a plea - let your damn tyres down!

PostScript to the above: Whilst in the Simpson we met a large group coming the other way, all in new, fully decked out (overloaded) Cruisers and Patrols. We picked up their UHF Channel and were privy to the leader berating someone further back for being bogged indicating that they should know how to drive in these conditions. It turned out that the leader was an Opposite Lock tag-along vehicle and that most of the group were running high tyre pressures (as well as very top-heavy roof rack loads of up to 8 Jerries). A member of our group, ex-ranger of the Simpson, told the group via UHF of the need for low tyre pressures. This was met with silence. So much for the benefits of joining a tag-along tour where the organisers will make sure you have knowledge before you leave (e.g. vehicle set-up, correct loading etc) as well as whilst on tour. I would have thought it is the responsibility of such tour leaders to make sure that vehicles (usually owned by inexperienced 4wders) are set up properly and running correct tyre pressures. I don't think it is exactly a positive first experience to be criticised publicly by someone you are paying for something that is due to poor instruction rather than personal fault.

Fair enough....lower tyre pressures on loose dirt/bulldust. But be careful. Harder gibber, clay, roots [Frazer Island] this may not be as appropriate. Driving style might be the more important factor. I don't think that it is cheaper on tyres though. The sidewalls suffer. Speed is more important. The right speed for the conditions. Do you reckon that the corrugations are caused by high tyre pressures. I got that impression. Cheers Royce

Pathfinder,
Tyres and tyre pressures always seem to be hot topics on any 4wd forum. With regard to appropriate tyre pressures, when we were on Fraser Island 2 years ago, we were talking to the local ranger and not having much experience driving in sand (being a Victorian) I wanted to check I was doing the correct thing with tyre pressures. (I had let them down to 20-22 psi cf "normal on road" of 34-36 psi). The ranger was a bit "reluctant" to formally advise, because, he was concerned about the legal implications. If he advised a low trye pressure, and we subsequently had an accident because a tyre rolled off the rim, then he felt he would be liable if we subsequently sued.
He did agree though, that the use of high tyre pressures does chop up tracks and cause deep ruts, but then again of Fraser Island, one decent rain, and the sandy tracks are flat again.
I think, that user education is something that needs to be done.
Rgds,
Mal58

Even though I've been driving a couple or few decades, with a fair bit off road, I don't reckon I know everything. However my experience is this. After a week on Frazer island where I deflated my tyres to around 20 psi ... I think 'cos it's a long time ago.... I had three of my tyres crack the side walls over the next few months. It has never happened since or again! Here's the thing...... I have a farm, live on a dirt road and drive down some fairly rough tracks. Yeah a lot of the time on the black top, but plenty of times across a soggy paddock or sandy ridge. The idea that you 'must deflate your tyres' is a bit dodgey. I reckon I saw plenty of locals travelling along the Tanimi Track a couple of years ago, who didn't worry about deflating tyres. I certainly wouldn't worry about deflating them for an hour's trip down a bit of bull dusty or sandy stuff. The idea that a softened tyre will for better to the shape of the road and corrugations is a nice idea. hmmmwell it would be very marginal unless you travel at 10ks an hour. If you are moving at anything over 30 ks it would make almost no difference.... okay.. tell me I'm wrong. I think maybe the case is being overstated. By the way. I WILL from now on deflate my tyres a bit more than I would have done on the advice of you blokes. By the way do you watch Russell Coight?? Damn near me! But then again, I bet we have all done some of his 4wd antics....Cheers Royce.

For what it's worth.. Having travelled "many many" times through Outback NSW and Q'land, I feel I can add my "two Bob's worth" With regard to steel belted radials of std size,ie 750x16. My experience has been as follows. Driving on rough stony country and Gibber plains, I have NEVER had a puncture or a damaged sidewall, with the tyres inflated to 35psi. However, I do DRIVE the vehicle with constant vigalance, looking for and dodging, large stones etc that are in the path of my vehicle. I also match my speed to the condition of the road. The corragation debate rages and I wish to put my "bit" in. Corragations are a function of the natural frequency of the vehicles suspensions that pass along the gravel road. You will notice that corragations caused by trucks are further apart that corragations formed by cars and 4WD's. Tyre pressure would have some effect to the severity (height) but not the pitch (distance apart).

As allways, there is the "pinhead" element that goes off on a tangent, expounding all their "pet" theories on all aspects of 4WD's etc, AND, trying to convince the public of their 4WD experience! To go on about the effects of tyre pressures and how this relates to corrugations is a bit "rich"! EACH and EVERY vehicle has it's own criteria regarding the ideal pressure in the tyres. Tyre SIZE (width) is especially important, along with the profile of the tyre ie; distance from the rim to the ground, where low profile tyres will be a "liability" in rocky country, and sidewall damage allmost certain. In sand, the "bagging" beneficial effect of tyres run at a low pressure, is well known. But, in stony country, the reverse is the case! Howeverin my experince, I am referring only to relatively narrow section, radial tyres. NOTE.A wider section tyre will have more chance of being damaged with the tyre inflated at high pressure, and probably should not be inflated to high pressures in stony country.(the balance is hard to achieve). The next time you venture into Gibber or stony country, stop and inspect the sidewalls of your tyres, especially the shoulders. If you are running yours tyres underinflated, then you will notice that the shoulders of the tyres will be "marked" or even "nicked" by the bulge of the tyre being exposed to the surface of the road. Ultimately it's your decision. But my advice is to err on the side of higher pressure.

4WD Tyres



What pressure do you put in these damn things?
Most of us probably ask ourselves this question as we try and fit the bent mangled service station doohickie that never seems to seal first try onto our tyre valve and press down the lever. It’s a good question and one I’ve never heard a definitive answer for. Of course there is no definitive answer for any category of tyre but I’ll do my best to shed some light on this shadowy subject.

First up, we have to grasp the concept that the amount of air we squirt into a tyre should be proportional to the size of that tyre and load applied to it. It is the internal air pressure that supports that load and allows the tyre to perform its intended function. Picture the tyre on your racing pushbike if you cut a section as long as your finger out of it. If you looked at it end on, its section thickness wouldn’t be much more than that of your thumb. If you could seal the ends and pump it up to 20 psi or 136.2 kPa you would find that you could crush the thing flat with your thumb and forefinger. Now go outside and cut the same section from your neighbour’s fat tyres on his Pajero (only kidding) and you’ll see it has the sectional thickness of a watermelon or thereabouts. Pump this up and try to crush it and you’ll need a hydraulic jack. This is a function of the physics of force and how it applies to cross-sectional areas. This is why your racing bike needs 70 psi and your car probably needs half that.

Now, if I haven’t confused you too much, how does this relate to your buckin’ and bouncin’ Hilux you may ask - as someone told you that 38 psi would do the job admirably. Well, it would on a heavier machine but assuming yours has been fitted with the almost standard after market fitment of 31 X 10.5 X 15 Desert Duelers or BFGs – its going to kick and jump around. The standard factory fitment for the Hilux utes was a 205R16 that was about an inch and a half shorter and 4 inches narrower. You have a much bigger tyre on a relatively light vehicle so the tyre pressures could easily come down to 26 – 28 psi. Now if you fitted the same rubber to an 80 series Toyota with its extra 600 kg mass and about triple the horsepower you’ve got a different situation altogether. The tyres would bulge and the car would ‘lurch’ on the corners as the sidewalls wobbled. If it were my machine another 10 pound would be added straight away maybe even 14 to make a 40 psi (272.4 kPa) total to get the machine to handle safely and enjoyably.

During my years of safety training we did a fair bit of driver training in sedans and 4WDs. The instructor we used a lot was a true gentleman by the name of John Van Leeuwen who runs Drivesafe Australia in WA and is an accomplished kart and Formula Ford state title holder. He loves his machines and understands the mechanics of them better than most. He made no secret of the fact that he likes people to err on the side of pumping their tyres up to the upper end of the manufacturers recommendations. After participating in many of the driving courses most of us got to dislike the heavy steering and sloppy handling of under inflated tyres and found it harder to modulate the brake pedal to achieve an emergency stop.

There is also the matter of tyre wear that increases with under inflation. To give you an idea of why tyres wear out with every day use – stand back and have a look at the round black things on the rims of your car and… you’ll notice that they’re not exactly round. Where they touch the road is flat and the sidewall bulges out noticeably at this point. If you were to let more air out of your tyre the flat section would get longer and the bulge bigger. As you are bowling down the road your tyre tread and sidewall are continuously bending and flexing and, where it meets the tar, rubbing against the road. This generates an enormous amount of heat which is the sworn enemy of tyre wear and degradation. If your heat a tyre up enough you could peel it apart with your hands. This flexing is the reason why manufacturers of tyres and cars recommend that you increase tyre pressures by 4 psi or 28 kPa when you are going on a trip to the country (or loading up your rig).

So what do we do about pumping up our tyres? I personally start with a median tyre pressure of 35 psi and work my way up or down from there depending on the situation that I’m faced with. If your interested – and most motor enthusiasts are – I’d follow a 5 step plan and add your own personal fuzzy logic factor just to make it more interesting. To add a personal touch, I’ll relate the logic to my work truck, which is a Patrol king cab GQ with 16 X 8 wheels and 265/75/16/ BF Goodrich Mud Terrains.

1. Tyre Size – Are they generally a large tyre in relation to what similar vehicles wear? Are they larger than what is on the tyre placard for that vehicle? Yes and yes – I might drop them to 33 psi.

2. Load – Is the vehicle loaded/heavy, likely to carry a load or tow a trailer in the near future? Nope, but it’s a 6 cylinder diesel and they are traditionally nose heavy – I might bump the fronts up to 35 again.

3. Speed – Are highway (freeway?) speeds likely? Are we in the middle of a warm season that will make the flexing heating problem worse? Yup yup – I’d better whack the drivers and steerers up to 38 all round.

4. Tyre Rating – You have to admire the quality tyres like BF Goodrich and Bridgestone that put useful information on the sidewall. Especially BFG that have paragraphs of info (just about!) with the most important for this application being: "Max load single 1380 Kg (3042 lbs) at 450 kPa (65 psi) cold". (Patrol 265 tyre) If I had these tyres on an F350 with a ton and a half in the tray, the tyres would be straight up to the 65 psi max as I know the manufacturer has tested it at these specs. The Patrol won’t be carrying anything like this so I’ll leave them where they are. 5. Tyre Life and Handling Vs Comfort – I like my comfort as much as the next person but I also like my machines to handle as well as they can. You’ll get a magic carpet ride if your boots are only carrying 22 psi but the extra rolling resistance is going to mean squeezing the accelerator that much closer to the floorboards and she’ll handle like the proverbial wet sponge. You wont be able to carry much speed through the corners with tyres wobbling around when they should be transmitting steering input to the road surface and all that flexing will be heating up the tyres. I’m not paying for the tyres but I don’t like wastage and the coil Patrols don’t ride too badly so I think 35 psi will be the lower limit that I run unless I’m on soft sand.

So … balance everything up and have a good look at the tyre placard and owners manual – it often lists alternative tyres that are fitted to different specifications of the model car that you drive and you can get a feel for what the manufacturer is trying to achieve. Be wary of the tyre and vehicle manufacturers' specs to a certain degree, as they are more likely to recommend low tyre pressures to give the impression that their stuff gives a lovely soft ride. They know that the average person is not going to push the tyre to anywhere near its limits and they are really not going to be broken hearted if you have to replace your tyres more often. Call me a cynic.

It really is one of those areas that a lot of people have no regard or understanding of, but if you were able to see how a decent pressure improves the handling, braking and general feel of your machine and makes it cheaper to run – you’d check them more often and keep them firm.

All this text applies to trailers as well and, in the case of box trailers, due to their lighter mass generally, they are one of the few areas where you may find over inflated tyres. The crude leaf spring suspension – never maintained – so the rusty leaves bind and this offloads a lot of the springing to the tyres in lightly loaded applications. If your tyres are rock hard you’ll shake the trailer to bits and everything on it. Conversely if you have a large boat that comes in around 2 tonne like mine does, or anything heavy for that matter, the tyres are often small to keep the load closer to the ground. This means they are working overtime and should be of the light truck (LT) variety and be pumped up to around 40 psi.

Now to muddy the waters a little…I’ve just read the Tyre Tips column in the Jan 2000 Overlander magazine by columnist Allan Finnie who was complaining bitterly about the fact that his new 4X4 was delivered with 36 psi in the tyres causing the ride to be ‘atrocious’. I would have been impressed myself but Allan did not state what sort of car he was talking about – only saying the tyre placard stated 26 front, 29 rear. He did say that the tyres were wide and low profile - and the car could have been a lightweight Honda CRV or something but he made no mention of handling or braking. He was right to say that lower pressures play a major part in how the car is perceived – especially on a trip around the block test drive – but its what he doesn’t say that worries me.

The modern 4X4s are getting more powerful, more comfortable with better brakes and handling. The driver these days needs good quality tyres and enough pressure in those tyres to transmit all this progress down to the road and give a bit of valuable feedback through the steering wheel and seat.

Enjoy your rubber

Jonesy

4WD Tyres



What pressure do you put in these damn things?
Most of us probably ask ourselves this question as we try and fit the bent mangled service station doohickie that never seems to seal first try onto our tyre valve and press down the lever. It’s a good question and one I’ve never heard a definitive answer for. Of course there is no definitive answer for any category of tyre but I’ll do my best to shed some light on this shadowy subject.

First up, we have to grasp the concept that the amount of air we squirt into a tyre should be proportional to the size of that tyre and load applied to it. It is the internal air pressure that supports that load and allows the tyre to perform its intended function. Picture the tyre on your racing pushbike if you cut a section as long as your finger out of it. If you looked at it end on, its section thickness wouldn’t be much more than that of your thumb. If you could seal the ends and pump it up to 20 psi or 136.2 kPa you would find that you could crush the thing flat with your thumb and forefinger. Now go outside and cut the same section from your neighbour’s fat tyres on his Pajero (only kidding) and you’ll see it has the sectional thickness of a watermelon or thereabouts. Pump this up and try to crush it and you’ll need a hydraulic jack. This is a function of the physics of force and how it applies to cross-sectional areas. This is why your racing bike needs 70 psi and your car probably needs half that.

Now, if I haven’t confused you too much, how does this relate to your buckin’ and bouncin’ Hilux you may ask - as someone told you that 38 psi would do the job admirably. Well, it would on a heavier machine but assuming yours has been fitted with the almost standard after market fitment of 31 X 10.5 X 15 Desert Duelers or BFGs – its going to kick and jump around. The standard factory fitment for the Hilux utes was a 205R16 that was about an inch and a half shorter and 4 inches narrower. You have a much bigger tyre on a relatively light vehicle so the tyre pressures could easily come down to 26 – 28 psi. Now if you fitted the same rubber to an 80 series Toyota with its extra 600 kg mass and about triple the horsepower you’ve got a different situation altogether. The tyres would bulge and the car would ‘lurch’ on the corners as the sidewalls wobbled. If it were my machine another 10 pound would be added straight away maybe even 14 to make a 40 psi (272.4 kPa) total to get the machine to handle safely and enjoyably.

During my years of safety training we did a fair bit of driver training in sedans and 4WDs. The instructor we used a lot was a true gentleman by the name of John Van Leeuwen who runs Drivesafe Australia in WA and is an accomplished kart and Formula Ford state title holder. He loves his machines and understands the mechanics of them better than most. He made no secret of the fact that he likes people to err on the side of pumping their tyres up to the upper end of the manufacturers recommendations. After participating in many of the driving courses most of us got to dislike the heavy steering and sloppy handling of under inflated tyres and found it harder to modulate the brake pedal to achieve an emergency stop.

There is also the matter of tyre wear that increases with under inflation. To give you an idea of why tyres wear out with every day use – stand back and have a look at the round black things on the rims of your car and… you’ll notice that they’re not exactly round. Where they touch the road is flat and the sidewall bulges out noticeably at this point. If you were to let more air out of your tyre the flat section would get longer and the bulge bigger. As you are bowling down the road your tyre tread and sidewall are continuously bending and flexing and, where it meets the tar, rubbing against the road. This generates an enormous amount of heat which is the sworn enemy of tyre wear and degradation. If your heat a tyre up enough you could peel it apart with your hands. This flexing is the reason why manufacturers of tyres and cars recommend that you increase tyre pressures by 4 psi or 28 kPa when you are going on a trip to the country (or loading up your rig).

So what do we do about pumping up our tyres? I personally start with a median tyre pressure of 35 psi and work my way up or down from there depending on the situation that I’m faced with. If your interested – and most motor enthusiasts are – I’d follow a 5 step plan and add your own personal fuzzy logic factor just to make it more interesting. To add a personal touch, I’ll relate the logic to my work truck, which is a Patrol king cab GQ with 16 X 8 wheels and 265/75/16/ BF Goodrich Mud Terrains.

1. Tyre Size – Are they generally a large tyre in relation to what similar vehicles wear? Are they larger than what is on the tyre placard for that vehicle? Yes and yes – I might drop them to 33 psi.

2. Load – Is the vehicle loaded/heavy, likely to carry a load or tow a trailer in the near future? Nope, but it’s a 6 cylinder diesel and they are traditionally nose heavy – I might bump the fronts up to 35 again.

3. Speed – Are highway (freeway?) speeds likely? Are we in the middle of a warm season that will make the flexing heating problem worse? Yup yup – I’d better whack the drivers and steerers up to 38 all round.

4. Tyre Rating – You have to admire the quality tyres like BF Goodrich and Bridgestone that put useful information on the sidewall. Especially BFG that have paragraphs of info (just about!) with the most important for this application being: "Max load single 1380 Kg (3042 lbs) at 450 kPa (65 psi) cold". (Patrol 265 tyre) If I had these tyres on an F350 with a ton and a half in the tray, the tyres would be straight up to the 65 psi max as I know the manufacturer has tested it at these specs. The Patrol won’t be carrying anything like this so I’ll leave them where they are. 5. Tyre Life and Handling Vs Comfort – I like my comfort as much as the next person but I also like my machines to handle as well as they can. You’ll get a magic carpet ride if your boots are only carrying 22 psi but the extra rolling resistance is going to mean squeezing the accelerator that much closer to the floorboards and she’ll handle like the proverbial wet sponge. You wont be able to carry much speed through the corners with tyres wobbling around when they should be transmitting steering input to the road surface and all that flexing will be heating up the tyres. I’m not paying for the tyres but I don’t like wastage and the coil Patrols don’t ride too badly so I think 35 psi will be the lower limit that I run unless I’m on soft sand.

So … balance everything up and have a good look at the tyre placard and owners manual – it often lists alternative tyres that are fitted to different specifications of the model car that you drive and you can get a feel for what the manufacturer is trying to achieve. Be wary of the tyre and vehicle manufacturers' specs to a certain degree, as they are more likely to recommend low tyre pressures to give the impression that their stuff gives a lovely soft ride. They know that the average person is not going to push the tyre to anywhere near its limits and they are really not going to be broken hearted if you have to replace your tyres more often. Call me a cynic.

It really is one of those areas that a lot of people have no regard or understanding of, but if you were able to see how a decent pressure improves the handling, braking and general feel of your machine and makes it cheaper to run – you’d check them more often and keep them firm.

All this text applies to trailers as well and, in the case of box trailers, due to their lighter mass generally, they are one of the few areas where you may find over inflated tyres. The crude leaf spring suspension – never maintained – so the rusty leaves bind and this offloads a lot of the springing to the tyres in lightly loaded applications. If your tyres are rock hard you’ll shake the trailer to bits and everything on it. Conversely if you have a large boat that comes in around 2 tonne like mine does, or anything heavy for that matter, the tyres are often small to keep the load closer to the ground. This means they are working overtime and should be of the light truck (LT) variety and be pumped up to around 40 psi.

Now to muddy the waters a little…I’ve just read the Tyre Tips column in the Jan 2000 Overlander magazine by columnist Allan Finnie who was complaining bitterly about the fact that his new 4X4 was delivered with 36 psi in the tyres causing the ride to be ‘atrocious’. I would have been impressed myself but Allan did not state what sort of car he was talking about – only saying the tyre placard stated 26 front, 29 rear. He did say that the tyres were wide and low profile - and the car could have been a lightweight Honda CRV or something but he made no mention of handling or braking. He was right to say that lower pressures play a major part in how the car is perceived – especially on a trip around the block test drive – but its what he doesn’t say that worries me.

The modern 4X4s are getting more powerful, more comfortable with better brakes and handling. The driver these days needs good quality tyres and enough pressure in those tyres to transmit all this progress down to the road and give a bit of valuable feedback through the steering wheel and seat.

Enjoy your rubber

Jonesy