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Why do tractors have smaller wheels at the front and larger wheels at the back?

08 May 2017

Each week, we ask you to share your questions about farming, food or Sacrewell with us on Facebook. The person who writes the best question wins a free tractor ride for a child and accompanying adult, as well as having their question answered on our website. This question came from Shaun Coulam’s little boy.

The word "tractor" is related to words like "traction" and "tractive," from the Latin word "tractus" meaning drawing (pulling): a tractor is essentially a machine designed to pull things along, usually very slowly and surely.

Tractors have large and powerful diesel engines and, in theory, that means they should be able to go incredibly fast, just like sports cars. But in a tractor, the engine's power is designed to be used in an entirely different way: for pulling big and heavy loads. The tractor's gearbox converts the high-speed revolutions of the mighty diesel engine into much lower-speed revolutions of the wheels, increasing the force the tractor can use for pulling things at the same time. A tractor's incredible pulling power must come at the expense of speed.

Most tractors have a two-wheel drive, with the large rear wheels driven from the engine (the rear wheels are the drive-wheels and are connected to the drive shaft) and the smaller front wheels are used for steering. We explore some of the benefits of a two wheel drive below.

Grip or Traction

Farm tractors spend a lot of their life working in muddy, bumpy fields. If you’ve ever been in car that has driven over a muddy field, you will know that the car slips around on the surface of the mud, or it gets stuck. A large tyre on a tractor has much better grip pads that can ‘bite’ into the ground, as well as a large surface area that distributes weight more evenly which means the traction is a lot better. Also, because a tractor is usually pulling things, the heavy weight behind it pushes the rear wheels down, increasing their grip by providing more contact and less slippage.

Steering

The two smaller wheels at the front have a much better steering radius which means it’s easier to turn sharp corners. This is really important to cover the maximum area of the field while carrying out different jobs like ploughing, sowing and harvesting. Being light weight and small is also really beneficial for ease of control.

Visibility

The large rear wheels of the tractor fix the driver’s seat at a higher elevation which ensures good visibility of the nose of the tractor and the corners of the field it ploughs.

Cost

Small tyres cost less than larger tyres, and therefore it is much cheaper to replace small tyres that the very expensive big ones! It’s worth noting that because the rear tyres have a much thicker tread that they don’t need to be replaced as often as the front ones.

Weight distribution

Having the driving axle higher above the ground means the tractor can pull more weight without the front of the tractor rising up. It works like a lever, where twice the height means twice the maximum pulling force before the tractor tips over (see the illustration below, image credit: Peteris)

It's also worth mentioning that this works the other way too. A tractor engine is very heavy and is located at the front of a tractor, potentially causing it to tip nose first. Larger heavier wheels at the rear of the tractor distribute this weight more evenly to counter a heavy front. 

Soil conservation

Tractors are used to prepare the ground before the seeds are sowed, as well as throughout the crops life to maintain healthy plants. A farmer needs to look after the soil as well as the crop. Because the rear tyres have a much larger surface area (they are larger in diameter and width), the weight of the tractor is distributed across a larger area and the tyres don’t compress the soil as much (plants won’t grow as well in compacted soil).

 

References

https://physics.stackexchange.com/questions/145571/dying-question-tractor-wheels-large-vs-small

http://www.explainthatstuff.com/tractors.html

http://megaanswers.com/why-does-a-tractor-have-different-sizes-of-wheels-in-the-front-and-back/

https://www.researchgate.net/post/Why_are_the_rear_wheels_of_a_tractor_made_bigger

https://www.quora.com/Why-do-some-vehicles-have-small-wheels-in-the-front-and-big-wheels-in-the-rear

https://www.crazyengineers.com/threads/why-do-tractors-have-big-wheel-at-rear-and-small-ones-at-front.60053/