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Everyone wants more torque and more power, right?
Well, the first step to getting there is to
understand what the terms actually mean, and many,
many people don't.
I've heard all kinds of crazy definitions:
"torque is what you feel, horsepower is just some
obscure mathematical concept"
"torque is what accelerates the bike, horsepower
is what maintains the speed"
"it's not how much torque you have that matters,
it's how much total torque AND horsepower you have"
"Horsepower is a formula, not a physical
sensation, where torque is a sensation!"
etc etc. Nice simple definitions that are easy to
get your mind around for sure. Unfortunately,
they also indicate a severe lack of understanding of
what the terms actually mean. It's not difficult to understand
torque and power and how they're related, but if you
read and believe stuff like this, you'll make it
harder than it has to be.
Forget about horsepower for a moment, let's define
torque and the other ingredient of power, which is
rpm.
Torque is just twisting force. One ft-lb of torque
is literally defined as one pound of force applied
at a one foot radius. Well, if torque is twisting
force, and twisting force accelerates the bike, what
else matters? Isn't torque the only goal?
Actually no, and the reason is that torque says nothing
about how fast something is moving, only how much
force is being applied. But how fast it's moving is
enormously important. And, as it turns out, how much
force you're able to apply and how fast something is
moving are interrelated. It's easy to push hard if
you don't have to push fast, and it's easy to push
fast if you don't have to push hard.
To illustrate this, let's take an example.
Say we have a handheld drill motor. It's
turning 1000 rpm and it has 2 ft-lbs of torque. If
we hook it up to a gear that has 10 teeth on it,
and mesh that with another gear that has 10,000 teeth on
it, what happens?
By doing this we've applied a factor of 1000 gear
reduction. Rpm goes down with gear reduction, in
this case it'll drop from 1000 rpm to 1 rpm.
Likewise, torque goes up with gear reduction. In
this case it'll go up from 2 ft-lbs to 2000 ft-lbs.
Wow, 2000 ft-lbs of torque from a handheld drill
motor. We've really accomplished something, huh? We
can move a mountain. But wait ... it's only turning
1 rpm! We have to time it with a calendar.
So what we've accomplished is that we've generated a
LOT of torque, but unfortunately, we've given up a
LOT of rpm. We're pushing very hard (torque) but
also very slowly (rpm).
What happens if we turn the gearing around, and
drive the 10 tooth gear with the 10,000 tooth gear?
The small gear will turn at one million rpm!
But it'll only have .002 ft-lbs of torque. Touch it
with your finger and it'll stop. We're now pushing
very fast (rpm), but also not pushing very hard
(torque).
The basic relationship is that if we gear deeper, the torque goes up and the rpm goes
down by equal factors. Likewise if we gear taller, the rpm goes up and the torque
goes down by equal factors.
Understanding this basic relationship between torque
and rpm and gearing is the key. Torque and rpm
are interchangeable entities! All you have to do
is change the gearing. Want big rpm? Gear it taller.
Want big torque? Gear it shorter. Hell, we just showed how we can make 2000
ft-lbs with a handlheld drill motor, but for that
matter, we could make a million ft-lbs with a drill
motor. All it takes is gear reduction. Making big
torque is no big trick if you don't care about
speed.
So to everyone who thinks torque is a performance
metric, well, put simply, torque is simply not a
meaningful number for evaluating performance unless
you also consider the rpm it's being made at. If the
rpm didn't matter, we'd all just gear gear our bikes
into the basement, and never take them out of first
gear. We have FAR more torque at the rear wheel in
first gear than any other gear. The torque and rpm
being produced by our engine go through gear
reduction in the primary, the transmission (except
in 5th), and the final drive. The gear reduction
through the transmission is deepest in first gear.
You want torque? Just leave your bike in first gear
all the time.
But nobody wants to ride
around in first gear all the time. Sure, you've got
lots of torque at the rear wheel in first, you can
probably even pull a wheelie you've got so much
torque. But you can't go very fast. Every time you upshift, you give up some rear wheel torque and
trade it for some rear wheel rpm. By the time you
get to 5th you may be able to go 100mph, but you
don't have enough torque left at the rear wheel to
pull a wheelie anymore.
Unfortunately, there's just no way to upshift and
gain more rear wheel rpm without also losing rear
wheel torque. So what we really want here, what
really will help the performance of the bike, is
to make more torque and more rpm at the same time.
Well, as it turns out, there's a term for that: it's
called "horsepower". Horsepower is literally torque
times rpm (divided by 5252, but conceptually you can
ignore that part, it's just to scale the number to
what Watt's horse could do). The term "horsepower"
describes the total combination of torque
and rpm, without specifying it's makeup. But for the
purposes of evaluating performance, it's makeup
doesn't matter. If it's not made of the combination
of torque and rpm that we want (and it's not), we just run it
through some gearing. That's what gearing is for.
In the
example above, the drill motor has .38 horsepower:
(2 ft-lbs x 1000rpm) / 5252 = .38. After we geared it down
to 1 rpm and 2000 ft-lbs, it still has .38
horsepower: (2000 ft-lbs x 1rpm) / 5252 = .38. The
gearing didn't change the horsepower, it only
changed the mixture of torque and rpm that makes up
the horsepower.
Let's
apply this to the real world in a simple example.
Roy Ricer has a 600cc inline 4 that makes 40 ft-lbs
at 15000rpm. He's up against Billy Biker with a
Buell making 80 ft-lbs at 6000 rpm. Who's going to
win?
Billy
Biker has TWICE as much torque as Roy Ricer. But Roy
Ricer's bike is turning two and a half times as many
rpm. That means he can gear his bike two and a half
times deeper than Billy Biker and still have the
same rear wheel speed. Well, if he can gear his bike
two and a half times deeper, that multiplies his
torque two and a half times more. Two and a half
times 40 is 100, and that's gonna put Billy Biker's
80ft-lbs on the trailer. At the rear wheel, which is
where it matters, Roy Ricer will have 25% more
torque than Billy Biker when geared for the same
rear wheel speed.
The bikes
I love and ride make
their power with relatively high torque and low rpm. OHC multis
on the other hand tend to make their power with
relatively low torque and high rpm. I like the
visceral feeling, the sound, the rumble, the ease of
pulling away, etc, of a high torque, low rpm V-twin.
But let's not kid ourselves. The fact that our power
is made of high torque and low rpm doesn't somehow
make it stronger than a bike with more power made of
low torque and high rpm. The makeup of the motor's
power isn't important for evaluating performance.
What matters is more power.
One point
of confusion, and I see it all the time, is when
people look at torque on a dyno sheet and call it
rear wheel torque. Seems to make sense, after all,
the measurement was made at the rear wheel, it's
rear wheel horsepower, must be rear wheel torque,
right? I even see this mistake made by veteran motor
guys as well as in magazine tech articles. Not
unusual at all to see a glowing report of "100
ft-lbs at the rear wheel", for example.
Well, let
me tell you, if someone really only has 100 ft-lbs
at his rear wheel, get a stock Blast and you'll blow
him into the weeds. Even in top gear the little
Blast has 4.97 of gear reduction between the crank
and the back wheel: 1.676 primary times 1.0 top gear
times 2.963 final. With 30ft-lbs or so at the crank,
that comes out to nearly 150ft-lbs at the back
wheel. When you're in first gear, you've got 13.35
of gear reduction between the crank and the rear
wheel giving you a whopping 400 ft-lbs!
The
confusion lies in interpreting the dyno's numbers.
It's not showing rear wheel torque, it's showing
engine torque as measured at the rear wheel, and
that's an important distinction. A Dynojet dyno
won't even show torque unless you use the tach
pickup, ever wonder why? It's because it needs to
understand the gear reduction that lives between the
drum and the crankshaft in order to calculate the
torque at the crankshaft, which is what it displays.
Notice how it plots torque against engine
rpm, not rear wheel rpm, and the torque crosses the
power at 5252 engine rpm, not rear wheel rpm.
That's because it's engine torque, i.e.
upstream of the gearing.
So now
that we know what horsepower is, how do we make more
of it? Make more torque and rpm at the same time,
that's how! How do we get more torque and rpm from
our engines? Well, the engine's torque is fundamentally
cylinder pressure and the mechanical advantage it
has on the crankshaft (bore and stroke both give it
mechanical advantage). Cylinder pressure comes from
filling the cylinder as completely as possible,
squeezing it tight, and burning it completely. Rpm
is how fast everything is happening. As rpm goes up
and things happen faster and faster, there's less
and less time to fill the cylinder. Hence the torque
wants to drop. If torque is dropping faster than rpm
is rising, stick a fork in it, it's done, because
our total combination of torque and rpm is lower.
So what
we do is concentrate on filling the cylinder at the
rpm of interest. Many people want that cylinder fill
at lower rpm so that their horsepower is made of
relatively higher torque and lower rpm, we
understand that. Cylinder fill is always the goal,
we just change the rpm of interest. Reducing losses through the intake
and exhaust tract, sizing the components for the
mixture of flow and velocity that gives maximum
fill, timing the cam events to match, and properly
utilizing wave travel effects in the exhaust system
are some of the ways we do this.
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