What determines a tyres pressure?

[Jersey Tom]

One thing to consider... while raising pressure may increase effective springrates of the plies and belt package... if it shrinks up your footprint size, think about what that does to the shear stiffness of the tread, and the implications of such.

[jadatis]

Peter, thank you very much for that Daws link. Very interesting articles. The only problem is that the 5th wheel is the only one that doesnt work.

I was surprised too that the 5th wheel didn't work anymore.
But I saved the article on my skydrive .
http://cid-a526e0eee092e6dc.office.live ... dloads.pdf
So you can still read it.

[memet883]

Thanks for the replys all, and thank you Peter for the excel spreadsheet! I'm going to play around with it now

[DaveW]

"changing the pressure split will modify the overall roll stiffness distribution"

Is that because you changed the tire spring rate or is there some other mechanism?

Brian

Tyre vertical spring rate is (usually) a function of tyre pressure. Tyres act in series with springs & bars, so changing the pressure split will usually change the roll moment distribution in a turn, which will change the mechanical lateral balance of a vehicle. To be fair reality is more complicated than my explanation would suggest, but mine is a reasonable "rule of thumb", I think.

[Belatti]

Another thing I dont read or listen to often is that tire vertical and horizontal stiffnesess are function of vertical loads and camber, so basically tires are springs with dynamically changing stiffnesses.

[Ciro Pabón]

Can they be modeled as shells? I mean, instead of horizontal and vertical stiffness I would think it behaves more like a membrane, only that the "distributed load" changes with the shape of the membrane. In that case you don't talk of horizontal or vertical but membrane forces in x and y and shear, as Jersey Tom implied. In this case, radius of curvature is VERY important to deduce stiffness and it behaves kind of a "vicious circle" in which, under load, the curvature varies which, in turn, changes the shape of the load which, again, changes the curvature, etcetera.

Anyway, if by any chance somebody asked me to figure out how a tyre works, I would take the dust off of my old Nervi books. The techniques are beautiful... and very elegant. Kind of the "Greta Garbo of engineering".

Nervi's example: "thread of tyre" (roof) on top, complete with "plies" (reinforcing beams); "rims" (rows of butresses) provide edge stiffness, to the right and left. To the right is visible the "wire" (that is, an edge beam) that goes around the "rim" and "anchors" the membrane (darn! I never remember its name in English!) allowing the structure to develop membrane forces


I can easily imagine the building in the previous image being hanged upside down, so gravity takes the place of the air pressure inside a chamber and any movement of the building (if you could move it upwards and downwards) providing the changes in load (or air pressure) proportional to displacement (well, actually, to the cube of displacemente, if I remember well). In that case, the building (or the tyre) changes its stiffness depending on the changing loads, because its shape varies.

The Cloud Gate also has a similar design, but the forces are half in compression (bottom half of the sculpture, so to speak) and half in tension (top half of the shell). It is supported by two independent rings, so it can float and change shape depending on temperature. It moves slower than a tyre over a kerb, but it "ondulates" in the same way, I would guess, because the floating suspension creates a load that is proportional to displacement in the same way.

Cloud Gate, aka The Bean

[Jersey Tom]

You are correct, Ciro, in that the shape of the tire (e.g. sidewall profile) is very important.

[g-force_addict]

What variables affect the optimal tyre pressure? Clearly it can't be over the max allowable pressure usually given on the tyre, but i was wondering if there's a formula that is used to find the ideal pressure? Thanks

Ideally they should run the optimal TIRE pressure for a given load and not the other way around.
I mean not first building the car, fitting the tires and then playing with pressures.

[jadatis]

For the spring-rate I can give the next info. For this the indentation of the tyre plays a role. This indentation relates to the surface on the ground.
Radial tyres give almost rectangular surface on the ground, wide stays the same, length relates to the indentation, so part of length is part of surface.
This length relates to the indentation in the next way.
If length surface gets 2 times as much the indentation gets more then 2^2 = 4 times as much. So the other way around if indentation gets 2 times as much the length of the surface gets root 2= about 1,4 times as much.
This is a fistrule, and I am busy working it out exactly.
In any case you cant calculate the indentation by using pythagoras with the radius of tire and half of surface length, as I first thought , and worked out in spreadsheet in the map "all about tire-pressure. but I also placed a paint-picture of how I think the tire intends in practice.

[Ciro Pabón]

What do you mean by indentation? Perhaps a drawing would help me to understand, I'm a bit confused.

[munks]

I think by indentation he is talking about vertical deflection.

[jadatis]

Vertical deflection is exactly what I mean with indentation.
But here the picture that is in the next map "all about tire-pressure"
http://cid-a526e0eee092e6dc.office.live ... e-pressure


This is the idea of how a tire indents or deflects to my idea.
The I is the indentation and Rt is the radius of the tire, R1 is a virtual radius and not the edge of the rimm.
I have to work it out still but mayby this gives an idea.
Edit: see that the image wont work,probably because I made it a png instead of jpg wicht the forum cant handle.

[hardingfv32]

1) You have a lot of control over what contact patch is going to do under different pressure conditions based on any number of tire construction variables.

2) Here is an interesting analysis of patch size vs weight and pressure.

http://performancesimulations.com/fact- ... ires-1.htm

3) How important are the tire spring rates when changing tire pressure? Why did the NASCAR teams check tire spring rate for every freshly mounted tire at the track before using them?

Brian

[Ciro Pabón]

Thanks, munks, you're very kind.

Well, I would say that indentation and tyre pressure are vaguely related, then.

I would also say that tyre pressure depends mainly:

- on the tensile strength of the tyre plies and rubber and
- on the amount of air inside
- plus dynamic effects (the whole thing is under centripetal force when spinning)

As we discussed earlier, a tyre does not support itself on the floor directly through the pressure of air. It is not a balloon. It is a more complicated structure.

A tyre does not push against the floor: the axle of the car hangs from the "upper" tread. The rim pushes down the orange bead. This bead pulls the walls in green. The walls are suspended from the top tread in red. This top part of the tread, which is the structural element in a wheel, I compare with an arch. This arch is made rigid by the pressure inside the tire. Like this (sorry for the copy/paste).




So, that's the interaction, between tread and air (air making the tread rigid), that controls the pressure inside. If the pressure is not large enough to make the tread rigid, then the pneumatic would "need" additional pressure inside to recover static equilibrium. The tyre develops this pressure by diminishing its volume, ergo, more air pressure (or indentation).

It's not the weight of the vehicle divided by the area of the contact patch.

I think that's the reason why the guy in the link posted graciously by hardingfv32 declares himself to be confused by actual pressure measurements. He's trying to correlate them somehow with contact patch size. Good luck with that!