Fuel mileage!!...why does my motorcycle get such lousy mileage?
©
Copyright, 2011, R. Fleischer
FuelMileage.htm-81
First, an overview...and later I will get into the mathematics:
All objects...cars, motorcycles, bicycles.....and
you!.....when moving through the air have friction and drag. Air is,
after all, a fluid, it just
happens that this particular fluid is a mixture of various
gasses. This also means that if you are moving through
the air, the air, relative to standing still, is also moving towards you.
The resulting air speed is your speed plus the air speed, that
is, if you are trying to ride your motorcycle INTO the wind, your effective speed, as far as
drag, air
friction and fuel and power usage, etc...is the TOTAL of the two
speeds.
On earth, the pressure of the atmosphere causes a higher pressure at sea level, than it does on a mountain top. Thus, the higher you go with your motorcycle, theoretically the faster you should be able to go...OR; conversely, the less power from your engine you would need at any same speed as at sea level; all due to the reduced air drag. In practice, the oxygen content of the air is less (by weight), and your engine will have less power as you go up in altitude, probably totally offsetting performance increase from less air drag/friction. Some things can offset that more or less, such as supercharging or turbocharging.
CARS are designed to slip through the air with minimum
friction and drag, so as to get the best possible fuel
mileage. Manufacturers do everything reasonably possible to
reduce friction and drag, due to various penalties for poor gas
mileage, not to mention sales points, etc. Of
course, there are
limits to designs that would be acceptable or practical.
Motorcycles, particularly large ones, are generally designed for
performance, not necessarily fuel mileage; although
that is or can be a considerable factor. Generally speaking, while there are
certainly
large differences between various motorcycles as far as what the
designers/engineers intended.....still.....some motorcycles are designed for
performance....and many are designed for more practical everyday
use in typical
paved road usage.
When an engine is designed for mostly performance, fuel mileage suffers.
Early motorcycles used carburetors with rather rich mixtures, for
best power and performance. Even with fuel injection, mixtures
on SOME high performance bike engines tend to be somewhat on the richer side, and
in particular and quite notable, camshaft timing and other such 'tuning' is such that
fuel tends
to be wasted. This wasting of fuel is much more pervasive
during fast acceleration, than at constant
road speeds. As
speeds rise, power demands greatly increase, and the fuel usage is
not on a flat curve with speed....rather, it is very steep.
This is particularly so on vehicles with lousy coefficients of
drag.
Motorcycles do NOT slip through the air with
low friction and drag. Even motorcycles with well-designed
full fairings are 'dirty' as far as friction and drag
goes....compared to a small car and many a larger car too.
Motorcycles have a huge number of exposed parts, each of which
presents its own
interference disturbances with
the desired smooth flow of air. These
air disturbances,
many of them very tiny, tend to interfere with each other.
Some parts will affect the air before other ones do. Thus, there is a constant
"interferences mixing". The over-all
effect is a lot of friction and drag. A motorcycle might
actually do better at fuel
mileage if it was a square ugly vehicle...a long rectangular
box! .. OR, at least an ugly non-streamlined
car.
Think about this in another way. Suppose a motorcycle has a
streamlined-looking fairing, and it was traveling through air
full of multi-colored smoke. Suppose you were some decent distance away, but able to stay at the same speed, and
look
sideways towards the motorcycle. You would see that smoke travel
around the motorcycle in very strange ways. It would
have all sorts of ripples and strange movements.
Typically, as it passed the
rear edge of the fairing, it would combine horizontally and vertically (air is
coming
over the windshield, and the rider...)....and try to rejoin the air doing
the same sort of thing on the other side of
the motorcycle. If
saddlebags, or a passenger, or a passenger backrest, or a tour trunk....were on
the
motorcycle...those air currents would strike these things in all sorts of
weird patterns.....and cause friction and drag.
If the fairing could be extended to a number of feet behind the motorcycle, and
the entire structure made to look
somewhat like a fish, then the friction and
drag would be GREATLY reduced. That is not practical for a
street-going motorcycle.
In the classic way of thinking about friction
and drag, one converts by one means or another the ongoing face
and structure of the
vehicle into a equivalent 'flat plate', of some number of square feet and
square
inches, and then
calculates using commonly known mathematical formulas, the
Coefficient of Drag, often expressed as Cd
You could do that yourself, by taking a photograph from the front, enlarging
it, and then calculating the area of
things and of no things, in the photograph.
If the photograph was full size, you could, with some effort, calculate
the
number of square feet and square inches, of effective frontal area.
The answer would not REALLY be completely accurate, however, as all those
air flows around and in-between everything DO interfere with each
other, making things WORSE than calculations by area alone.
But, you would get some decent information about Cd.
There are numerous ways to do this sort of calculation without all that manual labor, including wind tunnels with instrumentation, which give a much better and true idea of Cd. There are other means, such as with computer programs that do all the work. You would need to do it in three dimensions to get a reasonably accurate Cd. I can guarantee you that road-going motorcycles...all of them.....have lousy CD, compared to any modern small car. Thus, a motorcycle uses a fair amount of horsepower to maintain any speed, let alone accelerate. Using horsepower means using fuel. Increasing engine efficiency in burning of fuel has a VERY SMALL effect, compared to Cd. In the worst case, a large bike getting perhaps 38 to 44 mpg in steady riding at perhaps 60 mph, might get 42-48 mpg, by going from carburetion to fuel injection, with a few other changes....and maybe a FEW more mpg if the rear drive ratio was flatter.
All road-going vehicles with tires have friction with the road that produce heat. That friction is needed, but it is a user of horsepower. It could be thought of as power that the engine produces that ends up as heat from the tire friction that is transmitted into the road surface....and into heating the tire itself. There are other ways of thinking about this sort of thing too....such as the air resistance of the tire/wheel combination, and many many other small things...all of which tend to add up. These effects also use up horsepower.
Motorcycles offer a lot of drag and resistance, no matter their design, to the oncoming air.
The mathematics....AND THE CONCLUSIONS.....
First....you have undoubtedly heard about Cd, the Coefficient of Drag. I will
bet you don't have a good idea of what that
figure is for various vehicles.
Here is a nice list,
information derived from wind-tunnel measurements, so these can
be relied-on to be accurate:
Cd
Vehicle
5.54
Ferrari 308 GTB, year 1980
5.61
Mazda RX-7, year 1993
5.92
Porsche 911, year 1994
6.24
Toyota Prius, year 2004
6.27
Porsche Carrera, year 1986
6.81
Subaru Legacy, year 1989
7.57
Toyota Camry, year 1992
8.71
Buick LeSabre, year 1991
18.06
Hummer H1, year 1993
26.3
Hummer H2, year 2006
I will put two formulas below, and do a bit of discussion about each. These are not MY formulas, but well-known classic formulas...(nothing I made up!).
#1:
Fd = (1/2) Pv2
ACdV
For
the above formula, Fd is the drag force; P is the density of the medium...in our
example we mean air density; V is
the direction of the velocity; and A is the
area, usually taken to be an orthographic projection on a plane perpendicular
to
the direction of motion. If not 'simple', such as a sphere (your motorcycle
certainly is not a sphere!), then one needs
to calculate for each and every
area. Finally, v is the speed, relative to the medium.
For an object with well-defined points (HAH! for a motorcycle!), such as a
circular disk plate, perpendicular to the flow,
then Cd is a constant for
Reynolds numbers over about 3,500. Cd is a function of the
orientation of the flow.
There are many types of drag, one of which is FORM DRAG, which is pressure
variations around the object.
There is no need for you to get deeply into the formula details.
The
important part is quite simple. The drag force is
proportional to the
SQUARE of the SPEED. If the drag is proportional to the
square of the speed,
guess what that proportionality means to horsepower required (and
gasoline required!)!
You'd LIKELY BE WRONG!!
****It is at this point that confusion sometimes comes about, and
arguments can start. That is because fuel usage...and
horsepower
requirements...are NOT proportional to the square of the speed.
So, see formula
#2, below:****
#2:
Power required = Av + Bv2
+ Cv3
Let's take a look at that formula:
A is
the resistance....which includes the tire rolling resistance...this is a linear
function.
B is mostly concerned with internal engine friction components.
C is the aero-forces, including the coefficient of drag, air density, and so on.
v, as before, is the velocity, or speed
There is no need for you to get deeply into this formula. The important part is simple. The power required is proportional to the CUBE of the VELOCITY (speed, that is) as far as as the external things are concerned. POWER is directly proportional to fuel consumed.
SUMMARY of the math:
One formula says that drag force is proportional to square of the speed.
The other formula says that the power required is proportional to the
cube of
the speed.
Both formulas are effective at the same time for your motorcycle.
Power needs go up MUCH faster than drag force, as speed is increased.
What does all this really mean...well, think about it....if the actual power
needed is going up at a faster rate than the drag force
effect....just one way of several thoughts....then the FUEL need
is going up at a faster rate
too! Can you can see that
the final effect is
someplace between the square and cube functions?
All the things discussed are the reasons why your motorcycle gets comparatively lousy mileage, compared to a car, which is vastly heavier, with all THAT entails. If you were to put a sidecar on your motorcycle, the fuel mileage will GREATLY decrease....due to the greatly increased Cd.
NOTE!!......There can be many reasons for poor mileage. Alcohol-laced fuels (Gasohol); R80 engine with wrong jetting right from the factory (article is on this website: R80 fuel mileage problems); high speeds; higher ratio rear end ratio, etc.
Rev:
05/25/2009:
initial release.
01/03/2010: slight updating
06/01/2011: cleanup
© Copyright, 2011, R. Fleischer
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