What is horsepower and torque and how do they relate to a car being fast or a truck having a towing capacity?
In: Engineering
Horsepower is a product of speed and force (it also depends on the direction of these things aswell). Or of rotational speed (rpm) and torque (Nm, lbft, etc.)
Let’s look at them separately first.
rotational speed is how fast something is spinning, how many times it rotates every minute.
Torque is “how much force is behind that turning motion”. A Nm (newton-meter) is just 1 newton of force with a lever that’s 1 m long. (or 2 Newtons of force with 0.5 meter lever).
And one lbft (foot-pound) is the torque that 1 lb for force makes with a 1 foot long lever (or 2 foot long lever with half pound force).
These 2 units are essentially the same thing, only one is bigger than the other, like seconds to minutes.
There’s this thing called a gearbox that can speed up rotation at the cost of torque; or increase the torque of something, but at the cost of speed.
Doubling the torque means halfing the rotational speed.
Because we have this, it makes sense to talk about the product of speed and torque; something that stays constant no matter what kind of gearbox you put it through (ignoring the losses of course). That’s called power. And horsepower or kW (kilowatt) is a unit of power.
If you have an engine with a lot of torque and low speed you can put a gearbox behind it to make it “seem like” it has more speed and less torque.
Similarly, if you have a fast engine with low torque, you can still put a different gearbox behind it so it seems to have more torque, but less speed.
If you want a fast car then power (in horsepowers or kW) is the most important thing, since you need to put down a lot of force (or torque) at high speed. And that means a lot of power.
If you want to tow, then horsepower is still the most important factor of the engine. Yes, you need more torque at the wheels, but that’s exactly why the gearbox is there. It takes the fast speed of the engine, slows it down and increases the torque by a lot, so you can tow stuff.
However, with towing the mass (or weight) of your car is also very important, because a light car can have trouble pulling up a heavy trailer up a hill. Even though it has enough power (and torqe) to do the job, but the wheels would just slip.
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