For years I’ve carried in my wallet a small laminated 50 word paragraph from an article
written by the late great Gordon Jennings. I’ve quoted it before but here goes again.
To paraphrase; Mr. Jennings wrote that “the typical” motorcycle has 8.5 square feet of
frontal area producing 60 pounds of drag at 60 mph and a whopping 166 pounds of
drag at 100. He went on to calculate that it required only 9.6 horsepower to operate
such a motorcycle at the 60 mph level (no wind or incline factored into the numbers).
On the other hand one needs 44.3 horsepower to attain a speed of 100 mph on the
very same motorcycle.
In light of the above it’s easy to see why sport bike aerodynamics make so much sense.
Although dirtier than a car they are much cleaner than a so called adventure tourer
(whatever that is?) or a multisurface motorcycle. The huge downside of the sport bike
of course is the contorted body position and thin small seat one must endure in order to
achieve maximum fuel economy and speed per available horsepower.
On the least efficient end of the spectrum we have the so called adventure tourers.
With their huge square aluminum side and top boxes, higher wind shields, tool tubes,
tubular frontal guards and sit-up (or worse- stand-up) riding position; they require
substantial horsepower to get through the atmosphere. The increased power
requirements dictate larger and heavier engines and all the extra weight dictated by
increased power plus size.
We here at Top Gun Motorcycles have extensive off-road and enduro experience (Rod
Morris and I jointly have close to a century and mileage well into seven figures riding on
all types of surfaces) and we cannot believe any motorcycle weighing much over 400
pounds with half a tank of gas is anything more than a pavement bike that may be
occasionally used on firm well-graded, non-sandy dirt surfaces.
Kurt Grife, “Mr. Southern Baja” helped Clemente Salvadori research his definitive Baja
travel book, and although they featured a well known Adventure bike they pointed out
that heavy two-wheelers were less than ideal when the dirt roads got challenging. Kurt
and I both rode shaft-drive 4 cylinder Suzuki GS850G’s on some of the better dirt roads
in Baja; in fact, Pauline’s first off road two-up ride was behind me on an 850G in the early
1980’s. Clem, Kurt, Rod and many other long time experienced multisurface riders will all
tell you that when it comes to difficult dirt roads, “light is right”.
I guess I digressed some, but the point I’m trying to make is that the more you increase
your bikes frontal area and overall weight and the faster you ride, the greater the
horsepower and resultant fuel demand.
As I prepare to terminate this article I’d like to offer two personal experiences.
A couple of weeks before Pauline’s 85th birthday Baja ride she and I, in the company of
our principal riding companion Rod Morris, did our bi-annual two day 400 mile Big Bear
run. Since the ride is almost all two lane paved mountain roads with elevations ranging
from sea level to a little over 8,000 feet, I geared our KLR-A model 15/45 (next trip it will
be 15/46). Our frontal area is only slightly increased overall with no windshield and soft
rounded bags close to side racks.
The whole trip we ran mostly between 55 and 60 mph, but we kept our cornering speeds
high so we didn’t have to shift much. By staying primarily in 4th and 5th gears
on our stock tuned “A” model and not getting on the gas hard we were able to attain 55.8
mpg. for the two-up trip. Try that on a souped-up, over-jetted, 16/43 geared KLR
equipped with lots of extra drag.
The second example involves the author riding solo on his 2011 DR650SE. This 250
mile one day ride like the first one was on two lane mountain roads with elevations
ranging from sea level to 6,000 feet. By again gearing down to 14/42 (stock is 15/42),
being smooth and staying above 3,750 rpm in 4th and between 4,000 and 4,500 in 5th,
I was able to attain 59.8 mpg. for the day.
By minimizing wind drag, limiting overall weight, and shunning high speeds one can
expect better mileage especially if overall gearing is correct. Stock carburetor tuning is
helpful in mountainous southern California. You may run a little lean at sea level, but at
the higher elevations the engine will run significantly better than one with a dirty
over-jetted carb running way too fat at sea level.
The most you should have to do to a KLR or DR carburetor is open the pilot screw
slightly but only if it idles poorly. If the air box cover is removed and an open loud
muffler is installed all bets are off. You may never get it jetted properly and usually
your gas mileage goes down as your pollution output greatly increases.
I know two riders who tuned their DR650’s for more power and both have returned to
stock configuration. Neither of them liked mileage figures in the thirties or the loss of
power at high altitudes.
The prospect of a premature decarbonizing heads and cylinders at significant expense
is also not appealing especially in this age of the worthless dollar.
Running clean and quiet has it’s dividends.