Guidelines for off-road <span class="highlight">tyre</span> <span class="highlight">pressures</span>?
Submitted: Tuesday, Dec 22, 2009 at 15:04
ThreadID:
74572
Views:
7656
Replies:
8
FollowUps:
22
This Thread has been Archived
Member - Timbo
There is much talk on this site of the need to reduce
tyre pressures for varying terrain, but the question remains: BY HOW MUCH?! (How long is a piece of string?! LOL). Are there any guidelines for how much to reduce
pressures for different terrain, tyres, vehicle weights, etc...?
Having read a
recent thread about the Leyland Brothers' crossing of Australia from the most western point to the most eastern point, I quickly ordered the DVD set (since it's a trip I would like to do sometime). In the second DVD (the 40 year commemoration trip in 2006), the group is shown letting air out of the tyres as they prepare to enter the
Simpson Desert. It appears they are measuring the length of the contact patch to determine the required pressure...
The process is not explained on the DVD but they poke this plastic stick under the back of the
tyre and move it as far forward as it can go, then poke it under the front of the
tyre and move it as far back as it can go - this leaves marks in the sand.
Image Could Not Be Found
The next step, they measure the distance (in mm) between the two closest marks in the sand. In this case, they measured 300mm and seemed to be looking for 350mm, so more air had to be let out.
Image Could Not Be Found
This might be fine for a 'rule of thumb' say, for soft sand, as it will take into account the different weight of the vehicles (ie. a lighter vehicle will have more air let out of the tyres to achieve the same contact length) but what about for different sized vehicles/wheels/tyres? Obviously a 350mm contact patch is going to have different results in a 1t Suzuki with 15" wheels compared to 3t Landcruiser with 17" wheels... What about different types of terrain? Good quality dirt roads like
Gibb River Rd, Tanami,
Birdsville Tk etc.. will need different
pressures to soft sand like
Simpson Desert or Fraser Island etc.. and different again for rocky/shale found in parts of the Great Dividing Range.
In summary: what guidelines do people use for determining how much to reduce
tyre pressures from "normal" on-road
pressures for various types of terrain and vehicle loading?
Reply By: Mandrake's Solar Power- Tuesday, Dec 22, 2009 at 15:14
Tuesday, Dec 22, 2009 at 15:14
Don't know about guidelines as such - But Jeep specifies 33 psi for 16" 235 70 tyres - I dropped
mine to 25 psi on the gravel / dirt /
Oodnadatta and drove accordingly - Never got to play in the sand box so didnt get down to the next level of 15 - 18 psi ..... :(
Rgds
Mandrake
AnswerID:
396066
Reply By: Willem - Tuesday, Dec 22, 2009 at 15:22
Tuesday, Dec 22, 2009 at 15:22
Timbo
Many others will read a lot more into this and come to scientific conclusions but my rule of thumb is to let the tyres down to what the chariot can handle with ease.
In other words, I have no fixed rule.... I paly it by feel. In sand I have been down to 10psi and once or twice below that depending on the predicament I have found myself in.
The variables are neverending :-) If the chariot ( I have ceased calling it a truck...lol) is running hard on corrugations at 25psi then I will drop the
pressures to 22psi or below. Once down there one has to watch the speed however and desist on driving over 80kmh.
Cheers
AnswerID:
396067
Follow Up By: Member - Allan B (QLD) - Tuesday, Dec 22, 2009 at 15:40
Tuesday, Dec 22, 2009 at 15:40
Totally agree with Willem. Suck it and see. Depends on too many variables for formulas.
However, as starting points, and within your manufacturers specs: 40 psi on bitumen, 30 on firm unsealed and 18 on soft sand. Then adjust to suit yourself.
FollowupID:
664771
Follow Up By: Hairs & Fysh (NSW) - Tuesday, Dec 22, 2009 at 16:30
Tuesday, Dec 22, 2009 at 16:30
Sounds good to me Allan.
I run about 42 on road, 30 on gravel & 18 in sand.
And your right Willem, speed plays a big part too.
FollowupID:
664777
Follow Up By: Member - Robert G (WA) - Tuesday, Dec 22, 2009 at 17:48
Tuesday, Dec 22, 2009 at 17:48
Agree with Willem. That is pretty much my approach too. many variables to the equation so its hard to set something in stone. I do try and use the 4psi rule on the highway, especially since I got a TPMS which makes it a lot easier to keep an eye on
pressures and temps.
FollowupID:
664794
Follow Up By: Wilk0 - Wednesday, Dec 23, 2009 at 08:55
Wednesday, Dec 23, 2009 at 08:55
Hi Timbo
Totally agree with Willem, I dont measure how much air I need in the tyres by a gauge (they are to inaccurate anyway). I go by the look( how much they bag).
The Front usually has more pressure then the front. I then
check with the gauge purely to ensure the back to are similar and the front 2 are similar.
I've had my
mine down to 8psi ( dont actually think it was that prob 10- 12psi bloody gauges always inaccurate).
Its a matter of getting used to how your vehicle handles in different enviroments.
Cheers Wilko
FollowupID:
664861
Follow Up By: Member - Timbo - Wednesday, Dec 23, 2009 at 16:52
Wednesday, Dec 23, 2009 at 16:52
Thanks Willem, Allan, Jon, Robert & Wilko. I understand there are a lot of variables, and I understand that you have to experiment a bit, I was just looking for some guidelines as to
where to start and whether there is a better way to measure rather than a pressure guage (eg. contact patch or similar).
And I note (with some amusement) that you've started to call your personal transportation unit/vehicle "the chariot" and have ceased calling it a "truck" but you'd still better be careful - you might just end up inciting a thread about the proper use of the word "chariot" :-)
Have a great Christmas/New Year, and thanks again for your replies.
FollowupID:
664917
Reply By: signman - Tuesday, Dec 22, 2009 at 15:45
Tuesday, Dec 22, 2009 at 15:45
It is preferable that you use a footprint as this deals with other variables such as
tyre construction and load. Typically your
tyre footprint length will be around 180 mm on a hard surface. On gravel roads increase this to about 220 to 240 mm, In sand extend it to 300 mm. In dunes you may need 350 mm. However ALWAYS slow down and keep steering movements to a minimum- do not swerve hard. If you reduce
tyre pressures by 20 percent (typical gravel road) then slow down 20% ie to 80 kph max. If you reduce by 50% (typical sand) then slow down 50% ie max 45 to 50 kph max.
(Admittedly-' lifted' from another site...)
AnswerID:
396068
Follow Up By: Member - Timbo - Wednesday, Dec 23, 2009 at 16:45
Wednesday, Dec 23, 2009 at 16:45
Thanks Signman, that's the sort of info I was after, just some guidelines as to
where to start. I take the quoted lengths would be for a full-size 4WD and perhaps would use shorter contact patches for a smaller vehicle (eg. Suzuki) or longer for a larger vehicle.
FollowupID:
664915
Reply By: Member -Pinko (NSW) - Tuesday, Dec 22, 2009 at 15:58
Tuesday, Dec 22, 2009 at 15:58
Jol Flemming's website approaches
tyre pressures in detail. The plan is to get all tyres on a flat surface and measure each footprint and decrease or incease so that all four tyres or six in the case of trailers, have equal footprints. All vehicles will have differing
pressures dependant on weight /load distribution. Like most I once used hit and miss judgements but after using Jol's guide it is like chalk and cheese.
AnswerID:
396069
Follow Up By: iamian - Tuesday, Dec 22, 2009 at 16:16
Tuesday, Dec 22, 2009 at 16:16
Hi Pinko, where do we find Jol Flemings website...sounds a worthwhile read.
Cheers
IanW (QLD)
FollowupID:
664774
Follow Up By: Moose - Tuesday, Dec 22, 2009 at 16:32
Tuesday, Dec 22, 2009 at 16:32
Hey Ian - ever hear of Google?
FollowupID:
664779
Follow Up By: iamian - Tuesday, Dec 22, 2009 at 17:08
Tuesday, Dec 22, 2009 at 17:08
Thanks Bryan
Ian
FollowupID:
664788
Follow Up By: Member - Allan B (QLD) - Tuesday, Dec 22, 2009 at 17:10
Tuesday, Dec 22, 2009 at 17:10
Jol Fleming himself says...."These (footprint) measurements can change from car to car and
tyre to
tyre, you might need to do your homework first."
So where is the point of footprint measurements in the first place?
The only real criteria is how YOUR vehicle handles on YOUR surface with YOUR tyres and YOUR driving style and ability.
What happens after you apply all the fancy measurement techniques and it fails to satisfy? You experiment with pressure, that's what!
FollowupID:
664789
Follow Up By: MEMBER - Darian (SA) - Tuesday, Dec 22, 2009 at 18:03
Tuesday, Dec 22, 2009 at 18:03
Well, Jol has done a hell of a lot of driving in the bush (he does accredited training too) , and as I recall, his main thrust is that drivers should pay more attention to the footprint of the
tyre than the sidewall bulge - once that factor is recognised, traction is sand can be arrived at much sooner. As others have said though, continuous judgement by the driver on load, speeds, terrain,
tyre temperatures and potential staking of sidewallls etc. is a given.
FollowupID:
664797
Follow Up By: Member - Timbo - Wednesday, Dec 23, 2009 at 17:01
Wednesday, Dec 23, 2009 at 17:01
Thanks guys, was just looking for some guidelines for where to begin my experimenting. What Darian said makes sense though, the footprint of the
tyre is going to have a bigger affect on traction etc.
I remember on my mountain bike the tyres had a minimum pressure and maximum. I used to use the minimum pressure for gravel etc. and the maximum for hard/sealed surfaces, and I'd use very low
pressures for the beach. With that in mind, it's strange when I think that I was 4WDing for 7-8 years after that before I ever reduced
tyre pressures from the figures on the placard, and even then only after it was recommended by other travellers.
FollowupID:
664918
Reply By: Member - Phil G (SA) - Tuesday, Dec 22, 2009 at 16:56
Tuesday, Dec 22, 2009 at 16:56
I like Adam Plate's mudmaps (
oodnadatta roadhouse). Here's his blurb on
tyre pressures:
http://www.pinkroadhouse.com.au/Pink/graphics/docimages/tyre-pressure-pdf.pdf
AnswerID:
396076
Follow Up By: Willem - Tuesday, Dec 22, 2009 at 18:11
Tuesday, Dec 22, 2009 at 18:11
I wondered when you would surface....LOL
Hope you and yours have a good Xmas and all the best for Twenty Ten
Cheers
FollowupID:
664798
Follow Up By: Member - Phil G (SA) - Tuesday, Dec 22, 2009 at 18:21
Tuesday, Dec 22, 2009 at 18:21
Hehe - a magnet to a
tyre thread.
Hope you and Judith have a good Xmas too, and plenty of travel in 2010!!
Cheers
phil
FollowupID:
664801
Follow Up By: Member - Timbo - Wednesday, Dec 23, 2009 at 17:04
Wednesday, Dec 23, 2009 at 17:04
Yes Phil, Adam produces a lot of useful information which is a good starting point, at least for typical conditions on the
Oodnadatta Track, and much of this advice has been the result of a few years of fixing tyres for travellers!
They are 'rule of thumb' to cover a wide range of vehicles, I was just looking for some guidelines to fine tune it a bit based on various vehicles, loads, terrain etc.
FollowupID:
664920
Reply By: Peter_n_Margaret - Tuesday, Dec 22, 2009 at 17:25
Tuesday, Dec 22, 2009 at 17:25
I never tell anyone what pressure to run their tyres at. You are on a hiding to nothing.
It is too variable to generalise.
Tyre types and sizes vary and loads are critical to the correct answer.
Speed causes heat to build up in flexing sidewalls. Almost any pressure is OK if you are gentle and don't pull the
tyre off the rim and you don't get it too hot.
If you get it too hot it will fail. Maybe not this week, but invisible damage will result and make the
tyre unsafe.
If you can get it, the
tyre manufacturer's data is better than any advise you can get from anyone else.
I follow Michelins data for my tyres EXACTLY.
Some will have seen this before.
The numbers are from Michelin for MY
tyre (I calculated the %s).
Cheers,
Peter
OKA196 Motorhome
AnswerID:
396080
Follow Up By: Olsen's 4WD Tours and Training - Wednesday, Dec 23, 2009 at 09:24
Wednesday, Dec 23, 2009 at 09:24
As a tour operator, there are too many variables to tell people what pressure to run. I've used the contact patch method always, as it takes care of all the variables. When we tell people about this in our 4wd training you can see their eyes widen as they discover the errors the've made in the past using pressure.
Now lets start on on road "
pressures" for various loads :-) then we can move on to trailer tyres :-)
FollowupID:
664866
Follow Up By: Member - Timbo - Wednesday, Dec 23, 2009 at 17:13
Wednesday, Dec 23, 2009 at 17:13
Hi Peter, yes I understand it's too variable to start rattling off numbers for PSI/kPa, that's why I was just trying to find some guidelines that may help. I agree that a lot of the reason for reducing
pressures (at least in sand) is to increase the contact area, but how much should it generally be increased by for different conditions ie. mud, gravel, hard sand, soft sand, etc. ie. X % for gravel, Y % for sand, Z % for soft sand...?
There are plenty of people who say for gravel roads, reduce by X PSI and for sand, reduce by Y PSI - I agree, this is not particularly helpful, and doesn't take into account the various loads,
tyre types, etc. that there may be across even a convoy of half a dozen vehicles. The question is, how do I know
where to start? Do I let a bit out for gravel and then I get a
puncture I assume I didn't let enough out? Do I let out more for sand and if I get stuck I assume I didn't let enough out?
FollowupID:
664923
Follow Up By: Olsen's 4WD Tours and Training - Wednesday, Dec 23, 2009 at 22:35
Wednesday, Dec 23, 2009 at 22:35
Timbo
This is the problem I confront with my convoys. I can't be sure they start with the right onroad pressure. This is the beauty of the contact patch method- I don;t need to know where they started. 220 to 240 mm long is good for gravel. 300 to 350 for loose sand, 400 mm = oh bugger, and be very careful.
You then measure pressure and slow down by the same percentage you deflated from your starting presuure.
Your on-road pressure should be based on max pressure and max load per single
tyre. If we al started at the correct onroad pressure our
tyre footprint would be withing a few cooee of 180mm. we would get this by
onroad pressure= (actual load per
tyre * max pressure permitted / max permitted load per
tyre) + 4 psi
Summary
onraod around 180mm patch, pressure as calculated above
gravel 220-240mm patch or around 20% deflation
sand 300 to 350 mm patch or 50% deflation.
If you still get a
puncture on gravel running 20% deflation or 220 to 240 mm contact patch this is just bad luck.
20% deflation gives about a 16 fold increase in
puncture resistance, it doesn't make your tyres bullet proof. Slowing down 20% adds about a fourfld increase in
puncture resistance.- all good odds but not unbeatable :-)
FollowupID:
664966
Reply By: CJ - Tuesday, Dec 22, 2009 at 21:37
Tuesday, Dec 22, 2009 at 21:37
I have seen the measurement on another show. It is NOT intended to calculate the right pressure for you, it was ONLY used as information to demonstrate the effect
CJ
AnswerID:
396127
Reply By: Dean - Wednesday, Dec 23, 2009 at 13:36
Wednesday, Dec 23, 2009 at 13:36
Does this mean we leave the gauge at
home and take a ruler instead ?
Dean
AnswerID:
396185
Follow Up By: Member - Timbo - Wednesday, Dec 23, 2009 at 17:06
Wednesday, Dec 23, 2009 at 17:06
It's beginning to sound like that!
Oh, pack your pressure gauge anyway, then you'll know what to re-inflate the tyres to when you get back to the black-top! :-)
FollowupID:
664922
Follow Up By: Olsen's 4WD Tours and Training - Wednesday, Dec 23, 2009 at 22:38
Wednesday, Dec 23, 2009 at 22:38
No but it means asking your mate what "pressure" he runs is pointless as he has a different load, different tyres etc etc.
FollowupID:
664968
Follow Up By: Peter_n_Margaret - Wednesday, Dec 23, 2009 at 22:39
Wednesday, Dec 23, 2009 at 22:39
A much more valuable devise than a ruler is an infra red temperature measuring gizmo to monitor side wall temperatures accurately.
Cheers,
Peter
OKA196 Motorhome
FollowupID:
664969