Our readers are talking about two different things.
Torque and "torquey" have two very different meanings. Torque is a
physical measurement with a precise definition. But when riders say a
particular bike feels torquey, what they mean is not that it produces
high torque on a dyno, but that its engine can accelerate the bike strongly at almost any rpm in its operating range.
The opposite of torquey is peaky, which means instead of having torque
uniformly distributed across its operating range, its best torque exists
only at higher rpm, where it can make a lot of horsepower. When you
ride a peaky bike in its midrange or low-end, snapping the throttle open
gets you only moderate torque, resulting in weak acceleration. But if
you tap down two gears, putting the engine up at 9,000 to 10,000 rpm,
the bike rockets forward because now it is operating up where it makes
its best torque.
The result is that if two bikes—let's say a Harley-Davidson Big Twin and a Suzuki
GSX-R1000—make a top-gear roll-on from 3,000 rpm, the Harley pulls
smartly ahead until the Suzuki can rev up into its rpm range of best
torque, at which point it evens the score and then pulls away rapidly.
These two kinds of engines represent the achievement of different goals by differently slanted compromises.
A touring or cruiser
engine needs strong torque at low rpm to start and accelerate a heavy
bike, so it is given quite short valve timings with almost no (or in
some cases, negative) valve overlap. Such short timings run out of
breath as the engine revs up, but because nobody tours or cruises at 150
mph or tries to break 10 seconds in the quarter-mile, it doesn't
matter. It's a compromise. The touring rider needs and likes bottom
torque and doesn't mind if the engine runs out of breath as it nears its
peak rpm of 5,500.
A peaky engine results from the longer valve
timings that can continue to fill cylinders well at higher rpm, such as
12,000. Short valve timings close the intakes before the cylinders have
had time to fill at higher rpm, causing an engine to run out of breath.
To overcome this, intake closing is delayed after bottom dead center for
50 or 60 degrees, and the intakes are made to begin opening 20 or more
degrees before top center. At low and mid rpm, this late intake closing
allows the rising piston to pump back out some of the intake charge it
has just taken in, reducing torque at those engine speeds.
At
higher rpm, this pump back does not occur because intake velocity is
then high enough to just keep coasting into the cylinders even though
the piston is rising on compression. As a result, peak torque is moved
to higher rpm. It’s a compromise: giving away bottom-end and midrange to
move the torque to high rpm where it becomes high horsepower.
In
physical terms, torque is a force, tending to rotate something around an
axis. It is measured as a force—ounces, pounds, kilograms—acting on a
lever arm of a specified length (inches, feet, meters). This gives us
the familiar pounds-feet or kilogram-meters in which torque is specified
in manufacturer brochures or road tests.
When Cycle World tests a bike on its dyno, the results are presented in the form of two curves on a graph—one for horsepower, the other for torque.
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| © Ducati Big speed requires big power: Ducati and Honda MotoGP bikes ridden by (from left to right) Andrea Dovizioso, Marc Márquez, and race winner Danilo Petrucci surpassed 215 mph this past June on Mugello’s 1,100-meter-long front straight. |
The
ideal toward which all manufacturers strive is to have high torque,
constant over a wide range of rpm, resulting in a nearly horizontal line
on the graph. Indian’s dirt-track racing engine, the FTR750, achieves
this—flat, constant torque—from 7,000 to 11,000 rpm. Because horsepower
is just (torque x rpm)/5252, the horsepower curve resulting from this
flat torque is a sloping straight line, rising from left to right.
Real
life is not ideal, so actual torque varies because such things as
intake and exhaust-pipe resonance and airbox effects exist. They cause
local variations in the height of the torque curve. In the case of a
traditional touring or cruiser engine, the torque rises to a useful
amount at as low as 1,200 rpm, peaks somewhere in the usual range of
2,500 to 3,000, and then slopes gently downward after that. Why? This is
the “running out of breath” effect mentioned earlier. As the engine
revs up, there is less and less time for cylinder filling, so the short
valve timing and moderate port sizes of such engines cause cylinder
filling to become less and less complete as the engine revs up, so
torque falls.
