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Why do tractors have big wheel at rear and small ones at front?

Discussion in 'Mechanical | Automobile | Aeronautics' started by white_hat, Oct 20, 2012.

  1. white_hat

    white_hat Addict

    Engineering Discipline:
    Automobile
    Tractors used in farming,they have big wheels at rear and small wheels at front.Lets have a discussion on this topic and lets find out how many reasons we can come up with.
     
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  2. Kaustubh Katdare

    Administrator

    Engineering Discipline:
    Electrical
    I've had that question for long time and I used to ask that question almost all the times I spotted a tractor. First - not all tractors have the front wheels smaller than the rear ones. You'll have to observe the fields in which they're employed. The rear wheels, typically are the 'drive wheels'; to which the engine is connected. They provide the necessary traction to the vehicle in muddy lands. The front wheels are only for the direction. The bigger wheel solves the purpose of not getting stuck in the muds.

    Plus, the rear wheel helps in getting over smaller obstacles easily; imagine this when the tractor is moving on uneven land which is where the tractors usually move.

    However, if there's a different logic to the design, I'm all ears! This is quite an interesting question.
     
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  3. PuneetVerma

    PuneetVerma Maniac

    Engineering Discipline:
    Energy Systems
    In my opinion, we are more intrested in torque rather than speed in case of tractors. Mostly tractors are used for pulling purposes, so we need high torque at low speed at rear end. To get high torque, we use bigger wheel, larger the radius, larger will be torque..
     
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  4. Abhishek3349

    Abhishek3349 Newbie

    Engineering Discipline:
    Mechanical
    I am with Mr. Big K and wanted to add, It's difficult to steer a tractor if it has Big wheels in front too :)
     
  5. zaveri

    zaveri Star

    Engineering Discipline:
    Mechanical
    High torque and greater force to plough the land is a requirement.

    the smaller wheels in the front, makes steering easy, reduces the overall weight and moment of inertia of the machine and finally even the cost.
     
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  6. white_hat

    white_hat Addict

    Engineering Discipline:
    Automobile
    Tractors have a high displacement heavy duty diesel engine.So maybe the bigger rear wheels help in maintaining a 50:50 weight distribution.It helps in the overall stability of the tractor.
    Correct me if i am wrong.
     
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  7. Gurjap

    Gurjap Maniac

    Engineering Discipline:
    Mechanical
    You need maximum traction to plow and pull (hence the name 'tractor'). For that purpose, the rear wheels need to be large and wide enough. Why only the rear wheels? Because usually only they are powered by the engines.

    That being said,four wheel drive tractors have all their wheels tall and wide.

    While transmitting power (in mechanical engineering) you deal in two type of coins: speed and torque. You can sacrifice speed for more torque, and vice versa. How can you do that? Say a small diameter gear drives a large diameter gear ..for visualization purposes imagine a ceiling fan motor geared to a ferris wheel. The motor has more speed but less torque, the ferris wheel has more torque but less speed.

    So a diesel engine powers a shaft with small diameter, high speed and low torque. It is then geared to a wheel with a large diameter. The result? The wheel acquires more torque. What does that mean physically? It means that although the wheel has a slow rpm, it has enough "force" to do heavy duty stuff like pulling out of mud and plowing the field and pulling heavy things.
     
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  8. PuneetVerma

    PuneetVerma Maniac

    Engineering Discipline:
    Energy Systems
    Gurjap
    Very Well explained mate....cheers :)
     
  9. Amith Gupta

    Amith Gupta Apprentice

    Engineering Discipline:
    Mechanical
    yes. torque is the main factor for which the rear wheels are big. it helps in climbing different terrains with ease.
     
  10. Tyler Miller

    Tyler Miller Newbie

    Engineering Discipline:
    Aerospace
    Increasing the diameter of the rear wheel would decrease the force applied at the ground. The engine applies a torque to the rear axle, spinning the tire, which applies a force to the ground. If you wanted a larger force applied to the ground, you would need a smaller wheel. A large wheel requires more torque from the engine to drive the tractor forward.
     
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  11. msuresh444

    msuresh444 Enthusiast

    Engineering Discipline:
    Mechanical
    Basically, tractors are made for very high torque applications. The entire underbody is of transmission+engine!
    Large rear tyres, give a better grip in murky/loose soil conditions, front ones are only for steering.
     
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  12. nooneperfect

    nooneperfect Apprentice

    Engineering Discipline:
    Automobile
    If u put smaller wheel in rear wheel and bigger wheel in front since the rear wheel.is connected to engine power shaft imagine what will happen to rear wheel....it will rotate at higher rpm since wheel dia. Is less also the highest torque produce will not sufficiently drive a whole tractor It will just spin around, since we added lot more weight in frontal Axel Its no of use with low torque and high speed .
     
  13. G S Randhawa

    G S Randhawa Newbie

    Engineering Discipline:
    Mechanical
    1- Large dia wheels will rotate slowly (less rpm's) for a given speed w.r.t. a small dia wheel. That means when wheel have slower angular speed, slippage will be less. In tractor wheels we have large torque & to use the torque, wheels will tend to slip but to trap that torque to move large loads, we need to reduce wheel slip to gain productive use of engine power.
    2- We also need high ground clearance, so large dia wheels just do that most reliably
     
    #13 G S Randhawa, Jun 21, 2015
    Last edited: Jul 5, 2015
  14. Selvakumarchellu

    Engineering Discipline:
    Automobile
    Smaller rear wheels of tractor won't be pull the vehicle, and gives stability. And smaller rear wheels are not enough to take up the two different power and speeds.
    Bigger front wheels are difficult to steer.
     
  15. Maria Dass

    Maria Dass Newbie

    Engineering Discipline:
    Mechanical
    That was a good explanation, I would like to give an example of it . When we try to pull somebody we try to get a better grip on ground to pull or while pushing not only the grip power also matters . This is the reason why the rear wheels are made bigger and wider .coming the front wheels these are made smaller for easy steering purpose and for equal load distribution.
     
  16. Chirag Mehta

    Chirag Mehta Newbie

    Engineering Discipline:
    Mechanical
    Torque has nothing to do with the thickness of the tyres!!!!
     
  17. Anil Kar

    Anil Kar Newbie

    Engineering Discipline:
    Mechanical
    Large rear wheels of tractors facilitate the following
    1. High torque, less slippage
    2. High ground clearance
    3. Stability
    4. Pulling force applied is horizontal and upwards
    5. Ease of fitment of farming attachments.
     
  18. G S Randhawa

    G S Randhawa Newbie

    Engineering Discipline:
    Mechanical
    Gujrap has explained well but just to make it simple i add little more;
    1- Driving wheels are bigger in dia, it may be in front or rear. Commonly tractors have rear wheels bigger as majority of implements are connected at rear and the implement weight (in part) gets added to tractor rear weight and u get added traction through rear wheels due to increased weight on rear wheels.
    2- Whereas in combine harvesters, front wheel is larger than rear as most of m/c weight in concentrated in front. To enhance driving wheel traction, rear wheels are filled with water, add cast weights etc, as and when & how much is required.
    3- One aspect is common that smaller dia wheel will be the steering wheel. This is so that it is easy to steer smaller wheel, has lesser weight on top of it. More over, smaller wheels while in steering mode, do not come in contact with body parts and we get max turn angle for a smaller turn radius requirements.
    4- Now for driving wheels, we need to turn it slow in terms of angular rotation to get higher traction, but to achieve a particular speed it's radius need to be increased without increasing rotation speed. So larger wheel gives us Traction due to slow revolutions & we also get required speed due to bigger radius. High ground clearance which is one of the main off road operational needs comes as an added advantage.
    5- With 2WD tractors, only part of engine power can be utilized as wheels start slipping beyond a certain load being pulled (traction has limits) which is approx 35-40% of engine power.
    This is one reason that now 4WD tractors have gained popularity, which can trap approx 55% engine power.
    6- To utilize engine power beyond 4WD drive tractors, PTO drive implements have gained popularity which can utilize approx 80% of engine power. When PTO implements are milling the soil, traction power required through wheels is <10% and bulk of power is utilized by PTO (power take off shaft)

    G S Randhawa :happy::birthday:
     
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