Torque, in and of itself, does not make a bike go fast.  It is most often torque, however, that one builds to make their bike go faster.  Seems contradictory, doesn’t it?  Obviously, therefore, there are more easily understood forces than torque.  Hopefully I’ll be able to shed a little light on this topic.

To understand what torque is and what it does for you, it is first necessary to understand what a “force” is and how forces are measured.  Forces are measured in units that describe work accomplished.  They are called foot-pounds.  Quite simply, a force is measured by multiplying 1.) the weight of the object being moved by 2.) how far it was moved.  If I push on a 5 pound brick hard enough for it to move 2 feet, I have accomplished 10 foot-pounds of work.  That is, 5 feet X 2 pounds = 10 foot-pounds of work.  If I push the same 5 pound brick harder and it goes 3 feet, it would be 15 foot pounds of work.  Easy enough, right?

Notice that there is no time component in foot-pounds.

You probably assumed I whacked the brick and it moved instantaneously.  Maybe I slowly pushed it along.  It wouldn’t make any difference with regard to the amount of work being done.  If it takes 1 second or if it takes 1 hour, the force is measured the same.  Forces are not rates, they are a measurement of weight and distance alone.

In the example I gave, we are observing what is known as a “linear force”.  Any time the force exerted on an object results in that object moving in a line, the force is a linear one.  Torque is a force, and is therefore measured the same way.  Torque is NOT linear, though.  Instead, torque is rotational.  For instance, the force a screwdriver exerts on a screw is torque.  The force it takes to turn a knob is torque.  Turning a crank to raise a bucket from a well is an application of torque.  Unlike a linear force, the application of rotational force does not necessarily ‘move’ anything from point A to point B.  It makes something rotate.  This rotational force is often converted to a linear force by a mechanism to make something move – like the rope pulling the bucket up from a well.  So if torque just makes things spin, and since torque is measured in feet and pounds like a linear force, what distance are you measuring?  Unlike a linear force, the distance measured is the length of a lever arm.

Torque is the product of weight X the length of a lever.

For me, the easiest way to understand the elements of torque is to think about nuts, bolts, and wrenches.  Imagine a large nut on a large bolt protruding from a wall. Let's take a 2-foot long wrench and position it on the nut such that it is parallel to the floor.  Imagine the wrench looking something like the hand of a clock at 3 o'clock.  Now, suspend a 10-pound weight from the end of the 2-foot wrench.  The nut is experiencing 20 foot-pounds of torque.  If the wrench was 3 feet long, it would be 30 foot-pounds of torque (3 feet x 10-pound weight).  If a 3-foot wrench had a 20-pound weight, it would be 60 foot-pounds of torque (3 feet x 20-pound weight).

Note again that there is no time component to torque.  The wrench may turn the nut immediately or take an hour.  Either way, the same amount of torque has been exerted.

How does this apply to a motorcycle’s engine?  

In a four stroke engine like ours, the pistons accomplish different tasks at different times.  These are called cycles or strokes.  The first cycle is the “intake”.  The motion of the piston moving down the cylinder sucks fuel mixture from the carburetor(s) into the cylinder.  The next cycle is “compression”.  The piston moves back up in the cylinder and squeezes the fuel mixture to several times its original pressure.  The next cycle is “power”.  A spark plug ignites the compressed fuel mixture and the piston is pushed down the cylinder.  The final cycle is “exhaust”.  Valves open in the top of the cylinder and the piston moves up and pushes burnt fuel mixture – exhaust – out of the cylinder.

That third cycle – the power stroke – is where the engine generates force.  The burning fuel mixture exerts a linear force down on the piston.  Much like an old steam locomotive’s drive wheels, the piston’s up and down motion is converted to rotation with a connecting rod.  On the bottom of our pistons is a similar connecting rod.  This connects the piston to a crankshaft.  The distance from the center of the crankshaft to where the connecting rod connects to it is known as “offset”.  The length of the offset is the lever component in an engine’s torque.  Torque in a motorcycle engine is a product of how hard the burning fuel pushes the piston down and how much crank offset there is.  Hopefully you can see at this point that a long-stroke engine with a lot of crank offset would generate a lot of torque.

Torque, as measured in foot-pounds, is a measure of how much weight your bike’s engine can move over a given distance.  Torque is not a measure of how fast your bike is.  It is a measure of how hard the engine pulls.  The RoadStar’s engine, for instance, generates enough stock torque to whack a one pound object 98 feet.  Technically speaking, that is one hell of a lot of torque!

What does torque feel like?  Let’s use that example of a watermill’s wheel.  Picture a nice little stream rolling through the woods passing by an old watermill.  As the stream flows, it turns the wheel.  If the stream is small enough, you may be able to grasp the wheel and prevent it from moving.  Now imagine a larger stream pushing a wheel with wider paddles.  Assuming that the stream isn’t flowing faster, the wheel isn’t turning any faster.  What happens if you grasp the wheel, though?  Now make it a river.  These are examples of larger amounts of torque.  They are a measure of how ‘hard’ something is moving.

One can feel this in a motorcycle engine confidently chugging up a mountainside with a full load in top gear.  An engine with a lot of torque won’t even hesitate.  An engine with less torque will require more throttle to maintain speed.
 

Look for part three, a discussion about horsepower, in the next edition of Star Cruiser.  Part four will discuss methods Star owners can use to build torque and horsepower in their engines.