eli5 Why tractors have unproportionally big rear wheels?
In: 1
It’s because the heaviest part is on the back, so in order to have a larger surface area, the wheels are bigger
Because if they weren’t huge with huge treads, the tractor would have a *really* hard time moving itself, let alone on any challenging terrain (i.e. mud).
Better for going over uneven and soft ground.
Like if you look at tank treads (or for like heavy construction equipment) that’s really well supported, lots of flat areas that can bridge over holes.
A small car tire might fall into a hole and now you’re just perched on the car frame. Not so good.
The big wheels are in between. Nowhere near the complexity of a tread but wide enough and with a flatter curve in the outside to spread weight better.
That means less pressure on any single point on soft ground so it sinks less, and more ride height so that if it does sink you don’t get stuck.
Front wheels don’t really matter as much cause they are just there for a bit of stability and steering. It may make the thing more uncontrollable in some situations but you aren’t mudding with a lifted truck. Big tires are expensive. Mechanisms to turn them sideways as well. If you can get away with two big ones, why pay for four.
A reason not mentioned thus far is to clear crops. It’s easier to pass over the top of plants at certain stages of growth. Making the tractor narrow (like the front of a row tractor) would decrease side to side stability.
Because if you had large front wheels that are very hard to turn, they will hit the frame and engine of a tractor. So a large rear wheel for traction and load and a small one in front so you can turn is common. You can see the same for lots of horse-drawn wagons.
The wheel is large because it spread out the loaf on the soft ground better and makes it easier to drive over uneven ground. For a tractor the load is usually on the back having large wheels there is an advantage.
There are back loader tractors where the large wheel is in front, The design is a regular design. It is the drive compartment that is reversed. The design is used for digging and material moving applications not plowing fields, The loo like [like this mode](https://sv.wikipedia.org/wiki/Baklastare#/media/Fil:Volvo_Back_Loader_1976_front_q.jpg) The rear wheels turn and the front are fixed
There is also an articulated tractor [like this](https://static.agriculture.com/styles/width_550/s3/image/2021/04/02/International%20Harvester%204366%20hero%20image.png) where the wheels are all large and equal size. The wheels are all fixed to the frame. It is the frame that is split between the model so the front and rear can point in a slightly different direction. Because the wheels are fixed you can have large both from and back.
You can make the very large for farming with multiple wheels at each spot [like this](https://www.rbauction.com/cms_assets/category_images/1502501470/1502501470_W_S.jpg)
Articulated tractors have to a large degree replaced the backloaded design for large loaders. They look soothing [like this](https://power-equip.com/wp-content/uploads/2016/08/WLO_L90H_3_W_14394_01_Right.jpg). The articulation can be seen and is just below the driver’s cabin.
The back-loaded design is still used today in skid-steer loaders [like this](https://en.wikipedia.org/wiki/Skid-steer_loader#/media/File:Bobcat_S650_skid_steer_loader_(15084521732).jpg) the right and left side wheels can run at a different speed, that’s how you steer them. In applications where you have size limitations, they have a clear advantage over articulated tractors
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.
thanks for responses! eli5 sub is really cool!
To get lot of traction on soft terrain.
On mud you want big wheels or tracks. This makes your wheel “work” on more soil, meaning you destroy the soil less. If you destroy the soil, your wheel will dig a hole and you can actually dig on the spot until you sink completely.
Now, tracks tend to cost more, wear more and damage roads.
Big wheels are cheap, easy, and fuel efficient too.
About front vs rear, the front is less loaded when you work, as when you work with the tractor, the pull force will shift your weight to the rear. So the front has less troubles with sinking.
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