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

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.

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.

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

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.

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.

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...

Cheers,
Noel
exploreafrica.web-log.nl

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

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.

Tyre Pressures
 

:thumbup1: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.

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?

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

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

HZJ105 tire pressure
 

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.

Thank you all for the many responses, I’m amazed that so many have contributed.


The roads we have up here should perhaps be called dirt roads and not gravel roads, I don’t know if there is a difference between the two terms? But as Jean Visser writes the roads in northern Ghana are dusty red roads with plenty potholes, and also quite a bit of corrugation. I have never experienced a puncture here due to sharp stones…..but then again that might be because the grater rarely passes by. The one puncture I’ve had for the last year was due to a nail.

“If you are getting the mileage you want and not many punctures, who are we to say that you're doing anything wrong?” The tires have lasted 25.000 km now and need to be changed soon, I’m not sure if that’s sufficient, but it seems okay considering that they do so many Km’s on dirtroad. More important is that they are not worn unevenly on the sides or on the middle, so I guess that’s a sign that the pressure I’m running is ok.

“How heavily laden are you running?” – I usually don’t travel with significant load, besides the passengers. – Oh and the tires are tubeless!


Lastly I wasn’t clear enough in my first post - The argument between the Ghanaians and myself about high or low pressure was mainly about stability/safety on the road, and the fact that they believe high pressure will make the tires explode because of the heat (the last one I already ruled out).

As it looks for now, I think I’ll keep the pressure as the manual states, maybe experiment a bit with lowering it for comfort on the corrugation.
Once again thank you all for the many inputs, and thank you for a great forum.

Cheers
Thor

25 000km is not a lot! On decent off-road tyres you should be getting 2 to 3 times that at least...

So much of this is subjective! Speed, load, tyre construction, aspect ratio, road surface all play a part.

Ask the tyre manufacturer. they should know :)

Distance
 

I agree, we just finished going up and down Africa with one set of BFG Muds, 68 000 km's and a lot of abuse!