Member Since: 15 Jan 2009
Location: Melbourne, Victoria
Posts: 73
In what way is the balloon theory not relevant? A tyre is an air vessel just like a balloon. I have heard this said before but nobody has explained why it's not relevant. If I understand why it's not relevant I'll withdraw the analogy.
40psi or 38psi or 36psi....still too high.
I don't have sufficient evidence to comment on Grabber sidewalls vs any other.
Nothing special about the Flinders compared to many other dirt roads.
You don't find a difference between road and low -- that's fine, each to their own, I'm stating my point of view based on my tests, everyone is entitled to their own.
The problem with one person's experience is that it isn't statistically significant. That's why I didn't quote my own. If on the other hand I ran a fleet of the same cars across the same ground I could prove it statistically by having some cars at one pressure and some at another. Note that those fleets in that situation often carry heavy loads which require high pressures regardless.--
Robert
Life is better in low range
27th May 2009 10:59 am
Martin Krutli
Member Since: 17 Apr 2007
Location: Adelaide
Posts: 229
Tyre Pressures
In terms of difference surrounding 'punctures' at higher or lower pressures, the issue is not actually a punture (ie an object penetrating the tyre) but fractures of the tread caused by impacts of sharp rocks at too high speads on tyres that are too tight. Many of the 'older 4WD set do diagree with reducing pressures on rough dirt roads, but their comments and experience is invariably based on the older high profile tyres with very different characteristics than modern lower profiles.
On a trip around the Flinders a few years ago the benefits of reducing tyre pressures were well highlighted to me. I'd reduced pressures in our Jeep Cherokee (no laughter pleaae) and did not have one problem with tyres the whole weekend. A friend travelling in convoy with us (same roads, roughly same speeds) in his Pathfinder who refused to drop pressures blew 3 tyres. yes, he was running different tyres, so not completely apples with apples - but we were both on aftermarket AT's of some description (can't remember what now).
In our recent trip up the Oodnadatta Track, and thru the Mereenie Loop road and down the Finke Gorge, I ran about 28/34PSI with no problems - saw literally 1000's of destroyed tyres on the sides of the tracks - but of course I don't know what caused those problems. Down the Finke Gorge in heavy sand towing the Campomatic I was at about 22/24 and worked fine. I'd left enough in reserve so, if I got stuck I could go down further.
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27th May 2009 12:48 pm
rmp
Member Since: 15 Jan 2009
Location: Melbourne, Victoria
Posts: 73
Yes sharp stones can fracture treads and cause other sorts of structural damage. Tyres can be damaged without a puncture for sure. However, the puncture point and low pressures is still valid.
The lower profile a tyre, the less you can air it down. Airing down is even more applicable to high-profile tyres.--
Robert
Life is better in low range
27th May 2009 2:12 pm
Mr Andrew
Member Since: 28 May 2007
Location: Sydney, Australia
Posts: 128
I agree with everything that RNP says, but will comment that at lower pressures, the chance of breaking the bead (seal) of the tyre becomes more likely. A heavy D3 that lurches sideways (such as sliding sideways into a rut in mud) can introduce enough force to unseat the tyre. But I agree that at no stage has the sidewall become the tread.
As for the Commdore, I just don't know what to say. Maybe we have all been wasting our time with these silly D3's...
28th May 2009 2:00 pm
rmp
Member Since: 15 Jan 2009
Location: Melbourne, Victoria
Posts: 73
Agreed about the potential for sliding off the rim. However, at 26psi cruising on a dirt road, that is unheard of. It is a risk in the area you mentioned, muddy ruts at more like 20. On sand you can go as low as 15 and still really throw it around and the tyre stays on. I start to get worried about it at 10 though.
The potential to slip off the tyre is also related to the size of tyre, construction, how it was mounted, weight of vehicle and very importantly, what the driver does.--
Robert
Life is better in low range
28th May 2009 9:42 pm
Hopperoo101
Member Since: 19 Dec 2007
Location: Canberra
Posts: 20
Rmp, After having had more than thirty years in the tyre industry and seen most things done to tyres, I agree with most of your comments.
Many many years ago I sold some truck tyres to an old truckie out Lake Cargelligo way, who was mostly carting stock over rough dirt roads. We fitted them to his rims & inflated them to 100 PSI which was normal recommended pressure for his size tyres. On mentioning this to him he laughed - told me that was typical of somone with no idea of driving on rough dirt roads - and immediately reduced them all to 70PSI! A lesson I've never forgotten.
Further, in support to your comments, you will soon see if the tyres are running too much pressure for the load they are carrying on stony dirt roads, in that the tread blocks will start to chip, due to the continual bouncing over potholes / corrugations etc and actually slipping on the road surface - ie. your braking test - if a tyre is bouncing - it is NOT gripping. On rough stoney surfaces, reduce the pressures & reduce the speed for the conditions that prevail.
Regards, Dennis.[/u]
5th Jul 2009 3:33 pm
10forcash
Member Since: 09 Jun 2005
Location: Ubique
Posts: 16534
Couple of comments about inappropriate / extended use of low pressures...
Firstly, low pressures do expose sidewalls to ground contact, I have photo's somewhere of 19" MT/R's @10psi clearly showing the sidewall markings in the sand (soft enough to drop the pressure that low but firm enough to leave clear witness marks)
Secondly, running heavily laden heavy vehicles at low pressures for extended periods will cause the sidewalls to significantly heat up, thereby softening the rubber allowing easier penetration ( ) this also leads the sidewall ply cords to be continually stressed by folding and stretching on every rotation, low pressures can also lead to the sidewall being pinched between rocks and the wheel rim, also causing damage - fine at low speeds but can you see every rock ahead?
all of the above is from personal and observed experience over 20+ years
5th Jul 2009 4:05 pm
10forcash
Member Since: 09 Jun 2005
Location: Ubique
Posts: 16534
Not mine, but a D3 tyre running at 18psi
Clearly the sidewalls have been in contact with the ground, the damage is probably due to being pinched against the rim (no rim damage)
There was no option to run at higher pressures without endless recoveries, the terrain was mixed soft / hard surfaces - sometimes only a few metres apart so the most effective solution was to run at low pressure
running low pressures in ruts will also multipy the chance of sidewall damage, even if the pressures are not low enough to allow sidewall contact with the bottom of the rut, it will 'fill' the rut allowing sharp stones in the sides of the rut to slice sidewalls, the other factor with ruts is the ability of the driver to know what direction the front wheels are facing, it's possible to drive in ruts with full steering lock and still go straight ahead, clearly this will expose the sidewalls to potential damage regardless of pressure
5th Jul 2009 4:36 pm
rmp
Member Since: 15 Jan 2009
Location: Melbourne, Victoria
Posts: 73
The point about sidewalls not contacting the ground; even with 19s at 10psi the sidewalls do not touch the ground on a hard surface. The contact patch gets longer, not wider.
Now if we're talking soft surfaces like sand or mud that changes, and the sidewalls will touch at 10psi because the tyre sinks in. But they'll touch at any pressure, because the higher the pressure the more the tyre will sink in and the more sidewall is in contact. That would be why offroad tyres have tread on the sidewall. There is no point trying to avoid it, that's the way it is in sand and mud driving, the sidewalls touch and it's not a disaster.
Re heating up, rim pinching (or "rim crush"). Definitely. That is one reason why lower pressures must ALWAYS be run only at low speeds. It's a choice, low pressure and drive slow, or drive faster and run higher pressures. Definition of an appropriately low speed? Measure cold and hot pressures, look for around 4psi difference.
If a tyre is heated up sufficiently to allow easy penetration of an object then it has been abused in the first place, probably by a combination of too much speed and too little pressure.
Many logging trucks use central tyre inflation systems because of the huge variance in weight between empty and laden. These pay for themselves in road maintenance and tyre bills, plus the vehicle not getting bogged. As Hopperoo said, truckies may run 100psi in their tyres (laden) but take the trailer off on a dirt road and there may be 8 large tyres on two axles doing not a lot of work so 20 might be better. CTIs can often pay for themselves for trucks.--
Robert
Life is better in low range
5th Jul 2009 11:18 pm
10forcash
Member Since: 09 Jun 2005
Location: Ubique
Posts: 16534
But there'd be no need to air down on a hard surface... I accept that the sidewalls do not become 'load bearing', although by virtue of them being in contact with 'soft' surfaces they are still more prone to damage
An aired-down tyre will heat up the longer it's run at low pressure, an hour or so's driving in high ambient across sand (where it's often impossible to drive in a straight line for more than a few metres) will raise the tyres temperature sufficiently to soften the sidewalls - not abuse, just making progress
5th Jul 2009 11:25 pm
rmp
Member Since: 15 Jan 2009
Location: Melbourne, Victoria
Posts: 73
True, no need for airing down on the hard.
Yes, softer tyres do touch sidewalls more than harder ones. But my point is that a) the difference is negligible, and b) a softer tyre is in any case more resistant to puncture than a harder one, and c) the other benefits such as traction and less bouncing more than outweigh the tiny difference a tiny bit of extra sidewall bulge makes.
Should also point out the construction of the tyre makes a difference too, offroad light-truck tyres bulge much less in the sidewall than passenger constructions.
And finally the tyre setup on the D3 is, IMHO, not ideal for offroading. Given the D3 usually weighs around 3000kg when offroad a 55-60 profile tyre on an 18" rim isn't ideal. And as for the passenger construction 20" tyres, well I don't consider it advisable to air them down at all as the profile and construction just don't make it a realistic option.--
Robert
Life is better in low range
5th Jul 2009 11:33 pm
10forcash
Member Since: 09 Jun 2005
Location: Ubique
Posts: 16534
Can't agree about the puncture resistance, from experience the sidewalls on 17"STT's are tough as boots - never seen any damage, 18" AT2's sidewalls soft as baby poo, lots of damage 19" MT/R's sidewalls lots tougher than the AT2's but not as tough as the STT's - some minor damage but no punctures (except that one time in Morocco but that was driver error wasn't it Dom! )
5th Jul 2009 11:39 pm
rmp
Member Since: 15 Jan 2009
Location: Melbourne, Victoria
Posts: 73
You're comparing three different tyres and constructions there and three different profiles. The STTs are mud-terrain light-truck, AT2s mostly reinforced pax, 19 MTRs reinforced pax. They will have quite different puncture resistances for those two differences.
As for lower pressures = lower puncture resistance, well we'll need to agree to disagree. However I have intentionally punctured tyres at different pressures to prove this point and the lower pressures require more force.--
Robert
Life is better in low range
5th Jul 2009 11:54 pm
caverD3
Member Since: 03 Jul 2006
Location: Oberon, NSW
Posts: 6922
Depends on what you mean by airing down or 'low pressure', Letting a bit of air out on diert roads will make for a more comfortable ride and less chipping and punctures. Dropping tyres down to sand pressures and doing high speeds for a couple of hundred k's on hard dirt will cause tyre damage due to heat build up. LT tyres will take lower pressures due to the sidewall stiffness.
A matter of taking into account speed, pressure, road conditions and tyre type/construction.“There are only three sports: bullfighting, motor racing, and mountaineering; all the rest are merely gamesâ€
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5th Jul 2009 11:54 pm
rmp
Member Since: 15 Jan 2009
Location: Melbourne, Victoria
Posts: 73
That's true, "lower" is different for every vehicle and tyre combo. For sand in a D3 down 15psi works, but you'd never use a pressure that low on a high-range dirt road.
However one point re LT tyres; they are more strongly constructed and thus less able to dissipate heat, thus they require higher pressures than pax for a given load so they deform less and thus build up less heat. Or, drive at a lower speed. Heat kills tyres, not immediately, but very surely over time.--
Robert
Life is better in low range
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) this also leads the sidewall ply cords to be continually stressed by folding and stretching on every rotation, low pressures can also lead to the sidewall being pinched between rocks and the wheel rim, also causing damage - fine at low speeds but can you see every rock ahead?
