32 Cold All 'round
Submitted: Thursday, Jun 08, 2006 at 20:36
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Willem
I have been reading various posts lately relating to tyre pressures.
Why do most people have higher psi pressure in their rear tyres than in the front?
Is it because of a perceived weight factor?
Now big 4by wagons weigh in around 2800kg or thereabouts. Another 500kgs of stuff and humans is spread from the back through to the centre of the vehicle.
Has anybody weighed their vehicle on scales to see what the difference in weight is between the front half and the back half.....loaded?
Running less pressure on the front tyres must put an extra strain on the
suspension in normal bitumen and outback gravel road conditions. So why pump the back up to a higher psi level to make the vehicle bounce around more.
I run my tyres the same all around on cold pressures
32psi bitumen
28psi gravel roads
25psi bush tracks
20psi sandy bush tracks
15psi Simpson Tracks
13psi Off Road
Trailer on cold pressures
25spi bitumen
20psi gravel roads
10psi in the sand
It works for me
Reply By: Richard Kovac - Thursday, Jun 08, 2006 at 21:09
Thursday, Jun 08, 2006 at 21:09
Willem
As you may know we run 7.50r16 on splits
I did the 5 psi rule and run 40 psi in the front and 50 psi in the rear (could go higher in the rear as the rise is around 8-9 psi but left it at 50 psi)
weight of the troopy is 3.3 tonne at the moment (all way ready for a trip)
I dropped the rears to 30 psi in the driveway and they seamed flat????
Before the troopy we had a Prado and ran 35-40 all round bitumen and 29-30 psi on dirt and lower if needed.
Troopy I will drop 10 psi F & R for dirt and lower as needed
will be good to hear some answers
Richard
AnswerID:
177531
Follow Up By: Richard Kovac - Thursday, Jun 08, 2006 at 22:11
Thursday, Jun 08, 2006 at 22:11
Willem
after thinking about it, I also checked the height from the centre of the axle to the ground on both front and rear 40 psi front and 50 psi rear was around the same height.
Richard
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Follow Up By: Sand Man (SA) - Friday, Jun 09, 2006 at 12:05
Friday, Jun 09, 2006 at 12:05
5 psi Rule ?????
I am familiar with the 4 psi Rule.
I also ran the same tire pressures all around. Enough stuffing around as it is.
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Follow Up By: Richard Kovac - Friday, Jun 09, 2006 at 14:01
Friday, Jun 09, 2006 at 14:01
Sand Man
Sorry for being wrong
Richard
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Follow Up By: Sand Man (SA) - Friday, Jun 09, 2006 at 18:38
Friday, Jun 09, 2006 at 18:38
Richard,
Not criticizing you mate. But there may be other newbies referencing the
forum and come across reference to this rule. The 4 psi rule in commonly known. The 5 psi rule (same theory) perhaps not.
Personally, I think it is a guide only. Who knows, your 5 psi difference between cold pressures and hot pressures may even work better for you and your vehicle.
But, the "accepted" different is 4 psi. Anything over this and the tire is underinflated and getting too hot. Anything under this and the tire is overinflated and running too cold. The effect in both cases is abnormal tire wear and poorer performance.
Anyway Richard, have a good one.
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Follow Up By: Richard Kovac - Friday, Jun 09, 2006 at 20:50
Friday, Jun 09, 2006 at 20:50
That's cool Sand Man
Maybe the 1 psi is for the heat generated by the tube??? and 4 psi is for tubeless "LOL"
Richard
FollowupID:
433870
Reply By: Rick (S.A.) - Thursday, Jun 08, 2006 at 21:37
Thursday, Jun 08, 2006 at 21:37
Bitumen 100-110 kph, fully loaded (3.5 t) = 38 psi cold (tyre retailer rec'd 50 psi) - I'm thinking in this scenario about heat build up, so I keep plenty in there to make sure walls/tread not overheating.
Until you, Willem, applied the little
grey cells to the front /rear inflation bias, I had not considered it. Currently the truck has 34 in the front & 38 in the rear. Watch out, weighbridge, here I come.
Fast dirt - e.g. Oodna Track at 80 - 100kph, same load = 28psi cold
Dirt/rocky-ish but fair surface 60 - 80 kph, same load = 25 psi cold
Gibbers/station tracks e.g. up north - 15 to 40 kph, same load = 20 - 22 psi (now I have changed tyres, not getting sidewall punctures)
Sand, Bugger all load - e.g. at
Robe = 18 psi cold - if in trouble, straight down to 14 psi or so; always gets me out. Have seen tyre seals go at 12 psi or so, but it's always been a wrench of the steering wheel that pops a bead off - and always the front in my observations. Luckily I have not had this experience.
And yes, got a good compressor (Twin Blue tongue), as
well as an under bonnett mounted dinosaur, aka ARB.
Hope this helps,
Cheers
AnswerID:
177539
Reply By: Peter 2 - Thursday, Jun 08, 2006 at 21:49
Thursday, Jun 08, 2006 at 21:49
Willem
From my experience your pressures would be on the low side for a laden troopy running 7.50 x 16's on splits but with your tyres your pressures would be ok.
With bitumen pressures I've mostly stuck to the 4psi rule and it has served me
well for over 30 years of 4wd touring.
Inflate them to what you think they should be, then after an hours running at your preferred speed
check them, if they have increased pressure less then 4psi then you have too much air in them for the speed and load, if they have increased the 4psi odd you are pretty close to the right pressure and if they have increased more than 4psi then you do not have enough air in them for the given load and speed.
It works with all types of tyres and all types of vehicles.
Apparently it was 'invented' by Michelin.
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Follow Up By: Willem - Thursday, Jun 08, 2006 at 22:08
Thursday, Jun 08, 2006 at 22:08
Peter
I have found that the Coopers do not heat up more than 2-3psi
But then again, I tend to cruise at slower speeds
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Reply By: D-Jack - Friday, Jun 09, 2006 at 01:26
Friday, Jun 09, 2006 at 01:26
Me thinks that tyre wear contributes to people wanting to run higher pressures too. All tyre
places I have spoken to tell me that the vehicle manufacturers don't care about wear - just vehicle comfort, so they recommend the lower end so the drive is more comfortable. Me on the other hand find these pressures too low - the OEM tyres on the Jack would squeal around corners at recommended pressures when 30% tread.
As for the lower at the front debate, I think this is just a comfort thing too. I like to run my tyres at 38-40 on bitumen at the rear, but this in the front is pretty bumpy on the highway, so I lower it to 36ish. Works for me. Don't think I do it though because of a perceived idea that the front is lighter. HMMMM, food for thought.6
D-Jack
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Reply By: Frank_Troopy - Friday, Jun 09, 2006 at 08:22
Friday, Jun 09, 2006 at 08:22
The ratio of front pressure to back is determined by the way the car steers. If you have oversteer then the front should be lowered compared to the back. If the car understeers then the front needs to be increased relative to the back.
Oversteer is when the rear tends to hang out or break away around bends. Understeer is when the front tends to push forward and tuck under and the car wants to go straight rather than turn.
If you spend a lot of time in twisties your best pressures will be different than if you drive along straight roads. Also, if you drive faster you need higher pressures.
In order for my Troopy to beat the Ferraris and Porsches through
Sydney in the morning rush it is important that the pressures in my superb Hankook muddies are spot on. :-)
Cheers Frank.
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