Tyre pressure?

Submitted: Friday, Jan 20, 2006 at 09:21
ThreadID: 29906 Views:11389 Replies:13 FollowUps:9
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Why do people run such high pressures in their tyres? We read of people here who run over 40 psi, even 50 in radial construction tubeless tyres. Why?
I can understand in bias mult iply construction to keep the flexing down and hence the friction amongst the plies creating excessive heat, but why in radials?
Have never run a radial of any sort over 35 psi and never a problem. The Bridgestone Duellers on the Patrol wore out at 90 000 k running at 29 - 28 in all road conditions and no repairs.
I might add that we live in the 'bush' and drive on dirt and gravel roads a lot.
but, I seem to be missing something here, so just thought I would ask.
Might be a silly question, and I am sure you will know far more than me.

Mike
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Reply By: Rigor - Friday, Jan 20, 2006 at 09:34

Friday, Jan 20, 2006 at 09:34
Fair point Mike , I , within reason run the pressure that gives me even tyre wear across the tread . If I am carrying a particularly heavy load I will raise the pressure accordingly . I drive a 3 L Jackaroo and the optimum pressure is around 30 psi in the front and unloaded 28 at the rear , this gives me even wear and a good ride.

Dave L.
AnswerID: 149797

Reply By: D-Jack - Friday, Jan 20, 2006 at 10:28

Friday, Jan 20, 2006 at 10:28
When I ran those sort of pressures on the bitumen on my 99TD Jack I would be squeeling around all corners, that's using the OE tyres. But when I changed to LT Cooper ATRs I felt the need to have more air in them to maintain the same type of shape I wanted (flat across bottom to promote long even wear), and I feel they are designed to run higher pressures, and I like running on firm pressures anyway because I feel the car sticks a lot better on bitumen with pressures between 35-40psi, and I feel more in touch with the road and conditions. Of course it's a different story on gravel and off road.
AnswerID: 149809

Follow Up By: Sand Man (SA) - Friday, Jan 20, 2006 at 14:58

Friday, Jan 20, 2006 at 14:58
Yep, I agree D-Jack.

I run 35psi all round in the Jack using Dueller 694's.

I leant in a safety driving course I did many years ago that running a few psi above vehicle tyre placard specs will give a slightly firmer ride but improve tyre wear and overall vehicle stability.

I'm somewhat "lazy" in following the specs re different pressures front and back. I can't see how 2-4psi is going to make much difference and practice has confirmed an even tyre wear when running the same pressures all round.
Bill


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FollowupID: 403149

Reply By: Member - Barry B (WA) - Friday, Jan 20, 2006 at 10:33

Friday, Jan 20, 2006 at 10:33
Hi Mike,follow the tyre placard in the vehicle,the tyre manufactures should know.Mike L is spot on , only increase pressure to carry the load.As a new member,what a brilliant forum.Barry
AnswerID: 149810

Follow Up By: Shaker - Friday, Jan 20, 2006 at 11:04

Friday, Jan 20, 2006 at 11:04
Barry, ignore the tyre placard it is not the tyre manufacturers recommendations at all, it is the vehicle manufacturers & is for optimum ride comfort.
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FollowupID: 403103

Follow Up By: Member - Roachie (SA) - Friday, Jan 20, 2006 at 11:15

Friday, Jan 20, 2006 at 11:15
I agree with Shaker. I have heavy weight in my Patrol all the time.....even when it's "empty" it weighs just on 3T (I know.....too much chit !!!!! ;-))
I run my MTRs @ 42 all round and find I am getting even wear out of them thus far.
Also, as an aside, the higher pressures will provide less rolling resistence = better fuel economy; but harsher ride.
Cheers
Roachie
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FollowupID: 403104

Reply By: Deepat - Friday, Jan 20, 2006 at 11:32

Friday, Jan 20, 2006 at 11:32
Recommended tyre pressures are a compromised between comfort and safety. The lower the pressure the better the ride. Higher pressure gives a rougher ride but better control on bitumen.
When I did a driving course a couple of years ago the instructors got us to put 50psi into our tyres (standard road cars - not 4x4's) to reduce tyre roll on the skid pan and when cornering on the track. They recommended higher tyre pressures for safety at the expense of comfort.
Having said that, I find that if I run my AT Duellers at 40psi the centres start to wear, at around 35psi I get even wear, but if I'm on a lot of gravel at over 30psi I'm almost guaranteed a puncture from rock fragments.
So I guess the answer to your question is... find what works best for you and stick with it.
AnswerID: 149821

Reply By: Darian (SA) - Friday, Jan 20, 2006 at 11:54

Friday, Jan 20, 2006 at 11:54
In my case, 40 psi and over in the BFG's improves tyre life (only on the blacktop of course).... that's one reason why.
AnswerID: 149825

Reply By: desert - Friday, Jan 20, 2006 at 12:20

Friday, Jan 20, 2006 at 12:20
Every tyre and every vehicle is different. No two settings will be the same for every bloke. Tyre pressures should be experimented with for optimum wear signs, evenly across the tread surface. What annoys me though, is the fools who won't alter those pressures to suit the terrain and conditions. The fools who insist on maintaining 50 psi on the Canning stock route because the placard says so! Or, the idiots that will spit out rocks from under their wheels whilst trying to climb up Billy Goats track with 30 psi! Sure are some fools and armchair wankers in this industry, for sure
AnswerID: 149829

Reply By: Member - Barry B (WA) - Friday, Jan 20, 2006 at 13:39

Friday, Jan 20, 2006 at 13:39
Ignore the placard ? So if the placard says load rating 112s and your running 104s,how do you think you would get on with the insurance Co in the event of an accident.I can see this is a lively forum.Regards
AnswerID: 149839

Follow Up By: Wizard1 - Friday, Jan 20, 2006 at 14:31

Friday, Jan 20, 2006 at 14:31
I'm sure the insurance assessor is going to check the tyre pressures against the placard.....please...

I can't even believe someone even thought of that.
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Follow Up By: Member - Roachie (SA) - Friday, Jan 20, 2006 at 14:31

Friday, Jan 20, 2006 at 14:31
G'day barry,
Like most people, I've got my Patrol comprehensively insured and I've told the Ins Coy what I've got added to the vehicle. However, I am under no disallusions at all as regards my position in the event of an accident that I am deemed to have caused.....and I reckon many, many other blokes who frequent this forum should think likewise. What the hell am I talkin' about (I hear you ask)?
Well, the list (in my case) of things the Ins Co could "ping" me for goes something like this:
1. Wheels have 20mm more offset than standard
2. No internal mirror (replaced with LCD screen and rear camera)
3. Over weight (at around 3T before I load it up)
4. Non-standard headlights (I recently fitted HID inserts)
5. Impaired vision (I have a dashpod plus gauges on the dash in front of me)
6. New turbo charger (more power)
7. Non standard exhaust.....(dunno, but they'd soon find a reason to avoid a claim??)
8. 4" suspension lift (even though I've cleared this with them)
9. I've removed both anti-sway bars (for better articulation in rough terrain).

They're just the ones I can think of off the top of my head.

I keep the comp insurance up in case the things gets pinched or destroyed by fire etc.

Cheers

Roachie
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FollowupID: 403137

Follow Up By: conman - Friday, Jan 20, 2006 at 21:19

Friday, Jan 20, 2006 at 21:19
roachie,

off topic, but how do you find the HID inserts?

cheers.
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FollowupID: 403248

Follow Up By: Member - Roachie (SA) - Saturday, Jan 21, 2006 at 11:02

Saturday, Jan 21, 2006 at 11:02
G'day Conman,

Yeh mate, they're the duck's guts as far as I'm concerned. I still haven't been right out of town at night with them yet (only the 9klm between Kadina and Wallaroo the other night coming back from the beach).

I've stuck the Lightforce 240's up on the roof rack for the time being (as a bit of an experiment to see if the extra airflow into the radiator achieves anything on the heat side of things, as well as wanting to see what the light spread would be like).

The 240's are still "noticeable" and I will be wanting to keep them on, but at present thay are giving me a bit of glare through the windscreen. They are definitely YELLOW compared to the HIDs and I'm thinking of getting the blue covers for them as that is supposed to make them appear white/r than standard. They need adjusting as I've got them pointing too low at present and shining in the same spot as the HID's high beams.

Anyway, very happy with the HIDs

Cheers
Roachie
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FollowupID: 403326

Reply By: Wizard1 - Friday, Jan 20, 2006 at 15:02

Friday, Jan 20, 2006 at 15:02
I read an interesting post on another forum where the issue of tyre pressures was discussed.

In short there appears to be a guide called the Tyre Load Index that provides the PSI required for a given weight placed on a tyre in accordance with its load rating.

When do you follow the placard and when do you follow the tyre manufacturer or common sense.

I have 265/70/16 Cooper AT on my Prado which the maximum load is 1120 kg at 35 PSI. What that then means is you can carry more weight with more air.

1120 kg

So if the load on each tyre is 1280 kg then I need 40 PSI.

AnswerID: 149855

Reply By: Crackles - Friday, Jan 20, 2006 at 16:25

Friday, Jan 20, 2006 at 16:25
BFG have a measurement on their tyre specifications chart called the "Static loaded radius". This is the distance from the ground to the middle of the wheel. It doesn't matter if the tyre is fitted to a suzuki or an F250 the height stays the same. So for a big overloaded 4by heading up the desert yes the presures will need to be over 40 & possibly over 50 to maintain the optimum shape of the tyre to carry the weight. You may get less punctures at lower presures but that could be at the expense of road holding. Would you prefer a flat tyre or upside down on your roof? :-)
The key reason so many pump their tyres up above 45psi however is for longer life. Many brag on this site of 100 to 120 thousand KMs out of one set but what a rough ride it must be.
Cheers Craig.........
AnswerID: 149870

Follow Up By: Muzzgit (WA) - Saturday, Jan 21, 2006 at 02:28

Saturday, Jan 21, 2006 at 02:28
My understanding of the over-inflated tyre thing is mainly to gain better fuel consumption. This is only from knowing a couple of old timers who stick to it.
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FollowupID: 403290

Reply By: Kiwi Kia - Friday, Jan 20, 2006 at 17:46

Friday, Jan 20, 2006 at 17:46
There is an old 4x4 theory that says you select a lower pressure when on rough roads. How low? Well, you set your pressure drive for say 15 minutes and re-check the tyre pressure. If the pressure has NOT risen by over 4 psi then your pressure is ok. If the pressure HAS risen by more then 4 psi then the tyre is generating to much heat (flexing) so increase your tyre pressure and repeat the experiment. Note that this is how you sort out whether your tyre pressure to low for tyre safety and not for wear & tear, ride comfort etc.
AnswerID: 149884

Reply By: The Rambler( W.A.) - Saturday, Jan 21, 2006 at 00:18

Saturday, Jan 21, 2006 at 00:18
Why are there so many experts with differing opinions on tyre pressures----surely you can work it out for yourself and go with what works for you.I would probably be jumped on for saying that I never(apart from sand or mud) drop my tyre pressures below40p.s.i. amd have travelled thousands of kms all over Aus. on all types of outback roads with very few problems.But I am carrying 3.2t with split rims and tubed a.t. radials so the whole scene would be different if I were using tubeless on different rims.On the bitumen I run my tyres at 55psi all round.
AnswerID: 149936

Follow Up By: Member - 'Lucy' - Saturday, Jan 21, 2006 at 15:50

Saturday, Jan 21, 2006 at 15:50
Hoo! F***ing-ray, at long last some one with oodles of the old 'common dog'.

Congratulations Rambler

Its obvious that Sand Gropers in Troopy's have no trouble figuring out whats best for them and their vehicles.

I have had 4 sets of tyres on my Troopy that runs between 3.2t & 3.5t on trips.

(No need for anyone to point that I am over the GVM because a zillion posts ago I described in detail how I had it engineered up to 3.5t)

The second set of tyres were BFG's AT KO's which developed side wall cracks that were put down to a bad batch of tyres (KO's)

I am now now certain it was due to being sucked into the 'low pressure fetish' that seems to afflict many in the search for the 'perfect ride'. (apologies BFG for bagging you when it should have been the 'low pressure Ideologues'

Prior to the BFG's and the COOPERS & GENERALS afterwards, I start off at 36psi(F) & 46psi(R) and after about 50Kms at a 100KPH, stop, jump out and check the temp by placing a hand on the side wall.

If hot, add air, if not LEAVE THEM BE.

IF I am in mud, loose sand,hot dry powdery sand or large slow crawling rocky areas, then I'll deflate as required.

But as Rambler and some others have stated , you have to figure out whats best for you and your vehicle yourself.

The placard is a starting point only, because always keep in mind that the OEM has no idea what sort of tyres you have fitted, what you are driving on and most importantly how you have loaded your 'mother' of all 4WD's.

HEAT is the main destroyer of sidewalls and laminations, and is generally generated by under inflation whether by design or accidental.
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FollowupID: 403387

Reply By: P.G. (Tas) - Saturday, Jan 21, 2006 at 19:54

Saturday, Jan 21, 2006 at 19:54
G'day Mike, let's cut the hearsay and a mates, mates advice. The answer is simple. If replacing tyres, stick to the manufacturers plates rating or higher.

As far as tyre pressures go, check the side of the tyre for maximum load @ a given pressure, and if it's not there send an email to the distrubutor. They are only to willing to give you the maximum load pressure figures.

My GU III came with Bridgestone 693's standard. Nothing on the tyre, email Bridgestone Australia and was told the maximum load rating for that tyre was 36psi. DO NOT EXCEED that pressure under any circumstance!

I now have Cooper ST's and their maximum pressure at maximum load is 80 psi (on the side of the tyre).

Just a matter of home work, and find out how much weight is on the front axle, divide by then do the same for the back, and you can work it out from there.

Regardless of what figures you end up with, remember the correct pressure will be the 4psi rule!

Cheers,

Peter
AnswerID: 150045

Reply By: The Rambler( W.A.) - Sunday, Jan 22, 2006 at 20:02

Sunday, Jan 22, 2006 at 20:02
Thanks 'Lucy'
Its good to know that there's someone out there that agrees with what I consider common sense, as Ihave seen so many cases of expensive tyre failure due to people following other "experts" advice.
AnswerID: 150190

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