Tyre pressures - a different twist

Submitted: Thursday, Apr 21, 2005 at 21:59
ThreadID: 22271 Views:1999 Replies:6 FollowUps:8
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Hi all

Read a lot and had a fair bit of experience in experimenting with tyre pressures, but have a topic for debate.

At what temperature should we be taking the reading? I know that generically we take the temp. when tyres are cold, but what is cold?

I inflated both my front tyres to the exact same pressures the other day, and they were still exactly the same when I later tested them at a particular time. But at night when the ground is cold they measured 1.5 psi less than in the middle of the day before. The car had not been driven for hours prior.

And to add another twist, in the morning I got up and tested them, one of the tyres was in the shade and measured exactly 1 psi less than the other front one which was in slight sun.

So to make sure we get an accurate tyre pressure each tyre must be at the same temperature, this is almost impossible because the sun does not shine on each one at once.

And when we say we inflate our tyres to say 32 psi, that may well only equate to 30 psi if the temp. drops significantly overnight.

Not that I really care, but just thought I'd throw it up for discussion.
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Reply By: Richard Kovac - Thursday, Apr 21, 2005 at 22:24

Thursday, Apr 21, 2005 at 22:24
DJ
you check them at the same temp the MOB record the temp (in the shade) also you go for a mean temp and set them to it.
the 4 psi temp change will work what ever the ambient temp is, I would think?unless the energy waste is less than the ambient temp
I run my tyres at 40 psi front and 55 psi rear when cold (ambient temp in the shade of my car port) when traveling front will go up by around 4-6 psi the same for the rears when i hit the dirt I will lower them by 10 psi the next morning (cold or worm) they will by 10 psi lower than when I lowered them e.g. 35 psi front and 45 psi rears.
Regards Richard
AnswerID: 107734

Reply By: Knight Patrol - Thursday, Apr 21, 2005 at 22:32

Thursday, Apr 21, 2005 at 22:32
You really must get out more D-Jack!!!!!! Jokes aside you are right about the temperature variations as when you introduce outback weather conditions to the equation and then the necessity to infalte/deflate to suit changing conditions (usually in a hurry as everyone wants to get wherever you are heading) there can be considerable variations to what you see on the tyre guage. I always like to check where I am going with tyre pressures by simple having a good look at whichever tyre is on the flatest piece of ground and match that with what I am endeavouriung to acheive (deflate for soft stuff/ inflate for higher speed travel etc) then check the pressure and see if it makes sense. Nothing like being perfectly inaccurate.
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Follow Up By: Exploder - Thursday, Apr 21, 2005 at 22:38

Thursday, Apr 21, 2005 at 22:38
Hay all

Hear R my thought’s on the matter right or wrong what do you reckon

1/1.5 PSI as far as I am concerned is stuff all. The gage probably wouldn’t be accurate to that anyway.

As to taking a reading and setting pressure this is done when the tyres r cold as it says on the side of your tyre Max Load 990 at cold pressure 50 PSI. Now obviously after you have let the tyres down for 4WDing you will pump them up to a hot pressure and then put the rest in when the tyres cool down.

As for the air in the tyres getting hot enough to make a substantial deferent’s to the pressure reading it will take a bit of driving on a hot road or soft sand driving.

As for letting hot tyres down you could just go of you gage. Or at home when the tyres r cold let them down to different pressures say 35PSI, 25PSIand 15PSI and time it. Then when you let the tyres down in the bush you know how long it will take to reach the pressure you want.

Just my theory.
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Reply By: Lone Wolf - Friday, Apr 22, 2005 at 08:23

Friday, Apr 22, 2005 at 08:23
You reckon YOU'VE got it bad!!!

Whoa.... I live on a REALLY STEEP hill, and I am worried that my front tyres, because they are at a lower altitude, will be at a higher pressure than the rears. now, you come along, and change my day, telling me that when the sun comes up in the morning, and hits the left hand side of my Honda, I'm gonna have like..... 4 different tyre pressures......

...... I need to have a lie down........

Wolfie
AnswerID: 107774

Follow Up By: D-Jack - Friday, Apr 22, 2005 at 10:35

Friday, Apr 22, 2005 at 10:35
Don't believe me Wolfie? I would have though a technohead like yourself would appreciate the relationship between sun beating on black rubber, heat absorption, air expansion with heat heat a subsequent alteration of air pressure.

I hadn't thought of the altitude factor, I'm sure you're correct. The highter the atmospheric pressure, the lower the psi. Clever boy!

D-Jack
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Follow Up By: greydemon - Saturday, Apr 23, 2005 at 12:37

Saturday, Apr 23, 2005 at 12:37
A word of warning Wolfie - make sure that you always park the same way round. If you reverse into your parking spot then your front wheels will be higher and you could get yourself totally confused.

I suggest that you should get a really high lift jack and level the car before checking the tyres - which should only be done at midnight on the winter solstice when the tyre will be at their coldest.

8-)

Greydemon
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Reply By: Member - Jeff M (WA) - Friday, Apr 22, 2005 at 12:52

Friday, Apr 22, 2005 at 12:52
Just go for a 100km/hr blat with 10psi in the tyres, I'm sure they'll all be pretty close to the same tempeture then!! ;-)
(no don't do that, I'm just joking for all the people without a sense of humor).

1.5psi doesn't really mean crap, I wouldn't worry about it. If you are really losing sleep over it, take it for a drive and measure while hot. Sure the PSI may not be to spec, but I don't know anyone who actually runs 4wd tyres at 32psi anyway! At least if they are warm you'll know they are all roughly the same temp and they will be even. Being even is more important than the exact pressure IMHO. But personally as long as they are within 1 or 2 psi I don't care, that's good enough for me. I run mine at least at 42psi around town, I don't scrub out the sides of the tyres while corning around the burbs, use less fuel and get even wear.
AnswerID: 107805

Follow Up By: D-Jack - Friday, Apr 22, 2005 at 15:17

Friday, Apr 22, 2005 at 15:17
Fair call Jeff, but I am not worried about it I was merely bringing it up as a discussion topic. You obviously found it worthwhile to reply to so my mission was accomplished.

I hear what you way about 32psi, but every 4wd is different, suspension, type of tyres etc. I have experimented a lot with pressures and 32psi suits me just fine (that is 32 in the very cold of night, may be 34 during the day!)

However, if you don't have LT tyres then the maximum pressure is hugely decreased. The max for my Wrangler APs is 44 so there is no way driving around town at 42 would give me even wear. It would also be too skittish and breaking ability/wet weather handling would be much worse. I am going to a LT A/T tyre next, so that will be a different matter. I believe they need more pressure to bring them to the desired shape because of their extra robustness. Food for thought.

D-Jack
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Follow Up By: Member - Jeff M (WA) - Friday, Apr 22, 2005 at 15:40

Friday, Apr 22, 2005 at 15:40
Fair enough D-Jack, however in my "safe dirver courses" I have done over the years they have always said that higher tyre pressures were better in the wet as the tyre is shaped to cut through the water on the road and no aqua plain. Also it creates less of a shift in the rubber/sidewalls giving you better handling in general and less chance of spinning wheels etc.

I notice when my tyres are low I can chirp them taking off around a corner very easily but when pumped up it's almost impossible with the auto diesel to do that.

But as you say, everyone's configuration/needs are different.
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Follow Up By: D-Jack - Friday, Apr 22, 2005 at 18:38

Friday, Apr 22, 2005 at 18:38
Jeff- Yeh again I agree but in the context of this reply, and correct me if I'm wrong, but some tyres especially LT tyres require higher pressures to get the bottom of the tread completely flat to maintain maximum surface area connection with the road and to find a happy medium between comfort, shock absorption and road holding ability. Therefore when you are querying the pressures I use they are most probably comparable with the ones you use in a tyre that requires a higher pressure. It is actually an offence (In SA anyway) to drive with underinflated or overinflated tyres for the exact reasons we are talking about.

Anyway, have a great long weekend (that is if you are having a long weekend!).

D-Jack
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Follow Up By: Member - Jeff M (WA) - Friday, Apr 22, 2005 at 18:49

Friday, Apr 22, 2005 at 18:49
That's a bloody good idea, the amount of under inflated tyres I see on the road is scarey. Especially Excels and Barinas etc. They are almost rolling off the bloody rims some of them!!

I'm heading off for a little tour up north and around the east of Perth tomorrow, find some camp spots as we go, looking forward to it, should be good!! A virgin run for the new MTR's!

Hope you have a gooden too, D-Jack!
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Reply By: NedKelly - Saturday, Apr 23, 2005 at 17:26

Saturday, Apr 23, 2005 at 17:26
got one of these recently http://www.tyrealert.com/tyrealert.html and now I can see whats happening in real time you soon realise 5 or so psi above or below is nothing. There will be a temperature difference and even pressure difference even if the sun is shining on one side of the car over the other and once you start driving, especially offroad, the temp and preessure changes are quite large. 1psi is nothing. get one of these and youll never have to wonder again ;)
AnswerID: 107948

Reply By: Ken - Monday, Apr 25, 2005 at 18:03

Monday, Apr 25, 2005 at 18:03
I would be interested to know what sort of guage you used. The gauges at fuel stations and such aren't accurate and often will change by simply taking them off and putting them on again. You get a digital gauge (accurate but rely on batteries and don't like water but will allow for altitude changes), or I find a good quality pencil type gage with a steel shaft are usually accurate to about a pound or so. The dial type gages require regualar callibration and don't like being knocked around.
If you are on a trip and stop to check your tyes after a while and find that there is more then an 8psi increase in tyre pressurs then you are running you tyes to low and need to up the pressure a bit. A guy I used to work for did some tyre testing on a fully loaded falcon ute for a large company and found that the optimum pressure for the vehicle was 70psi. Raised a few eyebrows to start with but was accepted after a short trial.
The other gentlemen (i forget who it was) was correct the higher tyre pressures don't have to shift as much water as the footprint is smaller. Also you have more downforce as the same wieght of vehicle is pressing down on a smaller area.

Hope you found my dribble of some help and don't feel that I was telling you to suck eggs.
Cheers
AnswerID: 108098

Follow Up By: D-Jack - Tuesday, Apr 26, 2005 at 00:20

Tuesday, Apr 26, 2005 at 00:20
I use a cheap pencil type (about $10) from local auto store. Had it for years, works a treat. Is exactly in line with the commercial Digital compressor guages at BP - so happy with that.

I have to querie how many passenger car tyres would allow a 70psi inflation. Most 3 or 4 ply LT tyres won't allow that, are you sure you've got your figures correct?

As for the higher pressures in the wet, that is only a theory. Went through this with a guy from Tyrepower once, his theory was that decreasing the footprint gives you less surface area grip on the road (once you have overcome the water shifting factor) and decreases the shock-absoprtion factors that they tyres can give, making the tyre bump over small bumps and give opportunity for tyre traction loss. I'm not claiming to know the answer, just presenting the other side. Obviously underinflated tyres give less roadholding, but it would also make sense that overinflated tyres would work to the contrary. Surely, by your theory, skinny bicycle tyres would be best in the wet becasue more weight is being placed on a smaller footprint. I can't get my brain around that logic.

Anyway, thanks for your reply and food for though.

D-Jack
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