Common Sense Counterfactuals - Tire Width

[xpensive]

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This is explained by equilibrium of the external forces.

You have a tyre with an external force applied from the car weight of x, and the reaction from the ground also equals x. The force to the ground is applied by 28psi over an area of contact patch. The contact patch must equal force/pressure.
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I love this, words of wisdom from our esteemed moderator, read and learn members!

Thanx a whole lot Richard.

[WhiteBlue]

Richard's assumption that the contact patch pressure is somewhere near 28 PSI was later shown to be wrong by Ciro. That shows two things.
1. tyre pressures in F1 are much lower more like 12 to 14 PSI
2. he also thinks - for reasons unknown to me - that forces through the carcass are negligible

Perhaps it is common experience of F1 tyre engineers that contact pressure and tyre pressure are similar enough to neglect the deviation for this application. That does not mean they are equal in a mathematical sense. It would be helpful to the understanding to mention why certain assumptions are valid for the F1 application. The forum is made to learn about the technical side of F1. So even if certain assumptions are taken for granted by people familiar with the application it would be useful to elaborate for the common understanding.

[bhall]

So, WB, does that mean the pneumatic tire is a superfluous invention? After all, if it's the job of the sidewall to support the load rather than contain air pressure, why does a tire need air pressure? Seems redundant to me.

[rjsa]

None of that negating the basic fact that in a pneumatic, from the greek root pneuma (πνεῦμα) - air, wind, breath, soul - it's air doing the heavy lifting.

Yes, construction can shift some of this load from the patch area X pressure basic behaviour, but we have no clue by how much.

If anyone do have the numbers for current F1 tyres, please share with us.

[WhiteBlue]

So, WB, does that mean the pneumatic tire is a superfluous invention? After all, if it's the job of the sidewall to support the load rather than contain air pressure, why does a tire need air pressure? Seems redundant to me.

What is the purpose of this question? Trolling? Baiting? It does not seem helpful to me in order to progress with understanding of the consequences of tyre width. So if there is a legitimate content I do not get it. Please elaborate if there is. The current discussion is dealing with the forces in the tread and the carcass not with the side wall. That is one of the things that are confusing me about your question.

[bhall]

Based upon this...

[...]
3. The vertical component of the force around the edge of the contact patch in the tyre construction
So the first assumption is that internal forces around the edge of the tyre patch are negligible.
[...]

...I assumed you were referring to the sidewalls. But, when I looked at the rest of your posts on the subject here and in the other thread, I realized I have no idea what you're talking about.

My apologies.

[WhiteBlue]

No offence taken. I know a picture would be better but I have no drawing which would illustrate the force balance.

[xpensive]

No offence taken. I know a picture would be better but I have no drawing which would illustrate the force balance.

Oh but here are so many, contact patch will always be vertical load over internal pressure, only that it's not linear when pressure increases with deformation. For those who have seen a deflated F1 tire, that is very obvious indeed.

[bhall]

[WhiteBlue]

To visualize I have made a little drawing of the oval contact patch and the pierce of tyre that is above it.

You can see the three pressure or stress components that hold that piece of rubber in equilibrium. Now imagine that the same piece of rubber travels to the top of the tyre and look at the stress balance again. You have no contact pressure to balance the tyre pressure and the vertical force must be carried by the stresses only. It follows that the stresses are transformed when the piece of the tyre travels into the contact patch area.

Now think of a variable amount of pre inflation - let's say 50 PSI - and a much stiffer carcass and you suddenly have a much larger tyre pressure without a change in the dynamic load that would create an equal increase in contact pressure.

It may well be that for the F1 application the residual vertical stress in the contact patch is negligible because the tyre have such a low pre load, but it is not true for the general case. So I think I'm entitled to the view that contact pressure and tyre pressure are not strictly equal if you look at a force equilibrium.

[xxChrisxx]

None of that negating the basic fact that in a pneumatic, from the greek root pneuma (πνεῦμα) - air, wind, breath, soul - it's air doing the heavy lifting.

Yes, construction can shift some of this load from the patch area X pressure basic behaviour, but we have no clue by how much.

If anyone do have the numbers for current F1 tyres, please share with us.

The amount taken by the sidewall will be anything from 0% to 100% (I know, I know, I'm Mr Obvious ). We can't tell for F1 tyres, but Avon motorsport publish spring rate data for their motorsport tyres. I was playing about with this last night for another tyre thread.

http://www.avonmotorsport.com/resource-centre/downloads

The F3 tyre seemed to have contact pressures from about 30% to 100% of tyre air pressure. This indicated that at different conditions the tyre ranged from having most the load taken by the sidewall to all of the load taken by the air pressure.

The 100% condition seems to come from an overloaded condition, the vertical load is high about 490kg. The 30% was from an obviously underinflated low load condition. I would guess in a real operating condition, the sidewall would be taking some of the load.

I couldn't come up with any firm conclusions, so I didn't post it then. Maybe somoeone else can have more luck crunching the numbers.

[Lycoming]

1. tyre pressures in F1 are much lower more like 12 to 14 PSI

Just curious, Is that cold or hot pressure? Is that what they inflate the cold tire to, or what it develops once it gets some heat in it?

[WhiteBlue]

I have not seen a tyre pressure trace, so I have no way to know how pre heating, dynamic load (downforce) and heat from the work the tyres do raises the tyre pressure.The added pressure is probably well below the initial cold filling pressure which reportedly is around 1 bar over atmospheric pressure in F1. Another half bar would be a high estimate.

[langwadt]

1. tyre pressures in F1 are much lower more like 12 to 14 PSI

Just curious, Is that cold or hot pressure? Is that what they inflate the cold tire to, or what it develops once it gets some heat in it?

FIA recently mandated a starting pressure of minimum 16psi, and a running pressure of minimum 20psi front ,19 psi rear