Drag Coefficient, Range, and doing More with Less

reikiman's picture

Phase 2 of my Lectra project is coming. Phase 2? Read back in my blog in previous Lectra Conversion Diary entries for more details. In Phase 2 I have a full fairing which is meant to make a shell around the entire motorcycle and make it extremely aerodynamic. The idea is to explore vehicle efficiency as a way to increase range.

I've been doing some research .. Cabin scooter and faring design resources from around the Internet .. Go-one Velomobile .. Velomobiles .. 5th generation civic hatchback - improving aerodynamics .. Basjoos Tells All About His "95 MPG" Aerocivic .. and it seems that there can really be a lot of gain from aerodynamics. I came across a couple forum websites where people are discussing ideas on improving aerodynamics of gas cars and other ways to increase gas car efficiency, and they're getting good results. Yeah, they're still 100% addicted to oil but at least they're doing something about the underlying problem and they are getting good results.

Yesterday I did a bit of measurement so I could have a baseline to compare future performance. I found a simple way to ... Measure the drag coefficient of your car ... If you click through to the instructables site you'll find formulas and a spreadsheet. I haven't run the calculations yet, but I've got some numbers. The process is very simple. You take your car or motorcycle or whatever, accelerate up to approx 70 km/hr (45 miles/hr) speed, and then start coasting (in neutral) and measure how long it takes to coast to a stop and speed at various intervals during the coast-down. The formulas in the spreadsheet are able to tell you the actual drag coefficient number. But there is an interesting pattern even without having yet done the number crunching.

I went out with three vehicles (below) and made several runs with each. I have an ipod with a Belkin voice recorder attachment, and during the coast down I called out the speed at 40/35/30/25/20/etc miles/hr intervals. This afternoon I listened to the audio and wrote down the time points for each of the speeds. Making several runs lets me get an average and to account for differing wind conditions (there was little wind) or terrain (it's very flat around here) etc.

1999 Chevy Tracker: 40->10 miles/hr takes approx 60 seconds

2004 Honda Rebel 250: 40->10 miles/hr takes approx 40 seconds

Lectra: 40->10 miles/hr takes approx 25-30 seconds

The idea is that the more quickly the vehicle coasts to a stop, the greater the wind resistance, or put another way the worst the drag coefficient. But this also measures rolling resistance.

I'm a little surprised the Tracker came out the best here. It's shaped a bit like a box, with very little in the way of aerodynamic niceness to it. But on the other hand motorcycles do have these bits hanging out all over the place ..sooo...

The Rebel has a windshield.

The Lectra, since it doesn't have a transmission, that may have introduced some electrical or mechanical drag. With both the Tracker and the Rebel I could put them into neutral (both have manual transmissions) and coast properly, but the Lectra doesn't have a neutral because it doesn't have a transmission.

I think the weight of the Rebel and the Lectra is comparable. The Lectra is a lot shorter than the Rebel, but it has 220 lbs of lead acid batteries on board.

Oh yeah, doing more with less...

That is a phrase Craig Vetter said during his DVD discussing the motorcycle efficiency contest and the fairing I have .. he says he got the phrase from Buckminster Fuller, who he 'followed around' for a couple years. When I heard that phrase come out of Craig's mouth it crystallized for me an idea I've carried for years.

What makes compact fluorescent light bulbs interesting is they emit the same number of lumens by using less electricity. That is doing more with less.

A vehicle that's more aerodynamic can move and use less energy to achieve the same end. I think the Honda Insight, for instance, gets most of its fuel efficiency gain from being small, light, and aerodynamic. The Geo Metro after all got 60 miles per gallon without having a hybrid drive train. Mini-experiment: the wrath of roof racks is a meditation on the negative effect of roof racks on fuel efficiency.

before comments


davew's picture

I went out with three vehicles (below) and made several runs with each.

You have to be a little careful in using this to compare between vehicles. Just because your coefficients are low doesn't mean your vehicle is necessarily more efficient. In order to get the same lousy mileage out of a Geo Metro that you get from a Hummer you'd have to drag a sail. It's a mass thing.

"we must be the change we wish to see in the world"

reikiman's picture

You have to be a little careful in using this to compare between vehicles. Just because your coefficients are low doesn't mean your vehicle is necessarily more efficient.

Yeah, it's kinda like comparing apples and jalapenos. But I'm being inspired by the simplicity of the designs these ecomodder people have done to see what miles/gallon improvements I can get with my Tracker. Hmm, I didn't mean to get into a comparison between vehicles but I did that didn't I.

Mik's picture

If you want to compare BEFORE and AFTER of the same vehicle with and without fairings, then you could use a hill in the neighborhood and measure top speed, and distance traveled on the bottom of the hill when coasting down the hill.
Pick a small hill, or you'll run into trouble with the speed limit once you've got a good fairing going....

Mr. Mik

This information may be used entirely at your own risk.

There is always a way if there is no other way!

jdh2550_1's picture

David - bikes, in general, have terrible co-efficients of drag (Cd). Cd is greatly affected by non-smooth surfaces which disrupt airflow, not just frontal area. An exposed rider on a bike is a Cd nightmare!

Also, I don't expect a windshield to affect Cd very much - if anything it might make it better. On the one hand it's another "protuberance" to disturb airflow. On the other hand, depending on the design, it may make the air flow around the rider more efficiently than having no fairing.

The best shape for the lowest Cd, as you know, is the teardrop enclosure. The other day I had a kind of after the fact "brainwave" - it now seems "obvious" to me that this is the best shape. Nature shows us - look at a rain drop. It's malleable, and it falls towards earth with no force other then wind resistance stopping it - thus it will naturally form the most efficient shape.

Alas, I'm still vain enough that I just don't like teardrop enclosed bikes or cars - they just don't look good. I know, that's a lame reason not to use one, but there you have it...

John H. Founder of Current Motor Company - opinions on this site belong to me; not to my employer
Remember: " 'lectric for local. diesel for distance" - JTH, Amp Bros || "No Gas.

reikiman's picture

John, we're clearly thinking along the same lines. And you're thinking along the lines Craig Vetter is talking about:-




http://craigvetter.com/Movies/Vetter_DVD_preview.mov - this is a promo for a 70 minute DVD you can get from the above links.


Remember, I have one of his fairings leaning against my garage and this is phase 2 of the Lectra. On his DVD he talks about how certain vehicles from fish to airplanes have proved that a certain shape is the most aerodynamic -- in that they produce the best results in moving through air for the least input of energy. Since energy availability is a key achilles heel of EV's I think aerodynamics should be a fruitful way to improve EV performance.

What do you think of the Aptera? It follows the same shape. But for me looking at the pictures it looks like some kind of caricature of a vehicle, and it mostly looks like an airplane. But it also is clearly very aerodynamic.

Vetter hasn't invented anything new ... He just proved it in a contest and has publicised it a bit. On his DVD he describes how European motorcycle racers -- until the mid 1960's -- used full fairings over their motorcycles and were just going faster and faster and faster until the racing commission stepped in and changed the rules for fairings. Such as requiring the fairing leave the front wheel exposed and some other details. This increased the drag coefficient and had the effect of slowing the riders down to more rational speeds. But that fairing design is what became the standard fairing of performance bikes like the Ninja.

At the moment I'm in Belgium getting ready to attend an open source software conference. On the streets here I've seen a few instances of scooters that have a nearly full fairing. The front fairing goes to a full windshield, that goes to a roof, and the roof then mounts behind the rider. The sides are open but I think their legs are protected from the wind. I'll try to get a picture. Actually it looks a lot like the Quasar motorcycle

Dear David, I've known for a long time that a fairing is probably worth the effort, especially over 20 MPH. However, your very useful research and links have really opened my eyes as to how dramatically a well-designed whole-body fairing can effect fuel consumption.

Taking a 100+ MPG 250cc scooter and adding a comprehensive fairing, and then changing the gearing because of the lowered wind resistance, can result in over 200 MPG.

After some research I did a while back, I concluded that home-distilling ethanol was feasable, but for a 25 MPG car, it was more trouble than I was willing to commit to (am now a fan of WVO, attempting to brainwash wife...don't hold your breath).

However! at 200 MPG,...my work commute is now 14 mi/day (5 days/week), so, roughly one gallon of ethanol every two weeks.

Most aerodynamic bodies try to get as close as possible to the ideal "teardrop" shape, with as few minor compromises as possible. (Like the "Aptera")

Daimler-Chrysler engineers stumbled across a reference on the shape of the "Boxfish" which indicated that inspite of its fairly chunky (and useful) shape, it had a suprisingly low Cd of drag.


I just read (April '08) in Wired about the peraves fully enclosed motorcycle (BMW engine).


I think the price is $70K+, and only gets 57 MPG, but they claim hot-rod performance free from the weather

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