Public Service Bulletin #4 Streamlining

This picture is from the movie "World's Fastest Indian" (the motorcycle not the people who fought the cowboys in the movies). It is a motorcycle which has been shaped for moving at high speed, which is called "streamlining". I recommend the movie, I don't recommend streamlining your bike unless your only interest is a speed record. However although full streamlining is rarely used in motorcycles, streamlining does help in saving gas at normal highway speeds.

Given the amount of controversy on the Internet and on TV on the topic of wind resistance in cars and trucks (not so much with motorcycles). I thought it would be good to clear up a few misconceptions and apply some basic physics.

Simply put, to calculate the horsepower lost in punching through air, use this formula

power wasted  = cross sectional area X coefficient of drag X speed X speed X speed

I don't know how to write 3 (cubed) on this blog, so I thought what the heck, do it another way that emphasises how important speed is in the equation. If your goal is to beat a world speed record, then you cannot slow down to save gas, and you have to work with area and CD. But if you want to save gas in normal transportation, your best bet is always to slow down, as speed is factored in the equation not once, but  three times.

The cross sectional area is the area viewed from the front (or back) - in case of a rectangle it would be height multiplied by width. For more complicated shapes, its the sum of a lot of little rectangles. Note that anything that hangs out the side (Like mirrors) or sticks up at the top (like roof rack luggage) adds to the area. Anything hanging off the front (like a battering ram) or the back (like a trailer hitch) does not add to the area. The tires add to the area also, as seen from the front. Wider tires = more area.

The coefficient of drag is the most difficult part to understand. It is also the hardest to improve for you home tinkerers, but easiest to screw up. It is measured by wind tunnel testing of the shape of the vehicle, and it is very hard to apply some formula to calculate the coefficient theoretically from any given shape. The lower the number, the better. The number 1 represents the worst possible shape, which is basically a brick. Usually the shapes with the lowest CD would be like the streamliner above, although without the fins on the tail. If you are going over the speed of sound, the front would be better as a sharp point, but most cars are well under that speed.

The most interesting thing about CD is that the shape of the back is far more important than the front. Which is the exact opposite of what most of the public thinks. Especially if you check out the rooftop ski carriers that are shaped to a point at the front and squared at the back. Turn it backwards, you'll get better gas mileage.

And finally, speed. Basically, the slower you go in top gear, the better your gas mileage. I know a lot of propaganda out there says you shouldn't go any slower than x kph, or you will start wasting gas. Shhhhh! That's just to stop people from holding up traffic.  But generally, slower is better as long as you don't shift down and the engine still runs smoothly. Remember drag is mostly the speed cubed. And pedal a bicycle at 20 kph, you will know that measurable wind drag is starting to build already at that speed.

So now let's deal with some of the practical aspects. For example, you can almost never get better gas mileage by sticking something on to the the vehicle that adds to the area, no matter how nice the shape. The worst example of this is the huge wind deflectors put on the roof of transport trucks. If the truck is pulling a trailer taller than the deflector, no harm done. Unfortunately it's not doing much good either, because as I said it's the back end that's critical, not the front. But no harm done - until you take off the trailer and drive your truck down the freeway at 100 kph. Then that little gas saving gizmo becomes an air brake. In the last ten years, I have seen a decrease in this type of gas wasting, maybe people are starting to understand - especially when gas was more expensive.