conquerorforum.com
Sign up Calendar Latest Topics Chat Donate
 
 
 


Reply
  Author   Comment  
glynnp

Member
Registered:
Posts: 34
Reply with quote  #1 
I am planning a trip across the Savannah Way from Cairns to Darwin in July. This route has lots of river crossings and 100's of kms of unsealed road and they are probably corrugated.

I am wondering what tyre pressures I should use in my Jeep Grand Cherokee 265/60/18 Pirelli ATR (Jeep recommend 33PSI) (Pirelli Max 44PSI)and the UEV490 XMT Mudder Buckshot 8 Ply 285/75/16 (max 65 PSI). A tyre guide from Cooper recommends 32-36 for fast/smooth gravel and 26-32 for slow/rough gravel with a tip that higher pressures will be required when carrying heavy loads. Considering both the 490 and the Jeep will be packed up with camping equipment, I will be carrying heavy loads.
0
filbee56

Moderator
Registered:
Posts: 569
Reply with quote  #2 
Hello glynnp,
Have a read of this article.
http://4x4online.com.au/tyres.html

and this handy chart: 
http://www.l8ter.com/wp-content/uploads/2015/08/tyre-pressure-pdf.jpg


Cheers, Filbee56 

__________________
2013 LC200 TTD Sahara - Ex-Demo 10/2014 UEV-490 Evolution with the lot.
0
SVX17

Avatar / Picture

Registered:
Posts: 394
Reply with quote  #3 
Running 18's puts you at a disadvantage as the side wall profile is short.

For any prolonged gravel / corrugated road I would be dropping to around 22-25psi.

With that said these pressures may drop the rim to close to the road so you may have to work out what pressure works for vehicle/trailer and weight distribution.

So maybe drop them until they have a reasonable bludge or ballon. The idea is to have the tyre pressure low enough so that if or when you run over sharp or pointed rock the true can mold over it rather than having such a high pressure the tyre is forced to impact with it causing a rupture of the internal belts and tyre failure.

Hope this helps.
0
andrewhegerty

Member
Registered:
Posts: 30
Reply with quote  #4 
I ran 22 psi for my birdsville track run last year. 5000kms no flats and no tyre damage. I was running bridgstone dualer at
__________________
2014 Nissan Patrol Ti-L with air suspension, long range tanks, dual wheel carriers and some electronic tweaking to make it awesome.
2015 Commander S with all the fruit.
0
William the Conqueror

Avatar / Picture

Star Member
Registered:
Posts: 872
Reply with quote  #5 
Hi Glynn

There is no hard and fast answer to your question. The answer is based on a combinations of factors. Tyre construction, size, speed, road conditions and weight of load etc.

I used to run Pirrelli ATR's in a 285/75/16 on my cruiser. Unfortunately, they are the only tyre I have ever had that suffered a catastrophic failure (blowout). This was due to the soft side walls and lowered pressures on the Great Central Road.

The problem you have is (as SVX17 has said), you're running 18's which have a smaller side profile. With the softer sidewalls, you don't want to run the pressures too low or you face damaging your lovely alloy rims on the first washout you hit.

Ignore the Coopers guide - these are recommendations for completely different tyres.

Because the sidewalls are softer than a LT rated tyre, you won't need to go as low To get the same effect

I would suggest a trial and error approach to getting the right pressure. Drop your pressure by 5 psi to start and look at the ballooning and footprint. Then go done by 2 psi. I would expect that you will not need to go any lower than 8 or 9 psi.

The UEV tyres are a LT construction with a higher side wall so you can go lower. I lower my LT trailer tyres to 40 psi to start and then go down in increments of 5 until I hit 30 and then in increments of 2 psi. Generally, unless on soft sand or very slow rock hopping do I go below 30 psi.

Just remember to stop every couple of hours for 20 minutes or so. Feel your tyres, drums and bearings for excess heat build up (or use a non contact thermometer like I do). It gives you a break and let's your shocks cool down and gives you a chance to pick up problems before they go bang....

Cheers

Anthony

__________________
Vive Les Peuerels
0
filbee56

Moderator
Registered:
Posts: 569
Reply with quote  #6 
Anthony, you said - I lower my LT trailer tyres to 40 psi to start and then go down in increments of 5 until I hit 30 and then in increments of 2 psi. Generally, unless on soft sand or very slow rock hopping do I go below 30 psi. 

My question is: what pressure do you run your UEV tyres at for on-road?

Cheers, Filbee56

__________________
2013 LC200 TTD Sahara - Ex-Demo 10/2014 UEV-490 Evolution with the lot.
0
William the Conqueror

Avatar / Picture

Star Member
Registered:
Posts: 872
Reply with quote  #7 
Depending on weight of load - up to 55 psi.

I use a simple method of calculation for setting my tyre pressures. I have posted on it before. This is also a well documented formula elsewhere.

There should be no more than 4 psi increase between cold and hot tyre pressures. I set the pressure I think is correct, then drive for aprox 1/2 to 1 hour (depending on the ambient temp) then measure the pressures again.

If there is MORE than 4 psi increase tyres are too soft - add air.

If there is LESS than 4 psi increase tyres are too hard - release air.

I found that when I did the 8 months around Australia, I was very heavy so the pressures came down to anywhere between 40 and 45 psi (bitumen running). If I am doing short trips (without the weight associated with a remote trip), I could go as high as 50 to 55 psi. Again, ambient temperature has an effect on this as well.

There are no hard and fast rules with tyre pressures - it is dependent on make of tyre, tyre compound, tyre type (LT or passenger), speed rating of the tyre, speed travelled, weight of load, corrigations etc. etc etc.

Soft tyres get hotter than hard tyres due to the friction caused by the movement of air as it is compressed in the tyre. This is why I (and many others) recommend stopping every couple of hours to check things out and let the vehicle recover.

As I said above, I have only had one catastrophic failure which was caused by a combination of lower pressures and excess heat build up caused by corrigations in a soft sided carcass. Entirely my fault - I didn't check temps, pressures or stop regularly because I was in a hurry to get to Perth. The perfect storm for tyre failure.

Cheers

Anthony

__________________
Vive Les Peuerels
0
SVX17

Avatar / Picture

Registered:
Posts: 394
Reply with quote  #8 
Yep......just what WTC said

0
comjero

Senior Member
Registered:
Posts: 271
Reply with quote  #9 
I found these on an SA forum, this was from a bearing failure and towing at 120km, just a we bit too fast. Notice there is only half the axle and back plate left. IMG_20130812_153413.jpg  IMG_20130812_144438.jpg 
0
SVX17

Avatar / Picture

Registered:
Posts: 394
Reply with quote  #10 
Wow, how long did he drag it for....

The backing plate would wear away pretty fast but to grind that stub axel like that wouldn't have just happened !
0
comjero

Senior Member
Registered:
Posts: 271
Reply with quote  #11 
The article says 20 seconds but I would say it was more like 20kms which was on tar by the look at the photos . It probably took him 20 sec to stop it once he noticed it if he was belting along at 120km.
Surprising that the damaged to the body work was minimal and they were up and running again not long after swapping it for the replacement hub.
http://www.4x4community.co.za/forum/showthread.php?t=159066
0
glynnp

Member
Registered:
Posts: 34
Reply with quote  #12 
Thanks for the advice Guys. I also read the 4PSI change rule in the Coopers guide.
0
Previous Topic | Next Topic
Print
Reply

Quick Navigation:

Easily create a Forum Website with Website Toolbox.