The following excerpts are all taken from discussions on tyre pressures and
the 4psi rule on the 4WD Action Forum. If you are looking for info on the 4psi
rule or how to set your tyre pressures, then read on!
TYRE PRESSURES
Use the tyre placard pressures on your vehicle as a start point for normal
road driving pressures. To begin, set your pressures at the placard
recommendations one morning before heading off! (Maybe just on the way to work,
does it take 30 plus minutes?)
Drive for about 1 hour – 30 mins minimum, but try
not to go too much over 1 hour. Check the tyre pressures, if they have gone up
by LESS than 4psi, your starting pressure was TOO HIGH, drop 2 psi now, and
remember to use a 2psi lower start pressure tomorrow. If the tyre pressures are
still not going up by 4psi after the next hour of driving, you still shouldn’t
drop any more unless you change road surfaces etc,
otherwise you could end up damaging tyres. If they still don’t go up, you need
to start about 4psi lower tomorrow, and then see what happens.
If your tyre pressures go up by MORE THAN 4psi, your starting pressure was
TOO LOW! Add more air NOW. To work out how much to add, subtract the pressure
they should have ended up at from the end pressure you’ve just measured. Ie, they started at 20 psi & so should end up at 24 psi
if all is correct, but they are now 29psi, therefore 29 minus 24 leaves 5. So
add at least ½ of that 5 psi now; and use original start pressure plus your
final add amount as tomorrows start point, ie. original start = 20 psi plus the 2.5 psi added today, start
tomorrow on 22.5 psi.
You need to do that for every type of driving and road surface you drive on.
It will take some time, but the end result is that you will learn very quickly
to work out what pressure you should be running now, and what you should drop
it to when you change road surface/driving needs, ie.
4wd rock crawling versus 4wd high speed good condition dirt with odd sandy
patches!
Quote:
Originally Posted by Peter @ Aawen4×4
I use that 4psi rule everywhere, and for everything!! It works very well in ALL
circumstances and across all variations that you can think of and a whole heap
of others you never dreamed of! Use the tyre placard as a start point, drive
for an hour, stop, and check the pressure. If it’s gone up by MORE than 4psi,
your start pressure was too LOW, add pressure now! If it has gone up by LESS
than 4psi, then your start pressure was too HIGH and you need to drop 2psi NOW!
Those new pressures should become your start points for the next cold start!
And when you are off-roading, then use it the same way. Start with the
guidelines that come from places like the Cooper Tyre Book, the Mickey Thompson
guide to tyre pressure, even the prado4×4 guides above. Apply the 4psi rule,
and adjust your pressures as you go. You will take only a sort while to work
out what pressure is a good start for you, and by using the 4psi rule you can
adjust the pressures to what is the optimum for YOU and your driving style,
load, vehicle, conditions, temp, etc. AND remember that when you drop your tyre
pressure by say 20%, you should also drop your top speed by 20%!! Tyre
pressures lower, speed lower!
BTW, Cooper tyres tend to run a lower pressures than most people generally
think, probably because the extra layer of belting in the carcass reduces the
flex in the sidewall a little and reduces the induced temp increase as you
drive. Enjoy your driving, and don’t get too worried by it all, do what you can
without stressing, and live with it, it’s the best you can do at the time!
Shouldn’t let a little thing like tyre pressures take away too much from the
enjoyment of getting out there in your 4By!!
Quote:
Originally Posted by Peter @ Aawen4×4
All this tyre pressure stuff came (some years back) from a tyre guru who had
spent his working life in the back rooms of the tyre companies that spent mega
bucks supporting some of the race teams. He had a wealth of knowledge, and had
a career that included time with a couple of the large tyre companies, and some
of the big names in international racing, as well as successfully running one
of the more respected tyre development and testing departments in the world! I
was very impressed with his down to earth approach and knowledge, and he spent
some time explaining that despite all the computing power that had been bought
to bear on the subject of determining the ‘optimum’ tyre pressure for a given
tyre for ALL situations, they still hadn’t been able to get anything better
than the ‘4psi increase after an hours worth of
driving’ and it worked wherever they applied it!
Since then, I’ve seen it appear in lots of places, the latest being the
Cooper and Mickey Thompson 4WD Driver’s Guides, where they have a section on
tyre pressure, and they lay out the ‘4psi Rule’! They do suggest to use it only
for bitumen road use, and despite being told by ‘the man’ that it worked
anywhere, I too was a little slow to be convinced that it STILL works
fantastically for Off-road use, surfaces, and conditions; it’s just that the
pressures hafta be significantly lower to start with!
I suspect that is the reason for the rider in the Cooper and Mickey T
documents; either that or they are protecting their butt from some perceived
potential lawsuit. But it’s worked for me driving trucks & buses, cars,
4WD’s, and even tractors of various sorts, in all sorts of conditions, heat,
snow, rocks, sand, et al, so don’t be worried about applying it to whatever you
are doing with tyres, it’ll help get the pressures right for whatever you are
doing!
So try it yourself! If you seriously give it a proper try, and adjust your
pressures in the manner it suggests, you’ll get longer life from your tyres,
they’ll be less prone to puncturing than otherwise, and they’ll give you
traction, road manners, and ride characteristics that not only enhance the life
of the tyre, but make it easier on the vehicle and the occupants! Like they
say, use the tyre placard to get a good starting point, then fine tune it using
the 4psi rule from there. Use the guides suggested pressures for off-road
pressures as a start point too, and then do the fine tuning with the 4psi rule.
Again, it’ll enhance your driving in more ways than you expect.
One of the first things that I noticed when I started using this rule off
road was that when I went driving with others who weren’t aware of the 4psi
rule 2 things happened. 1 – they bagged me for
stopping and playing with tyre pressures; and 2 – they thought I was a much
better driver than the norm because I could go places that they couldn’t
without anywhere near the wheelspin or even at all!
The only thing that I could see that was different was that I was using the
4psi rule and they were generally not changing pressures much at all! Now that
was a long time ago, and it was when people generally worked on HIGH pressures
only for off roading, to allow the tyre to cut thru
the goop and get down to the firm stuff etc, and since then the whole ballgame has changed and
people are much more aware of using lower pressures to enhance traction, but
still, it was a telling point at the time.
What about all the variables that impact on optimum tyre pressures?
The thing to remember about the 4psi rule is that it is the ONLY method
readily available to all drivers that ACTUALLY takes into account ALL the
variables, cos it works off the driving you’ve just done in the last hour or so
– ambient temp, air temp, road temp, road surface, driving style, load, tyre
carcass, tread construction, etc, etc;
BUT it IS retrospective – it’s based on what you’ve just DONE – you hafta make a judgement call on what’s facing you in the
next hour, and adjust pressures accordingly!! (but the
4psi rule will let you fine tune that ‘judgement call’ into something quite
accurate fairly quickly ) So it is a system that you need to use pretty much
all the time until you get a good idea of what variables make how much
difference to your pressures, but I’ve noticed with all the people I’ve trained
over the years it hasn’t taken anyone all that long to get pretty good at
getting their judgement calls pretty close within a few weeks of starting
‘using’ the 4psi rule. So try it, vary your pressures accordingly, and modify
your start pressures accordingly, you’ll get better as you go along.
The ‘calculation’ for working out how much to vary your pressures when they
are out is just another ‘rule of thumb’, basically if you take 1/2 the
difference between what your tyre pressure SHOULD have been and what it
ACTUALLY is, and add/subtract accordingly, and you can keep doing that every
30mins/1 hour if you like, but it becomes a tiny adjustment eventually. The
suggestion mentioned earlier works too, you just need to be consistant
in what methodology you use rather than exact, same with your pressure gauge –
the most critical factor is consistency rather than accuracy.
Tyre Pressure Management Systems
Re the Tyre Dogs and their temperature/pressure variations, I’ve got a set
of them, and I’ve run them for a long time now. It has become very obvious to
me thru using them that the tyre temp reported by them is only very loosely
acquainted with what’s actually going on INSIDE the tyre, I believe it’s cos
they are externally mounted on the valves, and they reflect ambient & road
temp a lot more than internal tyre temp, AND they are very much effected by
external factors such as exposure to sunlight, wind, rain, snow etc rather than showing an accurate relationship between
the ‘real’ tyre temp and pressure. As such, I’m firmly convinced that while
their pressure advice is pretty good, and even usually reasonably consistant (altho not necessarily
all that accurate ) their temperature readings are not
something you want to rely on. And sometimes their pressure reporting can
reflect only what’s happening between the valve and the monitor, not what’s
going on inside the tyre, so if one Tyre Dog starts showing a higher or lower
reading that’s out of kilter with the others, it really IS a good time to stop
and look for a reason!! But don’t assume it’s necessarily always a puncture or
some other undesirable incident, it could be just that the rubber seal in the
base of the monitor has split or it’s jammed the valve stem in an odd position.
I also run a fairly sophisticated (and expensive) internally monitored TPMS,
and I’ve often run both systems on the same tyres over the same terrain – while
the Tyre Dogs have reported useful pressure data, often the temperature info
has been worse than useless – at best it’s confusing, at worst it’s downright
misleading, and sometimes you need to check the pressure readings against your
reliable tyre pressure gauge too. So don’t pay too much attention to the temp
info from any externally mounted TPMS.
BTW, digging thru the coding for the sophisticated TPMS, I came across an
‘increase factor’ that it uses to calculate the variation between what your current
pressure and the ‘correct’ pressure for that particular tyre is, and guess
what??!! Yep, it too uses the 4psi rule to determine the optimum pressure for
your tyres!!
Finally, (welllll, almost) your tyres (the rubber
bits mounted on the rims) need a certain VOLUME of air to hold up the weight of
your vehicle as you drive along. Skinny tyres that aren’t all that large in dia need higher pressures to get the same total volume of
air when compared with wider tyres or tyres with larger overall dia, so as you swap from the std
(& generally relatively narrow) tyres that were fitted by the manufacturer,
and fit wider/taller tyres to your vehicle, unless you’ve significantly
increased the weight of your vehicle, you are not all that likely to need
HIGHER pressures which would mean a greater volume of air – in fact, you are
more likely to need LOWER pressures to give you the same total volume and
therefore do the job. And that relates to why the footprint length system isn’t
quite as versatile as the 4psi rule – the optimum footprint length will vary
with the load, tyre construction, tyre size etc, so
it’s not quite as easy to cater for all those variables in determining exactly
what optimum is for YOU. The 4psi rule works on what the volume of air inside
the tyre is doing, so it DOES consider all those variables.
And now REALLY finally, some manufacturers have started recommending that
for 4WD’s we should be using a 6psi rule rather than the 4psi rule. No biggie,
a lot of the reasoning behind that is simply that 4WD tyres are generally of a
heavier construction than most passenger tyres, and MANY 4WDers have been
running cold start pressures that are WAAYY too high, so looking for a 6psi
increase means that you are starting with a pressure that’s a little lower, but
by using that rule you are still getting pretty close to the optimum pressure
for your tyres, and if you use the rule (either rule) as a matter of course,
you are spending more of your driving time at a better pressure than previously
– for almost all vehicles that weigh less than 3-5 tonnes. So if you are
running more than 40psi in your tyres, check the sidewalls for the MAXIMUM cold
pressure at full load – for most 15″ ‘flotation’ type tyres, the MAXIMUM
is about 35psi AT FULL LOAD (and unless you are off on a trip, you aren’t
likely to be fully loaded) and for most 16″ tyres with a sidewall listed
max of 55-65psi, MAXIMUM load is up around 2 tonnes per tyre or 8tonnes of
vehicle!! And the bigger the overall dia, then the
lower pressure you need to get the same total volume of air inside the tyre.
Simple really! Cheers
Quote:
Originally Posted by cac
thx for clearing that one up peter….when running low pressures like on sand
though, you really only want to drop your tyres so your footprint length gets
longer and not wider don’t you??
That’s pretty much what happens if you get the 4psi bit close to right, altho there will always be some width increase as the
footprint gets longer, simply because there is more sidewall flex possible (but
not necessarily ‘bagging’) As pressure goes down, the more recent
design/construction of 4WD tyres will (almost) always mean that you get greater
length instead of greater width, and tyres like Coopers, Mickey T’s, & even
BFG’s are made to optimise that feature.
But basically if you use the ‘footprint length’ method of determining
optimum traction/ride/wear characteristics, there is still no hard and fast SET
length for every (or even any) situation; the ‘optimum length’ varies depending
upon the same ‘whole heap’ of variables that applies to the 4psi increase, so
what works for YOU in your vehicle with your driving style and the load you
carry isn’t necessarily going to be correct for someone else in the same car on
the same track with the same load, while the 4psi rule is always going to allow
you (or anyone else using it) to be much closer to the ‘optimum’ because of its
‘reaction’ to all those variables, especially if you then continue to fine tune
your ability to get it right. Make sense??
Anything that gives you a set pressure (or length) for any given situation
won’t be taking the impact of all those variables into account, it will just be
giving you a (sometimes) reasonable start point that may well be seriously
wrong for today’s specific combination of variables. On the other hand, the
application of the 4psi rule still allows you to take the (same) reasonable
start point, and then take all those variables into account and fine tune the
pressure (and therefore the footprint length) to something much closer to the
optimum. And as you use it more, you get better at ‘estimating’ what impact
you’ll get from today’s conditions or what the track ahead looks like, etc, and you get better at picking what your theoretical
start point should be and what the resulting +4psi pressure should be.
That practice of taking into account the differences in variables each and
every time you drive, and the fine tuning you do by using the 4psi rule and
‘regular’ checking of tyre pressures will allow you to develop your skills in
deciding if you actually want to change (drop or increase) tyre pressures now
to suit the expected conditions, or leave them as they are
and vary one of the other ‘variables’ that you have control over, like speed or
cornering/braking etc. For instance, the fella who drives a loaded work vehicle
over a range of terrain every day might need to spend a little bit of time
initially in developing the skills to estimate what changes he needs to make so
that he’s not always getting out of the vehicle and changing pressures, but
once those skills are developed, he’ll be able to vary other things like speed
& cornering/braking techniques so that he can still get the best from his
tyres across a range of surfaces & under a range of conditions given the
pressures he’s currently running, and without adversely effecting his tyres. If
he doesn’t develop those skills, it’ll show up in the greater wear of his
vehicles’ tyres over those of other ‘more skilled’ drivers (greater wear as
well as chipping & cutting, losing tread blocks etc),
as well as showing up in the lesser overall traction obtained, the poorer ride,
& the greater stresses on the vehicle; and therefore the higher running
& maintenance costs for that vehicle. Just like a good race car or rally
driver develops skills that allow them to get the best from their vehicle and
actually finishing the race with a good placing rather than leading early but
destroying tyres thru trying too hard and eventually getting overtaken and
beaten, IF they manage to finish. It just takes longer to show when it’s a 4WD
that should be able to get 80,000km plus out of a particular brand of tyre
rather than a race or rally car that does its whole bit in less than 500km, but
the skills are no less as important.
Ambient Temperature & its impact on tyre pressures
Ambient temp certainly is one of those things that has an impact on your
tyre pressures. Basically, colder ambient temps means that ‘1 litre’ of air
will actually occupy less space than it will at higher temps, so while you got
away with x litres in Melbourne, you’ll need x+ some if you are further North
in the hotter country, ie you’ll generally need more
air inside your tyres to carry the same load when it’s hotter, and it’s the
volume of air that does the work. So you shouldn’t be just setting your
pressures in Brisbane and expecting the same pressures to work in the quite
different environment/ambient temps found in Vic, it’s a dynamic/pro-active
thing and it needs to be monitored and adjusted as the any of the variables
change, ambient temp being just one of them. BUT, by doing the ‘hard work’
early in the piece and paying attention to what’s going on around you and your
vehicle as you check and monitor your tyre pressures, it won’t take you long to
be able to make a good estimate EVERY MORNING as to what impact the ambient
temp of the day is likely to have on your ’standard’ bitumen road start
pressures, or your ’standard’ dirt road pressures, or even if you like, your
’standard’ wet weather fully loaded bitumen road pressures. So you adjust your
cold start pressures accordingly, then use the 4psi rule to fine tune those
settings during your day’s driving, as the tyres are subjected to all those
variables. Make sense??
And certainly your tyre pressure/temp monitoring system can and will help, altho I’ve previously mentioned the problems associated
with external (valve mounted) sensors, and how they are effected by external
air temps & conditions more than internal tyre air temps especially when
you compare the temps reported to those from internal sensors. Basically, I
wouldn’t be reading too much into the temps reported by externally mounted
sensors, altho any significant changes are certainly
worth looking at, even if just for your own peace of mind.
The pressure/temp variances you saw with changes in direction due to the sun
heating up one side of the vehicle are also quite common, but you don’t
necessarily hafta make adjustments immediately due to
that sort of ’slight’ variation – you need to allow the effect of the road
& load on the tyre to take precedence, hence the need to drive for an hour
so that those ’slight’ variations get swamped by the overriding effect of road
surface/load/driving, etc. I often park my vehicle such that one side gets
warmed (quite warm) during the early morning, but thru application and
monitoring of the 4psi rule I know that the 1-2 psi differences in pressure
between the sun warmed side and the shaded side will very quickly be ironed out
once I start driving, even just a few minutes at highway speeds. So it comes
back to using the ‘rule’ and getting to know how all those variables affect
pressure/temp etc. Actually do it for a while and it’ll fairly quickly start to
make sense, as you’ve already noticed.
And if you are running tyres that are significantly larger than those
designed for the vehicle, then you can run into the ‘problem’ of the tyres
never warming up enough to gain that magic 4psi increase, BUT, since others
mentioned the same effect to me, I’ve played around with big tyres on little
cars quite a bit, and the 4psi rule STILL WORKS, but there is a point of
diminishing returns, ie, your tyres can actually be
too big and hold too much air for the weight of your vehicle to actually heat
the tyres up to their optimum pressure/temp, so by default you are actually
getting less than optimum traction from them, ie not
exerting enough pressure on the ground to provide the optimum traction from
that tyre. But then if you are wearing tyres that big (or so heavily
constructed) under such a lightweight vehicle, then there are probably other
reasons that you’ve decided hold higher priority than the absolute optimum in
traction, or have you?? Still, when you run large tyres and get to that point
under a light vehicle, you can assume that “if the pressure doesn’t change then
the tyre pressure is correct for that sort of driving”, altho
I’d argue (and I’ve observed sufficiently to satisfy myself) that you can still
actually get better traction by using any one of a variety of methods to reduce
the volume of air carrying the vehicle such that the 4psi rule is satisfied, altho you might not want to add weight to the vehicle, fit
smaller tyres, fit larger diameter rims, or fit internal bead locks, et al.
Enjoy
Long Distance Travel
The 4psi rule pretty much applies everywhere, even on long distance travel;
BUT, just like your vehicle ‘glove-box’ manual will tell you, or maybe even the
tyre placard – long distance travel or fully loaded driving will need slightly
higher pressures to normal. So you still use the 4psi rule, but you need to set
your ‘cold start’ pressures in anticipation of the driving you are going to do.
So if your normal driving pressures work out at say, 26psi; then if you are
preparing for on a longer trip, a fully loaded trip, or today’s driving will be
on already hot bitumen, then you are likely to need higher start pressures –
say 28 or maybe even 30 psi. Just like when you are driving off the bitumen
onto a dirt road – you should really be looking at dropping your tyre
pressures, or at least slowing down so that you aren’t running pressures that
are too high for the surface/conditions (remembering that tyre pressure
responds to a range of variables, speed & track surfaces being just a
couple of them!) Make sense?
Stone guards
Re the flappy stone guard things right across the back of the underside of a
towing car, lots of modern vehicles use a ‘thermo-syphon enhanced cooling
system’, where the fan itself doesn’t actually pull enough air thru the
radiator to allow full cooling – the ’syphon’ system relies on the air flow
from the moving car adding to the total air flow thru the radiator, meaning you
can get away with less in the way of power sapping fans. And once the air flow
thru the front of the car is channelled thru the radiator, it then continues past
the engine, then down and out under the car, cooling other components as it
goes. That down and out bit is aided by the venturi effect created by the high
pressure area just in front of the firewall & under the bonnet actually
‘forcing’ the air down and out, and further enhanced by the air flow over the
top of the car and down the back creating a low pressure area at the back of
the car, so the air flow is basically sucked out from under – all aerodynamic
stuff that can get pretty hi tech; and not surprisingly, even a little change
under the car can make a big difference.
And like others, I’ve noticed that those large flaps across the back behind
the wheels can (and do) make a big difference to tyre temps (particularly the
rear tyres), and if the ambient/road temp is high enough, they can be
detrimental to other things as well. If you ever get a chance to play around
with temp monitors on other components, you’ll also see that those big flaps
play havoc with the temps in the diff & rear axles, and even the temp of
the fuel in a tank mounted behind or above the flap.
One vehicle we were running mainly on neat veg oil needed a pre-heater to
run properly until we set it up with a full width stone flap behind the rear
axle and in front of the fuel tank – saved the need for the pre-heater
completely but we couldn’t stop the shocks from ‘cooking’ and fading. When we
took the flap off, shocks lasted a helluva lot
longer, but we had to re-fit the pre-heater just to keep the engine running.
Had us puzzled for a while, until it dawned on us exactly what was going on
with the air flow and how much it actually cooled things down under the car.
So if you are already (or are thinking about) running larger than normal
‘mud’ or ’stone’ flaps under the car, just be aware of the potential for over heating problems, and not just necessarily the
engine!! But then again, not everyone works their vehicle hard enough or drives
where ambient/road temps make it an issue. F’rinstance,
I doubt that anyone who only ever drives in Sthn Vic would
even notice the difference, unless they could see it on remote temp sensors
(like those in most tyre monitoring systems).
Cheers