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What About Tyre Pressures

I have been playing around with my tyre pressures recently and it got me thinking about how little emphasis we pay to tyre pressures compared to all the other performance enhancements we make. So having done some research here's a few pointers for getting our tyre pressure right:

Under-inflation

If your 911's tires are under inflated by only 6 psi it could lead to tyre failure. Additionally, the tyre's tread life could be reduced by as much as 25%. Lower inflation pressure will allow the tyre to deflect (bend) more as it rolls. This will build up internal heat, increase rolling resistance and cause a reduction in fuel economy of up to 5%. You would find a significant loss of steering precision and cornering stability. While 6 psi doesn't seem excessively low, remember, it usually represents about 20% of the tyre's recommended pressure.

Over-inflation

If your 911's tyres are over inflated however by 6 psi, they could be damaged more easily when running over pot holes or debris in the road. Higher inflated tyres cannot isolate road irregularities very well, causing them to ride harsher. However, higher inflation pressures usually provide an improvement in steering response and cornering stability up to a point.

Effects of Time and Temperature

Tyre inflation pressures change due to time and temperature. Typically tyres lose about 1 psi per month due to air escaping through the rubber as it stretches and deforms. If you were to check your tires only every six months, it would not be uncommon to find them under inflated by as much as 6 psi. Tire inflation pressures also fluctuate with changes in the outside air temperature. This occurs at a rate of about 1 psi for every 10F (plus or minus). So the tires you set correctly with an 80F ambient temperature will be under inflated by 6psi at 20F.

If you add the variations of time and temperature together, it is easy to understand why a tire's inflation pressure should be checked frequently. Improper inflation can cause tires to wear irregularly.

Another advantage of checking tyre pressure frequently is that it allows a slow leak to be found and repaired before it permanently damages a tire. Tire pressures should be checked once a week, preferably before the vehicle has been driven. Spending about two minutes a week will help you get the optimum performance your tires can offer!

m911 forums for 911 owners and enthusiasts

Effect on Grip

It seems you really can't get a proper answer on this question. Generally speaking, if you don't really have a clue how to start, first start with the recommended pressure and work from there. Tyre manufacturers will tell you the correct warm operating pressure for your application, in our case road use primarily. Basically you would tend to run the lowest safe tyre pressure to maximise tyre grip over a longer period of time. Too high a pressure will deform the tyre and give less grip, but maybe you need the pressure to keep the tyre in shape due to high speed/high aero loads.

Some tyres really don't respond to tyre pressure variations, basically because of their stiff design, here you might not even notice the tyre being totally underinflated. Other tyres will show instantly they don�t like underinflation.

At the end of the day therefore tyre pressure is NOT perhaps the big question, it is overall performance. So the question really is: "what can I do to maximise grip laterally and longitudinally, and do I need this for one lap or the whole damn 50 laps of a circuit".

Therefore its up to us as drivers to KNOW exactly the behaviour of the car tyre pressure wise. This unfrotunately is a function of: Driver, Ambient /Tracktemp, Weather forecast, Tyre manufacturer, type, New tyre, used tyre, Setup of car, Geometric issues with the car, corner weights, amount of downforce, number of laps to be covered.....to name some variables! SIMPLE........

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By: Mistergee

Date: 04-07-15

Rated: 0

Interesting subject,

I cant comment on track driving as ive never done it (outside of local semi illegal racing)

theres so many different brands and ratings , when you mention (6psi) over, is too much is that too much over what??

Porsche recommend 29 front and 34 rear psi pressure, i assume they are taking their standard tire rating and conservastive pressures for longest life balanced with safety.

My rear tires rate to 45psi, so in my case 51psi would be over inflated but of cource i wouldnt even go to the tyre companies max rating.

i put 35 in my rears and 30 in fronts and if im feeling lazy il put an extra 4 or 5 psi in the fronts (for daily driving) as it makes a big difference in your arms being tired, with the higher psi in front turning is so much easier , (i only recommend this for dry driving as understeer is quite pronounced because of it and you need to be on your toes ,

i remember the first Porsche i drove and owned it was a 1976 carrera 3 Targa and i thought the front tires had no air in them it was sooo hard to turn.. takes a while to get used to when youve only driven power steering cars lol

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