Tire pressure

[ioan]

Switched to summer tires on Monday and since I've got the Espace decided to use another tire shop for the summer <> winter swaps.
The tire shop I used previously were getting worse every 6 months. Difficult to get an appointment, tire pressures were all over the place and so on, and they were doing burnouts when taking the car in, even though I was just 5 meters away and could see them through the window. :\

Well I am not really in awe of the new one either.
The positive was that they are friendly and would give me a good appointment time and did the job relatively fast compared to the old one.
The bad? Well Renault recommends maximum 2.5 bars on for the front tires and 2.4 for the rears, they've pumped them up to 3.0 on the front and 2.9 on the rears. That's 20% more. the car was as stable as a table tennis ball. :s
Tires are ContiPremiumContact2, came with the car, will see how they behave and eventually change them next year.

I'm curious what our resident rubber specialist(s) opinions are about tire pressure settings.

[ioan]

Thanks starter.
What I am really curious is just how much does 20% over the car manufacturer influence the dynamics of the car, in general. And is it really safe to do it?
In my case the car felt a bit like bumping up and down the road, and I didn't have the chance to take it to the highway yet, where the pressure will go up with temperature at least .3 bars.

I've driven the winter tires with 2.6 bar front and 2.5 bar at the rear as I felt it was better suited to my driving style and gave a bit better mileage too.

Just curious to hear what people experienced re tire pressures.

[Gregor-y]

I keep separate wheels and tires for summer and winter so there is less reliance on any shops. I'm not a tire maker, but I usually set the pressures five or so PSI below the maximum listed on the tire itself. In the US tires that come with a new car are typically based on a low bid from a supplier so the maker's specs aren't a good guide when you finally get your own replacements.

[Mark]

We generally use PSI in the UK too - another area metric has largely passed by. Ford recommends higher pressures on the back than the front - unless you have a diesel (which I do), in which case the same all around.

[Koz]

Running a little over on pressure will usually give you slightly better handling and mileage at a slight expense in longevity and ride comfort. Better than the opposite - under pressure, which you should never do.

Could you expand on that?
My father always has very low pressure in his tyres... Like in the mid 20psi... On 255-70-15.
I'm on 265-75-16 and I've always run on 38-40.

Most people consider this insane but he's been running like that for 20+ years without issue.

[BDunnell]

We generally use PSI in the UK too - another area metric has largely passed by.

Yes, isn't that interesting? More proof that imperial measurements will probably never go away entirely.

[schmenke]

We generally use PSI in the UK too - another area metric has largely passed by. ....

Ditto here.
I don't really pay much attention to tire pressure, except for verifying about one a month and using the recommended vehicle’s setting (the optimum pressure may vary from one vehicle to the next for the same tire). If anything, I’ll set to a couple of psi over the recommendation, as I’ve been led to believe that will provide better fuel economy.
When towing, I’ll set the rears to about 5 psi over the recommendation.

[Mark]

I'd say most only verify once a year at their MOT.

[MrJan]

There are several problems with running low pressure. You can run for a long time that way and only experience the first two of the following:
1) Lower fuel mileage because the rolling resistance is much higher.
2) Advanced tire wear because the contact patch is not square on the ground. The tires also tend to run much hotter.

The more serious issues are these:
3) Poor tread drainage in wet conditions because the slightly deformed tread does not "pump" water out of the tread as well, causing hydroplanning at much lower speeds.
4) Much less agility and a slower response in emergency manuvering to avoid accidents. In extreem cases, you could potentially break the tire bead seat on the rim causing rapid loss of air pressure. Also, more likely to break the tire bead in contacting objects like curbs, etc.

I thought that it was common in motorsport to run lower pressures in wet conditions? Also if you're talking about generating extra heat then the pressures will be up anyway won't they? I'm sure that when we've checked the pressures when we're doing motorsport they increase significantly when it's warmer.

[Dave B]

I thought that it was common in motorsport to run lower pressures in wet conditions? Also if you're talking about generating extra heat then the pressures will be up anyway won't they? I'm sure that when we've checked the pressures when we're doing motorsport they increase significantly when it's warmer.
Motorsport puts temperature into tyres which 99.99% of road users will never come close to (and it's best to avoid the other 0.01% on public roads).

[driveace]

Well 1 bar =14 lbs per sq in .In motorsport a lot of competitors use filtered air ,or hydrogen,so the pressures are more stable as the temperature of the tyre increases,and they can rise quite a lot.I always run higher pressure on front wheel drive cars in the front tyres,so maybe 35/37 in front 30 in rear,on my Fiesta diesel.And 4 wheel drive Sorento is 35 all round,45 when towing caravan.and rear wheel drive Merc 30 front 34 rear.But its over 50 years since I was fitting over 20 tyres a day,and then radial tyres were only just coming onto the market ,so all tyres were cross ply,s

[Mark]

Heat in tyres produces wear. You don't want hot tyres in a road car.

[schmenke]

....In motorsport a lot of competitors use ...hydrogen...

:?:

You mean nitrogen?

[driveace]

and under inflated tyres get very hot .The guy who said he inflates his tyres to just below the pressure on the side wall is not well informed as the pressure on the side wall is the very Maximum pressure the tyre is designed to run at,and not the recommended pressure for the vehicle the tyres are on.The pressure on most car tyre sidewalls willusually be about 65lbs per sq in

[driveace]

:?:

You mean nitrogen?
Correct I am tired again !

[ioan]

Correct I am tired again !

Too many tires?

[wedge]

I usually go up a couple of PSI as I personally like sidewalls stiffened but not too stiff that it dramatically affects the slip angle.

On my dad's workhorse he goes up a couple of PSI, too.

[Zico]

I generally go up a couple of PSI in the summer too, makes the steering a bit more responsive and improves mpg a tad.

Re- lowering tyre pressures, I 100% agree that in normal conditions its certainly not desirable for many reasons.... however there are occasional road conditions (Winter- very slippy, wet, slimy, salt covered roads only) where lowering the tyres by around 3-4 PSI can make the car less edgy/nervous, give much more feedback of the available grip due to there being more sidewall roll and larger tyre contact patch lets you put the power down a bit better and makes acceleration/cornering grip a bit more progressive when grip levels are limited even beyond wet conditions.
Some of you may dissagree and that may be true of your car/tyres combo.. all I can say is that it certainly improved things with the old Pug GTI/Pirelli P700z ditch-finders.

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[airshifter]

More reported spammage!

[schmenke]

This thread has got me deflated.

[ioan]

Went with 2.9 bar at the front and 2.8 at the rear and I am getting used to it. We'll see how the tires hold up to it.

[D-Type]

How long does it take tyres to get up to temperature, or to cool down? Some car handbooks quote two pressures - one for cold tyres and another about 20% higher when hot so the difference is significant.

I generally use the garage at the end of my road so I know the tyres are cold. If I'm doing hot tyres I check the pressures first and bring the low ones up to the same pressure as the one on the other end of the same axle.