Parking Camper On New Driveway Question

mattyj wrote:
Hi All, I just had an asphalt driveway poured at my house, its soft so Im waiting to drive on it for a week or so. I have parked my camper on the old driveway for 12 years on 3x8 blocks but the contractor said I should use a piece of 3/4 plywood under the jack legs to spread out the concentration of weight. I am wondering if anybody out there has done the same or has any other suggestions for under the jack legs. Thank you, Lou

A solid concrete block - just a 4" thick one - will distribute the load of the camper just fine. The other advantage is that they are cheap, already the perfect size, and don't deteriorate like wood will.

mattyj wrote:
Hi All, I just had an asphalt driveway poured at my house,

How did the "pour" asphalt?

Reality Check wrote:

valhalla360 wrote:
PSI in the tire is roughly the same as the PSI on the pavement. If you take load off the truck the patch of tire on the ground reduces proportionally so PSI stays roughly the same. (there is some minor variation as the sidewalls of the tire will hold a small bit of load but not a lot. Also the tread pattern plays in a bit but no a lot).

You have difficulty visually seeing the area of the tire patch on the ground but it is changing.

Having done pavement design in a former life, if you are only getting 29psi of strength, you got sold garbage.

Yes, yes and yes..and

I am NOT an engineer, but am required to understand loading, various materials, basic design criteria, etc, for cranes, temporary loads, etc. I think the very low psi rating often used for asphalt is due to creep, as related to it's flow under load. Flow has to do with the stability of the mix, the age and the exposed temperatures. I very well could be using a figure that is to low...but it has worked for us for many years. I couldn't tell you today when or where I settled on accepting that number, which engineer/designer, led us to it. But it works.

Simple example most will understand is a tractor trailer parked in a parking lot...lower the legs, good for a few minutes, hour, no visual effect. Asphalt seems to support it just fine. Come back the next day or week, and you very well might see two perfect depressions.

My guess is was a CYA thing.

They may have given you those numbers so if you messed up by a large margin, the company didn't have to pay for asphalt repair. Sure it works but that's because it's way below what the material can handle.

It's like saying your big crane can never lift more than 50lbs. In terms of failures due to overloading, you'll never have one but it's rather silly to buy a 10ton crane and never use it for more than 50lb loads. The difference here is most residential driveways are never expected to handle 40ton semi's.

Guys,

The inflation pressure in the tire is NOT equal to the pressure in the footprint.

Here's the wikipedia article on it:

Wikipedia: Contact Patch

However for low pressure tires (passenger car tires - about 30 psi), it's a good approximation, but for high pressure tires (100 psi), it is not.

HMS Beagle wrote:

Reality Check wrote:

Reality Check wrote:

but the load on the ground doesn't change unless the footprint does.

.... PSI in any particular tire only changes the profile of that tire (see above...) but the actual air in a tire does not change the load. ....

The rating of the tire makes no difference. How much pressure is actually in the tire is what makes the difference. Two tires with different psi ratings will have the same footprint if they have the same load and the same actual inflation pressure. I did not mention rating above, only actual pressures. You cannot change the total load by changing the tire pressure - but by changing the tire pressure you change the footprint, which changes the psi seen by the ground. Pressure and load are different things, mathematically related by the area over which the load is spread.

Knowing the load alone tells you nothing about ground pressure. But knowing the tire pressure alone does tell you what the ground pressure is, because the tire will deform until the footprint satisfies the equation: load = footprint x tire psi and since also load = footprint x ground psi, ground psi = tire psi.

From a lifetime wasted in engineering and physics.

Didn't I already say that???

Only problem with this whole side bar, is it has almost nothing to do with the pressure the legs exert when holding up a camper (unless of course the camper is still on the truck, with the legs down...now the load is spread out over...8 points....with the variable of the 'footprint' of the tire coming into play, which in the scheme of light weight campers spread over 8 locations, is almost negligible in world of the RV'er).

Reality Check wrote:

Reality Check wrote:

but the load on the ground doesn't change unless the footprint does.

HMS.. you make some good points, but I think confusing the subject a bit. PSI in any particular tire only changes the profile of that tire (see above...) but the actual air in a tire does not change the load. Two tires with different psi ratings but having the same profile on the ground (footprint) transfer the exact same load.

i.e. an 8" wide tire with 3" of contact with a 50 psi rating transfers the exact same load as a different, similar sized tire with a 110 psi rating would. The psi on the ground (receiving surface) would not change.

The rating of the tire makes no difference. How much pressure is actually in the tire is what makes the difference. Two tires with different psi ratings will have the same footprint if they have the same load and the same actual inflation pressure. I did not mention rating above, only actual pressures. You cannot change the total load by changing the tire pressure - but by changing the tire pressure you change the footprint, which changes the psi seen by the ground. Pressure and load are different things, mathematically related by the area over which the load is spread.

Knowing the load alone tells you nothing about ground pressure. But knowing the tire pressure alone does tell you what the ground pressure is, because the tire will deform until the footprint satisfies the equation: load = footprint x tire psi and since also load = footprint x ground psi, and since the load is fixed and the footprint common, ground psi = tire psi.

From a lifetime wasted in engineering and physics.

valhalla360 wrote:
PSI in the tire is roughly the same as the PSI on the pavement. If you take load off the truck the patch of tire on the ground reduces proportionally so PSI stays roughly the same. (there is some minor variation as the sidewalls of the tire will hold a small bit of load but not a lot. Also the tread pattern plays in a bit but no a lot).

You have difficulty visually seeing the area of the tire patch on the ground but it is changing.

Having done pavement design in a former life, if you are only getting 29psi of strength, you got sold garbage.

Yes, yes and yes..and

I am NOT an engineer, but am required to understand loading, various materials, basic design criteria, etc, for cranes, temporary loads, etc. I think the very low psi rating often used for asphalt is due to creep, as related to it's flow under load. Flow has to do with the stability of the mix, the age and the exposed temperatures. I very well could be using a figure that is to low...but it has worked for us for many years. I couldn't tell you today when or where I settled on accepting that number, which engineer/designer, led us to it. But it works.

Simple example most will understand is a tractor trailer parked in a parking lot...lower the legs, good for a few minutes, hour, no visual effect. Asphalt seems to support it just fine. Come back the next day or week, and you very well might see two perfect depressions.

PSI in the tire is roughly the same as the PSI on the pavement. If you take load off the truck the patch of tire on the ground reduces proportionally so PSI stays roughly the same. (there is some minor variation as the sidewalls of the tire will hold a small bit of load but not a lot. Also the tread pattern plays in a bit but no a lot).

You have difficulty visually seeing the area of the tire patch on the ground but it is changing.

Having done pavement design in a former life, if you are only getting 29psi of strength, you got sold garbage.

Reality Check wrote:

but the load on the ground doesn't change unless the footprint does.

HMS.. you make some good points, but I think confusing the subject a bit. PSI in any particular tire only changes the profile of that tire (see above...) but the actual air in a tire does not change the load. Two tires with different psi ratings but having the same profile on the ground (footprint) transfer the exact same load.

i.e. an 8" wide tire with 3" of contact with a 50 psi rating transfers the exact same load as a different, similar sized tire with a 110 psi rating would. The psi on the ground (receiving surface) would not change.

You must separate total load from pressure measured in pounds per square inch (PSI). 1000 lbs distributed over 100 sq inches is still 1000 lbs, but the pressure exerted is 10 pounds per square inch. The same load distributed over 10 sq inches is 100 psi. The total load does not change, but each square inch of reacting force from the sand or asphalt is 10 times in the latter case, compared to the former.

If you remove the camper from the truck and do not change the tire pressure, the tire footprint very definitely does change. Tires must transfer all of the weight of the truck to the ground - that is the load. Tires are a bit complicated because the tire carcass has some stiffness and will transfer some load regardless of pressure. Let's simplify it by saying we have a very flexible balloon type tire that will collapse all the way to the rim if you deflate it. Now, psi in the tire = psi on the surface. It cannot be otherwise. If you add or remove load, the tire will sag a bit until the area contacting the ground is sufficient to make that true. If you add or remove air pressure, the same thing will happen.

When you look at well inflated tire, the footprint is hard to estimate. It is the width of the tread times the flat spot created by the ground in it's otherwise round profile. If you flatten a round profile you pick up area very quickly. An 8" wide tire 30" in diameter inflated to 80 psi with no load on it resting on the ground has a single line of contact - no area at all. Load it with 3000 lbs and it will flatten at the ground less than 2/10 of an inch, the flat spot is now 4.7" long, picking up 37 sq in of area (4.7 x 8" width). 4 tires on a dually will carry 12,000 lbs in that condition. You are unlikely to notice the difference between an empty load of 3000 lbs and loaded at 7000 without some very careful measurements. The theoretical difference in rolling diameter is less than 0.050", though the contact patch is more than double the area. Most of what you notice is suspension sag.

When you air down the tires for sand, again the difference in ride height is very small. In the example above, going from 80 psi to 40 psi will double the contact patch of the tire, cutting the ground pressure (not the load remember, the pressure) in half. The ride height will only change by about 0.2".

So yes, if you remove the camper and do not change the tire pressure, the psi on the ground will be the same as the contact patch will shrink by a proportional amount. If the load is transferred to the ground through lumber, then the pressure will depend on the stiffness of the lumber - which is why you want to use thick dimensional lumber, rather than floppy 3/4 plywood.