Tires Loaded tires, problem.

[daTeacha]

I can see that this has the potential to become one of those topics which goes 'round and 'round with everyone thinking they are right, sort of disagreeing with others, and never resolving the issue.

If you have the capacity to accurately determine the internal volume of the tire when it is bearing weight and when it is not and allowing for the change in shape when the weight is on it, there may be a slight change in volume. As the Firestone people put it, volume is nearly constant. Based on that, they said increasing the weight will increase the pressure. Note that they did not say how much. Going up a few posts, the increase in pressure will be as miniscule as the decrease in volume and not something that will show up on a tire gauge of the type anyone is likely to have. You will not increase the pressure without decreasing the volume.

If said tiny decrease in volume occurs, there will also be a corresponding tiny increase in temperature, again adding to the pressure by a miniscule amount. However, within the range of values measurable by commonly available tools, the addition of any reasonable weight on the tire is not going to change the pressure by a measurable amount, as evidenced by the lack of change in pressure when the tractor with backhoe mounted was set up with 20 psi in the tires in the air and it remained at 20 psi when the tires were carrying the load of the tractor and hoe.

Did the pressure go up? Yes. Was it by enough to show up on a normal pressure gauge? No. Did the volume change? Yes, but only because the flat part of the tire on the ground has a different cross sectional surface/volume ratio than the portions not contacting the ground. Is that volume change enough to impact the pressure? Yes, but again only by a tiny amount not measurable by most of us.

For tires with liquid in them, the volume with which you start is considerably less. Thus any miniscule change in volume resulting from changing the profile of the bottom of the tire will be a bigger percentage of the original volume and should result in a more nearly measurable change in pressure. Liquids, however, are not compressible. Liquids in a tire will have the same volume regardless of the pressure and will have the same pressure as the air in the tire -- that's why hydraulic systems work. The only possible exception might be an again miniscule difference in the pressure of the liquid near the top of the tire when compared to the bottom due to the weight of the liquid in the higher parts of the tire -- just like the increase in pressure evident at the bottom of a swimming pool. Again, this is not something likely to be measurable by common tire pressure gauges.

Okay, gentlemen, fire away! /forums/images/graemlins/smile.gif

 

[Farmwithjunk]

48 years of farming and messing with tractors has taught me a thing or two.

One is that no tractor I've ever dealt with is heavy enough, and the typical tractor tire pressure is low enough that you won't be able to detect any increase or decrease in tire pressure using a conventional tire gauge from added weight. I'd also think the differential would change with tire size, smaller being effected more than bigger.

This is NOT a challenge of physics or engineering text books, but merely a "real world" observation.

 

[BadDog]

Doers do.... those who can't do, teach.

LCD pressure gauges at 20 paces.

'nuff said.

 

[john_bud]

OK, you guys are a bunch of pips. Man your slip-sticks, this is not a drill!

You want to know the change of pressure? Great! First determine the surface area of the tires. Don't forget the area of the wheel! Don't forget that there are two (2) tires, so double it!

Need data? 12.4-16 tire. 106 inch rolling circumference, 12.4" wide, wheel section is 11", 34" tire height, 16" wheel diameter. Not taking into account the thickness of the tires - surface area is about 26,700in2. Add a 1000# and the pressure changes by 1000/26,700 or 0.04 psi. If he has R4's they are larger, will have more surface area and the impact will be smaller.

But, that assumes the tire is perfectly rigid. It ain't. Ever measure a tire at various inflation pressures? NASCAR pit crews do. They add as little as 1/2 psi to change rolling circ to impact handling. Every watch a tire get inflated? It grows. It never stops growing as it inflates, it just slows down its rate of growth with rate of pressure increase. At some point, it pops when the elastic limit is exceeded. Then it gets smaller.


And who said the pressure was different stem down than stem up? Well, it is. In fact it looks like 1 psi is equal to a column of water 27.6" high or a column of CaCl2 filled water 22" high. So, measuring a filled tire with the stem up vs down would give about 2/3 of a psi difference, or 18 TIMES more impact than adding 1000 pounds of weight.

Do we REALLY need to go into this any more?

jb

 

[daTeacha]

Aww, c'mon. Don't you think this is kind of fun? I'm somewaht embarrassed to admit that I underestimated the difference due to fluid weight at the top and bottom of the tire. I should have known better since a water barometer needs to be about 400 inches high to measure one atmosphere pressure and tractor tires can easily measure 5 to 15% of that or more.

3RRL -- nice links! Now if only I had a handy portable vehicle scale I'd be in great shape.

On the soil compaction side of things, a relative was holding forth once upon a reunion about the European solution. It seems that the trend there is toward longer narrower footprints to localize compaction. He had charts showing the shape of the compacted area and the degree of compaction resulting from wide vs. narrow footprints. The narrow footprints compact less soil in the root zone than do the wide ones for the same weight per unit area and the degree of compaction is not linearly related to area compacted, favoring the narrow approach. It seems using doubles or triples is not as effective as going with tracks instead in that regard.

Now the tough question -- do you think anyone out there does the calculations and pressure adjustments for balancing the tractor prior to getting out and doing their field work? Somehow, I suspect that most professional farmers just set it once and check it now and then with little or no regard for the implement used and all the other factors the research types have worked over. I know off-road recreational users play with their tire pressures a lot, but I just have to wonder about the farmers. I'll try to ask some of the neighbors next time I talk with them. The guy who farms our land is pretty concerned about compaction, so he might. I'll give him those links and see what he thinks about it.

Oh yeah -- you guys have better pressure gauges than I do. My best one is a dial type marked in 1psi increments and I thought that was a really nice item compared to the stick types.

 

[john_bud]

</font><font color="blue" class="small">( Aww, c'mon. Don't you think this is kind of fun? I'm somewaht embarrassed to admit that I underestimated the difference due to fluid weight at the top and bottom of the tire. )</font>


Kinda fun? Sure! But, come on now, it's getting silly...

jb

 

[BadDog]

Very silly.

All because some ******* pumped too much CaCl in. /forums/images/graemlins/tongue.gif

 

[Farmwithjunk]

</font><font color="blue" class="small">( </font><font color="blueclass=small">( Aww, c'mon. Don't you think this is kind of fun? I'm somewaht embarrassed to admit that I underestimated the difference due to fluid weight at the top and bottom of the tire. )</font>


Kinda fun? Sure! But, come on now, it's getting silly...

Also the most frequently discussed issue on all tractor websites. Between "loaded tires", "What size mower for my__________________?", and "How many flood lights can I bolt to my ROPS"?, What else would we talk about?

Silly? Yes. Over with? I doubt it.

jb
)</font>

 

[daTeacha]

It's also educational to the newbies who don't search out and read old threads.

Wasn't the last "thread that wouldn't die" about where to place weights for the best results? I seem to remember stuff about torque moments and such being thrown around.

Now I'm wondering -- is there some basic formula that would enable someone without a scale, such as me, to estimate the weight on each end of the tractor based on tire size, pressure, construction, and amount of flattening as measured by comparing center to floor and center to top of tire when sitting on concrete? I ask this in all seriousness since balance is generally agreed to be more important than simple GVW.

I'd really feel if my first rollover was with the new tractor.

 

[john_bud]

Teach,

Probably is a way to determine weight distribution. At least qualitatively. Get 2 old tires and pump them up with air to the same pressure. Lay the tires on their side on a concrete or asphalt driveway. Get a couple bricks and put them on the sidewalls of the tires (one per). Get a 4x6 that is longer than the tread width of the tractor and lay that on top of the bricks. Lower the tractor wheels onto the 4x6 such that the tires are directly above the bricks. (I did say to raise the tractor and align the "scale", didn't I /forums/images/graemlins/wink.gif) Repeat for both ends of the tractor. The ratio of the weights will be the ratio of the compression of the tire sidewalls. You did measure the exact height of the bottom of the 4x6 before and after the tractor was lowered, didn't you? So if you get 1" of compression on the front and 2" on the back you have 2x the weight on the rear.

I haven't given it much thought, but it should work. You might have to diddle around with the pressure in the measuring tires to get enough deflection for an accurate measurement.

If you do it, let me know it it actually worked!
jb