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ACAD_Cowboy

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We’ve been fighting this fight since the beginning.

Jeep Gladiator I'm getting old. AE7C5B38-8A9A-46F4-AF12-D81C1FD2A2F0
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Ole Cowboy

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We’ve been fighting this fight since the beginning.

AE7C5B38-8A9A-46F4-AF12-D81C1FD2A2F0.jpeg
Dunno if that is for real or not, but if you will look at those old cars back in those of pre-WWII you will note the TALL and skinny tire.

WHY? The answer is easy, not many roads were paved, in fact, most were not even improved dirt roads. When it rained they turned to MUD so how did get around on them? That is where the tall skinny wheel/tire combo came into play. The tires would spin down thru the mud till they hit dry dirt and when they did they gained traction and that is how you drove thru those muddy roads.

Its ALL about the Contact Patch!!!
 

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Ian cj10

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ive got 15's on everything but my colorado i went down to 16's for that & when i get my jt it'll get the smallest diameter rims i can put on it
 

TampaJLU

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Am I the only one who likes the smaller rim bigger tire combo ?It seems all the nice looking rims are 20's and higher. Ugh!!! My hat brim is curved people.
Im doing 35x12.5x17 tires on 17 in wheels.
 

ACAD_Cowboy

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Its ALL about the Contact Patch!!!
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. :surprised::mad::swear:

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)


:computerrage:

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. :devil:
 

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Blown7

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represents brims ranging from

@ColdSRT - as you’re aware, SRTs/Trackhawks come stock with 20’s...I doubt anyone’s suggesting to roll some 285/65/17s on them ;)[/QUOTE]

Anyone who really knows that big brakes need big rims.
My Trackhawk has almost 16 inch rotors..
I'll take big rims and brakes any day compared to small diameter rims.
 

Ole Cowboy

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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. :surprised::mad::swear:

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)

:computerrage:

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. :devil:
Downloaded, will try to get to it in the next couple of days, will respond ASAP...
 

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Dallas91

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Anyone have a calculator I can borrow?
 

Lou3.6

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Anyone have a calculator I can borrow?
If there ain't one on your computer -? - then it's about (over) time to UpGrade . . . NO ? :LOL: :like:
 
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Dallas91

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Ok. ok... anyone have a computer with a calculator on it? I don't have either to figure it this equation.
 

nuut

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I bought my JT Sport S off the lot. My local dealer had installed 20’s with 35x12.50 Mickey Thompson Baja ATZ’s. Love the tires HATE the rim size. They are OK for casual trail riding but I found out yesterday that there isn’t enough rubber to air them down for traction on rock covered trails.
 

ACAD_Cowboy

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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?

Jeep Gladiator I'm getting old. 98405061-E117-42CF-9AFE-0B3D73A96BBC


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