Let’s Talk Torque: The Power To Turn That Wheel

BY - 27th January 2018
 

Let’s do a quick science experiment to explain torque. (If you’re already groaning because we used the words “science” and “experiment” in that sentence :-O don’t worry, we’re only doing this in our imaginations). So, imagine a rod hung horizontally in mid-air (anything is possible in the imagination, right?) with a one-pound weight hung perpendicularly from it with a string. Pictured that? Good. Now picture yourself grabbing one end of the rod in your hands like a wrench and twisting it, so that the string starts winding itself around the rod, thereby lifting the one-pound weight. Keep twisting until you raise the weight by one foot. How much time did that take, one second?

2

Torque is the twisting force – or rotational force – applied by your hand that causes rotation. In fact, we all use that twisting force all the time. For instance, you apply torque three times when you simply open a locked door: turning the key, turning the doorknob, and pushing the door open so it swings on its hinges!

In our experiment, imaging the rod as the crankshaft that turns the wheel, which moves the entire car (the one-pound weight) a certain distance (one foot) in a certain amount of time (one second). We have applied a force of one foot-pound per second or 60 foot-pounds per minute. That’s the torque that your hand was able to exert!

In metric terms, you would measure torque as Newton-meters instead of foot-pounds. But the principle is the same.
The best part of torque is that it can be physically measured – by a device known as a dynamometer. This means torque is a force that exists in nature. On the other hand, horsepower is simply a manmade or derived number, not a force of nature.

So, how does this apply to your car?

From the performance car driver’s perspective, torque is the boss. Because it is torque that determines how quickly the car will accelerate at any given point – whether from the standstill or from 20 or 30 or 50 kilometres per hour. A car can accelerate only at a rate that exactly matches its measurable torque curve. It will accelerate hardest at its torque peak in any given gear, and will not accelerate as hard below that peak, or above it.

Corvette torque

That’s the reason car manufacturers will tell you the torque reading for different rpm (revolutions per minute) rates of the engine. If you were really particular, you would hit the gas when the engine reaches that particular rpm, to experience the best acceleration.

Torque also has a lot of variables that either increase or decrease it.

Some of them are:
• The weight of the car, obviously
• The power of the engine that drives the rotation of the crankshaft, obviously
• The difference in height of the engine from the centre of the wheels; the smaller the difference, the larger the torque
• The diameter of the wheels; a larger diameter will increase the torque applied to them
Because torque has so many variables and a rather dense formula that determines it, marketing a car is easier using the derived term horsepower. If you look at the formula and the graph below, you’ll notice two things: one, that torque is used as one of the variables to calculate horsepower, and two, that torque and horsepower always intersect at 5,252 rpm, at which point, torque starts to wane and horsepower starts to move upward.

torque

T = F x r x sin(theta), where
• T is the torque
• F is the linear force applied
• r is the distance from the axis of rotation to where the linear force is applied
• Theta is the angle between F and r

And now here’s a graph that shows how horsepower and torque intersect for an engine that delivers a certain amount of power.

In this case, we’ve looked at GM’s old V8 LT1 engine that was used in the Corvette and the Camaro:

LT1 chart

Now that we are agreed on the fact that torque is a measurable force of nature and horsepower is a derived number from torque, the question is: how is horsepower calculated?

This way:

HP = T x RPM ÷ 5252, where
• HP is horsepower
• RPM is revolutions per minute
• T is torque
• 5252 is the radians per second, the constant at which HP and torque intersect, remember?

If you think you’ve got the concept firmly embedded in your brain, here’s a quick exercise:

How much torque is required to produce 300 horsepower at 2,700 rpm?

Answer:
Since HP = T x RPM ÷ 5252, then by rearranging the equation, T = HP x 5252 ÷ RPM
Therefore, TORQUE = 300 x 5252 ÷ 2700 = 584 lb-ft.

Did you get it? Let us know!

 
 

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