your velocity every second. My hope is to get to a point where I can measure improvements in rolling resistance and Cd. This means repeatable results. I've been working on a spread sheet that breaks that information down to real units of force, in this case newtons. The metric system is really easy to work with for this sort of thing. I also found that I can record my drive to work with the GPS. This gives me all the information I need to estimate all of the energy required. Like you mentioned above, knowing the Cd is good for constant speed, but it doesn't deal with stop and go, and hills. With a GPS recorder you get all of the information. I saw this thread and I thought I'd comment now, even though I'm not done with the investigation yet. I have a Soliton Jr controller which has a logging feature that allows me to log the energy use though it. My goal is to get so I can account for all of the energy that is reported by it with my spread sheet. So far that I still am missing too much in my spread sheet to feel comfortable about releasing it. The picture shows my drive to work. I have the velocity which shows the accelerations and stops that occurred. I also have the elevation change that I can compute the energy required to handle that. I have found that the elevation reported in a GPS isn't that accurate, so you have to be a little careful. If you look at the drag work VS acceleration, you get a feel for how much energy goes into stoping and starting. I get the drag work from knowing my drag forces that I estimate from doing the Cd measurements. That's where they become useful. As I take more data, I'll feel more comfortable sharing all of this so other can try it.

I have just posted my website about my car. This site is a work in progress, but it contains more information about using a GPS recorder, and the spreadsheet that I used to produce the graph below. Hopefully some other people can give this idea a try and comment back on the results.

Frank


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## fb_bf (Jul 6, 2011)

This is just so people can see the website destination in my new signature.


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## tomofreno (Mar 3, 2009)

fb_bf said:


> I've been working on measuring the drag on my car for some time now. I've used the approach listed on the site in this thread, and then I found I could get accurate speed versus time by using a GPS running watch...Frank


 Will be interesting to see how accurately you can predict results. Don't forget motor/controller and drive train efficiencies, and energy loss in the batteries if you use lead acid. Also work done in spinning up the flywheel and other rotating parts.


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## fb_bf (Jul 6, 2011)

Right now the route calculator is showing about 70% to 75% of the actual energy use recorded by my BMS. This would seem to be close if you assume 15% loss for the motor, 10% loss for the gear, and 2 % loss for the controller. You bring up a good point in that for the part that calculates the energy going into acceleration is only considering the F=Ma portion. (Mass being the mass of the car). If I could estimate the losses due to the rotating parts, I'd show even a greater amount of energy going into the acceleration losses. The acceleration losses already are surprisingly high in my short 5 mile commute. Like I point out, I have a very light car, so others are using even more. I'm starting to feel more comfortable with my modeling. I use the slowing down while coasting to get the deceleration forces (drag force at various speeds. I then use the Soliton Jr.'s logging feature to measure my real energy use at different speeds. At 50 to 60 mph, the Soliton is showing about 25% more energy required over the drag force, which seems right. At 35 mph I'm seeing like 40% more, which is bothering me. It would seem that I have a fairly large drag/friction force to overcome that is not very speed dependant. 
As DTbaker points out, knowing your Cd doesn't do you much good in predicting your range because it is VERY route dependant. I'm doing this because if I want to experiment with reducing my Cd or friction drag, I have to know what it is when I start changing things.


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## tomofreno (Mar 3, 2009)

fb_bf said:


> I then use the Soliton Jr.'s logging feature to measure my real energy use at different speeds. At 50 to 60 mph, the Soliton is showing about 25% more energy required over the drag force, which seems right. At 35 mph I'm seeing like 40% more, which is bothering me. It would seem that I have a fairly large drag/friction force to overcome that is not very speed dependant.


 Are you accounting for the rolling resistance force? It varies little with speed, and is larger than the drag force for speeds less than around 45 mph for a typical sedan.


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## fb_bf (Jul 6, 2011)

I am accounting for the rolling resistance. The coefficient of drag measuring technique has you curve fit your data with an equation that assumes a constant rolling resistance, and a air drag force that goes up as the square of the speed. You guess at the Cd and rolling resistance values until the calculated curve fits on top of the real curve. That curve being the rate that your speed slowed down at. Even though the rolling resistance does vary some as the speed increases it is not too hard to get the predicted curve to lie right onto top of my data. I'll admit that you can get pretty good fits with higher Crr values and lower Cd values, so there is a bit of judgment going on in this. For now I've settled on a Crr of.013, a Cd of .48, and a frontal area of 1.67 m^2. I'll keep at this, especially when the weather gets warmer. I might even take the car to a dyno. In one run on a dyno, I could answer most of the questions concerning efficiencies and drive train losses.


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## GizmoEV (Nov 28, 2009)

Remember that your BMS system will also include all other electrical loads which do not go through your controller.


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