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Drag Coefficient, Range, and doing More with Less
Submitted by reikiman on Sun, 02/17/2008 - 22:16
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.