Over inflated tyres

[Rocky]

This has been a useful thread that got me thinking about my approach to tyre pressures.
I have probably always tended to run high pressures in all my cars and the first thing I did when I picked up the Mazda 6 recently was increase the pressures to 36 cold for the long road-trip home. (Mazda recommendation:32psi) The ride was predictably a bit rough and after reading this thread I let them down to 32/33 and the car rides like a Jaguar.
Likewise I have been running 34 -36 in the MX5 (which produces a very rough ride) but have now reduced them to 30. I will drive my favourite roads at this pressure for a while and see how it compares.
I notice the recommendation for Son's Mazda 3 is 33F/32R which was higher than I would have expected.
(I would have to say that all the tyres wear very evenly at the higher pressures)

[The American]

Internet Forum (n), an electronic medium, comprised of text based threads, organised by topic and interest area, and predominately for the purpose of allowing strangers to fight about personal opinions, passionately. Aided greatly by the mechanism of misunderstanding.

7 = the average number of forum thread posts before a dissenting view signals initiation of the fight. Or, and acceptable pressure range to experiment with to arrive at personal taste choice.

36 = the average number of forum threads on the same topic that emerge before a 'sticky' is created. Possibly, an appropriate tyre pressure for some people in some cars in some situations, with some particular or no particular tyres.

Sticky = a special type of thread designed to settle once and for all the winner of a particular stream of fights about a particular opinion. The Established body of settled knowledge according to the prevailing views of the vocal and most convincing. Or, the tyre characteristic associated with track oriented tyres, at a given pressure.

Search Function = little known and disused internet tool.

[Magpie]

You forgot
Godwin's Law:
"As a Usenet discussion grows longer, the probability of a comparison involving Nazis or Hitler approaches one."
There is a tradition in many groups that, once this occurs, that thread is over, and whoever mentioned the Nazis has automatically lost whatever argument was in progress. Godwin's Law thus practically guarantees the existence of an upper bound on thread length in those groups. However there is also a widely- recognized codicil that any intentional triggering of Godwin's Law in order to invoke its thread-ending effects will be unsuccessful.

Formosa's Law:
“The truly insane have enough on their plates without us adding to it.”
That is, flaming someone with an obvious mental problem can't make it any better.

Rule 34:
“If it exists, there is porn of it.”

Rule 35:
“If no such porn exists, it will be made.”

Skitt’s Law
"any post correcting an error in another post will contain at least one error itself" or "the likelihood of an error in a post is directly proportional to the embarrassment it will cause the poster."

[The American]

magpie, was that intentional ^ ?

[Magpie]

Yes it was intentional humour...

By the way Hitler set his tyres pressures at 24psi

[The American]

Ok, carry on all, Hitler reference not genuinely invoked.

Phew!

[93_Clubman]

& don't mention the war...

[Rodger]

I am running Toyo Proxes 205/60/15 on an nb, the tyre supplier recommended 36 psi but I have dropped the rears to 32. seems to be ok.

[Odd]

https://www.toyotires.com/tires-101/tir ... ion-tables
http://toyotires.com.au/tyre-tips/tyre-maintenance

Seems your tyre supplier recommends something different to the manufacturer......
In the table provided the manufacturer say that tyre can handle a 1124lb load at 90mph.....510kg at 150kl/h....with a pressure of 26psi
twice the weight of an MX5.....why would you need to put in 36psi?
I also believe it is recommended to have the same pressure in front and back tyres....some cars have different recommended pressure due to a weight bias font to back
MX5 are 50/50

Perhaps I'm just an old cynic ......but I wonder how many retailers really understand that tyre pressure is a function of load?
the heavier the vehicle....the higher the tyre pressure

I've done a lot of searching and I cannot find one "Manufacturer" that recommends....in writing on a web site.....a tyre pressure of 36psi for ANY MX5

Please post link if you can find any recommendations over 30psi....;)

More than Happy for technical data or information that provides alternate opinions

[hks_kansei]

tyre pressure is a balance between supporting the weight of the car, supporting the tyre itself during driving, and keeping a contact patch on the road, and passenger comfort.


If you want max comfort and a wide contact patch you'll lower the pressures, the downside is that during corners the tyre has less support and can feel floaty, or even roll under the rim.

If you want maximum tyre support for cornering etc, or minimum rolling resistance for highway driving you increase the pressures, the downside being that the contact patch is less.


With cross ply tyres these points were important, since they tended to be pretty soft and easy to distort the shape of with the wrong pressure, which exagerrated the bad points of each pressure. But with modern radials, due to the far stronger casings thanks to steel belts, they can run a wider range of pressures with less of the downsides.


As for the tyre manufacturer reccomendations, they can only provide general recommendations of the tyre, depending on it's use, and the car, the needs may be different.
A 500kg trailer (250kg per wheel) may want only just enough pressure for the weight to help reduce bouncing.
Same pressure on an MX5 (1000/4=250kg per wheel) may cuase it to feel odd in corners and possibly even tuck a tyre under and pull it off the bead. The reason being that the MX5 can put a LOT of side load on a tyre during a corner (more than say a corolla), where a trailer generally cant.


At the end of the day, put whatever pressure you like in the tyre within the safe range (which on most tyres seems to be between 15psi and 50psi) and keep it if you like it.

It's air, it's free.

[Odd]

The adhesion patch is also changed by a change in tire pressure. Again, assuming the 1000 pound load but increasing the pressure to 50 psi results in 1000 lbs. divided by 50 pounds /sq. in. is equal to 20 sq. in. That’s a 66.7% increase in pressure resulting in a 40% decrease in adhesion patch size. Just as important as the total surface area of the adhesion patch is the shape of the adhesion patch.

Some of us who live with seasonal snow and ice have discovered that “skinny” tires provide better snow and ice traction than “wide” tires. That means that an adhesion patch that is longer (front to back) than it is wide (side to side) provides better forward traction and stopping power on snow and ice that an adhesion patch that is wider than it is long. Increasing tire pressure decreases the length (front to back) of an adhesion patch faster that the width. In fact, it takes a massive increase in tire pressure to significantly change the width. The tire “lifts” the front and rear of the adhesion patch with increased tire pressure much more readily than it lifts the sides.

Now, this combination of causing the adhesion patch to grow smaller at the same time that it is becoming shorter has a double impact on forward traction and stopping distance. So, as a rule of thumb, one should assume that a 10% increase in tire pressure above manufacturers’ recommendations results in a 20% decrease in forward traction and a 20% increase in stopping distance.

Q: If I were to increase the inflation pressure of my tires to improve fuel economy, what considerations should I be aware of? Can I run at the maximum pressure marked on the sidewall?

A: There has been much bunk posted on the Internet about how easy it is to save fuel by over-inflating the tires on a vehicle. The people never mention the downside: driver safety.

I have canvassed all the major tire companies and not one was willing to recommend deviating from what the vehicle manufacturer’s placard recommends.

Michelin, in particular, noted, “Michelin’s recommendation is, and always has been, that the correct and only inflation pressure for your vehicle is that which is specified by the OE vehicle manufacturer, either on the placard (inside the driver’s door jamb) or owner’s manual.”

Are the tire companies part of the cabal that killed the 100-mpg carburetor and just want you to wear out your tires faster?

In short, no. The thing that kills fuel economy and increases tire wear is underinflation. Driving with tire pressures 10 per cent too low will increase fuel consumption by 4 per cent. And the tires will give 30 per cent less mileage before wearing out.

The tire pressure numbers on the placard are not just a random whim by the car company. A lot of research went into choosing the tire size, type, speed rating and pressures for your car. The pressures have to take into account passenger comfort, vehicle grip, vehicle load, tire life, rolling resistance and fuel economy.

All car manufacturers would like to increase the fuel efficiency of their cars. But in dealing with tires, for everything you gain, you lose something elsewhere on the product map.

That’s why tire performance charts are spider graphs. The perfect tire would have a perfect circle graph to show its performance, but that never happens. The charts end up looking like spider webs.

The area where the tires touch the ground is the contact patch. By increasing tire pressure, the contact patch shrinks.

According to Michelin, “The first thing to understand is the tire’s interaction with the pavement is through contact-footprint forces, which are significantly affected by inflation pressure. Altering these contact-footprint forces from the intended design with an inflation increase will vary by vehicle and inflation pressure, but generally results in less than optimal performance for the tire application on the vehicle.”

Jon Bellissimo, Goodyear’s director of consumer tire technology, states, “On wet surfaces, lower inflation pressure generally produces slightly higher braking traction – until the onset of hydroplaning.”

On the lateral-grip (how well the car holds in the corner) aspect of overinflation, Bellissimo concludes,

“If pressures are increased without the corresponding increase in load (adding weight), lateral grip will generally decrease. For vehicles in the market place, the placard inflation generally provides optimum handling.”

The Michelin engineering team ran a computer simulation on a family car with pressures increased to 44 psi from the proper 35 psi.

They report, “Increasing inflation pressure can negatively affect performances such as handling, wear life, worn appearance and braking traction. For instance, our modeling simulations predict, with an increase from 35 to 44 psi, up to 6-per-cent degradation in stopping distance can occur on wet/dry surfaces.”

To put that in context, a Chevy Cobalt travelling at 110 km/h takes about 49.8 metres to stop. An increase of 6 per cent adds another 3 metres to the distance – three-quarters of a car length.

The Michelin report concludes, “The key learning is that inflation pressure affects the tire-vehicle interaction, and the only correct inflation pressure for proper balanced performance is what is specified by the vehicle manufacturer on the placard. Never exceed the maximum inflation pressure, as branded on the sidewall of the tire.”

- See more at: http://www.wheels.ca/news/over-inflated ... 5sRV2.dpuf

Makes sense to me ?

[hks_kansei]

It all makes sense, and tends to agree with what I suggested, that a lower pressure (within reason) will increase the contact patch in a straight line, helping acceleration or braking.

My point was that during cornering a higher pressure may be beneficial. (i'm talking a little here, not pump them up to 80psi)



The only mention there is that a higher pressure will only help lateral grip if there is an increase in load.
Now, I would be curious as to their testing, if it were real world cornering (ie: sling a car around a skidpan), or simply static grip tests (pulling a stationary car sideways and measuring how much force it took before it skids)

When cornering there IS an increase in load/weight on the outside tyres, as the weight of the car leans onto them.

So while a tyre at the optimum pressure for the 250kg load it may see on a static, or straight line dríven MX5, may be best at that load, when cornering hard that same tyre will be seeing far more than that regular 250kg (but below 500kg unless you've managed to get it onto two wheels)

What I am saying is that at a pressure optimal for 250kg, the tyre will deform and possibly (this part depends on the tyre itself, sidewall construction etc) reduce it's contact patch, where a higher pressure for the higher load would support it and retain at least the same contact patch as in a straight line.

[taipan168]

I recently took my car for the 1,000km inspection at a Sydney dealer, and when I checked the tyres a couple of weeks or so later, the tyres were inflated to approx 36-40 psi. No idea why they thought that it was necessary to do this given that the placard says 29 psi and that's what the tyres were inflated to before the inspection.

[Mr Morlock]

Its quite common to run highish tyre pressures. It supposedly improves fuel economy and wear. If you want to run lower pressures just let some air out and keep a tyre gauge handy. Many of us do our own tyres at home and using you own gauge is better for consistency rather than gauges in garages which may not be consistent. They say to check tyres cold but thats hard if you have been driving for some time.

[Alex 2550]

Manga_blue told me for the 195s on an NA8 28psi dry and 30psi for wet and touring. because he has been driving one of these things so long both on and off the track im sure he knows a thing or two and having used these pressures i can say they are better than the low to mid 30's i had before in fact i drove the same stretch of road back to back at the different pressures. both cars weigh in within 10kg of each other between 1020kg and 1030kg and south coast roads are crap which may also be why he told me those pressures. .
i spose its possible for understeer if the tyres were pumped up so hard that only the center of the tyre was making contact with the road and being dríven hard but im sure the ride quality would give the excessive pressure away.