Thread: Scary moment on general grabbers

on the dirt road at glasshouse, empty truck, I'd be on 28psi, maybe even 26

Originally Posted by Celtoid

Ok, so I'm confused. Maybe it's the concept of pressures being too low, being just right and being too hard. What you say makes sense with the exception of a now reduced contact surface.

So, walk me through this please. An overpressurised tyre will wear the centre out first and conversly too soft and you wear the edges (that's the point I was making about evidence). Most folks report maintaining the 'right' pressure gives them the most even wear on their tyres.

So focussing on over-pressured tyres (or is that the point, that 10psi isn't going to cause this), they wear in the middle. Wouldn't that suggest less tread is engaged on the road surface? How can that be good for traction?

And my own physical experience....why do the tyres at placard pressure feel more secure on the road? I actually experimented in the wet after they felt a bit skittish and gave the D4 a bit more welly on a corner at low speed. It came unstuck...the first time it's ever happened. After dropping to placard pressures, it was harder to replicate. Not very scientific I know but they do seem to behave better at the placard pressures.

A lot of folks seem to agree as they have been advising the OP to drop pressure (mainly through their own experiences) to the recommended PSI.

Is there something I'm missing?

Cheers,

Kev.

Kev, all I'm saying is that increasing the pressure over and above what you normally run in the dry, increases grip levels in the wet.
The increase in grip on the track can be staggering.

Case in point, many years ago I was asked to race engineer a car (Torana XU1) in a couple of historic races at Eastern Ck.
My experience was open wheelers which are totally adjustable, you can change bugger all on those old sedans, so I was pretty much there to fuel and water the car, swap tyres and play shrink for the drivers head.

Just before the first race it started to rain.
Not having used the Hoosier tyre everyone used, I asked Ross the driver what they did in the wet "just let the tyres down" he said, and watching around the pits, that's what everyone else did.
Hmm.

I said I'd be pumping them up, that's what we did on Formula Fords and production cars (bias and radial tyres respectively)
Nope, everyone let the Hoosiers down and he suggested that's what I should do too.
So I did.
Bear in mind this bloke hated racing in the rain and definitely didn't like a car moving too much underneath him.

I was abused as soon as he got back in from the race, I was trying to ****** kill him, how the eff could anyone drive something so diabolical, etc. etc. It was a full on verbal tirade.

Bugger this, I'm doing what I know works, so I pumped the tyres up about 10psi above our dry hot pressures (we always used hot pressures when racing) and told him I'd radically altered the car and it would be great now

In the second race you started from where you finished in the first, so our little XU1 was near the back of a full grid.
The lights went out and he proceeded to drive through the field.
I think he finished third or fourth ? but the main thing was he came back to the pits with the biggest grin on his dial, and the car looked great, he kept it straight where everyone else was slipping and siding everywhere.

Running higher pressures in the wet was what I'd been taught to do by a couple of very experienced racers, but that was the first time I'd seen the difference between lowering and raising them.


(Now I'll confuse things) In high speed dry weather bitumen running increasing pressures above what the tyre placard says on a radial tyre usually increases grip too, purely through increasing sidewall stability, but you do then tradeoff wear and comfort.
Way back when production race cars used street radial tyres, we used to run them really high, at least 45psi, depending on the car and end the tyre was on.
Anything below that and the tyre would wallow and distort and just slide everywhere.
With modern cars running really low aspect ratio tyres, I'm guessing there's less latitude in playing with pressures as you should have a lot less sidewall distortion. (haven't played with cars or low profile tyres for a very long time now)

It's hard to visualise what happens to a tyre when cornering, but the case distorts incredibly under cornering loads, the sidewalls seemingly rolling off the rim sideways.
If you can track down some Channel 7 footage of the V8's racing they have some shots in the wheelwell and it shows the massive distortion of the tyres under huge lateral loads.

Of course, what your 'ideal' dry pressure is depends on the case construction, volume, comfort, rim size, aspect ratio, etc.
The tyre placard is a recommendation and the ideal place to start, although different tyres in the same size will require different pressures too.

I only used to run about 34 psi all round in the dry when I ran 255/85 BFG MT's. My tyre placard says 45 and 65psi, front and rear, all loads and conditions with 235/85's. (I kid you not)
34psi gave me the best compromise of wear and grip for the conditions I ran, (a mix of bad, broken bitumen, gravel and highway) and funnily enough it was a lot lower than most everyone reported they used with the same tyre on here too.
When I went to Maxxis 762's in the same size I have to use 40psi for stability and wear. The 762's are much more compliant in the sidewall than the BFG's were.

I actually drop the pressures for the gravel around here too, (when driving only gravel)
Let the tyres conform to rocks and stones.
It used to be 26psi with the BFG's, a bit higher with the 762's although most of the time I don't bother dropping the 762's and the they are coping ok and still drive and steer well enough.
Bear in mind that most of the 'gravel' around here is crushed shale, super sharp and aggressive to tyres. High pressures on this surface generally means sliding which cuts the hell out of the tyre. (although I tend to induce sliding anyway )
A different type of loose surface might require totally different pressures.

For a scientific viewpoint-

Tire Tech Information - Air Pressure vs. Wet Performance

It applies due to the characteristics of liquids and not to be confused with driving gravel roads.

i doubt i would be raising my pressures.........

from what i read in the net prior to travelling outback roads and than what i experienced out there that dropping tyre pressures worked best for me

The article I linked to clearly shows that keeping your pressure up on wet TAR road gives better traction because water doesn't compress, cannot escape out the tyre grooves quick enough so aquaplaning is the result. All bets are off on gravel, lower pressure gives more traction on a dry surface however stability of the sidewall is an issue. I lower my tyre pressure (and speed) on gravel.

Originally Posted by weeds

i doubt i would be raising my pressures.........

from what i read in the net prior to travelling outback roads and than what i experienced out there that dropping tyre pressures worked best for me

The articles I have come across about driving outback roads such as the Oodnadatta track are concerned with a different issue from the one raised by the OP.

They are concerned with surfaces where the rocks are big enough to risk damage to the tyre. In that case, lower pressure makes the tyre more compliant and less liable to damage. Of course the speed needs to drop as well.

The OP wasn't describing that sort of surface. He was more concerned with grip.

I realise that you probably knew that and were just passing on another useful piece of information about different circumstances.

Originally Posted by vnx205

The articles I have come across about driving outback roads such as the Oodnadatta track are concerned with a different issue from the one raised by the OP.

They are concerned with surfaces where the rocks are big enough to risk damage to the tyre. In that case, lower pressure makes the tyre more compliant and less liable to damage. Of course the speed needs to drop as well.

The OP wasn't describing that sort of surface. He was more concerned with grip.

I realise that you probably knew that and were just passing on another useful piece of information about different circumstances.

umm.....surprised you didn't pick up everybody elses posts

ok maybe i shouldn't have mentioned simpson

all weather gravel road, slight bend doing 60. i reckon outback roads would fall into the OP'er category. i have driven glass house mountains and its full of gravel roads that you would find all over australia.

a few posts mentioning D3, wet weather, bitumen etc etc, TC, ABS which were way more off track from OP statement

just putting my 2 bobs worth in that probably assisted more than your post

Originally Posted by weeds

just putting my 2 bobs worth in that probably assisted more than your post

Something must have been lost in the translation. I thought I was supporting your information. It was intended as a clarification, certainly not as a criticism.