Changes in tire circumference under a load: Tractor Tires  -- Tractor Maintenance Discussion Forum and Review Changes in tire circumference under a load: Tractor Tires -- Tractor Maintenance Discussion Forum

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 06-21-2002, 21:29 Post: 39740
DRankin



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 Changes in tire circumference under a load

Some recent threads about tire pressure, tire loading, changes in tire height with loads and “axle wind-up” got me thinking. So I did a little experiment.
The front end of my Deere 4100 is currently equipped with 145-12 all season radial tires. For this experiment I loaded the FEL with 12 cinderblocks (about 330 lbs.) and I parked the tractor on a concrete slab. I measured the height by laying a level on top of the tire and with the bubble centered, measured from the slab to the underside of the level. With the FEL resting on the ground and all hydraulic pressure relieved, the tire measured 20+½ inches. Then I lifted the load of blocks and measured again and got 20+1/8 inches. So the tire compressed 3/8 of an inch under the load. Using Pi run out to 32 decimal places, it looked like this: 22 ½ equaled 64.42 inches circumference and 22 1/8 worked out to 63.25 inches. A difference of minus 1.17 inches circumference under a load.
Now the interesting part: I measured the actual circumference with a very flexible tape, being careful to stay in the center of the tread. The numbers look like this:
1)With the tire lifted off the ground 65 + 18/32 inches.
2)Tire on the ground carrying the weight of the tractor but not the FEL, again 65 + 18/32
3)Tire on the ground with the FEL load lifted, 65+17/32.
The actual difference in circumference was 1/32 of an inch, despite the tire losing 3/8 of an inch in height with the load.
So it appears that with radial tires, when the sidewall bulges under a load, it is simply laying more of the tread on the ground, and losing almost no actual circumference. How would this relate to bias ply tires? I am not sure, but common wisdom in the tire business is that bias ply tires tend to ‘squirm’ under a load. That would seem to indicate the bias ply tire is trying to lay more tread on the road also, but the sidewalls are not controlling the direction of the tread as well as a radial sidewall might. Input and comments, as always, are welcomed. Mark






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 06-22-2002, 06:46 Post: 39749
Art White



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 Changes in tire circumference under a load

Mark, the tires outer distance won't change. The length and width of the flat spot on the bottom is what will get you into trouble with gearing.






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 06-22-2002, 06:57 Post: 39751
TomG

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 Changes in tire circumference under a load

Mark: Lots for me to think through on the Board today and I can't do it this morning. I'm always much more interested in real observations than 'pat answers.' It's very good that you did the experiment.

I can say that somewhere in the older archives here I think is a method for calculating tire rolling circumference. It is more complicated than might be imagined and I don't know if the method allowed for different loads. Rolling circumference and tire circumference is somewhat different, and rolling circumference is what is speced for axle lead. I think that typical specs are for front axles to lead by several percent. It may not take huge differences to through a tractor out of spec.

Seems like the question of how load affects tire profile and rolling circumference has good potential here for another very interesting subject. Thanks for introducing the subject.






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 06-22-2002, 10:04 Post: 39755
Billy
2002-06-22 00:00:00
Post: 39755
 Changes in tire circumference under a load

The radius of an under inflated tire is less, thus will change the tracking speed. To get a true figure, you need to measure the radius from center of axle to ground. In other words the radius from axle up will be greater than from axle down.

Billy






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 06-22-2002, 10:15 Post: 39757
DRankin



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 Changes in tire circumference under a load

Art, now I am really confused. Clearly the tire footprint changes and therefore so must the amount of traction generated. So I reckon we have to consider there is some more work energy required to overcome that drag, but if the tire still takes 65.5625 inches to complete a revolution on the ground, how does this effect a change in relative gearing between front and rear axles?
Maybe what is happening is this: Because the front axle is set up to turn a bit faster by a couple of percent, when we increase the loading/footprint/traction profile the front axle has more trouble generating the slippage that is necessary to relieve itself of the extra rpm’s built into the system, and therefore can bind at the ring and pinion interface even though it has the same circumference as a rounder, less loaded tire.
Are you tracking with me? Or am I hopelessly lost in a cosmic cul-de-sac of ethereal vapors? Wait! There’s Tom G. He’s up here with me. Someone please find the strings to our balloons and pull us back to earth!






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 06-22-2002, 17:39 Post: 39764
dsg

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 Changes in tire circumference under a load

Mark,
You have WAY too much time on your hands Smile)

David






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 06-23-2002, 08:07 Post: 39773
TomG

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 Changes in tire circumference under a load

Yes, and I think it's time well spent--a luxury to have time to figure out how something works. I’m never too happy black-boxing things.

I started out thinking that the main thing about axle lead was that the tread surfaces of the front and back tires should have about the same speeds. The explanation was real basic. The surfaces of big circles move faster than those of smaller when rotated at the same RPM. So, the rear axles have to have lower RPM's than the front axles.

Somewhere along the way I also picked up the impression that load affects axle lead, and in my experience that seems to be true. I guess I'm guilty of black boxing this and I usually need my explanations. The main thing that seems to change with load is tire shape, and I don't have a ready explanation for how changing the shape changes the tread speed. It's probably pretty basic but I'm just not using the right set of concepts to try and come up with an explanation.






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 06-23-2002, 10:39 Post: 39775
Stan



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 Changes in tire circumference under a load

OK, at the risk of having to think this through the rest of the way with you guys, I'll just throw out a comment and run.

The circumference of the tire can't change (OK, it can stretch a little, but not substantially.

And the surface speed has to be the same, all 4 tires are attached to the tractor and the ground - (again, if you have slip in the mud, it changes a little.)

What does change is the effective diameter which - with a nice serving of pi for everyone - changes the rpm.

So, at "X" rpm out of the transfer case, through the front differential should give you "Y" rpm, and on the proper size tire with another serving of pi, gives you the ground speed that matches the speed of the rear wheels (w/in whatever lead the manfacturer built in). If the tire has a smaller diameter (squished or mismatched) it is trying to go at a different "ground speed", which it can't, so things start to "lock up".

Anyway - that's my off the cuff analysis - I'm sure I'm pretty close, but I don't know if I explained it all that well. Hope I have made a positive contribution to the thread - its nice to stretch the brain now and then.










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 06-23-2002, 10:41 Post: 39777
Stan



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 Changes in tire circumference under a load

...and somehow I missed the short post from Billy P - who is heading to the same place.






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 06-23-2002, 15:56 Post: 39782
DRankin



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 Changes in tire circumference under a load

Before I lowered a tire with a load on a tape measure, I would have agreed with Billy and Stan. The question I was trying to answer was this: If the tire gets shorter (which it does) where does the extra tread go? The answer is it flattens out on the ground and creates more traction, or turning resistance, or drag or whatever you want to call it.
Picture a tank or a dozer track. With nearly half of it on the ground it creates tremendous traction and in doing so absorbs huge amounts of horsepower, which in turn limits the ground speed of the machine. If you could construct a giant set of spokes and turn the track into a wheel, would you have the same amount of traction? I think not. But, as the traction/resistance/drag decreases, could you then expect to turn it thing faster (given the same horsepower)? I think so.
If we were dealing with rigid wheels, like a spoke wheel from a 18th century wagon or the steel wheels from a 1920’s tractor it would be a different story. With no flexibility built into the wheel it has no choice but to sink into the earth under a heavy load.
That is the circumstance Art was describing. He described a set of tires where the rears flattened out on the ground with a load, but the fronts were so rigid that they acted like a wagon wheel and sunk into the earth. If both ends had the same amount of flexibility (or lack thereof) in the tread then the problem would not have manifested as it did.
Here is another facet of the same issue most owners of four-wheel drives know. When you get stuck in the mud, many times you can get your self out by letting most of the air out of the tires. Why does that work? Maybe you are turning your wagon wheel into a sort of a modified tank track.
My tape measure is telling me that the rolling circumference of my tires is not changing with a big load even though the tire gets measurably shorter. So I think it is acting like a tank track and laying more rubber flat on the road. More rubber on the road will certainly increase traction and reduce slippage. By reducing the slippage, the drive train is not able to rid itself of the lead built into the front axle as easily and begins to bind the gears, especially on paved surfaces.






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