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  #1  
Old 9 Jun 2009
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High or low pressure on gravel roads

Dear all

I am working in the northern part of Ghana, where we mostly drive on gravel roads. We are driving a Toyota Landcruiser (HZJ 105 L).

My problem is that I'm unsure what pressure to run in the tires. The tire size is 235/85 R16 (Bridgestone A/T) and the manual states 2,6bar (38psi) in front and 3,75bar (54psi) in the rear tyres.

The Ghanians keep telling me that the nature of their roads means you need to run low pressure, and I keep insisting that low pressure is for sand, and that on the hard gravel roads here you should follow the manual. However I am now starting to doubt whether I'm right.

Is there anyone out there who can help me?

Thank you very much in advance
Thor
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  #2  
Old 9 Jun 2009
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You're right. Normal convention is high (standard) pressure on gravel/rocky surfaces. This will reduce the damage of pinch punctures/rim damage.

You may need higher than standard if the vehicle is heavily loaded.
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  #3  
Old 9 Jun 2009
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Low Tyre Pressure?

As with most questions there are multiple permutations that affect the answer.

Lower tyre pressures can result in fewer punctures due to sharp stones/gravel. Unless of course you are either travelling so fast or are unobservant such that you hit a large rock hard enough to cause a compression cut/puncture or even wheel damage.

You can avoid side wall damage or compression cuts by driving slowly over large rocks with the centre of your tyre.

On gravel I run my LWB G Wagen (no light weight!) at around 20 to 22psi all round for exactly the reason to try and avoid punctures. Sand around 12-14psi. Seems to work for me.
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  #4  
Old 9 Jun 2009
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In "normal" use (e.g., 2 wheel drive car on gravel roads in North America), pinch flats and dented rims are not a concern. Under these circumstances, higher inflation pressures run more risk of flats due to sharp rocks puncturing tire treads. The risk is particularly acute immediately after road grading, which turns a lot of stones on edge, and wherever the local stone tends to fracture into flakes (for obvious reasons). Back in the dark ages before radial tires became the norm, the feedback loop was quite obvious: hard tires + recent grading = lots of flats/soft tires = few flats. These days, with radial tires everywhere, it's less obvious but still operative. Obviously, speed is also a factor.

This might be what your Ghanian friends are talking about. Or not. I don't really know what this "normal" state of affairs might imply about the specific roads you're driving or the tires you're using. I'm just offering a different set of date points for your consideration.

enjoy,

Mark

Edit to add: cross posted with RussG
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  #5  
Old 10 Jun 2009
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One more for soft tyres on gravel. Definitely less punctures IMHO
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  #6  
Old 10 Jun 2009
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Quote:
Originally Posted by brethouwer View Post
One more for soft tyres on gravel. Definitely less punctures IMHO
Also a more comfortable ride, the soft tyres will absorb a lot of the bumps.
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  #7  
Old 11 Jun 2009
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With the tyre pressures you are talking about, it seems like your vehicle is very heavily loaded!

I drive a 80 series LC and use 2.5 all round for tar, or 2.2 for gravel. I drop it to 1.8 for rocky gravel and 1.2 for sand with 25% more in the rear if I'm loaded.

However, if you are not getting punctures, and your tyres are lasting sufficiently long, then there seems to be no need for change.
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  #8  
Old 11 Jun 2009
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different approaches

I know there is a group of people which find soft tires on gravel / stoney pistes the way to go to prevent punctures. The tires are supposed to form better around the sharp stones.

I personally do not believe in this theory. I only reduce pressure in soft sand and heavily corrugated pistes (for confort). On stoney pistes I allways keep pressure high because the tire side walls do not like sharp stones and with decreased pressure, the side walls become volnerable. I have driven many km through the worst hamadat (stoney) deserts and rocky and rough gravel pistes and have never ever had a puncture due to stones! Only due to Acacia needles. On our way from Netherlands to Cape town we had zero punctures. On our way back, we had a few on our worn tires but again only due to needles.

My 2 cents...

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Noel
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  #9  
Old 11 Jun 2009
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Quote:
Originally Posted by noel di pietro View Post
I have driven many km through the worst hamadat (stoney) deserts and rocky and rough gravel pistes and have never ever had a puncture due to stones!
I don't really know how to reconcile this experience with my own, which is directly contrary. Probably the difference involves the fact that I'm using lesser tires on a 2 wheel drive van, cruising forest roads and Alaskan highways under construction. In any case, I've had a lot of flats due to sharp stones over the years. Sometimes these are clearly attributable to excessive speed (of course: with hundreds of miles to go it's difficult to poke along at a speed that works well for my tires), but lowering tire pressure on such roads really helps prevent flats.

So does this apply equally to the far more rugged tires you'd use on an overlanding 4x4? I don't know.

Is the concern about lower pressures limited to sidewall penetrations? I don't know about this, either, having never had a stone puncture a sidewall...although I am old enough to remember the way bias ply tires used to self-destruct if you scraped a curb even at very minimal speeds. But it strikes me that on a graded, graveled road (the original question), the danger of puncturing a sidewall is probably pretty minimal. In true offroad driving, sidewalls are far more vulnerable.

Continuing to mull and hash, cognitively speaking.

Mark
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  #10  
Old 11 Jun 2009
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You need to add into the equation the tyre's aspect ratio when reducing pressures.

Dropping a few psi on a 100% aspect ratio tyre does not greatly expose the side walls to puncture, drop it 15psi and the sidewalls are at real risk. Likewise with a low aspect tyre, reduced pressures expose the rim to damage from large stones/rocks if you hit them at speed or at an angle that can dislodge the bead.

I dont think there is a magic formula that can be applied to all tyres and vehicle combinations, my own belief is that following the manufacturers advice is usually the best in terms of longevity for the tyres/vehicle, usually they have tested the vehicle under fairly similar circumstances or follow field experience.
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  #11  
Old 12 Jun 2009
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Tyre Pressures

I drove for 5 yrs on the gravel and rock tracks of the Omani central deserts.
We always drove on normal road pressures or a little higher in the back if loaded.

Softening the tyre will increasr heat within and ,with the percenpion that you can drive reasonably fast on gravel,leads to blowouts which were common driving reduced pressures.

A couple of other tips;never exceed 80km/hr on gravel;reduce speed with gears in oncomming traffic and oncoming is large(trucks)pull off the road to aviod driving blind in his dust.Driving in dust on gravel roads is one of the biggest hazzards you will face and you cannot rely on others for good safety.
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  #12  
Old 12 Jun 2009
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The theory of decreased tyre pressure for rock works like this:

If you inflate a balloon to capacity, and touch it with a needle... or acacia thorn, it will burst. Now if you take the same balloon and inflate it to 50% of capacity, you'll find that you need to apply much more pleasure with the same needle or acacia thorn to make it burst. So yes, the tyre is supposed to "mould" over the rocks instead of failing because of sharp points.

The mad men who do rock crawling competitions in 4x4's drive on 1.2bar tyres. This is for more grip and less tyre failures, including side walls. Surely they must know something.

On the gravel road argument... I'm sure it depends on what kind of gravel. My experience is limited to Namibia and the Koakoland in Northern Namibia. I have traveled that extensively and where people will tell you to not even considder it with less than 2 spares and a repair kit, I have never had a single puncture there. That wasn't even with re-inforced side wall tyres or anything special. In fact, that was with standard Goodyear ATX el cheepo pick up tyres.

However, to get back to Thor's question, I'll say again: If you are getting the mileage you want and not many punctures, who are we to say that you're doing anything wrong?
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  #13  
Old 15 Jun 2009
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Most the time we ran our 110 at normal tyre pressures given we were heavily loaded including throughout west africa. However when we got to Ethiopia, we did subscribe to the lower tyre pressures as their roads seem to be sharp hard packed stone. The other 110 we were travelling with kept their tyre pressures up and the wear was significant compared with ours that lasted quite well. I'd suggest it works in more extreme conditions such as that but I don't think I'd drop my pressures in most places
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  #14  
Old 16 Jun 2009
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Hi Thor,

again each to there own with this...but its worth a try to see how the TLC feels on the gravel.

how heavily laden are you running ? take a few psi off at a time front and rear and see if it feels any better or potentially worse.

I run the 90 & Disco with slightly lower psi on gravel, for a better ride on corrogations with tubeless tyres.
I would not run tubed tyres for long distances on a hard surface with low psi as you get a lot of heat build up between the tube and tyre sidewall a it flexes.

However dont go too low, especially so if you are running the TLC heavy, or you will get lateral flexing in the sidewall (more so with a fairly tall 85 aspect ratio) when turning at speed, making the vehicle feel 'wallowy' and unstable and pushing those sidewalls out - find the best compromise.

Cheers
Gipper
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  #15  
Old 17 Jun 2009
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HZJ105 tire pressure

Quote:
Originally Posted by ThorBN View Post
I am working in the northern part of Ghana, where we mostly drive on gravel roads. We are driving a Toyota Landcruiser (HZJ 105 L).
There are lots of opinions on this but I also run a HZJ105 and found on gravel sufaces it runs at it's most comfortable at 1.8 bar, no heat build up and still good handling. North of Ghana has those dusty red roads filled with potholes, the softer tires will absorb most of the impact giving you a more comfortable ride.

Just don't go below 1.8 on the front if you are travelling high speed, the 105 likes understeering and being permanent 4wd it can become a handful in a corner.
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