Tow trucks
Question: When racing cars, motorbikes or bicycles 鈥渟lipstream鈥濃�攆ollow
closely behind each other on a racing track鈥攖here is undoubtedly a benefit
for the trailing competitor of reduced wind resistance and the pull of the
vacuum created by the vehicle in front making a hole in the air.
However, I鈥檇 like to know if there is a corresponding cost to the leading
vehicle in having a competitor following closely behind.
Cars in a drag
Question: My car is small and so slow that even trucks and buses outpace me
on the local highway.
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When such a vehicle approaches from behind, I always experience a very
distinct drag at a point when the nose of the large vehicle is still perhaps a
metre or two behind my car.
It seems that a wave of air in front of the larger vehicle pulls me towards
it, instead of pushing me forward, as one might expect. The turbulence around
the sides of the large vehicle, on the other hand, does indeed push me sideways
and away from the passing vehicle. What鈥檚 going on? And is the shape of my car
important in this worrisome process?
Answer: The effect described is well known to motorcyclists. When you
approach a large, fast-moving vehicle from behind, you notice a number of
distinct zones of air pressure around it, each with its own properties.
The most famous is the slipstream, the low-pressure area just behind the
vehicle. This forms because the air that the truck pushes out of the way in
front of it lags a little when it sweeps round to fill the truck-shaped hole
behind the vehicle. If you are sufficiently young or foolhardy it is possible to
tuck your motorbike into this very still area, where there is no head wind and
the air displaced by the truck鈥檚 passage is actually pushing you forward as it
tries to rush from behind into the ever-departing gap. I used to be able to get
an extra 15 kilometres per hour out of my first 2-stroke motorcycle. This is
especially useful on long upward inclines, if you aren鈥檛 worried about riding
less than a metre behind your 鈥渢ow鈥� with absolutely no hope of surviving if it
brakes.
Reinhard Reading
Nutborne, West Sussex
Answer: A constantly changing, partial vacuum is always present just behind
any moving object, because air rushing in to fill the gap takes time to get
there. This vacuum pulls the object backwards. This is called drag. The faster
the object, the greater the vacuum and the more drag.
A bus slowly gaining on you, pushes a band of high-pressure air before it and
feeds this into the low-pressure area behind your car, decreasing the vacuum
temporarily. Slightly less drag means you will increase your speed a little.
Shortly afterwards, at a critical point (which is very sudden), the bus
experiences the pull of your vacuum and simultaneously increases this
dramatically by blocking off the in-rushing air with its large, flat front. Both
vehicles are 鈥渟ucked鈥� equally towards this extra-low pressure area. However,
because your car is much lighter, the net effect is that immediately after
speeding up slightly, you will decelerate much more than the bus
accelerates鈥攊t will feel as though you are being yanked backwards.
Then, as the bus pulls out to overtake, it first breaks the increased vacuum
in your wake, pushing you forward. Finally, it subjects your offside rear to the
band of high-pressure air spilling around its front near side, pushing you
away.
Dorothy Reich
Newchurch, Isle of Wight
Answer: Much of the energy loss experienced while moving through fluids
happens at discontinuities. This is particularly true at the front and back of
vehicles, so close coupling usually helps not only the following vehicle, but
also the leading vehicle.
If the vehicles are suitably close, the gap between them can be filled with a
pocket of air, making them behave almost as one long vehicle with the front
vehicle paying for the energy losses from the front and the rear vehicle paying
for the energy losses from the rear, and each sharing the cost of the relatively
small energy losses from the middle.
Keith Anderson
Kingston, Tasmania
Answer: A cyclist travelling at high speed (about 40 kilometres per hour) on
a flat road expends up to 90 per cent of leg power overcoming the aerodynamic
drag and is therefore quite sensitive to small variations in wind
resistance.
This resistance is determined by the density of the air the cyclist travels
through, the square of his velocity and also by his frontal area and general
geometric shape. In calculations of drag, these are taken into account by a drag
coefficient.
Everything else being equal and within reasonable limits, this coefficient
tends to decrease with increasing lengths of the body travelling through the
air. For a 2-metre-long cylinder with a frontal area of half a square metre
travelling with its axis parallel to the direction of motion, the coefficient
will be 0.93. This will be reduced to 0.83 if the cylindar鈥檚 length is doubled.
While a cyclist is hardly a perfect cylinder, body length is effectively doubled
if a companion follows close behind.
Radko Osredkar
University of Ljubljana, Slovenia
Answer: With a closed-up line of bicycles travelling at 25 miles (40
kilometres) per hour, all the slipstreaming cyclists will experience an energy
saving of between 26 and 27 per cent. None of the slipstreaming bicycles would
have any effect on the bicycle ahead of it.
With a line of four cyclists riding at 25 miles per hour and taking equal
turns at leading, the energy each would use would be the same as if cycling
independently at 22 miles per hour. This information comes from an article by
James M. Hagberg and Steve D. McCole in Cycling Science, September
1990.
Alex Rothney
East Grinstead, Sussex
Answer: By being very close at high speed, racing cars (notably the American
NASCAR stock cars that race on huge ovals with banked turns) share the drag,
allowing two or more cars in a line to ease up on the throttle while maintaining
speed. When accelerating to overtake, the second driver will pull well clear of
the front car, making that car experience all the drag alone and causing it to
drop back and be more easily overtaken.
Alex Taylor
Sans Souci, New South Wales