eli5 Why are tractors able to pull hevier stuff than cars/trucks with 3x more horsepower?
In: 7
Because tractors are lower geared and have more torque. Meaning more power goes to the ground. The more torque you have the easier it is to get something moving.
Its torque that counts not horsepower. Bets explanation ive heard about the difference is, horsepower is how quickly you hit a wall and torque is how far you push said wall.
There are two important measurements of how “strong” something like a motor is.
The first is “Horsepower” and the other is “torque”.
Imagine using a motor to pull a rope with a heavy weight at the end of it.
Horsepower determines how quickly the the weight gets pulled in, torque determines how heavy a weight the rope can pull. So high torque, low horsepower and the engine can tow in a really heavy weight, just slowly. High horsepower low torque the engine can tow in a small weight really quickly, but probably can’t pull in a super heavy weight at all. This is a bit of an oversimplification because engines and vehicles are more complicated and you often really want a good mix of both but it’s good enough for this concept.
A tractor can pull a heavy thing because it has high torque, but a tractor isn’t going to go 250 miles and hour down the autobahn because it’s design is for torque, not horsepower.
It’s due to the transmission. The transmission for tractors is geared for slower speeds, but in return they can pull heavier loads.
Weight and torque. Weight increases the coefficient of static friction between the drive wheels and the ground and the torque get’s the load moving and maintains speed (horsepower is the rate at which you apply torque).
Just as an example, a Ram 3500 with a High Output Cummins B6.7 (which is 6.7L of displacement) is spec’d at 420 hp (at 2800 rpm) and 1075 lb-ft of torque (at 1800 rpm). In comparison, the Peterbilt 378 with a C13 (12.5L) Caterpillar that a friend of mine just bought (used) is rated at 400 hp (at 2100 rpm) and makes around 1450 lb-ft of torque (around 1400 rpm). Power output between the two isn’t that far off…toque output is fairly significantly off.
Now the primary difference is in the final gear ratios. The Ram 3500 has uses an Asin 6 speed transmission that has a a 1st gear ratio of 3.74:1 with a 4.10:1 rear end gear which gives a final output of around 16,484 lb-ft (not accounting for losses through the transmission which will reduce the power output by 20%) at 1800 rpm. The Peterbilt has an Eaton-Fuller 10 speed transmission with a 1st gear ratio of 14.8:1 and a 4.11:1 rear end gear which gives the finial output of 88,200 lb-ft at 1400 rpm.
It’s all about the gearing.
A lot of comments about torque here, but there is a crucial factor that is mostly overlooked. Weight. Because a vehicle mostly uses its own weight to grip onto the ground, the force it can pull with is limited by its weight.
Cars are light. Trucks too. Tractors tend to be significantly heavier.
It’s all about mechanical advantage. Horsepower is a measure of power, 1 hp = 746 W, and power is a measure of energy (or work) over time, and work is force applied over a distance W=F•d
Mechanical advantage is how much you sacrifice the distance moved for more force. Since work is a measurement of energy, it must be conserved, so if we want to double our output force, we would need to use our normal input force over twice the distance the output would move.
You can apply it to any of the simple machines (pulleys, levers, ramps, etc) but here we are using gears. Calculating the mechanical advantage is as easy as calculating the ratio of teeth on two connected gears. A 20 tooth gear rotating a 40 tooth hear would spin the gear half as many rpm, but with twice the force, MA = 2.
If we do the reverse, 40 spinning 20, the gear would spins twice as fast, but only with half the force, and our MA = 1/2.
If you have lego technic stuff, this is super easy to play around with.
A tractor has so much more pulling force, because it uses a lower gear to get more mechanical advantage, but as a result, it can’t go as fast. Trucks can go 65 down the highway, but a tractor can’t for the same reason. It’s a tradeoff between speed and power. I answered a [recent post](https://www.reddit.com/r/explainlikeimfive/comments/vvbnr7/eli5_how_do_gears_in_a_bike_work/ifitybs?utm_medium=android_app&utm_source=share&context=3) about bike gears, and it’s the exact same thing, and can also apply to car gears. If you’ve ever driven a car with a manual transmission, you’ll know it’s very hard to go fast in a low gear, again, it’s mechanical advantage.
“With a big enough lever, I could move the Earth.” Archimedes
Fun fact: on roads you may see “steep grade, trucks use lower gear. This is so the truck, while coasting down the hill, doesn’t have to use the breaks as much. When the wheels turn, the internal resistance of the engine, multiplied by that mechanical advantage will help slow the truck down. This is because the pneumatic brakes on the truck don’t like being used rapidly over and over again in fast succession at high speeds, and this reduces the chance of failure.
Fun fact: if you shift your car down to a lower gear while moving very fast, the engine makes a very loud noise because it is now spinning much faster than it was a few seconds ago and you slow down a lot without your brakes. (This is very bad for your engine and transmission, but people do it to be “cool”)
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