Tyre pressures (again!)

[TheTree]

Hi

I recently had a conversation with a guy in a tyre place who stated that "tyre construction has been revolutionised over the last 5 years and you should run higher pressures than the compliance plate recommends"

I mentioned the the P38 recommendations are 10PSI less in the rear, and his response was "You should run at least 40PSI and I run 42PSI all round in my car"

I have been doing a little research and this 4si rule seems to be pretty well researched, though I have never heard of it before

4psi Rule (http://www.aawen4x4.com.au/index.php?option=com_content&view=article&id=1:4psi-rule&catid=2:current&Itemid=7)

Anyone used this rule?

Steve

[DiscoMick]

I think it depends on what tyres are involved. Tyre places are used to working with highway tyres and some ATs.
The placard on my Disco says 4 psi more in the rear than in the front. However, that would have been with standard highway tyres 18 years ago, so the pressures are different with my current tyres.
I currently run 32 in the MTRs, but pump the rear up to 36 when towing. No real theory, it just seems to work.
Every time I get the tyres balanced the workshop pumps them up to 40, which is just too hard, so I have to let them down.

[TheTree]

Hi

In reality the only sure way to tell is by the tread wear pattern, but by then your tyres are half gone !

I think I will try the 4psi rule and see where my pressures finish up.

These guys put almost 10psi over the compliance plate specs in the corolla :eek:

Steve

[redandy3575]

It's a strange theory, I have heard of it but never went with it.

The place I bought my tyres from who's past time was rally driving and extreme off-roading has mentioned to me that it is a comfort thing. Basically you don't want to exceed more than 5psi either way of the vehicles placard remembering that the pressure increases by about 4psi when it warms up. I.e in The P38s case 28psi in the front will run at 32psi at normal running temp, which makes sense condsidering such a low cold pressure rating in the first place. ATs & Mud terrains have harder sidewalls, so naturally it's ride is going to be harder, and they don't mind running a little lower in pressure like around 26psi (cold) in the front remembering the 4psi increase when warm.

It can be complicated, but highway tyres should run as close to placard as possible and higher if using lower profile like the 18' RR rims.

In the US there have been a large number of individual lawsuits towards tyre companies vs the tyre store vs the car manufactures due to their inconsistent guides on tyre pressures resulting in serious accidents if not fatalities for the wrong advise.

Many automotive Experts do agree that the manufactures placard is what we should be focusing on as that is what the vehicle has been tested on and signed off as.

[TheTree]

Hi

I am inclined to stick with the compliance plate, rather than take the advice of some guy from a tyre store !

They put 38PSI all round in the corolla, which is basically a rollerskate on wheels, and suggest I put 40 or 42 all round in the P38.

I suspect Land Rover and Toyota have done a lot more testing than anyone else regarding tyre pressures for their cars.

As far as the 4PSI rule, as Andy pointed out, that's the kind of pressure increase you would expect to see anyway.

The physicist in me says a higher pressure means a slightly larger volume of air than a lower pressure but that is unlikely to lead to a big variation in pressures.

Steve

[mtb_gary]

Steve

At one level I agree with the compliance plate. However, during advanced driver training courses I have attended they always suggest somewhat higher tyre pressures than the compliance plates (around 36 psi on a sedan) to improve emergency braking and improve steering responsiveness.
Then the other part of the equasion....load carrying. Going back many many years ago, I recall being advised that to achieve the correct pressure for the load being carried you first need a reference point of an unladened vehicle. With the car unladened and with the correct tyre pressure take a measurement in inches (yes it was that long ago) or mm of the ground to the centre of the wheel. When you load up the car the extra weight will give the apperance of having a flat tyre, simply pump up the tyre to achieve the height of the unladened vehicle (not pressure) and you will have the correct pressure for the load in the car.

Gary

[redandy3575]

Steve

At one level I agree with the compliance plate. However, during advanced driver training courses I have attended they always suggest somewhat higher tyre pressures than the compliance plates (around 36 psi on a sedan) to improve emergency braking and improve steering responsiveness.
Then the other part of the equasion....load carrying. Going back many many years ago, I recall being advised that to achieve the correct pressure for the load being carried you first need a reference point of an unladened vehicle. With the car unladened and with the correct tyre pressure take a measurement in inches (yes it was that long ago) or mm of the ground to the centre of the wheel. When you load up the car the extra weight will give the apperance of having a flat tyre, simply pump up the tyre to achieve the height of the unladened vehicle (not pressure) and you will have the correct pressure for the load in the car.

Gary

Yes that's true in theory, but over-inflating the load tyre can cause other issues such as an increase risk of a blowout with the load on the back, and that can't be good. If you're going down that path then again as long as you don't go above the tyre rating pressure then you should be ok.

Advance Driving courses may suggest that, and that's fine on racetracks in controlled environments with good Tarmac surfaces but unfortunately Australian roads especially in the country & outback areas are very ordinary. Like what happens if you suddenly end up on gravel road with tyres well above their vehicle's recommendation? It would be like driving on skates with every pot hole bouncing the wheels rather than absorbing the unevenness. I know these courses mean well, but in reality it can be a different case.

[Keithy P38]

I'll chuck my 2c in!

I've run 3psi higher in the rear than the placard in my P38 for the entire life of the BFG's it's currently wearing, and 7psi higher in the front. Have done 40,000km so far and they are wearing perfectly across the tread. I expect they'll go another 40,000km as well.

See attached pic taken yesterday. Might not show the entire tread, but that's what 40,000km on muddies looks like.
https://www.aulro.com/afvb/

Cheers
Keithy

[mtb_gary]

I'll chuck my 2c in!

I've run 3psi higher in the rear than the placard in my P38 for the entire life of the BFG's it's currently wearing, and 7psi higher in the front. Have done 40,000km so far and they are wearing perfectly across the tread. I expect they'll go another 40,000km as well.

See attached pic taken yesterday. Might not show the entire tread, but that's what 40,000km on muddies looks like.
https://www.aulro.com/afvb/

Cheers
Keithy

Keith, nothing to do with this thread but how stretched is the height sensor? From that pic it looks almost vertical?

Gary

[Keithy P38]

Still has a rearward angle! I took a pic of it but can't upload pics here via the iPhone :-(

It's virtually the same as it was before the lift.

[gusthedog]

Hi

I recently had a conversation with a guy in a tyre place who stated that "tyre construction has been revolutionised over the last 5 years and you should run higher pressures than the compliance plate recommends"

I mentioned the the P38 recommendations are 10PSI less in the rear, and his response was "You should run at least 40PSI and I run 42PSI all round in my car"

I have been doing a little research and this 4si rule seems to be pretty well researched, though I have never heard of it before

4psi Rule (http://www.aawen4x4.com.au/index.php?option=com_content&view=article&id=1:4psi-rule&catid=2:current&Itemid=7)

Anyone used this rule?

Steve

I reckon stick with the 4psi rule. I've been using it for a number of years now and I think it's great. I'm on placard in the rear of my D2 and 4psi over at the front (due to bulbar etc) and it's 4psi up when hot at both ends. Perfect :D:D

And tyres have not been revolutionised in the last 5 years IMHO. I would argue that hey have changed compounds and added stuff like silica etc but they are still round and black and haven't changed dramatically in the last 20.

[redandy3575]

I'll chuck my 2c in!

I've run 3psi higher in the rear than the placard in my P38 for the entire life of the BFG's it's currently wearing, and 7psi higher in the front. Have done 40,000km so far and they are wearing perfectly across the tread. I expect they'll go another 40,000km as well.

See attached pic taken yesterday. Might not show the entire tread, but that's what 40,000km on muddies looks like.
https://www.aulro.com/afvb/

Cheers
Keithy

Again, it's a comfort thing. If riding on higher pressure is your driving pleasure then you go with the flow.

I actually put 30psi (cold) in the front and standard 38psi (cold) in the rears and find a balanced mix of lesser roll resistance without compromising the ride comfort. Don't forget that it's not just the tyres that get affected but your shocks too, as they're made to work harder to absorb the extra vibration.

[redandy3575]

And tyres have not been revolutionised in the last 5 years IMHO. I would argue that hey have changed compounds and added stuff like silica etc but they are still round and black and haven't changed dramatically in the last 20.

I agree. If anything tyre technology is still behind the times.

I.e not catering Mud terrain for lower profile rims for RR sport, Discovery 4, L405 RR. In fact I reckon off-road tyre companies are still a good 10 years behind the times, thinking that we all still drive MQ Nissan Patrols, 80 series Lancruisers etc. when just about all new 4wd's are now going lower in profile and there are very limited choices out there.

[Dougal]

I agree. If anything tyre technology is still behind the times.

I.e not catering Mud terrain for lower profile rims for RR sport, Discovery 4, L405 RR. In fact I reckon off-road tyre companies are still a good 10 years behind the times, thinking that we all still drive MQ Nissan Patrols, 80 series Lancruisers etc. when just about all new 4wd's are now going lower in profile and there are very limited choices out there.

The problem with modern vehicles is the speed rating.

What sort of mud tyre gets a 200km/h speed rating?

[redandy3575]

The problem with modern vehicles is the speed rating.

What sort of mud tyre gets a 200km/h speed rating?

Not many, but then again Muddies aren't meant for speed, they're for off-road use usually with a 140kmh max rating.

Certainly ups the challenge for tyre companies eh!!

[Dougal]

Not many, but then again Muddies aren't meant for speed, they're for off-road use usually with a 140kmh max rating.

Certainly ups the challenge for tyre companies eh!!

Especially in countries where the speed limit is 130km/h or more.

[benji]

Steve

At one level I agree with the compliance plate. However, during advanced driver training courses I have attended they always suggest somewhat higher tyre pressures than the compliance plates (around 36 psi on a sedan) to improve emergency braking and improve steering responsiveness.
Then the other part of the equasion....load carrying. Going back many many years ago, I recall being advised that to achieve the correct pressure for the load being carried you first need a reference point of an unladened vehicle. With the car unladened and with the correct tyre pressure take a measurement in inches (yes it was that long ago) or mm of the ground to the centre of the wheel. When you load up the car the extra weight will give the apperance of having a flat tyre, simply pump up the tyre to achieve the height of the unladened vehicle (not pressure) and you will have the correct pressure for the load in the car.

Gary

So 65psi to achieve this was right! Hayman Reece probably didn't like the ball load, and the suspension didn't drop an inch. :D

The placard was signed of with Michelin xpc too. With my current road tires anything less than 32 cold and they loose grip, so I run 36 front and rear, though 40 in the rear when loaded up.
I used to run 36 and 40 cold with the bfg at's. Only because with the LT carcass I didn't want it flexing to much and building up heat.

These pressures are just what feels good.

So he's saying that tires now are more flexible in the side wall?

[Robsrod 58]

I run 39psi in the back and 36psi in the front cold, when warmed up they only ever move around 2psi according to my tpms . This is pretty close to what the installer recommended to me as he pushed a coopers tyre manual/tips and tricks booklet in my hand with all that 4psi stuff in it...

All good so far:D:D

Rob

[Hoges]

The increased rolling resistance is certainly noticeable with the Dueller 697s I now have instead of the previous Cooper H/Ts. So is the increased fuel consumption!!
Increasing the fronts up from 28 to 34psi improves the "rolling g" sensation, while the bumps and cracks in the bitumen are largely absorbed by the still new thick tread.

I had understood that the tyre pressure differential was also something to do with putting a slight preload on the viscous coupling in the transfer box due to the slightly increased diameter of the rear tyres due to their pressure. Perhaps this is a bit of a myth?

The blokes at the local Goodyear where I got the Coopers years ago were adamant that 35 psi front and rear was Cooper recommendation... damned if I know!

Edit: another thought regarding the VC. If there is a difference in overall diameter between front and rear tyres at the recommended inflation pressures, sufficient to preload the VC then am wondering if this may result in a degree of trasmission windup causing otherwise unexplained vibration at higher speeds? I have some experimenting to do!

[gusthedog]

http://www.aulro.com/afvb/attachments/p38a-range-rover/75043d1396328071-lifted-p38-air-questions-blue-range-rover-070.jpg

Cheers
Keithy

Nice home made extended bumpstop! :D What do you do when the cable tie breaks ?:eek:

[Keithy P38]

I get more wheel travel :-)

I'm hoping to up the tyre size soon, so until I've got the new boots on I'll leave the temporary bumpy's in there! Might need to add a bit if I get tyre scrub. They've already covered corrugations and plenty of flex without needing adjustment.

[Pete38]

I get more wheel travel :-)

I'm hoping to up the tyre size soon, so until I've got the new boots on I'll leave the temporary bumpy's in there! Might need to add a bit if I get tyre scrub. They've already covered corrugations and plenty of flex without needing adjustment.

Cable ties are good for 100 mile an hour and the Rangie doesn't go that fast very often hey Keith?

[Keithy P38]

Not any more!

[PaulP38a]

Seems that a few of us who did those advanced driver courses over the years have been told the same thing. I did one in the mid-90's and was told that I should be running 40psi all round, and I did for years. Great on a family sedan.

On the Rangies, for road driving, I'd go around 36-40psi for road tyres and 30-32 for muddies.

Hey Keithy, drill a hole in the plate on the diff to fit a bolt head, then drill the rubber block and recess the bolt thread with washer and nut, and lose the cable tie...

Cheers, Paul.

[Keithy P38]

Haha it'll come to that one day Paul, once I'm wearing 33's. I want to wait to ensure no scrubbing before i make it permanent.