Tyre pressures

Submitted: Friday, Mar 05, 2021 at 00:02
ThreadID: 141194 Views:1530 Replies:14 FollowUps:18
Good morning gurus,

Your mission, if you choose to accept it, is to advise what your recommended tyre pressures would be on heavily corrugated gravel roads (think CSR, GRR, AB etc etc). The vehicle is loaded at 3200kg (1400kg front axle, 1800kg rear axle). The tyres are LT265/70R16.The pilot drives like Granma :)

Appreciate all replies.

Regards

p.s. I have searched through the Forum but would like some more recent info.
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Reply By: Gbc.. - Friday, Mar 05, 2021 at 06:20

Friday, Mar 05, 2021 at 06:20
I have those tyres. Towing a camper on gravel I set the front and the trailer tyres to 28 cold and lower the rears until they look similar because they always take the most load. I have found the expensive way that any higher and the tyres will chip out on sharp, recently graded rock sections etc. - assuming there are any. That’s what works for us, but stand by for many different answers.
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Follow Up By: Dave(NSW) - Friday, Mar 05, 2021 at 08:10

Friday, Mar 05, 2021 at 08:10
And don't forget, The lower the tyre pressure the lower the speed.
Cheers Dave
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Follow Up By: Gbc.. - Friday, Mar 05, 2021 at 08:13

Friday, Mar 05, 2021 at 08:13
Yes, for sure. On fast highway tracks like the strzelecki I wouldn’t go that low. First, it isn’t sharp rocks or corrugated (too much) and second, you can do highway speeds on it.
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Reply By: Gronk - Friday, Mar 05, 2021 at 08:07

Friday, Mar 05, 2021 at 08:07
Whatever pressures you run, you have to reduce speed. I never go over 70k/ph with anything less than 32 psi.
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Reply By: axle - Friday, Mar 05, 2021 at 08:24

Friday, Mar 05, 2021 at 08:24
Hi Gramps. The time this thread finishes your head will be spinning...lol.


Id be more worried about axle housing failure on those roads, with that weight. (depending on vehicle)


Cheers,
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Reply By: Kenell - Friday, Mar 05, 2021 at 08:32

Friday, Mar 05, 2021 at 08:32
Hey Gramps, I reckon it depends on the tyre. My BFGs were different to my current Mickey Ts. The latter bag out more and don't chip. It also depends on the severity of the corrugations. 28psi is about right but amazingly 2 psi either way can make a difference. Then you get the heat effect. In my experience it is suck it and see. Start at 30 psi then adjust down as required. The real pita is when the corrugations are severe then they aren't then they are etc.
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Reply By: Alan H11 - Friday, Mar 05, 2021 at 09:06

Friday, Mar 05, 2021 at 09:06
I guess we've ended up with 30 psig cold all round.
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Reply By: Peter_n_Margaret - Friday, Mar 05, 2021 at 10:06

Friday, Mar 05, 2021 at 10:06
Speed, load and temperature are all interrelated.
A little less pressure means a lot less speed or you risk destroying the tyres because they get too hot.
Luckily for me, Michelin tell me that relationship, so I don't have to guess. This chart is derived directly from the published Michelin technical data.

EDIT: The Blue line is highway speeds, the red line is 65kph, the yellow line is 20kph.
Cheers,
Peter
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Follow Up By: Member - McLaren3030 - Saturday, Mar 06, 2021 at 08:28

Saturday, Mar 06, 2021 at 08:28
Hi Peter,

It is a pity other tyre manufacturers don’t put out a similar chart.

Macca.
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Follow Up By: Peter_n_Margaret - Saturday, Mar 06, 2021 at 09:48

Saturday, Mar 06, 2021 at 09:48
Macca, I have no doubt that the major manufacturers have it, but getting it is the problem.
I got the Michelin data 15 years ago from the Michelin technical bloke for Australia who lived in SA. He retired years ago and was not replaced as far as I know.
There is some very enlightening data included including how you can carry DOUBLE the rated load.
Even the dealers do not have this stuff.
Cheers,
Peter
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Reply By: Stephen L (Clare) SA - Friday, Mar 05, 2021 at 10:30

Friday, Mar 05, 2021 at 10:30
That’s a fairly easy one to answer.

Good gravel road like the GRR between 26-28 psi but you must remember that you must not exceed 80kph.

The next easiest one is the likes of the Anne Beadell.........

18psi as speed will be impossible as most times you will be sitting between 20 to 40 kilometres per hour.

As with any heavy corrugations, make sure you stop regularly to let your shockers cool down and do not overheat.
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Follow Up By: RMD - Friday, Mar 05, 2021 at 11:16

Friday, Mar 05, 2021 at 11:16
Stephen,
How regularly do you mean for the stopping and cooling of shock absorbers? Do you have a temp sensor on them or actually feel them to find the reality of it all? If for instance the road causes high heat, in the already warmed shocks fairly quickly after resuming travel, but you only stop every half hour, aren't they running hot as anyway? If the shocks get hot but are able to dissipate heat, albeit at an elevated temp, they won't get any hotter than a specific temp. Yes of course cool is better. A laser infrared temp gun would be good to use to monitor such events, then you know stopping every 5 mins and waiting half for an hour is the go.
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Follow Up By: Stephen L (Clare) SA - Friday, Mar 05, 2021 at 12:51

Friday, Mar 05, 2021 at 12:51
Hi RMD

Depending on what there is to photograph, on say the Anne Beadell, we would stop around every 30 minutes, stretch you legs, take a few photographs and then continue on. Sure you might lose 15 minutes every stop, but why rush a trip like that when there is so much to see.

In that way you are not pushing your car, and never giving the chance for them to not over work overtime rather than continually working, hour after hour.

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Reply By: Member - rocco2010 - Friday, Mar 05, 2021 at 11:29

Friday, Mar 05, 2021 at 11:29
Funny, when I first started looking at this forum a decade and a half or so ago people were still debating the need the reduce tyre pressures on corrugations.

On the CSR in 2019 I was down to 18psi front and rear, but probably not carrying as much weight.

Worrying about tyres overheating from speed was not a factor.

On good gravel starting point (on advice from 4wd trainer and guide) is 10 to 15 percent lower than usual highway pressures. And watch speed.

As road turns to track just keep going lower.

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Reply By: RMD - Friday, Mar 05, 2021 at 13:18

Friday, Mar 05, 2021 at 13:18
Gramps
It all depends on a few factors. Some tyres heat faster than others due to compound of tyres. The tyres are the FIRST line of suspension and suitably inflated for the conditions, speed aside, greatly reduces the amount of repetitive axle movement. RIght tyre, right pressure = less axle movement and shock heat generated. Large wheels used to have MORE air under the rim, ie, high aspect ratio, which can be traded for ride comfort and still carry the load at lower speed. Often, the vehicles today have larger dia rims but less aspect ratio to keep overall dia much the same and so those wheels/tyres if in your situation will be unable to perform as well, can't be deflated as much, may run hotter and still give the shocks far more action than previous models of the same vehicle. Combine a little outback dried sticky mud to the shocks to make sure they overheat through limiting heat dissipation and the shocks will suffer and fail. Only by careful monitoring, via stopping and testing heat of suspension components with a hand held infrared gun can you achieve what speed, inflation for the load suits your rig. One of your rear shocks and or tyres may be copping heat from muffler because of proximity, especially at lower speeds and so that side might be heat elevated above opposite side. We can all offer advice but it may not hone in on what you need.
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Reply By: GG7777 - Friday, Mar 05, 2021 at 17:32

Friday, Mar 05, 2021 at 17:32
Gramps
I find the old 4 psi rule gives a pretty good guide
If the pressure rises by more than 4 psi from cold, your pressure was too low.
If your pressure rises by less than 4 psi from cold your pressure is too high
Give it a good half hour to hour before checking and adjust accordingly.
Science tells us this system does not work, but, has always been around the ballpark for me.
Murray
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Follow Up By: Stephen L (Clare) SA - Friday, Mar 05, 2021 at 19:00

Friday, Mar 05, 2021 at 19:00
Trust me Murray

Yes it is the case for when you are on the bitumen, but not on heavy corrugations or good dirt roads.
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Reply By: Allan B (Sunshine Coast) - Friday, Mar 05, 2021 at 18:04

Friday, Mar 05, 2021 at 18:04
.
I have seen a number or recommendations for using some instrument measuring device to monitor the temperature of shock absorbers and tyres but I have yet to see any guidance as to what temperature is acceptable. I would also imagine that any temperature limit would vary with manufacturer and model.
I have seen non-authoritative comment on temperature limitations but without substantiation.
Just complying with someone's expression of arbitrary temperature limit may not be adequate as reliable advice.

So, just where would one find such data to enable prudent monitoring. My brief search of shock absorber manufacture websites produced no result.

At an earlier time I tried to determine temperature limits for operational tyres without success. Manufactures may make expressions of the hazard of excessive temperature but seem unwilling to put a number on it. The makers of TPMS units seem to commonly set their high-temperature alarms at 80C although I do not know where they obtained their wisdom. In any case, their valve-mounted sensors are incapable of gauging anywhere near accurate tyre temperature.
Cheers
Allan

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Follow Up By: Member - rocco2010 - Friday, Mar 05, 2021 at 19:27

Friday, Mar 05, 2021 at 19:27
Allan

A lot of my remote travel in WA has been in a tagalong situation and for quite a few years the leader has used a temperature device to check tyres and shocks in rough country like the CSR and the Gunbarrell.

I don't think there is any exact science to it, too many variables. Like different vehicles, different shocks, age of shocks etc.

But at times I have heard him advise a driver his shocks are a bit hot and to drop his pressures a couple of PSI. I guess over the years he has learned what is too hot and what is not.

As Steven says above generally there are stops every little while for photos or to look at something or morning tea and lunch and that seems to be sufficient for things to cool down. Depending on how long we have been moving and how the track has been that is when he is likely to do a quick check.

The only time I recall he called a halt specifically to check and allow a cooling off was on the longish stretch of washboard corrugations north of Well 33 on the CSR which are as bad as any I have experienced.

I'm probably tempting fate but on the trips I have done with him there has not been a shock failure.

Cheers

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Follow Up By: Allan B (Sunshine Coast) - Friday, Mar 05, 2021 at 19:57

Friday, Mar 05, 2021 at 19:57
.
Thanks Rocco, I would agree that your tagalong leader is not applying any "exact science" to it. But he could well do so if it was available. I do not agree that there are "too many variables". Apart from direct mechanical assault, there is only one variable that damages a shock absorber and that is temperature which will cause failure, primarily to the synthetic components such as seals. From other applications I have seen seal upper limits of between 80C and 200C so would expect shocker manufacturer's to be able to advise the limits of their products.

Furthermore, the temperature of the seal is certain to be significantly higher than that of the shocker barrel so allowance needs to be made for this. So, if you are going to use an instrument to gauge the shocker temperature, what is the number you are looking for? There is little point in using a scientific instrument if it is not related to date from a scientific source.
Cheers
Allan

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Follow Up By: RMD - Friday, Mar 05, 2021 at 22:11

Friday, Mar 05, 2021 at 22:11
Allan.
I had a bit to do with a Holden Calais test vehicle and it had hundreds of TC temp sensors on it, absolutely everywhere you could think of including shocks and axle bearings.
A few years back on large trail bikes I and a couple of others travelled on highway before the Vic high country. One rider had a NEW knobby tyre on rear and a bead lock inside. That weight unbalanced caused the swing arm to oscillate wildy, I noticed this after a time and the shock was SO hot I could not touch it and NO damping of the single shock rear end. Used fishing sinkers to balance the wheel and it ran cool as after that and and it returned to being a damper again. The out of balance whack to road wore off around half of the knobs in one area in around 150km. So I see the suddenness of axle movement, ie high heat generated, caused by speed or too much tyre inflation to be the killers of shocks. Using a temp gun allows monitoring from cold and knowledge of varying pressures used and corrugation. Just touching it doesn't indicate how hot it is, so some reading to compare is good I reckon.
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Follow Up By: Allan B (Sunshine Coast) - Saturday, Mar 06, 2021 at 00:15

Saturday, Mar 06, 2021 at 00:15
.
So "how hot" is it? What is the "reading" to compare with what?
Without knowing what temperature is acceptable, where is the point in measuring?
Cheers
Allan

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Follow Up By: Member - McLaren3030 - Saturday, Mar 06, 2021 at 08:40

Saturday, Mar 06, 2021 at 08:40
A few years ago, a had a Kimberley Kamper, the shocks had a temperature chart stuck on each one that was temperature sensitive. It would change colour at various temperatures so that you could read what temperature the shocks were at. From memory, the temperature gradient went red when the temperature reached a certain level. Not sure if it was accurate, but the Kimberley Kamper manufacturer and dealer noted that you should monitor these “charts” on a regular basis when travelling on rough roads. It was also specified in the owners manual.

Macca.
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Follow Up By: Allan B (Sunshine Coast) - Saturday, Mar 06, 2021 at 09:02

Saturday, Mar 06, 2021 at 09:02
.
Yes Macca, such devices are used in industry (Sample here). They are reasonably accurate, within a degree or two, and equal to the average infrared pyrometer.
Some change colour to indicate the highest-reached temperature then recover on cooling so are re-useable but others remain at the highest colour reached so are an event device, not a continuing indicator.
Available also in crayons. Some equipment manufacturers fitted single-point non-recovery indicator labels to indicate if the product temperature had exceeded a prescribed limit.
They can be a bit difficult to discern the colours with certainty especially if dirty.
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Allan

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Follow Up By: RMD - Saturday, Mar 06, 2021 at 10:12

Saturday, Mar 06, 2021 at 10:12
Allan,
Not sure why the obsession with how hot and what to compare it to.
Using a infrared temp gun allows anyone who is interested to monitor the temps of components and it can then be compared to other readings the operator has recorded. If you read a normal road temp on shocks, then gravel road with bumps, then mild rough stuff the situation changes re, temps and so it builds an awareness for the operator. If, during a corrugated section and the temp are well above all previous readings you know you are taxing the shocks and vehicle more than before. Just because they haven't a manufacturers max or failed yet isn't much of an indicator. As the oil in shocks gets hotter they dampen less and so vehicle suffers more. With continued usage at a certain max temp you have measured and all is well for a long time then it sets an awareness level of what is ok. Just because you haven't yet reached a critical temp doesn't mean life of components is going to be good. Using awareness you can reduce speed of decrease tyres a little. It all has an effect.
No measurements at all means flying blind.
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Follow Up By: Allan B (Sunshine Coast) - Saturday, Mar 06, 2021 at 13:19

Saturday, Mar 06, 2021 at 13:19
.
RMD, with respect I think what you have said above is nonsense.
On the basis of what you propose, i.e. simply comparing some elevated temperature with some previous lower temperature, provides no quantitative information. You have numbers but no relativity to what is acceptable to the equipment's safe limitations.
You may just as well not have calibrated tyre pressure gauges and manufacturers nomination of proper pressure. Or even a speedometer with meaningful values. No odometer to apply to map distances?
Being able to compare your actual operating temperature with a manufacturer's advice allows you to make valid considerations relevant to the conditions and driving conduct.
Cheers
Allan

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Follow Up By: RMD - Sunday, Mar 07, 2021 at 13:08

Sunday, Mar 07, 2021 at 13:08
Allan.
My nonsense has served me well for a long time as has the awareness of component stresses. No failed shocks or bushes etc. Having to have a benchmark from a manufacturer must be very limiting. I see comments such as, stopping for over heated tyres or shocks to cool down. as not addressing the problems at all. If they have reached an overheated level then they have suffered already. That is what overheated means to me. If you overheat your engine, do you wait until it cools and then continue so you can overheat it again and again with cooling stops in between? The monitoring allows you to detect a plateau in temps so you know what conditions cause that level and can continue all without overheating them or forced cooling stops. Yes, I stop and take pictures too.
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Reply By: Member - David P (WA) - Friday, Mar 05, 2021 at 19:38

Friday, Mar 05, 2021 at 19:38
Hi Gramps,

We crossed the Anne Beadell Highway a couple of years ago in a convoy of 4 Subaru Foresters and had no problems. One of the things we did was recommended by Beadell Tours was to use your hand to check the temperature of the shock absorber, if it was too hot to touch - stop and let it cool down. At the same time check the temperature of the sidewalls of your tyres, again if they are too hot stop and let them cool down.
It’s a bit of a balancing act to get the tyre pressures right for the speed you are travelling. If the tyre pressure is too low the tyres will overheat and if too high the shockies will overheat.
Cheers
David

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Reply By: Batt's - Friday, Mar 05, 2021 at 20:53

Friday, Mar 05, 2021 at 20:53
Leave them pumped up and let the suspension do the work the way it should if your travelling at speed on gravel roads with the usual corners dips etc the tyres will still be flexing a lot so you don't want to damage them when cornering I've never had issues with tyres or suspension doing that in the past 30 odd yrs. If you plan on running at lower speeds on rough tracks then around 30psi might suit your vehicle still remembering to be cautious on corners as the tyre will roll a lot more. Avoid letting them down too much like some of those 4wd magazine or vides guys do they bit a bit carried away some times.
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Follow Up By: RMD - Friday, Mar 05, 2021 at 22:16

Friday, Mar 05, 2021 at 22:16
Outback stations I have worked on, the people said they use a 30/70 rule. Nothing ever over 30psi in tyres and never over 70kmh. Sometimes less of course. They really rely on tyres to get them around their huge properties.
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Reply By: Michael H9 - Friday, Mar 05, 2021 at 23:22

Friday, Mar 05, 2021 at 23:22
The best thing I've done lately is get a set of tyre pressure monitors. The last trip in the North Flinders it saved me a brand new tyre. It shows tyre pressure and temperature and for around $150 on ebay it's money well spent in my opinion.
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Follow Up By: Allan B (Sunshine Coast) - Sunday, Mar 07, 2021 at 10:10

Sunday, Mar 07, 2021 at 10:10
.
Great for pressure Michael. Just don't believe the temperature.
Cheers
Allan

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