What is horsepower and torque and how do they relate to a car being fast or a truck having a towing capacity?
In: Engineering
Torque is rotational force. What keeps a wheel spinning is a force, and that force is called torque.
When you spin something, you are applying a torque to that object.
Of course, where there’s motion, there’s energy, so we know that where there’s a torque, there’s also some energy. Energy in a specific time frame is called power.
Horsepower, as the name suggests, is a unit of power.
Obviously, the more torque you give, the more power you get in return.
With that being said, there’s another factor that’s part of this – revolutions per minute (RPM). That is, how fast the object spins.
When horsepower stays the same, torque decreases at the same rate at which RPM increases. You can switch horsepower, torque and RPM in this statement in any way you want and it will still be true.
In other words, torque, horsepower and RPM are directly proportional to one another.
The car’s engine produces torque. Horsepower is then calculated based on the torque produced and the RPM at which it is produced.
However, as far as combustion engines go, the horsepower/torque figures you hear about cars are not fixed values. These are peak numbers. That is, at a certain RPM, the car will reach its peak torque and at a certain RPM, it will reach its peak horsepower. Google “(car name) dyno sheet” and you’ll likely find a sheet that shows this.
Now, how does torque affect how fast a car goes?
When traveling at a certain speed, there are forces trying to stop you, like air and rolling resistance. With that in mind, we know you need at least a little bit of torque to keep your current speed, or else you’ll eventually stop.
The more torque you have, the more resistance you can fight against.
If you have only the torque needed to maintain, not gain speed, you have reached your top speed. Giving the car more gas will have no effect.
The more torque you have than is necessary, the more potential you have to move mass. So if your car is lightweight, you will take less time to reach higher speeds. If you’re towing, having more torque means you can gain more acceleration with more weight being towed.
And last but not least, you might be thinking: how come a 700-horsepower sports car can’t tow like a 200-horsepower pickup truck?
Simple: refer back to horsepower figures not being a fixed value. The truck actually has MORE power at low RPM, which obviously means more torque. The sports car cannot start moving if there’s too much weight to tow because its torque is less than what is needed to start moving when towing X kilograms. It cannot reach 700 horsepower because the load being towed stops it before it reaches optimal RPM.
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