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A matter of pressure


Tony
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A huge topic so my intention here is to explain an overview not the entire package.... I think you will understand what i mean as you read this explanation.

 

For the domestic vehicle the standard "suggested" pressure is determined by the vehicles weight, the tyres aspect ratio and contact patch.

 

A predetermined internal pressure is suggested to compliment the tyres deliberate structure to enhance the handling properties desired by the manufacturer, tyre and car respectably.

 

So why the variants?

 

Air is not an efficient form of inflation!.... Within the composition of air some of the various molecules are susceptible to heat, in particular "water".

 

Since the holy grail of tyre technology is the transition of heat into the silica to accelerate traction properties the failure of air to maintain a constant value seems ridiculous..... But that's the way it is and the reason for this explanation.

 

Ok so what do we have then?..... Most manufacturing guides will offer three ranges

1> Normal pressure

2> Loaded pressure

3> High speed pressure

 

Why :D

As i have explained air is inefficient and the silica needs heat for traction, it's unreasonable to expect the owner to stop and change the pressure during any journey, although it is possible to predict how the air will expand and suggest a initial cold pressure to cover the fields above.

 

How

Manipulate the tyres contact patch.... All three fields have known parameters so it's possible to manipulate how hot or cold the tyre will be-become within those fields..

 

Pressure stages

 

post-2-1196429733.jpg

 

post-2-1196429768.jpg

 

 

If for example our imaginary car is intended for town use then the suggested pressure may be 28psi, this comfortably assumes 30mph and a full tyre contact patch with minimal temperature/pressure gains

 

If the same car was intended for motorway use an initial cold pressure of 34psi may be suggested to reduce the size of the tyres contact patch thus reducing the thermal internal expansion to a pre-calculated average level.

 

The loaded pressure is a harder topic

Since the guidelines for loaded "motorway" pressures is very vague the normal intention is to aid the tyres construction and maintain a full contact patch after the internal expansion.... It's also assumed the speed limit will be around the 70mph mark so centrifugal forces are expected to distort the higher pressure suggestion but the exact level is theoretical.

 

Non-standard tyres

Changes in the tyre width or aspect ratio will belay all guidelines offered by the manufacturer. It's reasonable to assume the replacements are not to distant from the OEM fitment so it's wise to adopt the original pressures as a template then read the handling/wear traits and adjust accordingly.

 

Reading the signs

Tyre wear due to inflation levels is easy to observe and correct (see image at top of page) but handling traits due to tyre modification needs a little more understanding.

 

A common modification is to go low and wide, so how do you judge the pressure now?

 

post-2-1196429924.jpg

 

If the pressure is to high at the front the low aspect ratio tyre will have a tendency to "tram-line". This is due to the inflexibility of the tyres sidewall. If the rigidity is to high then the transient grip can no longer be absorbed within the sidewall and perpetually steer the car.

 

Additionally if the pressure is to low then the pneumatic slip angle will allow the wheel to transit between Yaw and the actual tyre contact patch. The effect is a numb, lazy feel to the steering.

 

Finally there is a need to balance the tyres footprint and sidewall rigidity front and rear despite the obvious weight disparities.. If for example the rear footprint is low due to high pressure then the car will over-steer...To much footprint then the opposite can be said

 

All examples can be manipulated by small changes in the cold tyre pressure, examples of "over-under-steer" by manipulation.

 

In summery

As you may have noticed i keep saying "suggested" pressures? The reason for this is that no manufacture can cover all examples, so it's up to you to decide the correct pressures for your own unique example :)

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Great post Tony :)

 

I know what you mean about over inflating low profile tyres causes tram lining. I put mine on 34psi once and the handling was terrible. Dropped them to 32psi and all was well again. I was amazed at how 2psi can make that much difference!

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Thanks Tony, that's very helpful

 

Tyre pressure is a complicated subject and a great many of the calculation inputs are empirical rather than purely theoretical and are based upon extensive experimentation.

 

For the "domestic" passenger vehicle recommended pressures result from a range of compromises that recognise the impossibility of running at the "perfect" pressure in all circumstances.

 

Most of the catastrophic tyre failures that are recorded result from under-inflation and the resulting tyre wall damage. Many, many people fail to check their tyre pressures sufficiently frequently. I would suggest it should be done not less than weekly and before any long journey. For most people, regular checking is far more important than the precision of the inflation pressure to within a p.s.i. Note that I said for most people! Enthusiasts will have a far better idea and feel of the way in which a change in tyre pressure alters the performance of their vehicle, but most people are not enthusiasts and horror stories regarding inflation carelessness are easy to find.

 

The combination of pressure and temperature that exists in the inflating fluid in a tyre results from a complex set of differential equations, the parameters including:- initial inflation temperature and pressure; ambient temperature; road surface temperature; the working relationship between road surface and tyre surface, which will include whether the interface is wet or dry; road speed; axle load and what I will call the way in which the tyre "works". Google for the Pacejka formula if you want more detail about the way the tyre works.

 

Tyre manufacturers need to decide what "working" temperature and pressure will give the best "footprint" compromise for steering, braking, road holding and wear and the best "working" compromise for carcass life. From that decision they need to calculate the starting parameters (i.e. the "cold" pressure) that will result in "working" temperatures and pressures that are close to the ideal in "normal" use.

 

It is pretty obvious that the starting pressure cannot be specified in any very precise way for a domestic vehicle given the very varied way in which they are operated, but the empirical figures published by the manufacturers are an excellent guide for most people.

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I think we agree this area is endless in explanations and many people i feel desire a more finite solution to their curiosity but this is just not possible.

 

The tyre pressure arena exists within a dynamic realm during our journey.... The factors offered can only assist in a preempted formula and suggest a cold starting point.

 

It's a pants deal for the 21st century but for now it's the only one we have :)

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  • 1 month later...

Very interesting read. My car has a tendancy to "tramline" so it I'm going to lower the pressure on my front tyres (235x40x17) from around 30 to 28 to see if it makes a difference. It's difficult to gauge because the wheels and tyres are non-standard but thr recommended pressure in the fronts is 32-33.

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Very interesting read. My car has a tendancy to "tramline" so it I'm going to lower the pressure on my front tyres (235x40x17) from around 30 to 28 to see if it makes a difference. It's difficult to gauge because the wheels and tyres are non-standard but thr recommended pressure in the fronts is 32-33.

 

Your car is a perfect example for "finding" the correct pressure..... Remember though make small changes each time.

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Very interesting read. My car has a tendancy to "tramline" so it I'm going to lower the pressure on my front tyres (235x40x17) from around 30 to 28 to see if it makes a difference. It's difficult to gauge because the wheels and tyres are non-standard but thr recommended pressure in the fronts is 32-33.

 

Your car is a perfect example for "finding" the correct pressure..... Remember though make small changes each time.

 

There's been a post re. tyre pressures on the LOC UK forum. Unfortunately the conclusion/emphasis was on adhering to the recommended Lexus pressures as defined by the sticky label on the door pillar, instead of using those as the starting point and monitoring handling characteristics and tyre wear and adjusting as necessary.

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I think most times people are aware of a problem and seek guidance but fall back into the safe zone on the door pillar.

 

DJ wozza did some superb maths for his new wheels based on the corner weight data from TDi, the OEM rec's and realistic adjustments for the Supercharger.

 

This combined with the new tyre sizes offered a reasonable platform for the modifications.... Without giving actual data away the increments were a few PSI changes front and rear.

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DJ wozza did some superb maths for his new wheels based on the corner weight data from TDi, the OEM rec's and realistic adjustments for the Supercharger.

This combined with the new tyre sizes offered a reasonable platform for the modifications.... Without giving actual data away the increments were a few PSI changes front and rear.

 

Yes, I know :tired:

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  • 2 months later...

has anyone heard of the "max pressure -10% theory" that ive just read about on the tyre bible website? some people believe this to be a better starting point than the handbook figures. you take 10% off the maximum pressure of the tyre, and start from there. i have my doubts.

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has anyone heard of the "max pressure -10% theory" that ive just read about on the tyre bible website? some people believe this to be a better starting point than the handbook figures. you take 10% off the maximum pressure of the tyre, and start from there. i have my doubts.

 

I don't like the sound of that... I work in increments of up/down a pound or two. Basing a template from -10% of the max is down right dangerous in my opinion.

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I also think that sounds a bit dangerous..although i know people who have used it!! The thing is in a normal car with wallowy suspension pumping tyres to around 45psi you may not feel that much more harsh compared to 35psi say...HOWEVER once you go with a more firm and sporty suspension the difference is quite apparent.

 

Just as an example I run firmer lower sports suspension on my car a difference of 2 psi and the ride changes 34.5psi and it feels the tyre contact with the road when turning is too stiff and ride also suffers...at 33psi ride is more subtle and steering feel is better...... Now doing the same thing on a standard car running standard soft suspension i feel absolutely no difference.

 

What i'm trying to say is that upping the psi to extreme amounts makes your suspension work a LOT harder even though you probably don't feel it much with standard suspension....you also need to think about things such as road imperfections....hitting one at the wrong speed with such a rigid tyre is asking for trouble...

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  • 1 year later...
Arose such a question - should I raise the tire pressure for a long time parking?I have in view on a month or two.

 

Have you considered putting it onto axle stands? (Assuming it is in a secure place).

 

Reads logical to me.

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I've always wondered with the axle stands vs. on its tyres argument, which one is worse? Are axles designed to take the pressure of the whole car like that for extended periods of time?

 

There are no 'axles' are there? "Axle" stands will also take the load off the springs and the rest of the suspension joints, although I'm not sure what the effect would be on the on the shock absorbers.

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most new cars are stored for a good while before being brought and driven

I wonder if the manufacturers take any preventions

 

Mazda MX-5s are notoriously delivered with high tyre pressures. They set them high at the factory for shipment and the dealers are supposed :lol: to set them correctly as part of the PDI

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most new cars are stored for a good while before being brought and driven

I wonder if the manufacturers take any preventions

 

Mazda MX-5s are notoriously delivered with high tyre pressures. They set them high at the factory for shipment and the dealers are supposed :lol: to set them correctly as part of the PDI

 

Which they don't :lol:

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