Parking Camper On New Driveway Question
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 contra...
Forum Discussion
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.
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.
