my tyre pressure experiences

[prelude]

This last month I have had a nice vacation down to the mid south of france (auvergne) where I tackled some rough terrain and technical driving and I have been all the way up to the north of sweden mainly by avoiding bitumen.

This gave me several sorts of terrain, short of dessert and extreme mud holes to try and test the car, tyres and driver mind you :) Since reducing tyre pressure is something relatively new to me I have been experimenting with it for quite a bit and I thought I just share my experiences with you lot, possibly hear back and learn some more.

Firstly I had to figure out what my normal tyre pressures on bitumen should be. Since I changed the tyre size on my car from the standard 255/65R16 to 235/85R16 I was at a loss for quite some time as to how to get to the optimum tyre pressure. I did not just want to mimick the standard "40 psi" setting I see and hear a lot. Turns out, there is a way to calculate the optimum (minimum) tyre pressure which I used:



take the load rating of a tyre
take the maximum pressure of the tyre
take the weight of the car and preferably

take the weight per axle (since front and rear often are different)


divide that number by 2 (2 wheels per axle) if you know the weight per axle else divide by 4 if you only have vehicle weight
find the ratio between the max weight rating and the actual weight per tyre
apply that ratio to the max tyre pressure to find your tyre pressure.


ie I have a tyre that can handle 1380kg per tyre at 5.5 bar or 80 psi. My front axle is rated at 1380 kg and my rear at 1820 for a total of 3.2 tonnes. (yes this would make me 400kg over GVM). I used these figures as an absolute max, knowing full well that I am probably right up to GVM.

This would give me the following numbers:

1360/2 = 680 | 100/1380*680 = 59% so 5.5 / 100 * 49 = 2,7 bar or 39 psi (well wouldn't you know! nearly 40 psi)
1820/2 = 910 | 100/1380*910 = 66% so 5.5 / 100 * 66 = 3,6 bar or 43 psi (funny, that is really close to 40 but almost a full bar of difference... freedom units, they always catch me out!)

Since I do not weigh THAT much I settled on 3.2 in the rear for my trip.

End of the section that would possibly fit more in the technical section of this forum [bigwhistle]

On to my actual experiences. I reduced my tyre pressures to 20 in the front and 25 in the rear for most of the work I did. I did not drive hot dusty sand or sloppy mud but mostly muddy ground with a firm base, rocky ground with a single spot of rock crawling and gravel/dirt roads where the gravel could best described as 5cm rocks.

What I found is that on slow going, often even low range driving in the auvergne on all sorts of terrain these pressure performed well and I had no problems with the handling of the car when we had to do a very short stretch of bitumen between the offroad lanes (top speed 60kph for no more than a few minutes). What did seem to happen however is that due to the weaker side wall I had trouble with a particularly off camber rut that I wanted to straddle. The tyre wanted to form to the bank quite well and the increased footprint made for a lot of extra grip but due to the flex of the tyre, it was also pushed inwards eventually forcing the smooth sidewall (past the sidewall "biters") to end up on the dirt and I slid right in. It would seem a higher pressure would have worked better, contrary to popular believe.

In regards to the gravel and dirt tracks I have found that 25 front and 30 rear reduced corrugations discomfort noticeably but going further down to 20 front and 25 rear I was able to find the sweet spot of minimal discomfort and reasonable speed (70kph) but it had the added effect of making the car a lot more prone to wandering from rut to rut. I also chalk this down to the tyres themselves. I used to run wrangler MTR kevlars in the same size and they behaved quite a bit more than my current falken MT's. I did not feel the need to go down even further as a test but I did go (or start rather) higher with 30 front and 35 rear which was hardly an improvement in anything.

Thus far my limited experience with tyre pressures. It leaves me wondering if there is such a thing as the right tyre pressure :)

What are your experiences? Did I miss anything?

Cheers,
-P

PS mods, There is some techincal stuff in here but I felt that this was a campfire yarn to be spun, feel free to move it though.

[DiscoMick]

Interesting calculations.
What vehicle?
Our Defender on 235/85/16LT Hankook ATs runs 46 rear and 36 front on bitumen.
I lower them by about 30% on easy off road, but go down to low 20s on sand.
Strong sidewalls are important if pressures are being significantly lowered.

[350RRC]

.................
Strong sidewalls are important if pressures are being significantly lowered.

As is consideration for ambient temps speed and terrain.

If you run them too low on corrugations on a hot......tulip killing.........40C+ day in Oz you'll run the risk of catastrophic sidewall failure methinks.

DL

[Tote]

Heat is the killer of sidewalls if you are running lower pressures at more than 70-80 KM/H then you need to consider speed. Mightn't be an issue if you are driving back to camp after a trip in Tassie but not so good if you decide that 100KM/H on the Oodnadatta track is appropriate when you have dropped your pressure to low 20s.
On the 130 with 265/70/16 I generally drop to 24 front/26 rear on most off-road tracks and adjust from there if I need more traction. On the Disco 3 28psi was my go to pressure off-road running 265/60-18 tyres. It's pretty easy to check your sidewall temperature, if it's too hot to touch slow down or add more air.

Regards,
Tote

[JDNSW]

Tyre pressure is always a compromise, between ride, rolling resistance, ground pressure, handling, and tyre life. So there is no ideal pressure.

Of course, you could always go by the driver's manual. But even there, there are problems knowing what to do. I have in front of me, owner's manuals for Series 2a 1963 and 1970 and Series 3 1981. (these are essentially identical vehicles)

Tyre pressures for the same radial tyre, same size for a 109. Pressures in psi, in the order 1963, 1970, 1981

Front, normal use, load under 250kg - 20,25,25 Load over 250kg - 20, 25, 25 Emergency soft - 15, 15, 15

Rear, normal use, load under 250kg - 20, 25,25 Load over 250kg - 35, 35, 42 Emergency soft - 26, 35, 35

For my 1986 110, similar vehicle, same radial tyres, they have a more complicated schedule. (pressures psi front/rear)

Load up to 500kg up to 105kph 35/35 Up to 120kph 45/45 Over 120kph 50/50

Load 500-1000kg " 35/45 " 47/55 " 55/62

Load over 1,000kg " 40/50 " 50/60 " 60/65

And tyres may be reduced from these pressures for cross country operation by 15psi.

So for the same tyres on similar vehicles we have a recommended pressure range from 15 - 65psi depending on load, speed, conditions - and when the recommendation was made!

On my 2a, mostly used on unsealed road, but short on bitumen trips, I use 30/30psi or 30/40 loaded - but 35/35 for long highway trips, and as low as 12/12 when the paddock is very soft.

On the 110, I routinely use 35/35 unloaded, but 35/45 when fully loaded, increasing to 40/55 on long trips with temperatures over 40. And I have used as low as 15/15 when the road is very bad, especially deep dry sand, although it also helps a lot on soft mud with no bottom.

Low tyre pressures should be avoided where the ground is hard and there are sharp stones or sticks, to avoid sidewall damage, although low pressures do help adhesion on steep rocky surfaces.

[prelude]

Interesting calculations.
What vehicle?
Our Defender on 235/85/16LT Hankook ATs runs 46 rear and 36 front on bitumen.
I lower them by about 30% on easy off road, but go down to low 20s on sand.
Strong sidewalls are important if pressures are being significantly lowered.

It's a 1998 P38 range rover.

I agree on sidewall strength but that also determines tyre pressure in a significant way since very stiff sidewalls tend to bulge less and to get the same footprint length you need to air down more. In reducing tyre pressure I tend to do two things: look at the sidewall bulge visually, I have a MT with sidewall biters that offer good protection but I never run them so soft that they already make significant contact with the surface when stationary. I also stop after driving a while and feel the tyre temperature by hand, both sidewall and tread. If I can't touch it properly, it's too damn hot :) (and thus soft).

Regarding the different tyre pressures per year, that is an interesting little nugget :) I guess that is because tyres have evolved and thus the manual had to keep up? I have also noticed a trend like that with oil type and change interval on the rover V8 engine. That was more of an environmental and marketing though I guess since afaik there was never any change in the internal tolerances that it warranted thinner oil and a longer change interval.

Anyway, I digress... All in all I found the experiment to be worth while since it gave me a bit of a basis to go on when I am on my trip. I guess I ended up a bit too soft in the end and I should run a tad more. With temps in northern sweden already down to freezing at night and around 8c during a sunny day, my tyres stayed very cool.

Thanks for sharing!
Cheers,
-P

[RANDLOVER]

It's a 1998 P38 range rover.

I agree on sidewall strength but that also determines tyre pressure in a significant way since very stiff sidewalls tend to bulge less and to get the same footprint length you need to air down more. In reducing tyre pressure I tend to do two things: look at the sidewall bulge visually, I have a MT with sidewall biters that offer good protection but I never run them so soft that they already make significant contact with the surface when stationary. I also stop after driving a while and feel the tyre temperature by hand, both sidewall and tread. If I can't touch it properly, it's too damn hot :) (and thus soft)................
Cheers,
-P

Have you considered fitting a TPMS Tyre Press. Monitoring System as they can show temperature as well? They do work as I was on a dirt road one hot day, nearly 40 degrees and was looking at a map deciding where to go next with the engine idling, and a breeze blowing in the direction of one of the rear tyres and the TPMS alarmed at the 50 degree setting.

[prelude]

It's on my todo list :) It is a bit of work to figure out what system to use. I think the type that sits on top of the valves is a bit of a risk but the same goes for the internal type, when changing tyres you could well have them damaged... I will have to look into this.

-P

[gavinwibrow]

It's on my todo list :) It is a bit of work to figure out what system to use. I think the type that sits on top of the valves is a bit of a risk but the same goes for the internal type, when changing tyres you could well have them damaged... I will have to look into this.

-P

Like any number of others I have the external screw in type (Safety Dave version). Never had a problem towing or off road - but I do add a little piece of garden hose on each stem for added protection. Externals of course is easier to use if changing wheels.

[350RRC]

Have you considered fitting a TPMS Tyre Press. Monitoring System as they can show temperature as well? They do work as I was on a dirt road one hot day, nearly 40 degrees and was looking at a map deciding where to go next with the engine idling, and a breeze blowing in the direction of one of the rear tyres and the TPMS alarmed at the 50 degree setting.

Didn't know they could do temp as well...........got a link?

DL

[RANDLOVER]

Didn't know they could do temp as well...........got a link?

DL

Tyre Pressure Monitoring Systems (TPMS) | Tyre Pressure Monitors & Sensors (safetydave.com.au) (https://safetydave.com.au/product/tyre-pressure-monitoring-system/)

TYREDOG - Wireless Tyre Pressure & Temperature Monitoring Australia – TYREDOG TPMS Australia (https://www.tyredog.com.au/)

[jadatis]

As "pigheaded Dutch selfdeared Tirepressure specialist, I think I can add to the discusion here.

In opening post you come to 59%, but calculate pressure using 49%, a miswriting I think.

But the lineair calculation you use is not that bad, gives slightly higher pressures then the official european calculation from ETRTO, wich I got hold of end 2007, and went running with.
Now use my own determined formula ( better ofcource) closer to linear calculation.

Have an old list of Michelin with pressure/loadcapacity lists for different speeds on normal roads and for on track and in mud/sand.

Rule is about, so I figured out:
Mud/sand 50% of pressure for 20kmph( 12.5mph) on road, wich then is max speed to use.
On track,( must be that on gravel), 80% of pressure for 65kmph(35mph) on road.

TMPS sensors are not that good in giving temperature, especialy external sensors, because give pretty acurate the temperature on end of valve, and not of gascompound in the tire.
Beter then is to calculate it back from the pressure change.

For that I made a simple to use list.
Look in that the cold filled pressure at 70degrF( but also can make a list for Celcius and bar or psi), and look behind that for degrF/psi.
Then you can calculate by head acurate enaugh the temp in tire. Or reverse.
If you use it for cold filled at 65 degrF, it wont give dramatic differences.

70degrF./degrF/psi

20 psi/ 15,5F/psi
21 psi/ 15F/psi
22 psi/ 14,5F/psi
23 psi/ 14 F/psi
24 psi/ 13,5F/psi
25 psi/ 13,5F/psi
26 psi/ 13 F/psi
27 psi/ 12,5 F/psi
28 psi/ 12,5 F/psi
29 psi/ 12 F/psi
30 psi/ 12 F/psi
31 psi/ 11,5 F/psi
32 psi/ 11,5 F/psi
33 psi/ 11 F/psi
34 psi/ 11 F/psi
35 psi/ 10,5 F/psi
36 psi/ 10,5 F/psi
37 psi/ 10 F/psi
39 psi/ 10 F/ps
40 psi/ 9,5 F/psi
42 psi/ 9,5 F/psi
43 psi/ 9 F/psi
45 psi/ 9 F/psi
46 psi/ 8,5 F/psi
49 psi/ 8,5 F/psi
50 psi/ 8 F/psi
53 psi/ 8 F/psi
54 psi/ 7,5 F/psi
58 psi/ 7,5 F/psi
59 psi/ 7 F/psi
63 psi/ 7 F/psi
64 psi/ 6,5 F/psi
70 psi/ 6,5 F/psi
71 psi/ 6 F/psi
77 psi/ 6 F/psi
78 psi/ 5,5 F/psi
86 psi/ 5,5 F/psi
87 psi/ 5 F/psi
96 psi/ 5 F/psi
97 psi/ 4,5 F/psi
109 psi/ 4,5 F/psi
110 psi/ 4 F/psi
126 psi/ 4 F/psi
127 psi/ 3,5 F/psi
148 psi/ 3,5 F/psi
149 psi/ 3 F/psi
177 psi/ 3 F/psi

[jadatis]

Niw see this is australian forum so list for degrC needed. And read tyre where I wrote tire.

DegrC/psi/psi20C70F/degrF/psi
18,5C/psi/ 1 psi/ 33,5F/psi
17,5C/psi/ 2 psi/ 31,5F/psi
16,5C/psi/ 3 psi/ 30 F/psi
15,5C/psi/ 4 psi/ 28,5F/psi
15 C/psi/ 5 psi/ 27 F/psi
14 C/psi/ 6 psi/ 25,5F/psi
13,5C/psi/ 7 psi/ 24,5F/psi
13 C/psi/ 8 psi/ 23,5F/psi
12,5C/psi/ 9 psi/ 22,5F/psi
12 C/psi/ 10 psi/ 21,5F/psi
11,5C/psi/ 11 psi/ 20,5F/psi
11 C/psi/ 12 psi/ 20 F/psi
10,5C/psi/ 13 psi/ 19 F/psi
10 C/psi/ 14 psi/ 18,5F/psi
10 C/psi/ 15 psi/ 18 F/psi
9,5 C/psi/ 16 psi/ 17,5F/psi
9 C/psi/ 17 psi/ 16,5F/psi
9 C/psi/ 18 psi/ 16 F/psi
8,5 C/psi/ 19 psi/ 15,5F/psi
8,5 C/psi/ 20 psi/ 15,5F/psi
8 C/psi/ 21 psi/ 15 F/psi
8 C/psi/ 22 psi/ 14,5F/psi
8 C/psi/ 23 psi/ 14 F/psi
7,5 C/psi/ 24 psi/ 13,5F/psi
7,5 C/psi/ 25 psi/ 13,5F/psi
7 C/psi/ 26 psi/ 13 F/psi
7 C/psi/ 27 psi/ 12,5F/psi
7 C/psi/ 28 psi/ 12,5F/psi
6,5 C/psi/ 29 psi/ 12 F/psi
6,5 C/psi/ 30 psi/ 12 F/psi
6,5 C/psi/ 31 psi/ 11,5F/psi
6,5 C/psi/ 32 psi/ 11,5F/psi
6 C/psi/ 33 psi/ 11 F/psi
6 C/psi/ 34 psi/ 11 F/psi
6 C/psi/ 35 psi/ 10,5F/psi
6 C/psi/ 36 psi/ 10,5F/psi
5,5 C/psi/ 37 psi/ 10 F/psi
5,5 C/psi/ 38 psi/ 10 F/psi
5,5 C/psi/ 39 psi/ 10 F/psi
5,5 C/psi/ 40 psi/ 9,5 F/psi
5,5 C/psi/ 41 psi/ 9,5 F/psi
5 C/psi/ 42 psi/ 9,5 F/psi
5 C/psi/ 43 psi/ 9 F/psi
5 C/psi/ 44 psi/ 9 F/psi
5 C/psi/ 45 psi/ 9 F/psi
5 C/psi/ 46 psi/ 8,5 F/psi
5 C/psi/ 47 psi/ 8,5 F/psi
4,5 C/psi/ 48 psi/ 8,5 F/psi
4,5 C/psi/ 49 psi/ 8,5 F/psi
⁸4,5 C/psi/ 50 psi/ 8 F/psi
4,5 C/psi/ 53 psi/ 8 F/psi
4,5 C/psi/ 54 psi/ 7,5 F/psi
4 C/psi/ 55 psi/ 7,5 F/psi
4 C/psi/ 58 psi/ 7,5 F/psi
4 C/psi/ 59 psi/ 7 F/psi
4 C/psi/ 63 psi/ 7 F/psi
3,5 C/psi/ 64 psi/ 6,5 F/psi
3,5 C/psi/ 70 psi/ 6,5 F/psi
3,5 C/psi/ 71 psi/ 6 F/psi
3,5 C/psi/ 75 psi/ 6 F/psi
3 C/psi/ 76 psi/ 6 F/psi
3 C/psi/ 77 psi/ 6 F/psi
3 C/psi/ 78 psi/ 5,5 F/psi
3 C/psi/ 86 psi/ 5,5 F/psi
3 C/psi/ 87 psi/ 5 F/psi
3 C/psi/ 91 psi/ 5 F/psi
2,5 C/psi/ 92 psi/ 5 F/psi
2,5 C/psi/ 96 psi/ 5 F/psi
2,5 C/psi/ 97 psi/ 4,5 F/psi
2,5 C/psi/ 109 psi/ 4,5 F/psi
2,5 C/psi/ 110 psi/ 4 F/psi
2,5 C/psi/ 115 psi/ 4 F/psi
2 C/psi/ 116 psi/ 4 F/psi
2 C/psi/ 126 psi/ 4 F/psi
2 C/psi/ 127 psi/ 3,5 F/psi
2 C/psi/ 148 psi/ 3,5 F/psi
2 C/psi/ 149 psi/ 3 F/psi
2 C/psi/ 152 psi/ 3 F/psi
1,5 C/psi/ 153 psi/ 3 F/psi
1,5 C/psi/ 177 psi/ 3 F/psi
1,5 C/psi/ 178 psi/ 2,5 F/psi
1,5 C/psi/ 198 psi/ 2,5 F/psi

[jadatis]

Now see this is australian forum so list for degrC needed. And read tyre where I wrote tire. Or do you use Bar?

1,5 bar/ 11,5 C/0,1bar
1,6 bar/ 11 C/0,1bar
1,7 bar/ 11 C/0,1bar
1,8 bar/ 10,5 C/0,1bar
1,9 bar/ 10 C/0,1bar
2,0 bar/ 9,5 C/0,1bar
2,1 bar/ 9,5 C/0,1bar
2,2 bar/ 9 C/0,1bar
2,3 bar/ 9 C/0,1bar
2,4 bar/ 8,5 C/0,1bar
2,5 bar/ 8,5 C/0,1bar
2,6 bar/ 8 C/0,1bar
2,7 bar/ 8 C/0,1bar
2,8 bar/ 7,5 C/0,1bar
3,0 bar/ 7,5 C/0,1bar
3,1 bar/ 7 C/0,1bar
3,3 bar/ 7 C/0,1bar
3,4 bar/ 6,5 C/0,1bar
3,6 bar/ 6,5 C/0,1bar
3,7 bar/ 6 C/0,1bar
4,0 bar/ 6 C/0,1bar
4,1 bar/ 5,5 C/0,1bar
4,5 bar/ 5,5 C/0,1bar
4,6 bar/ 5 C/0,1bar
5,1 bar/ 5 C/0,1bar
5,2 bar/ 4,5 C/0,1bar
5,8 bar/ 4,5 C/0,1bar
5,9 bar/ 4 C/0,1bar
6,8 bar/ 4 C/0,1bar
6,9 bar/ 3,5 C/0,1bar
8,0 bar/ 3,5 C/0,1bar
8,1 bar/ 3 C/0,1bar
9,6 bar/ 3 C/0,1bar
9,7 bar/ 2,5 C/0,1bar
12,0 bar/ 2,5 C/0,1bar

[DiscoMick]

Psi here.

[PhilipA]

What does the tyre placard on the car door say? It seems nobody thinks that Land Rover know what is best for their own products.
something like 33 front and 38 rear on low profile tyres.
235 85 r16 tyres have far more internal air volume than the licorice stick low profile tyres so should probably have lower pressures than stock.

You are just punishing the car suspension and yourself by running excessive pressures and risk what is in this quote. Tyres are a part of the suspension.


I have not long owned my P38 and took it for MOT yesterday. Everything was fine apart from needing 2 rear tyres. They had previously had too much pressure in as the sides have no wear but the centres are illegal.

The wheels on it don't look like the originals, and the tyres are 255 / 55 / R19.

I have trawled the net trying to find the correct pressures, but everyone seems to be running them differently.

Currently have 27 in the fronts which are fine, and 37 in the rears. Before I go and put 2 new rear tyres on I would like to know what the recommended pressures are

I run pretty stock cold pressures even in 40C temperatures as the temps raise the internal pressure anyway. About 30 front and 42 rear and 45 rear towing in my D2 TD5 . Tyres are Bridgestone 245 70r16 AT.
Regards PhilipA

[Slunnie]

What does the tyre placard on the car door say? It seems nobody thinks that Land Rover know what is best for their own products.
something like 33 front and 38 rear on low profile tyres.
235 85 r16 tyres have far more internal air volume than the licorice stick low profile tyres so should probably have lower pressures than stock.

You are just punishing the car suspension and yourself by running excessive pressures and risk what is in this quote. Tyres are a part of the suspension.



I run pretty stock cold pressures even in 40C temperatures as the temps raise the internal pressure anyway. About 30 front and 42 rear and 45 rear towing in my D2 TD5 . Tyres are Bridgestone 245 70r16 AT.
Regards PhilipA

I think the problem, especially with tyre advice is that everyone has different tyres with different constructions and with different priorities with the tyres - and short of having OE tyres with no thought about tyres (like 99% of people) then the placard may not suit. You can see that in the post where factory tyre pressure recommendations varied over time, and I would assume that is because the tyre fitted varied.

Airing down offroad, my tyres all go down to different pressures. Simex on sand might be 10.5psi, GoodYear MTR 12 psi and Pirelli ATR 15psi. Measure the foot print length to get your pressure and when loaded it will probably be different from front to back.

On road my old Cooper ST needed to be run at a minimum of 35-36psi as the carcass was so unstable, but the STT could be run at 32. The GoodYear MTR needs to be run at 32 as over that the carcass stiffness gets too much but it is stable even at 28. The Pirelli ATR can be run anywhere from 30 - 40 and its fine (under that just understeers a bit, and this I think is why LR have low front pressures - to induce stability).

You can see the variation and thats why the factory pressures sometimes need to be varied from. I actually think the factory settings generally are set for max comfort with a stable rear end and leading to understeer when pushed with the OE tyre - but vary the carcass and the pressures will change.

On my Audi which has 21" rims and low profile tyres, the pressures are set to give even wear and to protect the shoulders, these tyres are really sensitive to pressure and get run above placarded tyre pressures otherwise they get chewed out, and at $500-600 tyre, I don't want them chewed out.

[vnx205]

Log in to Facebook (https://www.facebook.com/TheSprings4X4Park/photos/a.10151794900427063/10158458832037063/?__cft__)[0]=AZV8lCr2xB3iUBe46gMyroPHmDTz6IoKuzNPCuk_5EOONzlu0 VrqTwNfcj1bAdlc5_YTF1DK19CRXpyt5wNuy46JFCOFb3hWoHl 8Mgcxfo93orcESvvpVJ_E2--9Ds6_ZPrrYlI_jIfsFo_4Y2CMYouH_WTD27_D30DU2JW7jXyGQ DkW_FFRmKikzw8XIAeZtl1HR4pl44DFoG0vaPYCvWDx&__tn__=EH-R

This is one manufacturer's graphic representation of the effects of airing down. I don't know if it the result of actual tests or just calculations.