Candid Cameron
Candid Cameron
Q After re-reading David Edwards’ editorial comments regarding Craig Vetters attempts at modifying a Honda Helix to do 70 mph into a 20-mph headwind carrying four bags of groceries and get 100 mpg while doing so (“Freedom Machines,” September, 2008), I visited Vetter’s site. I was fascinated by the development process he has bravely put out to public view, warts and all, but I am puzzled by his idea that aerodynamics is the major factor. I am no expert, but my gut feeling is that power-to-weight ratio and gearing have at least as much to do with mpg (to say nothing of the rider’s control of her/his right wrist). Can you shed some light on the science of engine efficiency and gearing as they relate to economy? At what speed do aerodynamics become a significant factor? Kevan Smethurst Stoneham, Maine
A A rule of thumb is that about 25 percent of a car’s power goes into overcoming rolling resistance, so aero is a substantial item. The frontal area of a motorcycle or scooter can be measured by projecting its shadow onto paper by means of a very distant light source, at night. Aero drag force is proportional to frontal area, multiplied times aerodynamic coefficient, times velocity squared. Aero drag power is proportional to velocity cubed. Bikes generally have considerably worse aero coefficients than do cars, but they have smaller frontal area.
Every engine has a point of minimum specific fuel consumption, which for a four-stroke engine is something close to 0.5 pound of fuel burned per horsepower, per
hour. That being so, if it takes 15 hp to drive a particular bike at X steady speed, and if the engine is geared to operate at rpm and throttle angle close to that point of minimum specific fuel consumption, the bike will burn 0.5x15=71/2 pounds of fuel per hour, or about 1.2 gallons. As four-stroke engines are throttled down, their pumping loss increases, so their specific fuel consumption increases as well. As they turn higher and higher rpm, their friction loss increases.
Ideally, you’d provide an engine that was about twice as powerful as needed for the desired cruising speed; but in general, motorcycle engines are much more powerful than this because their mission is to provide pleasing acceleration. That makes them less efficient on the highway because they are then cruising at something like 10 percent power, which puts the engine in an inefficient zone of specific consumption. This is the basis of hybrid vehicles-to run on the battery electric system at road speeds that would be inefficient for the piston engine.
The problem of reducing aero drag is complicated by the short length of twowheeled vehicles, making it very hard to close their wake. For this reason, motorcycle “streamlining” really isn’t very streamlined; it is smoothly shaped in front but is then chopped off before it can “put the air back where it was” and thus avoid wake drag. Adding a tapered tail makes the vehicle vulnerable to side-wind effects-something Craig Vetter is trying to deal with. A small engine of high compression ratio, turning moderate rpm, is a good beginning.
Kevin Cameron
