ACAD_Cowboy
Well-Known Member
Cocaine Cowboy special?
Must make a hell of a noise on the trail as all the bling drags and scratches.
Must make a hell of a noise on the trail as all the bling drags and scratches.
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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 .Cocaine Cowboy special?
Must make a hell of a noise on the trail as all the bling drags and scratches.
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.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.
Hey RTR I like how you roll, but you left out hydraulics. Got have juice to make that puppy JUMP!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
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.Give me some 15x10 D Windows and 37's. Takes me back to the early 2000's NEUROC competitions. Those were the days...
OMG the CHROME!!! Me eyes!!!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.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?
That thing is crazy looking. Like if a skeleton had a car. No not Maria Shriver, a real one.Cocaine Cowboy special?
Must make a hell of a noise on the trail as all the bling drags and scratches.
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.Hey @ACAD_Cowboy love to know who makes these step bars. Thanks
Come on Man.... I just ate lunch....Hey @ACAD_Cowboy love to know who makes these step bars. Thanks
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.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.