I'm getting old.

[ACAD_Cowboy]

Cocaine Cowboy special?

Must make a hell of a noise on the trail as all the bling drags and scratches.

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[Ian cj10]

just dont hit a pothole

 

[Lou3.6]

Cocaine Cowboy special?

Must make a hell of a noise on the trail as all the bling drags and scratches.

200 years from now . . . after the Earths Frozen Landscape has thawed sufficiently for Extraterrestrials to "visit" -- though I don't know why they would -- They may come across "THIS" Contraption (as they would describe it) and wonder . . . WHAT WERE THEY THINKING ? They'll "turn around" ( that's "space talk" for Rapid Ascent ) and head back to their own Planet . . . laughing all the way . I know it's true I saw it in my crystal ball .

 

[Ole Cowboy]

So about contact patch.

Being a horrible infovore I came across perhaps the most concise way of calculating the contact area for any given tire. Sad and shocking news; the size of the tire doesn't effect contact area in any way you'd think.

So grab a big cup of coffee (or beer) and check this out.

First off from Boeing:
Calculating Tire Contact Area - http://www.boeing.com/assets/pdf/commercial/airports/faqs/calctirecontactarea.pdf

Calculating Tire Contact Area
The tire contact area for any aircraft tire is calculated by dividing the single wheel load by the tire inflation pressure. If the load is expressed in pounds, and the tire pressure in pounds per square inch, then the area is in inches squared. The same thing works with kilograms and kg/cm2 - the result will then be in square centimeters. The shape of the footprint area is usually understood to be a 1.6 ellipse (as referenced in the US Corps of Engineer's S-77-1 Report), wherein the major axis is 1.6 times the minor axis. The calculation to solve for the minor axis is .894 times the square root of the contact area. Note that the major axis runs parallel to the normal direction of motion of the aircraft, and the minor axis is perpendicular to the major axis.

Example: 777-300 Main Gear Tire Contact Area
For this case, use the maximum taxi weight of 662,000 lbs configuration of the 777-300 as shown in Figure 7.2 “Landing Gear Footprint - 777- 200/300” and Figure 7.3 “Maximum Pavement Loads - 777-200/300.” Figure 7.2 provides the main gear tire pressure of 215 PSI. Figure 7.3 shows the V(mg) per strut / maximum load at the static aft center of gravity for this airplane configuration of 313,900 pounds. Given that the 777-300 has six wheels per main gear as shown in Figure 7.2, to calculate the contact area first determine the load per tire (313,900 / 6 = 52,317) then to calculate the contact area, divide the load per tire by the PSI (52,317 / 215 = 243.3 in2 contact area). The footprint area is a 1.6 ellipse determined as follows: Minor axis is .894 x square root of the contact area (0.894 x sq root of 243.3 = 13.94 inches minor axis) Major axis is 1.6 x minor axis (1.6 x 13.94 = 22.30 inches major axis)



To break this down into JT terms
JT Sport automatic - 4672 pounds, 245/75R17 @20/30/40psi
4672/4 = 1168lb/tire = 58.4"^2 / 38.93"^2 / 29.2"^2 total contact area

minor axis sqrt(58.4) = 7.64*.894 = 6.83"
major axis = 6.83" * 1.6 = 10.93"
contact patch @ 20psi = 10.93" x 6.83"

minor axis sqrt(38.93) = 6.24*.894 = 5.58"
major axis = 5.58" * 1.6 = 8.93"
contact patch @ 30psi = 8.93" x 5.58"

minor axis sqrt(29.2) = 5.40*.894 = 4.83"
major axis = 4.83" * 1.6 = 7.73"
contact patch @ 30psi = 7.73" x 4.83"

So despite Boeings rather fancy math to tell us the shape of the contact patch the real down and dirty math is contact area = load/inflation pressure.

There is some limiting parameters the chief of which is minor axis should be limited to wheel width, if the minor axis gets too wide then the tire will pull off the bead face of the wheel but this is countered by ply rating , if you put a higher load rated (stiffer sidewall) tire on you can run less pressure with less sidewall deformation but that's a different discussion.

But as we can see, your actual tire size or wheel and tire size doesn't drive your contact area or patch dimensions so much as your area/patch drives your wheel and tire size.

Thank you for the data and the PDF, enjoyed that albeit it was touched on when I was going to flight school. Sorry, it has taken so long to get back to you, got a lot going on in the summertime.

AhHa, I am going to assume YOU are getting it. Understanding the point I make. However, you cannot compare this to Jeeps and here is WHY? The airlines know exactly how much the plane weights and they also know how much the passengers and baggage weighs.

I have been on many planes where they adjust passenger loading before we take off, move some passengers to the front or rear and into the center. Now lets look at a Jeep, we have the weight as noted by Jeep on the door jam. Using my TJ that ran on 16-18 lb BFG KM2s on 37x12:50 x 17 Walker Evans Bead locks + rocker guards made from Schedule 40 4 in Pipe and a rear bumper make from Schedule 4 in pipe, Duynatrac ProRock 60's F/R and a lot of other weight distributed across my Jeep to = +/- 25 lbs weight at each tire.

You can apply the Boeing math and come up a psi based upon the input. But plane tires are not auto tires and that goes in spades for off-road tires. Plane tires are not driving tires, their singular mission in life is to land safely. Planes move by thrust not by driving axles.

Autos that off-road, on the other hand, have more than one set point for tire psi. For the street it is the WIDTH of the patch that is key, you seek the Goldilocks psi, not too much, not too little, too much you drive toward the center of the tire, too little you drive on the other edges-sidewall. Either one = premature tire wear. Off-road, we air down for max traction, but even then we face challenges. In Moab and high traction, I ran 3-5 lbs. In less traction such as dirt like found on logging or unimproved trails, I would run 6-8 maybe a bit more. In Shale and areas where rocks can slice a tire open, I ran higher to get my sidewalls off the ground. I did not do mud so not sure on that. Sand varies, due to compaction, one needs to play with the psi to get it right. In the soft sand like found in the S of the Sonoran desert or the N end of the Sea of Cortez in Mexico I ran about 10 lbs.

Boeing math works great on planes, the rest is up to you, your Jeep your wallet, your tires and if you understand that the contact patch if determined by the load and the psi then you can find the size of the contact patch. Bear in mind, Radial and Bias ply tires are different and perform differently. So does the sidewall ply and its ability to flex. I could run 16-18 in KM2 on the street but on the same size GY MTR on the same wheels, I ran 24-26.

 

[Link Pin]

Man...... I wan some 26s, maybe wid some spinnas and da thinnest tires possible and Imma gonna get some of the those stick on number from Autozone and put them on the fenders right below the Jeep sinz

Hey RTR I like how you roll, but you left out hydraulics. Got have juice to make that puppy JUMP!

 

[Wraith]

Give me some 15x10 D Windows and 37's. Takes me back to the early 2000's NEUROC competitions. Those were the days...

 

[Ole Cowboy]

Give me some 15x10 D Windows and 37's. Takes me back to the early 2000's NEUROC competitions. Those were the days...

Were you running bias ply's at that time? There are still a lot of off road guys who run them, making the case the sidewalls are stiffer.

I have considered that, but not sure where that applies, MUD???? NOT Rocks IMO, I want max wrap and contact.

 

[TheITGuy]

I’ve done the math and going to some serious baller rims with low pros wouldn’t require regearing or hurt articulation as the OD would nearly the same.

Who’s ready to ride the donkey?

OMG the CHROME!!! Me eyes!!!

 

[bangolia]

I’ve done the math and going to some serious baller rims with low pros wouldn’t require regearing or hurt articulation as the OD would nearly the same.

Who’s ready to ride the donkey?

you found my rig! Im going to keep the tires when I trade it in for my JT in 4 weeks.

 

[Jake]

Hey @ACAD_Cowboy love to know who makes these step bars. Thanks

 

[Slapping_Rabbits]

Cocaine Cowboy special?

Must make a hell of a noise on the trail as all the bling drags and scratches.

That thing is crazy looking. Like if a skeleton had a car. No not Maria Shriver, a real one.

 

[5JeepsAz]

Another awesome thread. I like tires.

This a 100% jeep 100% truck problem also. No plane changes tires. So do you want manly footwear for a DD, a workhorse, or a crawler?

And there is also overkill. You would be surprised what a weenie tire will get you through if it's all ya got!

 

[ACAD_Cowboy]

Hey @ACAD_Cowboy love to know who makes these step bars. Thanks

It looks a little home made, to my eyes I see some corrugated metal deck painted blue and some plate and tube chromed. What I don’t see is any triangulation to resist front to back smash damage. I moved my step bars back about 2” on each side and never even noticed it happening.

 

[RedTRex]

Hey @ACAD_Cowboy love to know who makes these step bars. Thanks

Come on Man.... I just ate lunch....

 

[ACAD_Cowboy]

Thank you for the data and the PDF, enjoyed that albeit it was touched on when I was going to flight school. Sorry, it has taken so long to get back to you, got a lot going on in the summertime.

AhHa, I am going to assume YOU are getting it. Understanding the point I make. However, you cannot compare this to Jeeps and here is WHY? The airlines know exactly how much the plane weights and they also know how much the passengers and baggage weighs.

I have been on many planes where they adjust passenger loading before we take off, move some passengers to the front or rear and into the center. Now lets look at a Jeep, we have the weight as noted by Jeep on the door jam. Using my TJ that ran on 16-18 lb BFG KM2s on 37x12:50 x 17 Walker Evans Bead locks + rocker guards made from Schedule 40 4 in Pipe and a rear bumper make from Schedule 4 in pipe, Duynatrac ProRock 60's F/R and a lot of other weight distributed across my Jeep to = +/- 25 lbs weight at each tire.

You can apply the Boeing math and come up a psi based upon the input. But plane tires are not auto tires and that goes in spades for off-road tires. Plane tires are not driving tires, their singular mission in life is to land safely. Planes move by thrust not by driving axles.

Autos that off-road, on the other hand, have more than one set point for tire psi. For the street it is the WIDTH of the patch that is key, you seek the Goldilocks psi, not too much, not too little, too much you drive toward the center of the tire, too little you drive on the other edges-sidewall. Either one = premature tire wear. Off-road, we air down for max traction, but even then we face challenges. In Moab and high traction, I ran 3-5 lbs. In less traction such as dirt like found on logging or unimproved trails, I would run 6-8 maybe a bit more. In Shale and areas where rocks can slice a tire open, I ran higher to get my sidewalls off the ground. I did not do mud so not sure on that. Sand varies, due to compaction, one needs to play with the psi to get it right. In the soft sand like found in the S of the Sonoran desert or the N end of the Sea of Cortez in Mexico I ran about 10 lbs.

Boeing math works great on planes, the rest is up to you, your Jeep your wallet, your tires and if you understand that the contact patch if determined by the load and the psi then you can find the size of the contact patch. Bear in mind, Radial and Bias ply tires are different and perform differently. So does the sidewall ply and its ability to flex. I could run 16-18 in KM2 on the street but on the same size GY MTR on the same wheels, I ran 24-26.

So in keeping with the rules of protocol, I do whole heartedly concede that this bit from Boeing is a bit specific to their chose tire and their methods are tuned to their work however the methodology is sound as a tool for tire selection.

Were you armed with corner scales on a built rig you could Devine the patch size for nearly any psi down to ambient air pressure.

To me the real takeaway from it is the realization that the size of the tire carcass doesn’t drive but is driven by patch as a function of inflation pressure and that ground contact pressure == inflation pressure.

In my typical trails I have glacial moraine and screw mixed with loamy soil that may or may not be mud. Rock features tend to be large and relatively smooth. Inflation pressures for my stock MT with a reasonable amount of gear and people (and food because what better to do watching other people than eat a sandwich or two?)
Is around 12-15psi which results in a compliant sidewall that’s not too sloppy and a compliant tread face that can still hog down. The fee OTR portions are fuuuun as it rises like a caddy from the 70’s, just smooving.

When out in deep snow I will on occasion drop to 20-25psi, closer to 20 if the plow is on to soak up ice clots stuck to the road. Again you gain some flotation but don’t lose your dive and dig.

Sand driving is very limited for me, when last on the outer banks I think I went right to 20 and left it alone as we were drive right after the tide went out. My state will hang you, shoot you, crucify you then take you down to draw and quarter you then burn the whole mess should you even steer a wheel towards the dunes on the little beach you can ride, god help you if you go near beach nests. As such dry dune driving isn’t something I’ve gotten in to.

For the record I’ve never lost a bead and never lost forward momentum due to lack of tractive effort. I have been unable to climb up the scree gravel filled muddy creek with the 60* incline, made it about 2 lengths up until it bent left and just could t make it happen. Front was just kissing and the back was just digging deeper and deeper.

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