Tyres

[2luxes]

The 4psi rule is apparently aimed more towards passenger vehicles driven on road surfaces only.

While discussing tyre pressures with Goodyear's head office a few years ago, I asked about the 4 psi rule. They said their technical people have tested it extensively and found there are far too many variables out on the road for it to be accurate.

They also mention about being aware of older vehicle tyre placards recommending low tyre pressures which is aimed at comfort .

I have heard that said many times but I don't agree. It is the pressures that determine the tyre slip angle in corners. That is the angle between where the wheel is pointing and where the distorted tyre is pointing. Just about all cars for the last fifty years or so have been designed to understeer in corners because very few drivers can handle oversteer. In order to make the car understeer, the front slip angle must be slightly higher than the rear slip angle. If you increase the front tyre pressure to the point where the rear angle exceeds the front, the car will oversteer which is swinging its tail out in corners.

I noticed the pressures in the handbook for my single cab Lux are the same front and rear when the car is empty. The rear pressures increase by up to 9 psi depending on load. This would suggest that when the car is fully and evenly loaded, there will be no increase in weight on the front wheels. This would result in the front tyre slip angle remaining constant.

Increasing the rear tyre pressure as the car is being loaded would progressively reduce the slip angle resulting in the front angle always being higher and the car would still be understeering as the suspension engineers intended.

Goodyear also told me during that conversation to use the factory specified pressures and if I wanted to raise them for whatever reason then never exceed them by more than 4 psi. I would imagine that would have a lot to do with tyre wear. If I increased the front and rear pressures by the same amount, the front slip angle would still be higher than the rear but the centre of the thread would be pushed out further which would increase the wear rate in the centre of the tyre.

Some may think this is all BS but it works. I have used the pressures in the book for years and have never hand any handling problems or uneven tyre wear. Those pressures would have maintained full tread contact with the road. They would have also kept the sidewall flex point in the right position. If it is too high or too low, it has an adverse effect on tyre temperatures.

 

[Batts88]

When I took the disco for a drive before changing the pressure it didn't want to stay in the lane on corners at one point in an 80k zone it crossed over the white lines on a bend that it should have handled easily. After I pumped the tyres up I done the same run and it cornered fine and handled a lot better.
I don't doubt that some manufactures have got the correct tyre pressures on their placard but the old disco was way out I also forgot to ask how much his econ improved by

 

[Sean Logue]

All terrain tyres have a more open tread pattern and small lugs on the side wall. They are designed for 50% on road use and 50% offroad use. In theory, they should be the perfect tyre, and for some people, they are. It all depends on what type of 4WDriving you do. They have slightly more road noise than road tyres and have predictable handling on road. Offroad they are better suited to medium 4WDriving on dry tracks. In muddy situations the tread clogs with mud and offer little, if any traction. If operated at road pressures on gravel tracks, the tyre can develop cuts across tread pattern.

Mud terrain tyres have a very open pattern and aggressive lugs on the side walls. On the road they have bad manners; they are very noisy and have a tendency to aquaplane on wet roads. Offroad they afford maximum traction, especially when deflated. Mud terrain tyres use the lugs on the side wall to bite into the mud and the tread pattern is designed to be self cleaning.

So does that mean having M/T would have less grip on very steep climbs in the dry then A/T because of the very open pattern ?

I can see why some people do get M/T just because they do look tough, but that's a waste unless you do go mud bashing.
I just want practicality, so I just use A/T because I never go out looking for mud and go exploring when weather is beautiful.

One day Ill be deep in the VHC and it may piss down rain on me, but ill deal with it then. Has not happen yet and I go up there every 3 weeks.
Photo 27-1-18, 8 30 10 am by Sean Logue, on Flickr

 

[phs]

So does that mean having M/T would have less grip on very steep climbs in the dry then A/T because of the very open pattern ?

I can see why some people do get M/T just because they do look tough, but that's a waste unless you do go mud bashing.
I just want practicality, so I just use A/T because I never go out looking for mud and go exploring when weather is beautiful.

One day Ill be deep in the VHC and it may piss down rain on me, but ill deal with it then. Has not happen yet and I go up there every 3 weeks.
Photo 27-1-18, 8 30 10 am by Sean Logue, on Flickr

I found MT do get more slippage than AT on all surfaces.

The thing is when you encounter Mud even the aggressive AT still don't compare.

 

[Swaggie]

I found MT do get more slippage than AT on all surfaces.

The thing is when you encounter Mud even the aggressive AT still don't compare.

Hi mate
That's an interesting observation..
I've only ever run M/T in The High Country...
Was thinking about getting A/T this time around.
I don't come across that much mud too be honest, keep out of there over Winter... The clay is just crap and dangerous on steeper stuff...

Don't expect long life from M/T in The High Country either, mine have only lasted 35k max but there on the vehicle all the time.

 

[phs]

Hi mate
That's an interesting observation..
I've only ever run M/T in The High Country...
Was thinking about getting A/T this time around.
I don't come across that much mud too be honest, keep out of there over Winter... The clay is just crap and dangerous on steeper stuff...

Don't expect long life from M/T in The High Country either, mine have only lasted 35k max but there on the vehicle all the time.

Hey swaggie,

It would be good to hear your opinion if you go down that path but that is the way I have found it, I could get similar traction out of the MTs on harder surfaces ie gravel and Rock, but it would be roughly 10 or so PSI lower.

MTs will still get you through hard surface tracks even if they encounter more slippage

ATs however may not get you through some tracks with clay and mud

 

[03hilux]

Just a self claimed observation through both personal usage and the best part of 25 years in the tyre game listening to real world feed back.
A/T tyres are a lot better as an all rounder. Even is sand. Their down side is deep mud or long muddy tracks.
M/Ts tend to dig sand up on the front making the rears work harder, and as a result, increased fuel usage. Worn M/Ts that have below 40% of life left will work the same as an A/T on sand
The true value of M/Ts are in mud or on surfaces with a firm base where the crust may be loose such as a sandy bush track (non beach type track)

 

[Sean Logue]

Awesome feedback guy's. Ill stick to my A/Ts then, perfect for what I use them for. More grip and last longer, cannot go wrong there.

 

[jacnden]

you may have made up your mind now but have a look at bridgestone mt d64. ive hade them for a few years now and find them great more like an aggressive a/t than a muddie , quiet and ok on road
you know you could probably get more argument on tyres than any other subject

 

[hiluxdriver]

I found MT do get more slippage than AT on all surfaces.

The thing is when you encounter Mud even the aggressive AT still don't compare.

Not quite what I have found, though I'm not saying you're wrong. I had much better grip on loose gravel roads with my previous mud tyres than I do now my ATs. Sand is much of a muchness and I haven't driven mud yet so can't compare that one. ATs of course are much quieter which is always good.

 

[Sean Logue]

you may have made up your mind now but have a look at bridgestone mt d64. ive hade them for a few years now and find them great more like an aggressive a/t than a muddie , quiet and ok on road
you know you could probably get more argument on tyres than any other subject

Ill check them out, Cheers.
Haha, i would believe that you probably would get more arguments on tyers. In the end it's what ever make you happy.

I never care if I am wrong, only way you will learn.

 

[Superdad]

I have had Cooper AT3 in 265/70/17 and now run Cooper STMaxx in 285/70/17.

The STMaxx's leave the AT3's for dead off road...

 

[Sean Logue]

I have had Cooper AT3 in 265/70/17 and now run Cooper STMaxx in 285/70/17.

The STMaxx's leave the AT3's for dead off road...

STMaxx's are the first 4WD tyers I have ever had. No reason why i would not be getting them again.

 

[Marck]

Have a look at the Mickey ATZ P3. More aggressive than an AT. Not quite a muddy . All the noise of a Muddy. Great wearing. And great on road stability and thick side walls. I love them but will not go with them again because of road noise. Only another 60000 k to go.

M

 

[itlldoo]

Only another 60000 k to go.

800 k's a day for 75 days should see the job done ! that would be a three month Barra Expedition all across the top end....

 

[Marck]

I have 6000k coming up in august it’s a start.

 

[Rhett HS]

A lot of people have two sets of tyres, one set for the road and one set for offroad use. Whilst this may seem extravagant to some, after the initial cost of rims and tyres, it is no more expensive to operate than having a single set of tyres. You can only drive on one set of tyres at a time, so you actually double your tyre mileage. In fact, it may save you money; as you can operate a tyre that is designed for a particular environment and not be as susceptible to damage as a road tyre may be.

True, sometimes. Problems are, if you go offroading a lot the 'cost' of changing wheels before and then after each trip should not be underestimated. Whether that cost is at the local tyre shop, or to your back and tools and wheel nuts, etc.

If you dont go offroading much, the tyres may get too old before you have got full use from them, or you might sell the vehicle and get little money for the second set.

Some people get cheap muddies as their offroad set, then suffer from punctures, and cracks from low pressure running.

I reckon its a modern idea, that rarely stacks up.

All terrain tyres have a more open tread pattern and small lugs on the side wall. They are designed for 50% on road use and 50% offroad use. In theory, they should be the perfect tyre, and for some people, they are. It all depends on what type of 4WDriving you do. They have slightly more road noise than road tyres and have predictable handling on road. Offroad they are better suited to medium 4WDriving on dry tracks. In muddy situations the tread clogs with mud and offer little, if any traction. If operated at road pressures on gravel tracks, the tyre can develop cuts across tread pattern.

Mud terrain tyres have a very open pattern and aggressive lugs on the side walls. On the road they have bad manners; they are very noisy and have a tendency to aquaplane on wet roads. Offroad they afford maximum traction, especially when deflated. Mud terrain tyres use the lugs on the side wall to bite into the mud and the tread pattern is designed to be self cleaning.

My experience is that mud tyres can clog too, but that both AT and MT tyres can still do a decent job if it is mud, but not so much if it is clay. MT is more an advantage in clay, and in climbing out of ruts.

What often stops people in mud is diffing out. Thus, a 33" AT will often outperform a 32" MT.

Also, while MT tyres can be the best at paddling up steep soft dunes if you have enough power, they have the problem that they need to be run at slightly lower pressures due to their stiffer construction to achieve the same flotation (as an AT), and yet that stiffer construction can't handle as much flexing. Its a double whammy. My AT tyres have spent heaps of time at 12psi and up to 60km/h on sand. If i had MT, i wonder if they could have handled that.

 

[Rhett HS]

Who's got an answer? Tyre pressures.
The car Bundera, tyres 285/75r16 Silverstone MT 117 Sport.
What pressure for highway & what for Off Road?
Let's open the discussion.
Running 35psi currently, and the back keeps trying to pass me !!!!!!!

Bundera's were known for having somewhat dodgy handling. Having a short wheelbase never helps. It may be that you should try 33psi in the rear, that should help when unladen. What modifications have been done to the vehicle? Are all the bushes and shocks etc in good order?

 

[Rhett HS]

I have recently come across a conversation about tire and rim width's. Apparently a 10" rim is to big for a 12.5" tire. I have been running my mates rim/tire set up for a few weeks and she has been running them for about 3 years with out any issue. I am told that running such a wide rim can cause the tire to come off the bead easier when at low pressures..

Anybody have any info as to why this would be the case?? Because from what i can see it would be the other way around.

I have 285's on 9" rims, which is similar. I have done heaps of sand driving, including dunes and mucking around. I have never lost a bead. Most has been at 12psi, with zero issues. A small amount has been at 10psi, for extra fun, and i got sand in beads, which caused slow air leaks. If i drove conservatively at 10psi i wouldnt expect to get any sand in the bead.

 

[Rhett HS]

I have heard that said many times but I don't agree. It is the pressures that determine the tyre slip angle in corners. That is the angle between where the wheel is pointing and where the distorted tyre is pointing. Just about all cars for the last fifty years or so have been designed to understeer in corners because very few drivers can handle oversteer. In order to make the car understeer, the front slip angle must be slightly higher than the rear slip angle. If you increase the front tyre pressure to the point where the rear angle exceeds the front, the car will oversteer which is swinging its tail out in corners.

I noticed the pressures in the handbook for my single cab Lux are the same front and rear when the car is empty. The rear pressures increase by up to 9 psi depending on load. This would suggest that when the car is fully and evenly loaded, there will be no increase in weight on the front wheels. This would result in the front tyre slip angle remaining constant.

Increasing the rear tyre pressure as the car is being loaded would progressively reduce the slip angle resulting in the front angle always being higher and the car would still be understeering as the suspension engineers intended.

Goodyear also told me during that conversation to use the factory specified pressures and if I wanted to raise them for whatever reason then never exceed them by more than 4 psi. I would imagine that would have a lot to do with tyre wear. If I increased the front and rear pressures by the same amount, the front slip angle would still be higher than the rear but the centre of the thread would be pushed out further which would increase the wear rate in the centre of the tyre.

Some may think this is all BS but it works. I have used the pressures in the book for years and have never hand any handling problems or uneven tyre wear. Those pressures would have maintained full tread contact with the road. They would have also kept the sidewall flex point in the right position. If it is too high or too low, it has an adverse effect on tyre temperatures.

The recommendation to raise the rear pressures for higher loading is primarily because the higher the load, the higher the pressure needed to ensure the tyre doesnt overheat.

Handling characteristics of understeer vs oversteer are primarily tuned through the size of the front anti roll bar. The size of the rear anti roll bar, front and rear spring rates, suspension design, etc etc etc, and the tyre pressures, also play a shaping role.

Increasing tyre pressure does not increase ultimate grip in a simple inclined graph scenario. In fact, for any given loading and situation, there is a pressure at which optimum grip will be obtained. Go above or below that, and you will get less grip. Go too high, and very little rubber will be on the road, and the rubber will shear.

I once had michelin 31" tyres fitted to the rear of my modified holden one tonner. I went through the first roundabout and felt the rear slide out a few inches, even at low speed, like i was on ice. I immediately went back and investigated. The guy had simply looked at the max pressure rating of 60psi and put that in. The car weighed about 1450kgs total, with a shortened alloy rear tray. We let a chunk of air out and it worked great. I ended up settling on 28psi for the rear. The 350chev needed a good stab to get the rear sideways at all at that pressure.