how slow is slow enough?

Submitted: Thursday, May 07, 2009 at 15:02

ThreadID: 68608 Views:3058 Replies:9 FollowUps:5

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jeff360

' reduce tire pressures, reduce speed', by how much?

My question came about because i am about to do an outback trip,birdsville tk ect, if tire pressures are down to 26 ish and road is good enough to do 100 ks, is the pressure to low to do that speed? are we talking about knocking off 5ks, 10 ks 20 ks,
i know there is alot of variables, tires, load, just want to get an idea so i dont 'melt' the tires for no reason. 4b is loaded landcruiser.

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Reply By: Member - Mick O (VIC) - Thursday, May 07, 2009 at 15:10

Thursday, May 07, 2009 at 15:10

Jeff,

I think common sense will dictate your speeds. Every vehicle is different and the circumstances dictating tyre pressure such as load, tyres, temperature and track condition will vary every time. 26 psi sounds a bit low for my liking on a solid track like the Birdsville, especially down the rear yet if it was extremely corrugated like the gunbarrel, I'd be even lower than that and travelling slow to help the shockers. As to speed, I've seen an awful lot of people get on these tracks and tear along them at 100 kph as if it were a sealed ride. That's just asking for trouble (having said that, I've travelled on sections of the Kalumburu and Gibb River Roads where it was like bitumen and 100 kph was an easy and safe speed.

You'll know your vehicles capability. In a loaded landcruiser or Patrol I'd be at 32-ish psi in the rear and 28-30 psi up front, travelling at max 80 kph or slower as road conditions dictate.

Cheers Mick

''We knew from the experience of well-known travelers that the
trip would doubtless be attended with much hardship.''
Richard Maurice - 1903

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AnswerID: 363694

Follow Up By: Willem - Thursday, May 07, 2009 at 17:09

Thursday, May 07, 2009 at 17:09

Hmmmm..Common Sense....have you seen any lately?....LOL

Cheers

FollowupID: 631390

Follow Up By: guzzi - Thursday, May 07, 2009 at 17:47

Thursday, May 07, 2009 at 17:47

Everytime I look in the mirror.
Its sadly lacking anywhere else though :)

FollowupID: 631393

Follow Up By: DIO - Thursday, May 07, 2009 at 18:52

Thursday, May 07, 2009 at 18:52

I thought that it was generally regarded that 4 wheel drive operators were over-endowed with consideration and heaps of common sense. William, are you seriously suggesting otherwise.

FollowupID: 631397

Reply By: Member - Kiwi Kia - Thursday, May 07, 2009 at 15:16

Thursday, May 07, 2009 at 15:16

Hi Jeff360, You need to reduce speed for both comfort and safety. As the sidewalls of the tyres flex heat is generated. If you travel at a reduced speed the heat can dissipate but if you travel to fast the heat build-up may such that you destroy the tyre. Also, if you should need to swerve with an under inflated tyre you could roll it off the rim. I have not had any problems with 30 psi and 80 kph. After lowering your tyre pressure stop occasionally and feel the sidewalls. You will soon get to know if you are over heating the tyres. :-)) So, the problem is the flexing of the sidewalls and not really just a number (pressure). Some brands of tyres with very strong sidewalls and not overloaded can be run at a very low pressure and not get very hot at all.

.

AnswerID: 363696

Reply By: Tenpounder - Thursday, May 07, 2009 at 15:19

Thursday, May 07, 2009 at 15:19

Hi there. Three or four years ago, I did the Birdsville at around 85-95 kph and 25psi with two punctures - one terminal (blow out) the other repairable. An old hand said 70 kph max at that pressure. The next year, I kept the speed down, and no punctures; last year, same low speed, same pressure, one minor puncture (repairable). I have similar feelings about the Oodnadatta track and lesser roads around that part of the world: gibber and other sharp stones don't mix with speed. And lowering the pressure MUST be accompanied by slowing down, in my view. I remember one guy who blew three tyres between Marree and Mt Dare, running 50 psi and cruising at 100kph
But my experience has been that I'd not try to mix 25psi and 100 kph on any road. And, if I was determined to stay on 100kph, then I'd be running 30-35psi min and have two spares, and also tyre pressuure monitoring as well. But that's just me. Others will say different things, I am sure.
Finally, I have found that LT tyres are better than HT tyres, and even Coopers say not to run HT tyres too low.
FWIW
Chris (SA)

AnswerID: 363697

Reply By: Rossc0 - Thursday, May 07, 2009 at 16:03

Thursday, May 07, 2009 at 16:03

We travelled from Adelaide to Whitsundays last year, via Birdsville and Winton.

Only one road in the 3500kms that we needed to reduce tyre pressure on between Monkira and Diamantina NP. Alluvial sand and down to 18psi and not more than 60kmh mostly 40kph.

Birdsville track 40psi (what I normally run) and generally 80kph. There were areas where we slowed more than this. Not a single puncture in the whole trip.

If it's good gravel road then it's unlikely you need to reduce pressures.

More important to drive to the conditions than picking a particular speed for the pressure.

Cheers
Ross

AnswerID: 363703

Reply By: Member - Tony & Julie (FNQ) - Thursday, May 07, 2009 at 16:08

Thursday, May 07, 2009 at 16:08

I left 36psi all round in my tyres last year, road was in good condition. Was able to sit on 100k and slow down for the occasional bad spot and the 1 car that came the other way. No tyre problems at all. Cheers Tony

AnswerID: 363705

Reply By: RobAck - Thursday, May 07, 2009 at 18:41

Thursday, May 07, 2009 at 18:41

The Birdsville track can be broken into three distinct sections. Heading South from Birdsville the first 75 or so kms is normally well graded dirt but this is never a constant like all outback tracks. Then until Mungarannie it is gibber with plenty of creek crossings that combine sand and rocks, nice. From Mungarannie to Maree is normally graded dirt and again the condition varies considerably.

We run a 120 Prado which weighs in at 2950 kgs and choose to operate tyres at 26 psi and run at 90 kph maximum. We have used Bridgestone D694 AT tyres with no problems but have now moved to the LT tyre as we are operating in these conditions almost constantly so want a stronger tyre for that reason alone.

They way you drive, how the vehicle is loaded, suspension setup and all other variables aside we have taken enough vehicles over all outback tracks to know that firstly there is no such thing as a puncture proof tyre, next the higher the pressures the greater the chance of a puncture and finally 26 psi and 90 kmh, no more and sometimes less, depending on track conditions works for every tyre type and every vehicle we have had on a tour

Regards

RobA

AnswerID: 363722

Reply By: Member - Smiley Bill - Thursday, May 07, 2009 at 19:21

Thursday, May 07, 2009 at 19:21

Hi jeff360,

I use the the following as a guide, then modify as necessary:

Dirt road in good condition, low to mid 30's in tyres and 80 kmh max,

Loose surface on dirt road (gravel etc) max. speed even lower,

Corrugations, washouts etc, maybe drop a few more psi"s (4-5) and if things are bouncing around the dash board i am travelling too fast.

Remember a loaded Landcruiser weighs well over 2 tonne and can't be pulled up or thrown around on dirt too easily.

If i ever tow a trailer i apply the same rules minus a few k's an hour.

Have a good trip.

SB

AnswerID: 363731

Reply By: troopyman - Thursday, May 07, 2009 at 22:02

Thursday, May 07, 2009 at 22:02

Wise man once say he who lets tyres down on outback roads is asking for stakes in side wall bulge .

AnswerID: 363768

Follow Up By: Tenpounder - Thursday, May 07, 2009 at 22:21

Thursday, May 07, 2009 at 22:21

Sure, troopyman, you get a lot of stakes on the Birdsville Track! (and how much bulge do you get with LT ATRs at 26psi?
That wise man says many things, often before he thinks.
Chris (SA).

FollowupID: 631435

Reply By: Bob of KAOS - Thursday, May 07, 2009 at 22:06

Thursday, May 07, 2009 at 22:06

In my experience, while it feels safe and comfortable travelling at 100 on dirt roads, the likelihood of damage is very much less if you drop speed to 80.

The extra travelling time is negligible, but savings in fuel, tyres, wear and tear, windscreens is significant.

Bob

AnswerID: 363770

Follow Up By: Bob of KAOS - Thursday, May 07, 2009 at 22:44

Thursday, May 07, 2009 at 22:44

I just found this in the archive and thought it made a lot of sense:
S&N replied:
what is the "pressure/speed/load equation"? is this an actual equasion or just experience?
Reply 8 of 10
FollowupID: 437522 Submitted: Friday, Jun 30, 2006 at 10:19
Robin posted:
Hi S&N

There is an equation, but as technical details of commercial tyres are kept out of the glossary ads it is effectivily empirical from our point of view.

I think its pretty well universally excepted now that on roads like Birdsville track which have lots of stones , there are two prinicpal factors that lead to tyre loss. One being fracure of tyre by hitting a stone to hard , and the second being heat build up.

Lower tyre pressure reducing fractures but increasing heat , and speed increasing likely hood of either failure.

So to go faster you need to increase pressure to avoid fractures but increasingly move into the likely hood of failure via heat build up.

The question being , what is best combination. Here I guess with lack of information it becomes experience based.

Starting point being a little lower pressure than normal for your load and dropping speed back to around 80 to compenstate for more heat from lower pressure.

Not doing both , increases the likehood of failure , if your car is overloaded/lack of suspension travel /or tyres not so good travel then you need to go lower/slower again than the rest of the mob.

In this trip the 60 series due to suspension and loading was likely to break first.

One tyre shredded first , then the others were pumped up a bit more then a second failed soon after.

Unfortunately I wasn't on the spot , and don't know if a judgement could be made as to failure mechanism

Robin Miller

FollowupID: 631440