Small Wheels & Big Wheels - Why?

[KennK]

OK guys, if this is a stupid questions then please ignore me, but ...

I have a John Deer 3320 that is 4WD. I have a beautiful picture of it hanging in my office. I can't help but looking at it and wondering . . .

Why are the rear wheels is so much bigger than the front wheels? The rears are almost twice the diameter of the fronts.

My first though is that for 2WD tractors the bigger rear wheels gave it better traction over very rough surfaces, but with 4WD that might not be needed - or at least both wheels should be about the same size. The large rear wheel might also provide more axle-to-ground clearance, but the front wheel wouldn't have that same clearance.

Ken K.

 

[RobS]

This seemingly benign topic is liable to blossom into a major discussion! I think it was this question that got Mark Chalkley re-thinking his tractor needs way back in the earlier days of TBN.

Here's my take: The big tires provide a larger contact patch to the ground thus reduce soil compaction. Important for agriculture. Large front wheels take a lot more engineering to steer so the small wheels kept costs down.

Same today, even with 4wd. Large fronts put a lot of stress on a steering axle. You do see more large ag tractors with bigger fronts and even dual fronts and of course, an articulated tractor solves the weak steering axle problem by eliminating it altogether. The articulated machines normally run large tires front and rear and often duals all the way around, sometimes even triples. All in the name of reduced soil compaction and increased traction.

There are other factors such as ground/crop clearance, front/rear weight distribution, force distribution when pulling etc.

If you want some interesting reading, search for Mark Chalkley's threads leading up to his purchase of a new TLB. I can't even remember the name, it was made in eastern Europe I think and was eventually bought by Ingersol/Bobcat. (edit) It was an Earthforce EF-500. Here's one of the more recent posts: earthforce

 

[RobJ]

Traction I guess, but there is more weight hanging off the back with an implement too. And of course gear ratio.

Dunno..

 

[mjarrels]

Dang... I thought the biggest mystery was why do hotdogs come 10 per package and buns only 8? The world may never know.

"When you have nothing to do... how do you know when your done?"

mark

 

[elalexander]

Cause guys made 'em and guys like 'em that way...Kinda sorta I guess

I like mine, EL

 

[Whitey]

I went through this when I tried to find new R4 tires for my 770 that came with ag tires. I went through six months of varied opinions from everyone. From what I can remember it all came down to rolling circumference or loaded circumference and the gear ration of the tractor. Real touchy with 4WD. I got alot of info from the technical guys at JD after my local dealer e-mailed them when I posed the question to him. Ultimately an ag tire supply guy gave me all I needed to know. I ended up buying new wheels for the front to get the right R4 tires for my tractor.

JD 770 / 70 FEL / 7 BH - 425 AWS / 54 FMP

 

[john_bud]

I don't KNOW but my thoughts are as follows.

The rear tires have to take up weight of about 1/3 the tractor's total non-tire-wheel weight. They also have to take up the weight of the 3pt implement. But when that implement is picked up, weight is transferred to the back, so the total on the tires is larger than the 3pt dead weight. For simplicity call it 2x. But there's more, when transporting the 3pt implement, and you hit bumps the dynamic loading is even greater.

So, that means that either a high pressure tire or a tire with a large air chamber. As there is no suspension on the rear axle, high pressure is not optimum. Taller tires also have better ability to roll over obsticles and have a longer contact patch.


Front tires on some 4wd tractors were (and are) the same large diameter. But turning corners is a chore as you can't turn the tire to much of an angle before it hits some part of the tractor. Those tractors are said to require a "40" to turn around. Some have articulated bodies to gain mobility. Much easier to gear up the front axle, shrink the tire diameter and increase the tire turn angle.

So to sum it up. Rear tires are large to accomodate the large loads and front tires are small to allow for tight turning radius.

Who knows, that might even be close!

jb

 

[Superduper]

Because they can?

Sorry guys, I just couldn't resist.

 

[RalphVa]

I think they put the small front wheels on for access reasons. One is access to the engine. The other is access by the FEL, without interfering with the front wheels. The smaller front wheels allow them to drop the differential down a bit. Otherwise, the tractor just couldn't be lowered as much, unless the differential was placed out in front of the engine. This would make the tractor longer, and it wouldn't turn as sharply.

The smaller front wheels actually slip a little bit because they're designed to turn slightly faster than the rears (this is what I've garnered from reading this forum anyway). Think this slip enables them to steer through turns better without putting too much stress on the front end.

Ralph

 

[Z-Michigan]

I guess I had always assumed that the rear driving tires were huge because by maximizing their diameter, the contact patch is much flatter than it would be with a smaller diameter, which makes it behave more like a track (which is basically flat) and therefore less likely to slip and spin in low traction conditions.

From a traction perspective tracks would be ideal, but they cost a lot more and change how steering is done, for the worse. I think the use of huge tires is a compromise between traction, on one hand, and cost/simplicity/ease of use on the other.

There must be a good reason because big rears require very big reduction ratios in the transmission and final drive gearing to keep travel speeds low, in the normal tractor working range.