What determines a tyres pressure?

[memet883]

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

[hardingfv32]

1) Tire spring rate

2) Contact patch load distribution: Is the contact patch providing max grip.

It is generally a compromise between the two depending on the cars wheel rates.

Brian

[memet883]

1) Tire spring rate

2) Contact patch load distribution: Is the contact patch providing max grip.

It is generally a compromise between the two depending on the cars wheel rates.

Brian

Thanks Brian, do you know the formulas that link pressure to tire spring rate / contact patch, or where i can find them?

[memet883]

Would it be as simple as

Ideal tyre pressure = load on tyre / contact patch area ?

I can't think how to incorporate tyre spring rate though.

[Jersey Tom]

1) Tire spring rate

2) Contact patch load distribution: Is the contact patch providing max grip.

It is generally a compromise between the two depending on the cars wheel rates.

Brian

Hell of a lot more than that.

[Belatti]

Im waiting for JT comment

EDIT: I was some minutes late

[Jeffsvilleusa]

I'm waiting for JT to elaborate [-o<

[hardingfv32]

1) "do you know the formulas that link pressure to tire spring rate / contact patch, or where i can find them?"

For the most part these are really two separate criteria. The tire spring rate having a relationship to the suspension design and the contact patch an issue mostly restricted to the tire itself. There are suspension design models and tire models, but I have not made an effort to research them. The better ones will not be public knowledge and their application and interpretation beyond my skill set.

2) "Hell of a lot more than that."

So please enlighten us. A discussion gets nowhere with answers like that.

Brian

[munks]

Some things that tire pressures affect:

1) Rolling resistance: Generally, more pressure leads to less rolling resistance because the tire is stiffer and doesn't deform as much. Ideally, of course, you want lower rolling resistance.

2) Temperatures: Generally, more pressure leads to lower internal temperatures because again, the tire doesn't deform as much. But in some cases this could make the tread slide more which would lead to more temperature at the surface. Also, you need to know what your goal operating temperature is (and where that is attained on the track, it's no use to be at the perfect temperature in the middle of a straight-away).

3) Vertical spring rate: usually increases with tire pressure.

4) Slip angles: will generally decrease slightly with more pressure. This may help tune the balance of the car front-to-rear.

5) Contact patch load distribution: of course you don't want to be crowning or the reverse, whatever that might be called (riding on the sidewalls).

I'm not a setup expert, but I'm guessing their main concerns would be the slip angles and the temperatures. Spring rates are for the suspension guys to deal with, rolling resistance isn't going to gain you much time compared to having a good tire for the corners, and finally the contact patch load distribution will likely be fine for a reasonable range of pressures as designed by the tire manufacturer.

Hope this helps.

[hardingfv32]

Maybe contact patch load levels is not the best statement. How about over performance of the contact patch of the tire.

Under contact patch performance:
1) Rolling resistance
2) Temperatures
4) Slip angles

1) Rolling resistance: How does tire deformation (heat generation?) create drag? Does the additional heat make the contact patch softer?

4) Slip angles: By what mechanism are the slip angles changed by a pressure change? This relationship is generally not provide by the tire manufactures in the tire data packages. I have never seen research on the relationship.

Brian

[jadatis]

I once got hold of the formula that the tyre-manufacturers use in Europe and since 2005 also in America. Comes from the ETRTO.
Worked it out in spreadsheets and translated a few to English to go worldwide with it. Now I am googling often for tyre-pressure to see if I can give info about it. Registered to this forum to give you it.
The formula gives the minimum pressure for the load and speed you fill in , at wich the tires wont get damaged by dryving. At this pressure you have maximum save gripp and comfort . You can play with this to influence over or under-steer.
http://www.dawsengineering.com/dewebsite2_003.htm
At this page several articles from J.C.Daws. Click the fifth wheel for a new suggestion to calculate the pressure, and comparison with the power-equation that is used by the tyre-manufacturers. The goal of this formula and also of the power-formula is to get the same indetation of the tire for reduced loads, as for maximum load at reference-pressure ( as it is called in the formula I got hold of).
I myself introduced the load or Gripp% . this is what the real weight is of the weight you calculate the pressure for. 100% gives maximum gripp and comfort, I concluded bij reactions that under 85% load% discomfort begins by bouncing.
So for racing keep it close to 100% but not over it, for maximum gripp.

You can navigate my public map of hotmail skydrive for articles and spreadsheets about tire-pressure.
http://cid-a526e0eee092e6dc.office.live ... px/.Public
And If you then still have questions, ask me here. Download and open spreadsheets in Excell or compatible program to use it, in the browser wont work because of data-validation and sheetprotection I used in it, and cant be handled.

Greatings from Holland
Peter

[DaveW]

Munks: Excellent post. Thank you. I would only add that changing the pressure split will modify the overall roll stiffness distribution, which will affect mechanical lateral balance (e.g. increasing front tyre pressures will nudge balance towards understeer).

[hardingfv32]

"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

[Belatti]

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.

[munks]

1) Rolling resistance: How does tire deformation (heat generation?) create drag? Does the additional heat make the contact patch softer?

Deformation is resisted by the internal friction in the material (and yes, friction causes heat). As the rubber in a rolling tire enters the contact patch, it resists deformation which increases the load near the front of the contact patch. As the rubber leaves the contact patch, it resists restoring its shape, which decreases the load near the rear of the contact patch. The difference in load front-to-rear causes a moment about the tire ... which causes some 80-90% of the measured rolling resistance.

And sure, any additional heat is going to tend to soften up the rubber. But again, you need to know your optimum temperatures for the track & conditions.

4) Slip angles: By what mechanism are the slip angles changed by a pressure change? This relationship is generally not provide by the tire manufactures in the tire data packages. I have never seen research on the relationship.

Slip angles arise from at least three different sources: a) the sidewalls allow the entire carcass to change angle w.r.t. the rim, b) the belt bends at the contact patch, and c) the tread shears in the contact patch. Increasing pressure will change the first two of these stiffnesses directly.

Munks: Excellent post. Thank you.

You're welcome. Being a lurker for a few months up until now, I'm probably just regurgitating what you or JT has written; I always read your posts with great interest.