18 inch tyre pressures

[brad]

For my GTi on 18" rims it was 260kpa ( est at 37 lb ) for all tyres for highway driving. Didn't have an issue for a few months then one day driving from Sydney to Canberra the low tyre pressure warning light came on. Inspected the tyres all looked ok but just to be on the safe side drove the last 75km @ 80km/hr. *groan*

Checked the tyres when cold no loss of pressure but warning light stayed on. I booked the car into the local VW dealer and was advised issue caused by having the pressure too high. The recommended pressure was to run 34 lb/ sq in all round.

Since then have no further issues with the warning light and the ride feels better.

Dealer has fed you a load of bollocks. Hope they were tasty.

[kash_0]

Mine are on 38 all round...no issues to report

[RosscoMK3]

Interesting thread.
I'm running mine(17inch I think) highend and they are showing signs of wear on the middle of the tire. No ****e Sherlock.
At high running pressure the road feels closer and the tires slide out easier under hard cornering and acceleration.
When I had my motorbike I ran really low pressures and had great response and grip once they were warm. They shredded pretty quickly though.
IME, run tires as low as possible without going below the manfacturers advice. Way better handling and comfort.

[random]

Interesting thread.
I'm running mine(17inch I think) highend and they are showing signs of wear on the middle of the tire. No ****e Sherlock.
At high running pressure the road feels closer and the tires slide out easier under hard cornering and acceleration.
When I had my motorbike I ran really low pressures and had great response and grip once they were warm. They shredded pretty quickly though.
IME, run tires as low as possible without going below the manfacturers advice. Way better handling and comfort.

Its about contact area. The contact area reduces as the pressure goes up. Think about it, the 20 psi has double the contact area as 40 psi as it has to support exactly the same mass of car. So at some point this side of rock hard, the contact area reduces and the tyre compound is exposed to high wearing loads, usually in the centre. Too low an bad things happen too, wear on the outer edges being one. So for every load and temperature there is an optimum pressure. Given that most of us only have one person in the car, the driver, that sweet spot must be somewhere on the low end of the recommended range.

On the sticker it has 240 for two people and one or two bags. Sooo, with just me going to work I rounded it down to 230 and it seems great. So I probably have the same type contact area as someone running 40 psi with four in the car and some shopping in the back. So for the warriors running 40 on light loads I'm not sure that either the types or the ride will thank you. But I know that my GTD and I are smiling now.

[RosscoMK3]

Elementary my dear Watson.

[Candyman]

It is not as simple as "this is the best pressure", even within a model

Your comment about those of us running 40 psi, I ran that for 3 years on my two sets of tyres with 45,000 km from each set. Golf GTI

Currently on 40 psi and 15,000 km. A Golf R.

The manufacturer placard will always err on the side of comfort ie low pressure compared to optimum for handling.

It isn't just load and temp, it is tyre compound, tyre sidewall depth, tyre tread, road surface and so on.

Running a low pressure to provide a better ride is not wise. Suspension provides ride quality.

Set tyres to provide the optimum for safety in handling and stopping.

[RosscoMK3]

Mr Holmes, I presume.
Lower pressures( within range) win out any day as far as stopping and handling are concerned.The lower pressures increase the contact area and increase the interface temperatures of a tire very quickly.
I remember my brother inlaw trying to tell me off years ago for running the tires in the lower range.
He is a breeder.

[Candyman]

Lower pressures, qualify please. Lower than what ?

Lower pressures can mean longer stopping distances, poorer cornering, sidewall rolling, tread malformation

[RosscoMK3]

Lower Pressures within the range specced by the manufacturer.
A lower end pressure can also mean the tire heats up faster when under strain. Heat equals increased pressure.
One thing I know for sure is that less pressure means more money on tires.

[Candyman]

How quick is the differential ?

Heat generated from surface contact, irrespective of pressure, can be regulated and managed in other ways and more effectively.

One thing I know for sure is excessive braking, sidewall rolling and poor alignment strips tyres much quicker

[RosscoMK3]

Jesus candyman. Im trying to watch the crow.

[random]

It is not as simple as "this is the best pressure", even within a model

Your comment about those of us running 40 psi, I ran that for 3 years on my two sets of tyres with 45,000 km from each set. Golf GTI

Currently on 40 psi and 15,000 km. A Golf R.

The manufacturer placard will always err on the side of comfort ie low pressure compared to optimum for handling.

It isn't just load and temp, it is tyre compound, tyre sidewall depth, tyre tread, road surface and so on.

Running a low pressure to provide a better ride is not wise. Suspension provides ride quality.

Set tyres to provide the optimum for safety in handling and stopping.

Yep, got that, so what Kms would you have got at some other pressure? After all it depends on tyre compound, tyre sidewall depth, tyre tread, road surface and so on, doesn't it?.

After my playing with pressure these last few days, I now know that ride quality is partly a function of tyre pressure. I bet my 34 psi GTD tyres have more grip on breaking than your 40 psi set, but just my opinion. Contact area.

The Manufacturer recommends changing the type pressure for the load in the car, not having a one pressure fits all mantra. Concerns about rolling sidewalls and deformation apply less to 18 inch tyres I suspect. Over-riding all of this is the driving style of the owner. The car is way more capable than I am am I just do not have the opportunity to push it anywhere near its limits, so I guess what works for some does not for me. This thread has really helped me, I had not even checked tyre pressure on my new machine before this, but I'm thinking now and its working for me.

[Candyman]

You will push it in an emergency, well beyond your skills

The time when you will regret not using tyre pressures optimal for handling as opposed to a slightly softer ride

Go with what you prefer. I'll go with what provides performance and safety and rely on my suspension to provide the ride

[MACCAA]

Mr Holmes, I presume.
Lower pressures( within range) win out any day as far as stopping and handling are concerned.The lower pressures increase the contact area and increase the interface temperatures of a tire very quickly.
I remember my brother inlaw trying to tell me off years ago for running the tires in the lower range.
He is a breeder.

Seriously?
You obviously haven't done much Driver Training or Tyre Testing.Or Motor Racing.....[Over 20 years worth]
I do both-and there's no way in Hell that lower pressures improve handling or reduce braking distances-quite the opposite.
We've done tests with wet and dry braking and lower pressures and the results are irrefutable.Not to mention the increased rolling resistance and higher fuel consumption.
The first thing we get people to do at a Track day is increase the pressures,otherwise tyres get destroyed in no time.
Candyman is spot on-the pressures on the placard are on the low side for comfort,not handling.
A couple of pounds extra can sharpen things up noticeably.
Lower pressures certainly increase the temperature-which is not what you want.Any increase in contact area is well and truly lost due to lack of sidewall stiffness,and rapid overheating.
There is not one tyre manufacturer that I know of who doesn't recommend more pressure for higher speed running.
We've got a Track Day at Marulan tomorrow.
The cars making the most tyre noise and attempting to leave the track in the process will be the ones that didn't increase their pressures as advised.
Your brother-in-law is right.

[RosscoMK3]

So you get them to run a higher cold pressure to achieve a lower running pressure while subject to extreme friction on the track? Am I on the right track here?
It sounds like you have plenty of experience and I'm always keen to learn something new. My only track experience is with the two wheeled variety.
I tend to drive my car on the road for the most part and usually for no more than 30 minutes at a time. How would this affect the running pressure and grip?
Would it be fair to say that the cold pressure of a tire is less important than the running pressure during the intended use?
I was under the impression that maximum contact, while maintaining the shape of the tyre, was best for handling and feel.