This question has stumped me for over a month now:
Why is it that a tractor has large wheels at the back and small wheels in the front?
Current ideas:
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small wheel in front –> lower center of mass–> less likely to tip over, moment.
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large wheels in back provides more torque, since friction is the driving force of the wheel.
Let's assume that there's a load behind the tractor and a cable is connected to the tractor and cart that carries the load. FBDs and moment equations for the wheel and load are highly appreciated.
How does the extra torque help pull the load? and how does it provide more torque? if the engine has a CC moment of 100, the torque from the friction opposes that, and why would I want more torque from the friction? Wouldn't that slow down how fast I'm able to pull the load?
Sum of Moment at the wheel's center=Applied moment- F_f*radius of wheel.
Best Answer
Leverage
Often, a limiting factor in tasks required from a tractor is the amount of pulling force a tractor can apply without tipping over (the front rising up) - the engine is strong enough to do so.
Having the driving axle be high from the ground helps by simple lever action - see the illustration; twice the height means twice the maximum pulling force allowed before the tractor tips over. The other thing for the same problem that's commonly done is to attach a large heavy object far to the front - https://www.google.lv/search?q=tractor+front+weight&tbm=isch
[edit] - a more accurate but more complex illustration of the actual leverages.