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Old 12-01-2010, 04:57 AM   #21
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Rock, I was going to let this pass, but I think you might have misunderstood my post.

1) My concern with the backing/pulling forward process requires split second coordination between the spotter and the driver to avoid overshooting the optimum lift point. This risk is eliminated with a jack.

2) I have a Diesel TV and power is never a problem. It is the application of said power that I was referring to. I have a 6 spd Allison automatic transmission and, while a manual transmission might be easier to manage with the clutch (though you might need three feet to do it), a two foot technique is required with the brake and throttle. I don't know about you but I have never been much good doing that since it is not something I practice.

3) Most of our tires are running pressures pretty close to their maximum just to carry the rated weight split up among the four tires.

Example: My 5th wheel is rated at 9300 pound max gross weight. I have 2 axles each rated at 4000 pounds for a total of 8000 pounds. That leaves 1300 pounds that has to be carried by the king pin.

The tires that came with the camper are C rated tires rated to carry 2150 pounds each at 50 PSI (the maximum air pressure).

With one flat tire, the 8000 pounds carried by the tires must now be split between the three tires in contact with the ground. (You might think that some of the weight would be added to the king pin, but the moment arm of the load will not change with a flat and the king pin weight will remain unchanged). As such 8000 pounds now split 3 ways means that each inflated tire must carry 2,666 pounds. While overloaded by 516 pounds each; since you are stopping and then stopped, the tires should not fail outright. If you tried to drive any distance you would find the other tires starting to delaminate and fail as well.

Using a bottle jack to lift the frame, the jack will carry the entire load that was supported by the two tandem tires on the side that had the failure.

In this way, the two remaining inflated tires on the other side will only have to support the original 2000 pounds per tire (4000 pounds on that side).

Make sense?

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Old 12-01-2010, 08:09 AM   #22
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You can loosen the lug nuts -slightly- and pull forward a foot
onto planks without harm.
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Old 12-01-2010, 09:50 AM   #23
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Originally Posted by herk7769 View Post
With one flat tire, the 8000 pounds carried by the tires must now be split between the three tires in contact with the ground. (You might think that some of the weight would be added to the king pin, but the moment arm of the load will not change with a flat and the king pin weight will remain unchanged). As such 8000 pounds now split 3 ways means that each inflated tire must carry 2,666 pounds. While overloaded by 516 pounds each; since you are stopping and then stopped, the tires should not fail outright. If you tried to drive any distance you would find the other tires starting to delaminate and fail as well.
Lou, I am going to have to disagree with your statement. If you have 8000 on 2 axles, equally distributed with 4000 lbs each side, then a flat tire on 1 side will probably make the remaining tire on that side carry 4000 lbs., or close to it. I don't think the 2000 lbs. each tire carries on the opposite side of the flat would go up to 2666 lbs. automatically to take up the weight of the flat. There would probably be some type of weight transfer, but for the most part the good tire on the flat side should take most of the weight. That is why I have read that when you have a flat, the remaining tire on that side should be replaced as soon as possible......it has been grossly overloaded for x amount of miles.

I understand your reasoning. A center weight distributed between an equilateral tripod should have the same weight on all three legs. But since 2 tires are close together (the opposite side of the flat) then some weight is probably transferred to that side, but the majority would be transferred to the good tire on flat side of the camper.

Depending on whether the front or rear tire went flat, I would think the weight shift on the hitch could increase or decrease. If the rear tire goes flat, the hitch would probably lose a little weight due to the weight shift all now being on the front axle on the bad side.

No geometry major here.....please someone correct me if this reasoning is wrong.
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Old 12-01-2010, 10:10 AM   #24
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I am no Physicist so I am only guessing here.
Going back some 40 years now since I last looked at this stuff.
My reasoning went like this:

I assumed that the Center of Gravity (CG) with 4 wheels would be equidistant between the axles and a big "X" through the centers of the 4 tires. With a flat; the CG calculation diagram would look more like a right triangle with a very short base between the two good tires. This should shift the CG toward the "fat area" of the triangle until the load is equally distributed among the 3 apexes of the triangle.

In the case of the jack lifting the opposite frame (side) there would be almost no CG shift and thus no load increase on the tires in contact.

If this is wrong, someone needs to let me know why...

http://en.wikipedia.org/wiki/Weight_transfer Has an explaination.

http://www.loadxpert.com/lx/en/lx.htm

has some info on CG and load distribution. I am trying to "finger it out" as we write.
Here you go... This will explain it!
Definition
The center of mass R of a system of particles is defined as the average of their positions, , weighted by their masses, mi:
For a continuous distribution with mass density and total mass M, the sum becomes an integral:
If an object has uniform density then its center of mass is the same as the centroid of its shape.
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Old 12-01-2010, 10:16 AM   #25
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Originally Posted by herk7769 View Post
I assumed that the Center of Gravity (CG) with 4 wheels would be equidistant between the axles and a big "X" through the centers of the 4 tires. With a flat; the CG calculation diagram would look more like a right triangle with a very short base between the two good tires. This should shift the CG toward the "fat area" of the triangle until the load is equally distributed among the 3 apexes of the triangle.
Hmmm, now you got me ciphering on this.
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Old 12-01-2010, 10:20 AM   #26
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Sorry, I really have to apologize. I really have NO idea what that equation is saying. Just had to do it.
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Old 12-01-2010, 10:29 AM   #27
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Sorry, I really have to apologize. I really have NO idea what that equation is saying. Just had to do it.
Before you fessed up, I was duly impressed. Shucks, I am still impressed that you could even find that formula, and know it relates to weight transfer.
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Old 12-01-2010, 10:45 AM   #28
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So what happens if you blow one tire on each side.....SOL?
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Old 12-01-2010, 10:59 AM   #29
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I just phone AAA - saves tryin' to figure out that formula !!!

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Old 12-01-2010, 11:19 AM   #30
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I would agree with load being twice normal for the good tire on the side with the flat. I also agree on changing the other tire on the blowout side. We do that on commercial aircraft as well if we have a tire blowout on dual tire nose or main gears and on 4 tire mains we change the tire across from the blowout.
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