Anyone know definititively what the pressures should be?
,28 psi rather than 29 seems about right, have been running at 28.5 and the wear pattern indicates over inflation.
Hi Chris, Sorry, can’t help with personal experience as mine runs on 205/45x17. However, provided all your alignment is correct and you’re confident of all the preceding tyre pressure monitoring, I’d go with your observations of the wear pattern.
Don’t take this the wrong way, but do you not think you are over thinking this? It would surprise me a great deal if 0.5 of a PSI, particularly on a modern radial tyre made any difference at all to any wear pattern or even if you could feel the difference. If you have a wear pattern that you do not think is right, then I would be looking for an issue elsewhere or even the type of tyre. Understand that it is the air pressure that supports the car, unless you have changed to different width rims then the pressure you want is the same regardless of width. However there is tolerance on everything and the margins you are referring to, even if your gauge etc. is super accurate will vary by far more than 0.5 just depending on the ambient air temperature. Park in the sunshine and the tyres in the sun will likely gain 3 or 4 PSI just from that. variances will occur all over, drive a bit faster, go through a puddle, take 3 roundabouts in a mile it all changes it.
I understand where you are coming from but I know my alignment is spot on as a specialist set it up recently and there are other factors at play.
The air chamber is significantly larger than on the 205 section and logic determines that it needs less pressure to support the same weight.
0.5 psi is indeed a small increment but I always have been penicity about tyre pressures and wear indicates that 28.5psi is too much.
Even though in my 72 years I’ve rallied and done a significant number of track days including at the Nurburg I would always defer to a specialist which is why I posted.
With respect, alignment is always a compromise and therefore cannot be spot on in the general sense. An alignment set to deliver quick turn- in at lower speed may not be much good for high speed stability. Tyre wear depends on the bias of the alignment and therefore fairly uniform wear across the centre of the tyre in an overinflation type pattern may be preferable to severe edge wear. You can reduce air pressure to try and compensate and even out the wear.
As Paul Roddison did the fast road set up I am confident that the settings are good. Not extreme.
Have been running 28psi locally (on 7.5" rims) but was hoping to get a response from someone who has fitted the same size tyre and had definitive info
But, if the rim width is the same, the actual pressure holding the car up acts on that, not the balloon holding the pressure, in that the pressure on the Tarmac is lower due to the bigger width. The true question is what are you determining as Wear due to over pressure?
Paul did mine also and I’m running 215’s but not noticed any significant wear but only got 4k of use in regular road driving.
Let me explain it better as it is way past lock down beer o’clock.
Imagine a cross section through the tyre and rim. The air inside the tyre is contained by the rim which is the “lid”. Regardless of the volume of the tyre it is the pressure acting on the lid that supports the weight of the car. Unless you increase the size of the lid, you need the same internal pressure to support the weight.
Now to the tyre. You have a bigger contact patch potential. There are many schools of thought that will say that for a given psi, the contact patch area will remain the same regardless of width. So various things come into play. It is reasonable to assume that an increase in tyre width is a desire to increase cornering performance. Droping pressure while increasing contact patch, will reduce contact pressure and reduce grip. The decrease in pressure will give more flex in the sidewall which will increase carcase temps increasing pressure. It is swings and roundabouts. (Cutting this very short). I would very much maintain that half a psi will have zero noticeable effect and if for whatever reason you do have over pressure issues then 26 would be a starting point and that any centre wear issues, especially when considering 0.5psi would be more likely from pinching on the rim. Again, bact to what is considered the wear problem.
The rim is 7.5 wide not 7 as the original.
The centre of the tyre is worn more than the outer edges, classic over inflation.
0.5 psi is negligible indeed but if I run at the recommended 29 that will give a (slightly worse)over inflation problem.
Most of my mileage last year was on two trips to the Algarve , about 5k fully loaded and I would normally have increased the rear pressure by a small amount to allow for that but I didn’t and it’s the rears that show more central wear. The geometry can’t be far out as the inner and outer edges of the tyres have worn the same without any sign of scrubbing.
I can’t agree that you need the same pressure for a greater volume of air and too much pressure will reduce the effective contact area and adversely affect the ride and therefore the handling.
I would agree that I probably need a lower pressure which is why I’m running 28 cold. All recommended pressures are from cold(ambient) and will increase very little round Oslo in January and be 4 or 5 psi more after driving round the Mojave desert in July, this I understand but you have to have a starting point.
I would say rim width is immaterial unless it is physically deforming the tyre. 640lbs a corner acting down regardless of rim size will require 640lbs/29psi = 22.07 square inches of contact patch opposing it. 7 inch or 7.5 inch rims cannot alter the laws of physics. A wider tyre would probably have more of that contact patch widthways and a slight reduction in the circumference contact.
Have you checked the gauge you are using for accuracy as it should need more than 0.5 or 1 psi extra to cause significant wear…
On the subject of gauges, i relied on a Michelin Eurodainu for years until it suddenly went about 10 psi off. I replaced it with a PCL airforce mk4 made locally in Sheffield and its proving to be a nice piece of kit.
You are looking at it the wrong way around. It is not about the contact patch area but the weight of the car. You are looking to hold the car up and so need the same pressure.
However, using some very man maths, the width of 7.5" and the slight increase in width would result in a 10% increase in air volume in the tyre, therefore if you want to change the air pressure to suit, you are looking at a 3psi (2.9) reduction so, 26psi. The issue then comes in that the sidewall will flex more and create more heat, result in a more stoggy turn in etc. etc. etc. All that said, I would not expect that 26psi would in anyway compromise the performance of the tyre.
I’ll try one of my son’s gauges, he races a couple of cars(one with the aforementioned Mr Roddison) so has several.
Interesting to note my Octavia VRS shows different pressures for 17" and 18" wheels, both with 225 section tyres and therefore have to use both, one for my summer tyres and another for my winters.
Due to lockdown boredom I’ve just googled and read some articles about this and realise that my opinions are fundamentally wrong in some areas. Please disregard those and concentrate instead on those areas in which I’m fundamentally right🤔
“Paul did mine also and I’m running 215’s but not noticed any significant wear but only got 4k of use in regular road driving”.
Hi Curiosity killed the cat and all that. You said you haven’t noticed any significant wear, but ONLY got 4K miles of use in regular road driving?
I would be extremely disappointed in any set of tyres only giving me 4000 miles of use.
Or did I miss something?
“Paul did mine also and I’m running 215’s but not noticed any significant wear but only got 4k of use in regular road driving”
I think he means had
I read it as 4k miles isn’t enough to notice any significant wear pattern.
Ahhh, yes that would make sense. Ta.
You are correct Mick, my grammar had lapsed. Only 4000 on the tyres from new so far. They did a good portion of that in a road trip to Valencia and back last September.
Those are very much my thoughts as my MK2.5 ran 26 psi on a s.imilar weight?
My main concern is running too low pressure at high speed for hours on end in hot temperatures and the effect on the integrity of the carcass.
My pet theory(don’t we all have one?) Is
That the standard 205 tyre on a 7 inch rim needs “,too much” pressure to get a reasonably flat contact patch and this is distorting my perseption of what is normal.
I think I will continue at 28 psi and measure the wear depths relative to now which are 4mm centre and 5.5 outer.
I’ve been running 215/45/17’s on my 5 for years now. I upped the pressure from 28 to 29 psi. My thinking was that there’s a slightly bigger volume inside to fill, so the extra 1 would offset tyre wear. So far, I have been proved right and experienced reasonably even wear across all tyres.