I Thought it might a good idea to build a data base of real tire diameters. To be clear, for example a 34" tire on a stock 17" wheel is not 34". Tires are expensive. It would help to know how they fit on our Jeeps. So, tell us the brand and size of the tire you have, same with the wheel. Take a horizontal diameter dimension and a pic would be awesome. If you have a lift let us know. Tell us if you have any rubbing and where. Thanks much.

 

Quite a few people have tried doing this over the years. They get a handful of replies, then it gets lost and just fades away. You would probably need a mod to sticky the thread, then be willing to monitor and update the original post on a continuing basis.


Here is one.

https://www.jk-forum.com/forums/modi...meters-291259/

 

I think this is a great idea and I've considered attempting to do this same type of thread. However, the most important part would be to define how the measurements are taken first and foremost. Without establishing that, the information will be useless.

I've also seen this done on another forum where they did define how the measurements needed to be taken. However, I don't agree with their defined measurement method.

I'll post it here and mark what I consider to be good vs bad:
And here's why I consider these items bad.
1st off, you are performing the measurement with the weight of the vehicle on it. While this is good for actual tire height for YOUR vehicle, it tells very little about the actual tire's height, so you are NOT getting consistent and accurate data for any particular tire. YOUR vehicle does not weigh the same as everyone elses. YOUR vehicle may have heavier bumpers, winches, axles, skid plates and other accessories on it which places more weight on the tires and thus reduces the overall height compared to someone that is riding with stock equipment and seeing as though the Wrangler is one of the most modded vehicles with a lot of different accessory options to chose from, the weight on the actual tire will vary drastically from vehicle to vehicle.

They also did not define which wheel to take the measurement on. Wouldn't a wheel located in the front where the engine is sitting on top of it, have more weight on it than a rear wheel? And wouldn't the amount of weight vary as to the different springs each vehicle has installed on it? How many different lift/spring options are there on a JK again?? Quite a few right? What about the difference of weight between a 4 door and a 2 door? Wouldn't all those differences in weight, have an impact on the actual amount of air in the tire, as it would apply to the tire pressure. Try inflating a tire to 30 PSI with it being off the vehicle and then put it on the vehicle and then measure the PSI and then tell me what that tire pressure difference is at. With the height measurement taken from the ground to the shoulder edge, what is used to ensure you are measuring at the crest (0 deg point) of the top of the wheel shoulder and the 180 Deg point bottom of the wheel point to ensure you're measuring the actual max height?

What about the differences of the wheel's width, that the tire is mounted on? Would a tire mounted on a 6.5in width wheel, measure the same as the same tire mounted on a 10in wide wheel? What about the ply rating of the tire? Doesn't the exact same tire with a Type C rated ply sidewall flex more than the exact same tire which has a Type E ply sidewall? Some manufacturers do provide that option of different ply load ratings for the same tire.

I could go on and on. The key would be to establish a measurement method that minimizes and/or eliminates the potential variable differences and at the very minimum making a note for the measurements of the particular variable which can't be eliminated such as wheel width differences, so it can be useful.

 

Yup... this ^^^

 

You'll have to explain this one to me.

 

Leave it to Ron to call me out on the one item I wasn't 100% sure on. It's been a long time since I did the calculus equations used in Static mechanics engineering analysis classes. I could be wrong and over thinking it but what I do remember is that since your vehicle is "stationary" (ie it isn't falling through the ground), when weight of the vehicle is placed on it, then the sum of the forces is equal to zero. How that weight gets distributed between all the upward forces (your springs + tires), would always depend on the spring coefficient (ie the stiffness of the springs)


So with that thought in mind, the weight of the vehicle will distribute itself differently to each of the wheels depending on the stiffness differences of each of the springs. For simplicity sake, I like to give extreme examples so everyone can draw a better visual image in their mind of what is going on. Try to imagine if your front springs were very stiff and you could not flex them when pushing on them with your body weight and pushing down on them with your hands. Then imagine if your rear springs were very squishy and you could fully push them down easily when flexed by hand. The weight of the vehicle applies a downward force. The springs are an upward force, as well as your tires and are pushing in the equal and opposite direction of that downward vehicle force. (ie the sum of the forces=0) With a stiffer spring in the front, it will apply more upward force than the softer squishy springs in the rear. Therefore, what ends up happening is that more of the vehicle's weight gets distributed to the rear springs since the front springs are pushing upwards more than the rear springs.

It's a Newton's law static (not moving) equation, where the sum of the forces will equal zero, and the forces will be distributed differently in the X direction, as well as in the Y direction and will be distributed differently depending on the spring coefficients.


Too deep for a Jeep forum? LOL......I know.

 

I believe we are talking about static weight on the scales; one scale under each tire.

Variations in spring stiffness won't change the scale readings as long as the relative heights among the four corners don't change.

Take the springs out and let it sit on the bump stops, (simulating very stiff springs), place the springs on each fender to account for the weight. The scales will read the same whether the springs are installed or on the fenders.

Are you talking about something different?

 

Weight is weight. My 6,500 Jeep weighs that at stop and at 65 mph. Weight transfer based upon acceleration, aerodynamics, etc, would all figure in at velocity, but that's way too much math for this group.

 

You also have take account for side wall ratings because tire height at 30psi will be different for the same tire in C, D or E side wall rating. This also effect if you lower your pressure to 15psi for 4x4ing, to achieve the same footprint all 3 sidewalls will be at different pressures.

 

I'm referring to "static" as in "Stationary/not moving".

The Newton's Law defined equations we're working with become defined as.
Fx=0 (Read, the Sum of the forces in the X direction are equal to Zero): Your Jeep isn't moving up and down when parked.
Fy=0 (Read, the Sum of the forces in the Y direction are equal to Zero): Your Jeep isn't moving Foreward and backwards when parked.
Fz=0 (Read, the Sum of the forces in the Z direction are equal to Zero): Your Jeep isn't moving Side to side when parked.
If all those equations are true, then this equation becomes true.
Fx+Fy+Fz=0 (read, the sum of all the forces are equal to zero.)
F=mA (Read Force equals, Mass times acceleration) Mass being the weight applied by the downward forces and acceleration being gravity.

If the spring stiffness changes, then this impacts the Forces applied in the X direction (up/down), which therefore also impacts the Forces that get applied in Fy and Fz which can be more easily understood just by looking at the equation I have in Bold. This is the weight distribution which impacts the Weight distribution in the Y and Z directions.

I'm trying to find a application example. Not the best, but this is kind of what is going on. Imagine the front spring is A, if A pushes up more than the rear, then more weight will be on point D.

https://www.youtube.com/watch?v=XaFpT4YiA0s Also think of a simple playground See-saw. If you push up on one side more than the other, what happens to the opposite side? It pushes down more right? So there's more Force on the opposite side and in our equations of F=m*A, acceleration is a constant (ie Gravity). Therefore the Mass (weight) increased in the opposite side.

This should also give you what I'm describing. Are all JK's heights among the four corners the same? I believe they would be different heights depending on the springs used or are you saying we can make an assumption that all JKs sit level, which we already know they don't? And why don't they all sit level? Difference in spring stiffness and heights?