Tyre Pressure

Submitted: Tuesday, Feb 01, 2005 at 01:31
ThreadID: 19985 Views:2762 Replies:9 FollowUps:19
This Thread has been Archived
I know this will get some conflicting replys but here goes. I have Goodyear MTR's on my vehicle when I go onto the dirty stuff. I noticed in a thread a few months back that a couple of guys were discussing tyre pressure and it seemed they thought that around 34psi was good with this rubber whilst on the black top. A mate has just fitted them to his Patrol, as his only set. He was adviced by the dealer to never run them on the black stuff at anything less than 40psi. Just wanted to know if anyone had any ideas and or experience with goodyear MTR's
Back Expand Un-Read 0 Moderator

Reply By: Eric from Cape York Connections - Tuesday, Feb 01, 2005 at 06:26

Tuesday, Feb 01, 2005 at 06:26
Glenn I run mtrs 32*11.5*15
On the black stuff 40 psi
On the ruff stuff depending on load front 20 rear 24

All the best
Eric
AnswerID: 95964

Reply By: DukeAtty - Tuesday, Feb 01, 2005 at 07:34

Tuesday, Feb 01, 2005 at 07:34
Ditto...When you run less you can tend to wear out the centre first...
AnswerID: 95968

Follow Up By: Member - JohnR (Vic) - Tuesday, Feb 01, 2005 at 07:52

Tuesday, Feb 01, 2005 at 07:52
other way round Duke, when the pressures are high, the middle, lower pressures the walls take a hammering and the outsides wear. The reason being that the weight isn't being carried so much with the air pressure, across the tread.

I tend to use less most of the time as I do a bit of gravel and dirt on and off, and heaps of broken edges in country Victoria as Bracksy reduces the revenue paid for repair.
Cheers,
Who?
John

Member
My Profile  My Position  Send Message

0
FollowupID: 354744

Follow Up By: DukeAtty - Tuesday, Feb 01, 2005 at 12:43

Tuesday, Feb 01, 2005 at 12:43
It is a fact that when the tyre is under-inflated the middle can throw out with centrifugal force causing premature wear.
You can get away with low pressures on dirt or bitumen if your speeds are not great but if you travel at high speeds then you could have problems.
I stand by what i said.

0
FollowupID: 354803

Follow Up By: Member - JohnR (Vic) - Tuesday, Feb 01, 2005 at 13:45

Tuesday, Feb 01, 2005 at 13:45
centrifugal force with modern radials, on a 4by, you must be kidding. What a joke. At 100 kmh my tyres are at 670rpm+/- one or two a minute. Especilally going to extremes you will find wearing in the middle at higher pressures, and on the outside at lowers, eben to the extent of cracking of sidewalls at the extremes. That has been discussed here in the past.

Cheers,
Who?
John

Member
My Profile  My Position  Send Message

0
FollowupID: 354811

Follow Up By: DukeAtty - Tuesday, Feb 01, 2005 at 14:04

Tuesday, Feb 01, 2005 at 14:04
I wouldnt call 100kmh a high speed. I didnt even say 100kmh anyway.
I would think 120kmh+ would be where centrifugal force would start to play a roll.
I once tried fitting a bicycle speedo onto my 4x4 to check my speedo because those things are very accurate,,, you know where you have to measure the circumference of the tyre and place the electronic pickup on the body......... Well anyway i noticed that when my speed got over 140kmh+ the error blew out heaps on the bike speedo because of tyre growing....
Anyway, thats the plain fact... I wont reply again..The fella that asked the original question just wanted a simple answer,,, not us arguing about splitting hairs...
Sorry ExplorOz...
0
FollowupID: 354813

Follow Up By: warthog - Tuesday, Feb 01, 2005 at 16:28

Tuesday, Feb 01, 2005 at 16:28
Why would centripetal force be increased in a tyre with less air? Air also has mass.
0
FollowupID: 354846

Follow Up By: Tuff60 - Wednesday, Feb 02, 2005 at 00:34

Wednesday, Feb 02, 2005 at 00:34
Sorry I must now how this "centrifugal force" force is lifting a 2+tonne 4WD, if the diameter is getting bigger the 4WD is getting higher. Yes/No???
0
FollowupID: 354946

Follow Up By: Member - JohnR (Vic) - Wednesday, Feb 02, 2005 at 10:58

Wednesday, Feb 02, 2005 at 10:58
Tuff60, I could not possibly add to your knowledge here. hehehe I could not possibly comment. I thought the bound in air pressure was holding it up. Otherwise the flatter your tyre, the faster it could go with the centrifugal force. My neighbour always said that tyres were only flat on the bottom, all you needed was to rotate it.

warthog; I think your theory combined with DA's has a limited advance for future. I am very hazy on centripetal forces though having not studied physics formally.

For my understanding would you tell me please. Is centripetal force is created by the torque of the engine through the transmission I would imagine, and the gyroscopic tendency of the wheel to want to continue, and in downhill conditions, the weight of the vehicle. All offset by the rolling resistence, gravity, and air resistence properties. Because the circumference of the wheel is bound by the elasticity of the wheel/tyre combination, we get forward propulsion as the tyre gets grip.

How is your physics warthog?
Cheers,
Who?
John

Member
My Profile  My Position  Send Message

0
FollowupID: 354995

Follow Up By: warthog - Wednesday, Feb 02, 2005 at 15:37

Wednesday, Feb 02, 2005 at 15:37
Hi John my Physics is very average having not studied it since year 12, now 18 years ago.I did enjoy it back then but I'm sure there must be people on this forum who can go were my rudimentry knowledge will not allow me to follow. However I remember my physics teacher explaining that centrifugal force was a combination of centripetal force( the force acting on an object moving in a circular motion that acts to force that object out radially from the centre of its motion) and the momentum carried by an object in the direction of its travel, so it is centripetal force that acts to force the tread outwards as the tyre rotates and the air within it. This force is related to speed of rotation and mass so that a tyre with more mass will have a greater centripetal force acting on it. This is pretty irrelevant of course when you consider the mass of the air compared to the mass of the tyre and the pressure of the air exerts a much greater force in forcing the tread outwards.... and I now wish my physics teacher was here to dig me out of this mess.
mercy please, Chris.
0
FollowupID: 355032

Follow Up By: Member - JohnR (Vic) - Wednesday, Feb 02, 2005 at 16:26

Wednesday, Feb 02, 2005 at 16:26
Hi Chris, as I said I didn't study physics, so you have it all over me that way. Year 12, hmmmm what was that? Not that I forget, just didn't get there. LOL Not that I'm not educated, just in a different direction.

I really didn't know about centripetal forces til I read it yesterday, so you have taught me a thing or two. Understood centrifugal and centrifuges and all that to some extent. I was just reasoning or rationalising above that was what I expected. I couldn't be a Dr Karl any other way. Then I'm not anyway. LOL
Cheers,
Who?
John

Member
My Profile  My Position  Send Message

0
FollowupID: 355035

Follow Up By: warthog - Wednesday, Feb 02, 2005 at 18:41

Wednesday, Feb 02, 2005 at 18:41
Phew, that was close, think my physics can go back to its mouldy old cupboard where it came from.
0
FollowupID: 355058

Reply By: Utemad - Tuesday, Feb 01, 2005 at 09:37

Tuesday, Feb 01, 2005 at 09:37
I'd think it would depend a lot on weight.

I have 31in MT/Rs on a Rodeo but I wouldn't use the same hway pressure as a Cruiser. Try the 4psi method. I did and ended up settling on 34f/32r for bitumen.
AnswerID: 95982

Follow Up By: Member - Roachie (SA) - Tuesday, Feb 01, 2005 at 11:41

Tuesday, Feb 01, 2005 at 11:41
G'day mate,

Like Pauline once said.........."Please explain"

ie: "TRY THE 4PSI METHOD"

Thanx
0
FollowupID: 354789

Follow Up By: Utemad - Tuesday, Feb 01, 2005 at 12:18

Tuesday, Feb 01, 2005 at 12:18
I was going to explain it but then Bradley beat me to it. See reply below.
0
FollowupID: 354798

Reply By: Member - Bradley- Tuesday, Feb 01, 2005 at 11:47

Tuesday, Feb 01, 2005 at 11:47
on my jackaroo i was advised by dealer to run 40 psi all round, well it was good in a straight line but hopeless cornering and couldnt be stopped in the wet, just hopped and skittered everywhere, now using 34 all round and it is heaps better, no real difference to the noise. And when running they go to 38/39 hot, so its pretty spot on.

4 PSI RULE - basically if you run the correct pressures there will be a 4 psi difference between cold and hot pressure. Check out the Sime tyres website, they have a really good explanation on the subject.
AnswerID: 96022

Follow Up By: Member - Jeff M (WA) - Tuesday, Feb 01, 2005 at 14:46

Tuesday, Feb 01, 2005 at 14:46
Actually in heavy rain having a higher pressure will help avoid aqua plaining as the tyres cut through the water rather that floating on top of it.

I used to run my sedan at 40psi and it stopped on a 50c peice in the wet. I run my Bridstone DD's at 44psi all round and find it handles supurb on the open road compared to mid 30's. I dropped my tyres down to 29psi on some gravel down SW of WA over chrisy and (fully loaded) the car felt like the shocks had died. Bouncing all over the place, pump them back up and she was cruising very happy again. We were down Margeret River way and "caves road" is hilly twisty and bumpy sealed road, you could really tell the difference.
0
FollowupID: 354823

Follow Up By: Member - Bradley- Tuesday, Feb 01, 2005 at 15:08

Tuesday, Feb 01, 2005 at 15:08
ya for 'normal' rubber this is right, i used to run my 693's at 38-40 on road and they were fantastic, But these mtrs sit up on the centre blocks at 40 psi and its pure physics in the wet , it just doesnt have the contact area to provide enough braking force, no probs with aquaplaning with such an open tread, but it didnt sit the tread flat enough to gain enough retardation and load transfer to get the front tyres to squash down and give proper braking, if you know what i mean.

at 34 its a fair bit better stopping in the wet, taking off, well thats another story, anything more than 1/2 boot and it will spin through 1st and 2nd , sometimes it breaks out on the shift to third as well.
0
FollowupID: 354825

Follow Up By: Member - Jeff M (WA) - Tuesday, Feb 01, 2005 at 15:13

Tuesday, Feb 01, 2005 at 15:13
Fair enough, that makes sense.

Hmmm, my Automatic Diesel doesn't seem to have that problem in the wet for some reason??? :-(
Nah, it's not a lack of power, it's all that traction my old weathered bridgesones have! LOL (keep telling yourself that Jeff)
0
FollowupID: 354826

Follow Up By: Member - Bradley- Tuesday, Feb 01, 2005 at 17:06

Tuesday, Feb 01, 2005 at 17:06
Hmm how to ruin Jeffs day totally,

Tell him that aged tyres harden through heat and sunlight, and therefore have a lower coefficient of friction and are easier to spin - especially in the wet.

Na, wont tell him that.......

;-)))))))
0
FollowupID: 354850

Follow Up By: Member - Jeff M (WA) - Tuesday, Feb 01, 2005 at 17:48

Tuesday, Feb 01, 2005 at 17:48
Arrrrrgh!

Well on the upside I used to spin the wheels alot until I fixed the rear suspension, new coils and airbags.... Does that count!! :-)
(just shows how much play the Yota LSD has hey!).
Man it's only getting worse, I'm gonna quit while I'm getting slautered!
0
FollowupID: 354856

Follow Up By: Member - Bradley- Wednesday, Feb 02, 2005 at 10:38

Wednesday, Feb 02, 2005 at 10:38
Ahh airbags eh, well the solution to all your anti-social wishes is to pump the airbags up to full height, whack 60psi in the rear tyres and do the 2 pedal shufle :-))

Not that i condone such behaviour though................

take it easy buddy. Brad
0
FollowupID: 354990

Reply By: Bob Y. - Qld - Tuesday, Feb 01, 2005 at 14:56

Tuesday, Feb 01, 2005 at 14:56
Glenn,

Run MTR's(235.85R16's) on 79 series tojo, with skinny rims. Run at 34-36 psi, on bitumen, gravel and station roads. Would run them around 32-34 if had them on 7" rims.

We kick 'em up to 40 if it's loaded, or hot, and going for a quick trip on the black stuff. Only time I'll let them down is if it's wet, and I want to get home!!!

Am on 2nd set, and the original ones wore evenly, at those pressures. In fact, got about 50K out of most of the first lot. Good for this country. Like the TG wranglers as well, in 750R16.

Hooroo...

Seen it all, Done it all.
Can't remember most of it.

Lifetime Member
My Profile  My Blog  My Position  Send Message

AnswerID: 96047

Reply By: Member - JohnR (Vic) - Tuesday, Feb 01, 2005 at 16:20

Tuesday, Feb 01, 2005 at 16:20
Glenn, I know this page does not answer your question directly but will dispel some myths supported elsewhere, It is a Canadian site discussing tyre pressures and the potential costs of under/over inflation. Just click the Link and scan to see the wear versus contact area. I think it should correct a couple of ideas.
Cheers,
Who?
John

Member
My Profile  My Position  Send Message

AnswerID: 96069

Reply By: greydemon - Tuesday, Feb 01, 2005 at 18:12

Tuesday, Feb 01, 2005 at 18:12
Hi Glenn,

The link to the Canadian site looks fairly good. My company sends most of it's employees on a specialist driving course. The advice there is to always inflate tyres higher than the vehicle manufacturer recommends. This is because the manufacturers go for lower pressures to increase ride comfort. Lower pressures lead to increased side wall flexibility which reduces handling and increases heat.

Most vehicles run with under inflated tyres, just go and have a look at the old tyres behind any tyre outlet, I doubt that you will find many, if any, worn out down the middle through over inflation. Some people have mentioned that they run lower pressures on gravel, well each to their own, when in the Kimberley I was advised that the local police and prison service always run on high pressure tyres on gravel, most blowouts on gravel are due to side wall failure from flexing and heat build up. Of the blowouts I saw on that trip - about 10, I would say that 7 of them were side wall damage.

The opinion of the course instructors was to always run on high pressure - in your case at least 42-44psi, and even for small passenger vehicles 40psi was recommended. The ONLY exception was of course for driving in sand where very low pressures were recommended, though not so low that the tyre comes off the rim.
Sorry if this just adds to the confusion!

Greydemon
AnswerID: 96088

Follow Up By: greydemon - Tuesday, Feb 01, 2005 at 18:29

Tuesday, Feb 01, 2005 at 18:29
Me again,

Go and have a look at post 19953. It concerns tyre blow-outs at high speed. Several of the examples quote the cause as a slow puncture causing low tyre pressure causing overheating and side wall failure. It's bad enough when this happens due to a slow puncture, you would be pretty upset to cause it yourself through under inflated tyres on the gravel or blacktop.

Greydemon
0
FollowupID: 354861

Reply By: Member - Sand Man (SA) - Tuesday, Feb 01, 2005 at 18:35

Tuesday, Feb 01, 2005 at 18:35
Yep,

I agree with Greydemon in that the "Drive Safe" Instructors recommend a higher pressure. I too, have done one of these courses.

However, in relation to this and also the Canadian site, I can't help feeling that the logic and comments relate specifically to your standard passenger vehicles with standard "black top" tyres.

I would support the comments made by Utemad and Bradley.
I am also a believer in the 4PSI rule. This will work for everybody and every car.

Bill


I'm diagonally parked in a parallel Universe!

Member
My Profile  My Blog  My Position  Send Message

AnswerID: 96094

Reply By: Truckster (Vic) - Wednesday, Feb 02, 2005 at 16:13

Wednesday, Feb 02, 2005 at 16:13
EVERY CAR IS DIFFERENT... TRIAL AND ERROR.

How much weight does your mate have in his car?
How many accessories are weighing down his car?
Does he tow?
What size tires?
Where does he live - eg Temps of the road/ambient etc?
What roads does he drive on?
What suspension does he have - firm or soft?

MTR's in a 33x12.5's as my daily tires on my Patrol... 40psi is like driving on solid rubber blocks! No give at all. IMHO.. IYDGFY :) I run 36 all round at moment on road and its good.

Down to 22-14 offroad if needed they bag out well.

Will Go MTR"s again when Royce buys these off me.
AnswerID: 96260

Sponsored Links

Popular Products (14)