eli5 Why tractors have unproportionally big rear wheels?
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Ground pressure. Tractors used to be extremely heavy when they were steam powered, and have remained very heavy ever since. To prevent the wheels or tires (they used to have steel wheels with no tires) from sinking into the soil under all that weight, they had to have as much area as possible in contact with the ground. Ideally this means treads, like on a tracked vehicle. But treads are complicated, expensive, and hard to maintain. So they just made their wheels, which are relatively cheap and easy to maintain as big as possible. The wider and bigger in diameter the tires are, the lower the ground pressure they put on the soil, the better. They are larger on the rear as that’s the driven wheels on most tractors and where most of the weight is concentrated when pulling equipment. The front wheels hold up a smaller amount of weight and are not driven, and so do not need to be as big to distribute as much weight, and it’s easier to turn smaller wheels, requiring smaller and cheaper steering gear.
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