Does anyone know why the rear wheels of tractors are
so large?
Is there anything more to it than (a) being able
to drive over reasonable high obstacles and (b) not sinking
in into the ploughed field?
Isn’t it a problem that they have less torque, since they
are so large, or is this maybe a feature so as to avoid
spinning and throwing up mud and earth?
I would like an engineering type answer I guess
H
An educated guess:
I am not going to try to address any of the torque issues (I’ll leave that to someone more knowledgeable), but the sinking issue is very important. Larger wheels mean larger “footprints” which in turn means that the weight of the tractor is distributed over a larger area than if the wheel was smaller or narrower. This helps prevent the tractor from sinking into the soft soil, and increases the amount of traction by putting a larger area of rubber in contact with the ground.
By minimizing the wheels tendency to compress, dig into, or toss up the topsoil, not only do the larger wheels aid the movement of the tractor across soft soil, but they help to minimize the damage the tractor does to the field itself.
It really doesn’t have much to do with higher obstacles since the front axle would have to travel over the same obstacle. It does have to do with getting a larger footprint on the ground to spread out the weight and get better traction witrhout tearing up the field. In fact many of the larger tractors now being made will have shorter duals for the rear. Most of the real large tractors nowadays will have equal size tires (sometimes dual) at all four corners. Also treaded tractors are being used more and more, pictues crawler type tracks made of rubber in place of the usual tractor tires.
[engineering nitpick]
Torque produced by the engine is independant of wheel size. It is measured in units of length x force (ft-lb, for example). What you’re referring to is motive force at the tire surface, which is inversely proportional to wheel radius (and calculated by dividing the torque at the axle by the wheel radius).
All else being equal, a wheel with larger radius will transfer less motive force to the ground than a smaller wheel.
[/engineering nitpick]
This is strictly a WAG. I do think it has to do with the two things mentioned; ground clearance and making the tires big to avoid sinking.
Yes, the front of the tractor has to go over the same obstacles but the wheels are mounted on simple spindles which can be mounted on a fork that drops down from the tractor body. That wasn’t done with the rear wheels because of the added complexity of making an out-of-line drive train. Having once decided on that approach the companies simply continued it because, well, why fiddle around with something that works?
Beeble is spot on. The rear of the tractor is usually where the implements are mounted, hence it carries most of the weight. If the tires were of a small diameter, they’d tend to push into the ground more easily. Large diameter (and often large width) tires are used to reduce the ground loading. And the tracked vehicles like the Cat Challenger 75E follow the same theory; larger surface in contact with the ground means less weight per square inch. Hence, a tracked vehicle can get into a wetter field and plant and harvest at times a non-tracked vehicle can’t. The Challengers are a blast to drive by the way. And they are way faster than you’d think!!
b.
i dont know if i can explain this properly, but i’ll try. the large rear wheels give the operator a larger selection of speed of travel in the range of 2-10 mph without spinning. you can maintain a more precise forward speed. this is very important for doing a good job of spraying, mowing, planting etc. the weight of the large tires helps in traction. they can be run at lower air pressure, this improves traction. finally, many implements that are used behind a tractor are lifted clear of the ground by the hitch in the back, the rear tires carry the weight of the lifted implement( which can be thousands of pounds) this weight is too much for the smaller tires.
You simply don’t see this much on modern (even two wheel drive tractors) because it isn’t as heavy duty a design as a straight axle front end. Simpler is better and more reliable. Besides how high does a tractor really have to be ? It’s not like we keep waist high rocks or stumps in the middle of the field and by the time corn is high enough to be damaged by driving over it then there is no reason to be driving in that field unless you’re harvesting it.
thanks so far for the information
As Kamandi said,
So now I ask, doesn’t this have a consequence? I think it must
be the fact that this way you avoid spinning.
If Kamandi means that a larger tire effectivly increases the gear ratio I think this could be made up in the gearing of the tractor.
A larger wheel also takes more energy to rotate, but a the low speeds a tractor operates at I don’t think it is an issue.
WAG, with older tractors it was probably a cost issue using two large tires to carry the load. Four large tires would be better but tires are expensive and small front tires are easier make steerable. The tractor can also turn much tighter with small front wheels.
Of course, you have to push on the ground hard enough to move the load irrespective of the size of the tire.
Small tires can be made just as strong as big ones. The tires on a jet fighter aren’t particularly big, but they are sure tough.
Exactly. Think of the wheel as just a great big gear at the end of the gear train.
Could it also be that having large wheels means that the tow bracket is then easier to set below the axle height, which would then mean that the front wheels were forced downwards, rather than if the towing point was above the rear axle, in which case any heavy loading such as plowing etc would have a tendency to make the front end go light?
It would make even more of a differance if one were dragging earthworking tools uphill.
BTW, don’t discount turning radius in the thinking of most farmers. Your average farm tractor can turn a very tight corner, and a lot of farmers like to try to squeeze every bit out of their fields by making as tight a turn as practicable at the very end of the field. Being able to make a tighter turn was one of the selling points of “tricycle” style tractors with the two front wheels close together, rather than the front wheels spaced at the same width as the rear. This was considered an important selling point in spite of the fact that the design was prone to tipping over on slopes. IIRC, the tricyle design isn’t being manufactured anymore because of the safety issues.
Your classic farm tractor is also equipped with a split brake pedal which allows you to lock one wheel to spin around on it. I don’t think that system would work as well with four equal sized wheels.
(farm tractors in my memory had a clutch pedal on the left side of the transmission hump, which your feet straddled, and the split brake pedal on the right, with a hand operated throttle on the steering column)
