eli5 Why are tractors able to pull hevier stuff than cars/trucks with 3x more horsepower?
In: 7
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.
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