Tyre Pressures

[The Poet]

Thought there would be more previous posts on this but couldn't find them so after some advice....

After driving my 996 for two months now thought I should check the tyre pressures
so down to Halfords for a pressure gauge and after checking had about 33 in the fronts and 41 at the back. Checked the manual and it recommends 36 and 44 psi for 18" wheels (was surprised that for 18" wheels the rear pressures are 8 psi more than 17")
postamble();
so gave them a good pump up,

Noticed the ride got a bit firmer, can't comment on the handling because I haven't had chance to try it yet.

Are recommended pressures best, anyone run them with less air?


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[Maxy M]

IIWU I would:

- Get a tyre pressure gauge (done;good)

- Get a foot pump

- Check your cold TPs every week

- Use the recommended pressures per the handbook - they're recommended for a good reason

- Don't vary them by much if you want to experiment (you don't want your insurer to refuse to pay up if you have an accident - unlikely but you can't rely 100% on insurers...)


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[Elite Detailing]

Personally I would experiment. I have never gone on standard pressures on any of my previous cars. As long as you dont go silly.


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[Used Porsche]

Softer tyre pressures increase sidewall flex which increases temperature which eventually can lead to a blowout which can wreck the car and you.

The higher pressure is to stop the tyre tread deforming at higher speeds and causing uneven wear.

Nice video on Sky recently of a police patrol on a motorway having a blowout at 90 - 360 degrees spin, clouts the barrier and scares the sht out of the crew. You can bet they had that car in tip top condition too.

Not worth the risk - use the recommended pressures.

Very brave to confess to not having checked tyre pressures on a supercar for 2 months. Might create an impression of IQ smaller than shoe size with some people.

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[Seagull]

I've never reall understood the idea of cold tyre pressures. The harder you drive, the hotter your tyres become and therefore the air inside the tyres. According to Boyles law, the pressure of a gas increases with temperature. This means that someone pottering about driving like a bible basher on vallium would have a lower (actual) pressure in his tyres than someone really giving it some.

Personally, if I'm just going to be driving normally, I run them at the recommended 36/44 psi but if I'm going for it, I drop them to 34/41 psi because that's what I've found works best for me. This may not work for you/your car so in the end, it's a case of experimenting and as has been said above, it's a risk that you have to assess for yourself.


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[Maxy M]

Quote: Originally posted by Seagull on 22 January 2006

I've never reall understood the idea of cold tyre pressures.

Blimey! Boyle's Law - I'm dead impressed. :wink:
I guess the 'cold' pressure is a benchmark because, as you say, if you drive the tyres warm up and so do the pressures... And the pressures can go up a lot on a fast/hard drive. I imagine that 'hard' driving creates the 'ideal' pressures, and just when you need them.

However, it's all awfully hit-and-miss. It depends on how 'cold' it is when you check them, eg first thing after sun-up or at 11.00. In the summer the early am air temp can be a LOT lower than in mid-pm, whereas in winter the temp variation is less throughout the day. Suppose you check your pressures on a 'warm' day and it then gets colder; should you recheck and pump up a bit? And vice versa to avoid over-inflation? (I do.)

Conceivably, on a warm Sept day, your LH tyres could be harder than the RH ones if you're travelling W down the M4 cos the sun is shining directly on them!

I try to check the tyre pressures on any car - and especially the 993 - at least once a week.


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[Mark911]

The reason for cold tyre pressure settings on a road car is that this provideds a fixed datum, and the car/tyre manufacturers know how much the pressures alter with the the change in temperature and allow for this. Unless you also measure the tread temp as per the racing guys, your pressure readings will always vary.

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[Shed17]

Now we get into an area which I have wondered about, mainly because I used to live abroad.

Setting your tyre pressures to manf. rec. What will the difference be here where it is 4 degrees and where I used to live 40 degrees within 10 minutes of driving?

Personally have never driven at recommended pressures as they are merely a guideline for the average car and have always experimented.

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[Used Porsche]

"Personally have never driven at recommended pressures as they are merely a guideline for the average car and have always experimented."

Ahem!

So why do car manufacturers go to the trouble of testing tyre and wheel combinations and finding the safe pressures which give best tyre life and grip under wet and dry conditions?

And if your sticker on the car and its manual says 36 lbs in the front and you run 28 psi what's the rozzer going to say as he books you? (one offence per tyre for incorrect pressure).

I suppose the next thread will be how many miles do we get from a set of tyres.....




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[Used Porsche]

To quote Bridgestone

Contrary to popular belief, tyre pressure is not determined by the type of tyre or its size but
upon your vehicle's load and driving application i.e. speed

To find out what your car's tyre pressure should be, consult the manufacturers tyre placard usually found inside the driver's door sill, glove box, fuel filler cap or under the bonnet.

The placard also displays the manufacturers recommended tyre sizes.

Tyre pressures should be checked when the tyre is 'cold', as pressure increases as the tyre becomes 'hot'.

Take the "cold" reading and check them against the recommended tyre pressures from your placard.

Heavy loads or towing puts an extra strain on your tyres. So if your vehicle is fully loaded with passengers and luggage, the general rule is to add 28kpa (4PSI or 4lbs).

At high speed, (defined as driving at 120km/h for over one hour), your tyres will wear out twice as fast as when you drive at 70-80 km/h. If your tyres are under-inflated by twenty per cent tyre life can be reduced by thirty per cent. The rule here is to add 28Kpa (4PSI) from your Minimum Compliance Plate Pressure. Don’t inflate your tyres above 40 psi or 280 kPa. When the tyres get hot from driving, the pressure will increase even more.

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[Shed17]

Stevo,

I think the first sentence sums it up.

"Contrary to popular belief, tyre pressure is not determined by the type of tyre or its size but
upon your vehicle's load and driving application i.e. speed"

Therefore load and driving style determine tyre pressure which means they have to be changed, which means that you are not running at the recommended manufacturers tyre pressure, which means that you run at non-standard pressures, which is what I do. :wink:

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[Used Porsche]

To find out what your car's tyre pressure should be, consult the manufacturers tyre placard usually found inside the driver's door sill, glove box, fuel filler cap or under the bonnet.

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[Used Porsche]

Quote

Porsche 911 Performance Handbook.... Bruce Anderson...

Overinflation and underinflation each have their own problems, when tyres are oveerinflated the absorb less of the shock of driving and that puts an extra strain on suspension systems. Also because extra pressure lifts the shoulders off the road, traction can be substancialy decreased. On the other hand underinflation can make tyres sluggish and unresponsive. Tyres are designed to have proper balance of rolling resistance, cornering and resistance to hydroplaning when correctly inflated.

Why constantly ask yourself whether the change you have made to the manufacturers recommended pressures is really safe and an advantage when you know you have no scientific basis for your opinion, which is more likely than not founded on the fact that you don't like the hard ride of a performance car and would be better served by a Mondeo?

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[Shed17]

OOOOOOOOOOOOO!

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[Used Porsche]

Truth hurts....

But not as much as hitting armco at 100 mph because you thought you were clever.....

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