Tires Loaded tires, problem.

[3RRL]

I'll try.
Picture a tire and rim it's filled with air and completely round. (No load whatsoever on it)
Now picture that same tire with the same amount of air in it except it's now under a tremendous load that makes the tire completely flat at the bottom up to the rim.
The cubic inches has been reduce (since the tire is no longer round) so the air is being compressed to occupy the now smaller volume. Result = increased pressure.

 

[JoeL4330]

right you are ...as long as the tire is "pneumatic" (i.e., not on the rims, still able to flex) ...more weight means a bigger contact patch so that the no. of sq. in. of contact times the pressure equals the weight.

 

[3RRL]

Another example of this is a scuba tank filled up. It is under pressure. If you were to empty it into a giant ballon, the air in the scuba tank would now occupy a much bigger volume (the balloon) at atmospheric presssure.

 

[Jibber]

Well, I aired up the tires to 20 pounds while the tires were still off the ground (using the hoe stabilizers). I just went out and rechecked the pressure with the full weight of the hoe on the back and they are still 20 psi.. sooo. I guess you folks who paid attention in Physics class are correct! It is sitting a little lower so my guess is they had quite a bit more pressure.. and calcium solution in the tire than the manual calls for. I'll check that out with the dealer when they open monday.

of course.. the manual also says when you add a backhoe you should remove any ballast in the tires. As if you can just pump it in and out any time you like.

I do most of my loader work with the hoe attached.. but on big loader projects sometimes I drop the hoe because it's easier to move around in tighter quarters not worrying about that bucket hanging off the back.

 

[daTeacha]

</font><font color="blue" class="small">( ... As if you can just pump it in and out any time you like.... )</font>

That's one reason why wheel weights are superior to filling the tires. They cost more, but you can have the weight when you want it or more importantly, you can remove it and reduce compaction of the soil when you don't need it.

For counterweights, I'd rank wheel weights first for effectiveness and balance, then a ballast box or similar since you don't get the rotational inertia effect like you do with weights but it's easier to put on and off, and loaded tires next since you get the balance of weights but adding or removing the weight is so awkward that most will just put it in there and forget it. Then, too, a CaCl2 leak is corrosive to your wheels and would also be hard on your plants if you puncture a tire.

I'd rate the backhoe last as a counterweight, so if you don't want yours, I'll take it off your hands at no cost to you. /forums/images/graemlins/smirk.gif

 

[john_bud]

Jibber,

First off, congrats on getting the valve stems changed. Rusty stuff is nasty sometimes. Now the bad news, you did put stainless steel valve cores back in, right? If not, the new ones will get nasty pretty soon too.

Next, your dealer (or who ever filled your tires) was a bone head. They over filled and over pressurized the tires. Bad on both counts, but the over fill can actually destroy the tires. When you hit a bump, the pressure jacks up thru the roof as the non-compressible liquid tries to compress. (bad expensive noise waiting to happen). Keep the fill in the tires. It gives the hoe the extra azz needed to dig properly.

On the physics and air pressure with and without the tractor on the ground. The pressure probably did go up, it just went up such a small amount that your gauge couldn't detect it. The collapse of the tire volume due to the weight is so small, probably on the order of 1-2 cc's, that the change in pressure is around 0.00013 to 0.00026 psi. And that assumes that the sidewall doesn't expand at all...

jb

 

[BadDog]

Make sure you take your tire air pressure with the valves at 12 o'clock.

Otherwise you're taking the pressure of the liquid in the tire.

You should see the air pressure vary a little bit, not a huge amount (probably less than 5%) with a full load, since there isn't a lot of air in there to begin with. The air will be compressed differently by the liquid as additional weight pushes the tires into the ground, increasing the size of their contact patch. By contrast, the liquid pressure should stay constant.

You're only supposed to fill the tires about 70-80% full... otherwise you're asking for a bit of trouble, but nothing that can't be fixed, as you've found out.

 

[3RRL]

There's a reference chart available that helps with tire ballast. The amount of ballast depends largely on the type of usage and implements, but you should also keep the tractor "balanced" between front and rears, especially with 4wd.
CLICK HERE For information about balance and the correct "footprint" you should have and HERE to determine the weight you want to achieve.

 

[Paddy]

This was a quiz when I was in engineering school. the trick is tires have a max volume, or nearly. As you add weight, you reduce volume. Unlike a baloon, the tire does not expand some where else to compensate. PV=NRT, the chem formula presurexvolume=Constants of the gas(NR)x Temp. The change is not so big with small loads.

I got the question correct on the quiz. Some sudents were so convinced they were right them contacted firestone. Their reply was V was nearly constant, add weight-> add Presure.

 

[BadDog]

I have an engineering degree as well, but don't remember too much other than:
1. you can't push a rope
2. water doesn't flow uphill
3. 40 beers doens't sound like much until you've downed 39 (perhaps this has something to do with my memory?)