18 inch tyre pressures

The specified OEM pressures are fine for most people...
... provided the tyres are inflated accurately. But the trouble is, they often aren't.

Why? Because of the ideal gas law.
And it's because of the ideal gas law that this happens:
The pressure in a tyre changes by approximately 10 kPa (1.5 psi) for every 10°C change in temperature.


Example 1:
Say you adjust the pressure in your tyres to 240 kPa inside your garage where it's 20°C. As you drive out to work the next morning when it's 10°C outside, you will find the pressures will drop to 230 kPa. Hence, your tyres are underinflated.

Example 2:
This is a more common situation - say you adjust the pressures at a service station after you've driven some distance. The trouble with that is that a tyre normally heats up from usage, and so does the pressure (lets say, from 240 kPa cold, to 260 kPa warm for this example). So if you readjust them to 240 kPa in this state, the pressure will drop to something like 220 kPa when the tyre cools down. Hence, your tyres are underinflated.

Obviously, to get around the problem in both examples, you simply compensate by adding 10 - 30 kPa to your desired pressures.


This is why some people say the OEM pressures makes their car feel soft and think they're overly biased for comfort - they've unwittingly underinflated their tyres.
These are the same people who also say it's better to add a few kPa - they're unwittingly compensating for temperature, inflating them back to the proper OEM pressures.


The best way to ensure you are inflating the tyres at the pressures you want, is to adjust them at home when the tyres have completely cooled down - after all, the OEM pressures apply when the tyres are COLD. And of course, compensated for temperature.

From here, you may add 10 - 30 kPa to the OEM pressures to suit personal preferences if you wish. However, I recommend that you never go below OEM pressures.

Quote:

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Originally Posted by RosscoMK3

Would it be fair to say that the cold pressure of a tire is less important than the running pressure during the intended use?

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On a track with racing tyres, yes.

On a track with road tyres, no. Use whatever pressure that will save the tyres from oblivion.

On the road, no. Stick to OEM pressures or higher.

DV.There are plenty of studies that would support your advice whole heartedly. Most carried out by insurance companies, transit authorities and tyre manufacturers.
Thanks for your advice.
An "under recommended pressure tyre" will likely cause more problems than an over inflated one. That's something I think everyone agrees on and I hope no one thinks that's what I'm suggesting. At the end of the day I was just interested to read about others experiences with these little hatchbacks.
They are not high performance race cars last time I checked (including the R crowd)and most of us don't drive to work via a 20min session at Eastern Creek.
Now that would be a fun way to start the day.
IME with normal road driving, I found the upper ends of the manufacturers range to be great on the 13 hour drive interstate but, completely useless for city driving.
The wear pattern suggests that the tires are over inflated and that I need to stop turning the wheel as sharply as I do (regardless of how much fun it is). This wear pattern is in line with the feeling of the front tires skipping more than they need to and if that is my mistake then so be it. Let me know what I should try.
MACCA.Please don't tell me I need to run my tyres at a pressure suited to a track day unless I'm doing one. Also, lets not confuse safe running pressures, with pressures that extend the safe use of a tyre under extreme conditions.

Note that the OEM pressures are different for each variant.

Front and rear, normal load:

118TSI
230 kPa (33 psi)

GTD
240 kPa (35 psi)

R
270 kPa (39 psi)

Quote:

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Originally Posted by RosscoMK3

IME with normal road driving, I found the upper ends of the manufacturers range to be great on the 13 hour drive interstate but, completely useless for city driving.
The wear pattern suggests that the tires are over inflated and that I need to stop turning the wheel as sharply as I do (regardless of how much fun it is). This wear pattern is in line with the feeling of the front tires skipping more than they need to and if that is my mistake then so be it. Let me know what I should try.

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Stick with the OEM pressures (230 kPa all-round). Add 10 - 20 kPa all-round to suit personal preferences if you wish. Make sure they're accurate.

Only use the full-load pressures when you are actually carrying a full load of passengers or cargo (or both). 300 kPa at the rear is a bit much otherwise.

Quote:

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Originally Posted by RosscoMK3

Also, lets not confuse safe running pressures, with pressures that extend the safe use of a tyre under extreme conditions.

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Though highly impractical for most situations, you can inflate the tyre up to whatever the figure is on the sidewall (usually 350 kPa or 51 psi for high-speed passenger tyres) without undue risk to safety.

Thanks for that.
Unfortunatelty there arent too many studies in to results from over inflated tyres. Even less into the differences caused while running within the recommended ranges.

For all the track stars out there.
There is however, much information about finding a balance between the lowest pressure and causing mechanical damage.
Even track tires are designed with this in mind and it would make sense to run a road tyre above maximum recommened OEM pressures to conserve rubber over a prolonged period of extreme friction.
I'd love to hear some more input from MACAA. It sounds like he has a lot of experience with road tyres being pushed to extremes.

MACCA.Please don't tell me I need to run my tyres at a pressure suited to a track day unless I'm doing one. Also, lets not confuse safe running pressures, with pressures that extend the safe use of a tyre under extreme conditions.[/QUOTE]

Er -I didn't tell you any such thing.The example given of Track Day pressures was for increased grip and less wear for cars at the Track-nowhere did I tell you to run these pressures on the road.
Just as an example,the Golf R's and Audi S3's on road tyres were running at around 2.7 front and 2.5 rear[cold]
Some running adjustments are often needed with hot pressures,especially with the hardest worked front,which is the left one at Marulan.
It's not unusual for the hardest worked front to be 10% higher than it's opposite number.
Rears stay much more consistent,in keeping with these cars being more front wheel drive biased than anything.
Cars would come in with the LHF quite hot,RHF a fair bit cooler,and both rears feeling like they'd just been driven on the highway,with very little pressure increase over cold pressures.
Manufacturers pressures are reasonable close,but it's always best to err slightly on the high side.

Sorry MACAA. I didn't mean to put words in your mouth.
It just seems that whenever you talk to poeple about cars or bikes being setup for the road everything turns into a track day or high speed driving( usually defined as being over 120km/h). Most of the time it's a struggle to get the tyres hot on a public road without handing over your licence.
You did tell me that my brother in law was right. I dissaggree with that as the tyres were within the recommended range for the load and conditions. At no time have I suggested that anyone run there tyres below what is recommended on the car.
That said, I'll get off the back foot now.
Thanks for sharing.
I've gotta go train but I'll be back with a couple of Qs. Cheers.

Quote:

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Originally Posted by Candyman

Which VW dealer ?

Just so I don't use them.

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Gerald Slaven Belconnen

Quote:

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Originally Posted by csf6701

Gerald Slaven Belconnen

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Hi Gerald.

MACAA,
I noticed that you said the R's run at 270kpa front and 250kpa rear. Is that before the track and then you add more? I don't have a R so I'm not sure what the VW recommendations are.
Is the 250kpa lower than oem recommendation on the rears? I always thought the fully loaded pressures for the rears on my placard were overkill. Unless of course the car is seriously loaded up.
I'm not familiar with Marulan. How long does it take for average road tyres to get up to temp? I've noticed with all the cold weather lately that the tyres feel like rubbish for a good few minutes.

Cheers.

Quote:

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Originally Posted by Diesel_vert

Note that the OEM pressures are different for each variant.

Front and rear, normal load:

118TSI
230 kPa (33 psi)

GTD
240 kPa (35 psi)

R
270 kPa (39 psi)

Stick with the OEM pressures (230 kPa all-round). Add 10 - 20 kPa all-round to suit personal preferences if you wish. Make sure they're accurate.

Only use the full-load pressures when you are actually carrying a full load of passengers or cargo (or both). 300 kPa at the rear is a bit much otherwise.

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The reason I started this thread was a country drive with my wife ended badly when her neck started aching. Had completed about a 25 km section of typical country road averaging 120, the first decent cruise in the car after doing 50km daily round trip to work. More than ten years ago we had an accident, pushed off the road by an aquaplaning 200B, hit a tree and we both sustained whiplash injuries, mine is ok now but hers can be aggravated. The last time was a ride in a friends Hi-Lux 4wd. Anyway the trip was spoiled by her discomfort. She normally drives an air suspension Land Rover.

Sooooo, after searching the net for adjustable suspension mods, as you can see here I went to the extreme measure of checking tyre pressures. :-) The rear had 43 psi by the servo gauge, a BP station digital unit. Now with 34-35 all round, when hopping over small speed bumps/joints in driveways, small potholes etc, the rear bump is felt slightly less than the front. It was the otherway around. I'm convinced that the jolt from the very hard rear set caused her problem and that it is markedly different now, feels balanced.

So much so that on the way to work there was a coarse surface section that I learned to pick a line through, I still avoid the worst of it or course but the need is not there now, the car just tracks over it nicely. So after all the discussion here we get back to something as simple as me following the manufacturers recommendations. The sticker on the flap says 240 unloaded, when I did that the car performed as I needed it to.

I will take the advice offered by more than one here and get an accurate gauge to use at home.

Quote:

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Originally Posted by RosscoMK3

Unfortunately there aren't too many studies in to results from over inflated tyres. Even less into the differences caused while running within the recommended ranges.

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I don't think we really need a study on the consequences of overinflated tyres. Most likely, the manufacturer will have plenty of data on this anyway (even if it isn't available to the public).

Quote:

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Originally Posted by RosscoMK3

MACAA,
I noticed that you said the R's run at 270kpa front and 250kpa rear. Is that before the track and then you add more? I don't have a R so I'm not sure what the VW recommendations are. Is the 250kpa lower than oem recommendation on the rears?

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As mentioned previously, OEM pressures for the Golf R is 270 kPa or 39 psi (front & rear, normal load).

Quote:

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Originally Posted by RosscoMK3

I always thought the fully loaded pressures for the rears on my placard were overkill. Unless of course the car is seriously loaded up.

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You should only use the full-load pressures when you are actually carrying a full load of passengers or cargo (or both).

Quote:

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Originally Posted by random

The rear had 43 psi by the servo gauge, a BP station digital unit. Now with 34-35 all round, when hopping over small speed bumps/joints in driveways, small potholes etc, the rear bump is felt slightly less than the front. It was the otherway around. I'm convinced that the jolt from the very hard rear set caused her problem and that it is markedly different now, feels balanced.

So much so that on the way to work there was a coarse surface section that I learned to pick a line through, I still avoid the worst of it or course but the need is not there now, the car just tracks over it nicely. So after all the discussion here we get back to something as simple as me following the manufacturers recommendations. The sticker on the flap says 240 unloaded, when I did that the car performed as I needed it to.

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Yes, the car won't be as balanced if you inflate the tyres using the full load pressures - and there's no full load of passengers or cargo to actually carry around.

And the reason why the rear full load pressures are so high is because of the potential cargo volume. It's a hatchback after all.

Quote:

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Originally Posted by random

Now with 34-35 all round

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Did you measure that when the tyres were cold? And 240 kPa = 35 psi, not 34 psi.

Quote:

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Originally Posted by random

... as simple as me following the manufacturers recommendations.

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Who would've thought? ;)
As long as it's for normal use on normal roads, that is.