923 ground pressure on 1400's

[acme66]

I am sure the ground pressure per axle/tire exists in a neat chart in a TM some place but some half hearted search on my part didn't turn it up. Anyone care to shortcut me? I need to know the empty psi per tire as it could come up with the roads in spring breakup. I have the 1400 goodyear tires on it.

Ken

 

[doghead]

What axle weight restriction do they place on the road?

 

[plode]

I too was worried about this. We have a pretty cheap/thin asphalt driveway...I'm nervous about driving on it during the warmer months. I keep it at my fathers house...he's already threatened me with having to replace the asphalt with concrete if the truck leaves ruts in the asphalt.

 

[Scott88M]

I don't know specifics but in general on hot days my garbage truck at work will tear up asphalt, but primarily only if i'm turning the wheel on the asphalt. If I'm driving straight it may or may not leave a slight impression on the cheapest thinnest of asphalt. My garbage truck IS heavier but it's a decent approximation.

 

[Artisan]

Wouldn't that roughly be 4000 pounds per tire if we have 6 tires
and you weigh 24,000 pounds?

Now determine your footprint w/ maybe a string wrapped around
the footprint of the tire at rest, roll over and off the string and get a
good estimate of the area in sq inches where the tire touches the ground.
(Be sure to subtract the spaces between the tires lugs)

144 Sq Inches is one square foot right? So if you had 1 square foot
of tire touching the ground you would have 4000 pounds per square foot (roughly)
on Dad's driveway.

This will all change quickly w/ loads and varying tire pressures.

 

[acme66]

These are gravel roads and as far as limits it varies as to how bad (wet) the road is. Most times around here the limits are psi per tire so I am just trying to be proactive. Not everyone is going to be thrilled to see us so I would like to have my ducks in a row. As far as parking the truck in the drive way on hot days it will leave tread impressions and divits in some places but then so does the pickup. I think the base of my drive is some sort of playdough. This winter where the truck sat parked in the back the front tires sank 4 inches and the back 2. It is my yard and I am the one who has to mow it so I don't care but I could see where others would. Maybe some tire web page would have the footprint for the 1400's and I could do the math myself.

Ken

Artisan: I didn't see your post before writing, that is an idea to let me know and if I had to I could prove it alongside the road if the officer is in a listening mood.

 

[Gunzy]

Getting the weight per axle wouldn't be hard. Go to a certified scale and they should be able to get you that then just divide by two and that is the per tire. Once you know the footprint contact area the rest is basic math.

 

[porkysplace]

Wouldn't that roughly be 4000 pounds per tire if we have 6 tires
and you weigh 24,000 pounds?

Now determine your footprint w/ maybe a string wrapped around
the footprint of the tire at rest, roll over and off the string and get a
good estimate of the area in sq inches where the tire touches the ground.
(Be sure to subtract the spaces between the tires lugs)

144 Sq Inches is one square foot right? So if you had 1 square foot
of tire touching the ground you would have 4000 pounds per square foot (roughly)
on Dad's driveway.

This will all change quickly w/ loads and varying tire pressures.

On a empty truck the front axle will have more ground pressure than the rear axles.

 

[MtnSnow]

Yep need to do what Gunzy said. Go get the weights of each axle at a scale and do the simple math.

 

[Squirt-Truck]

Actual ground pressure in PSI is the pressure in the tire, unless you have so much pressure that the tire stands firm and does not compress.
OTR truck, 110 psig in front tires = 110psi ground pressure. Or are you looking for tire loading?

 

[Neophyte]

A tires "footprint" is an elipse in shape.......so figuring the surface area will give you PSI at each tire once the axle weight is known (heavier in front for our trucks, though not sure about the wrecker). I know when we did our concrete in the pole barn it was all based on PSI rating. A scale on front axle then back axles (if it can be done without a portable unit) would be the way to go.

I know in Wisconsin it's based on per axle load for the spring thaw (type II roads).

 

[MtnSnow]

I don't think that is what he is after Squirt...cause I don't have "air pressure" in my shoes but I do exert pressure on the ground. IE I weigh 240 lbs I wear size 9 boots which are approximately 3.75" x 12" each = 45 sq inch's of area so x 2 (since I have two feet) = 90 sq inches total so 240lbs divided by 90 sq/inches = 2.66 psi
I assume you want to know how much pressure is exerted by a number of tons?
For example, a steel block weighing 10 tons sitting on a floor covering 10 square feet exerts:
10tons *2000lbs/(10ft*144inches) = 13.88 psi on the floor

Edit...Missed calculated the total sq inches in the second example...as pointed out in the following post.. I rushed and did just 10 feet not sq feet so the 13.88 psi was based on 1,440 sq inches not the correct 14,400 sq inches! lol .

 

[Neophyte]

I don't think that is what he is after Squirt...cause I don't have "air pressure" in my shoes but I do exert pressure on the ground. IE I weigh 240 lbs I wear size 9 boots which are approximately 3.75" x 12" each = 45 sq inch's of area so x 2 (since I have two feet) = 90 sq inches total so 240lbs divided by 90 sq/inches = 2.66 psi
I assume you want to know how much pressure is exerted by a number of tons?
For example, a steel block weighing 10 tons sitting on a floor covering 10 square feet exerts:
10tons *2000lbs/(10ft*144inches) = 13.88 psi on the floor

Close, but that would be 1.388 PSI (120 inches x 120 inches for square inches)

I did a quick calculation assuming 14R20's........14" wide. A square "footprint" would be 14x14 = 196 square inches at each tire (not exact but likely close). So say the front axle is measured at 16,000 Lbs .....the PSI footprint at each tire would be:

(16,000 Lbs / 2) /196 = 40.81 PSI (the lower the tire pressure, the bigger the footprint, the less PSI the tire exerts on surface)

 

[MtnSnow]

Ahh yes 1.388 psi....see how quick simple math can screw us up! lol

 

[MtnSnow]

Well I just walked out to my 923A2 and the data plate says the truck empty has the following weights
Front Axle = 9,450
Intermediate Axle = 6,050
Rear Axle = 6,050
for a total of 21,550

and Loaded with 5 tons of cargo
Front = 9,390
Inter = 11,080
Rear = 11,080
Total = 31,550

So using your 196sq inches of contact patch per tire (for a total of 392 sq inches per axle) the front tires would be at 24.107 psi empty and 23.954 psi full
Rear tires would be at 15.433 psi empty and 28.265 psi loaded. ballpark-ish

 

[Julian]

I weighed my 925A0 (with winch) and the front axle was 11,140, and both drive axles were 12,160 (total 23,300 empty).

Theoretically, the tire internal pressure in lbs/squ inch is the same as the force exerted on the surface in lbs/squ inch, regardless of the load. This works when the tire behaves like an inner tube- smooth surface with no structural strength in the tire itself. Once the tire structure starts being a factor, the lbs per square inch on the road surface gets more complicated. Tire carcass stiffness, tread pattern, speed of rotation, and road surface properties need to be factored in. For instance- if the road surface is so soft that it completely fills in the gaps in the tread, then the tread doesn't matter anymore. So, a precise calculation is complicated (and beyond my abilities), but generally, pressure in the tire equals the average pressure on the contact patch on the road regardless of the weight the vehicle.

 

[wreckerman893]

Our road is "tar and chip" which means they grade a dirt road, pack it down, spray it with tar and put small limestone gravel on it. My neighbor brings his Freightliner tractor home and there are pockmarks where he maneuvers in and out of his driveway. I have to make a three point turn to get the M927A2 into my driveway and I don't have similar marks out front. I think his skinny tires do more damage to the road than the super singles on my truck. I know when I last maneuvered the gooseneck into the driveway there were some scuff marks where the trailer turned. The super singles may be better than skinny tires on those type roads.

 

[5tonman1971]

I dont know if this means anything but I have my trucks parked in my yard next to me barn and maybe move them once a month on average and they don't sink in to the ground; rain, snow, hail, flooding. My 813a1 has the 16.00s and the m939s have the 14.00s the wrecker is in the barn.

 

[ThatXJGuy]

I weighed my 925A0 (with winch) and the front axle was 11,140, and both drive axles were 12,160 (total 23,300 empty).

Theoretically, the tire internal pressure in lbs/squ inch is the same as the force exerted on the surface in lbs/squ inch, regardless of the load. This works when the tire behaves like an inner tube- smooth surface with no structural strength in the tire itself. Once the tire structure starts being a factor, the lbs per square inch on the road surface gets more complicated. Tire carcass stiffness, tread pattern, speed of rotation, and road surface properties need to be factored in. For instance- if the road surface is so soft that it completely fills in the gaps in the tread, then the tread doesn't matter anymore. So, a precise calculation is complicated (and beyond my abilities), but generally, pressure in the tire equals the average pressure on the contact patch on the road regardless of the weight the vehicle.

I think you have the idea spot on, realistically the stiff tires we use do have some serious structure to them and would affect the calculation a lot, along with the tread. For instance, an empty truck's rear tires can sit at 70PSI or 35PSI and have a very similar footprint, and a similar PSI ground contact but the more air doesn't magically increase the PSI to ground. On semi hard surfaces the best way to get accurate would be to jack up a tire off the ground, paint the tread, put the tire back down with the paint still wet, then measure the surface area of the paint on the ground, and divide that into the weight. Like you said though, on soft surfaces, it would change for the better if it engulfed more of the tire.

 

[Bighurt]

The front tires of my M925 are 4" in the ground.

They were fine till I rolled 2' forward, spring thaw...

I get maybe 1/2" of indentation on compacted ND soil, but our fertile soil does some weird things when wet.

After reading this article;

http://road-transport-technology.or...E TYPES AND ROAD CONTACT PRESSURES - Yap .pdf

I think it's safe to assume your average contact pressure is equal to the tire PSI, and for me that would mean the unladen vehicle with manufacture recommended PSI. The maximum vertical contact pressure is somewhere between 1.5 and 1.6 times greater. Doesn't appear possible to have a single formula as there are to many factors.

In the end if you were to place 160 lbs on on one sq inch of asphalt drive the resulted effect will be your answer if it will support the weight.