Tire pressures in split rim wheels?

[gswest]

I've just got around to checking tire pressures in my HJ47 split rim wheeled tires. Wow! I had only 15-20 psi all around (I know,, should have checked when i got the truck). The 10 ply tires say 75 psi at max load. These wheels are for heavy loads and I'm assuming they are fine at 35 psi like regular wheels. Am I making an A#* out of myself "assuming" this? 75 psi is what we run is our Fire trucks, so I'm hoping that's for for heavy vehicles and not our 40 series trucks under normal conditions . Anybody know what's a safe/normal psi for these split rims?

 

[Kynot62]

No, you don't have to run max pressures. 75Psi is only if you load up the tires to their maximum weight rating which would probably exceed the GVWR of your HJ. 35 should be just fine. Sorry can't tell you what the minimum is.

Tony

 

[bigbrowndog]

Hj47 Owners manual says 7.50x16 spr lt below and above 80KPA [50MPH]
Front 34 psi Rear [below 80KPH [50MPH] 50psi above 80KPA 50MPH] 57 lb.
MHO 50 to 57 in the rear would be a rough ride I think.

 

[herbs]

34-36 psi all round, air up the rears to the mid 40s or even 50 if you're really loaded heavy. When off road, I knock them all down to a bit under 20, or even lower in sand.

 

[gswest]

Thanks guys!

 

[Dieseler]

I would not air them down to much or you may end up pinching the inner tube also being 10 ply they need sufficient air pressure so the side walls dont start to seperate or delaminate I would keep them at 50 psi for the road they will ride stiff but that is what you get with a heavy constructed tire

 

[DieselFumes]

I Run 35 cold in my HJ47 10 ply 16"splits...

 

[Flatdeck]

Um... I have always assumed that you should have even pressures front and rear ... since that is what it says on the original sticker I have on the dash I have always run 45psi front and rear on the advice of my local tyre shop, the owner of which competes in 4x4 comps ... fwiw

 

[lostmarbles]

35psi in them all. (BFG 235/85 R16)



Drop down to 15psi (& sometimes lower) is sand/mud. (Will get mudgrips again oneday soon!)

 

[bigbrowndog]

 

[lostmarbles]

Those charts are interesting Gazza.

But I can see no logic at all on why the rear tyres should be pumped up higher than the fronts. And I suspect this is because the charts are formulated on the basis of "these vehicles are trucks and as such should carry a payload in the back".

I believe the weight of my BJ40 (in normal use) is pretty evenly distributed between front and rear. (Certainly close enough to avoid wasting time/effort with using different pressures.)

And I think the tyre manufacturers also skew their recommendations (for tyre pressures) towards "full-load-carrying figures" with the aim of protecting themselves against warrenty claims.

My 35 psi figure comes from "ride feel", "visual appearance" (how much they bulge at the footprint), and "tyre wear pattern".

In fact 35 is such a great figure that it is also the pressure I run in my trailer tyres and in the wife's car tyres too ... even though they have vastly different sizing.

Using the same figure everywhere avoids confusion for me and reveals the fact that I think there is room for variance and that you don't have to be too fussy. (It isn't an exact science.)

But then I'm sure "driving style" plays a part. If you are the type who carries a massive amount of gear and "increases speed to smooth out corrugations" (rather than slowing down like me) ... then I think that'd be a different ballgame and higher pressures would probably be in order there.

And the "original-equipment-type cross-plys" certainly always need higher pressures than what I run. And even then (no matter how you treat them or what tread-pattern they have) - they'll always wear out at least twice as fast as a radial.



PS. I'm not about to lower my on-road pressures. Because to my way of thinking that would give me too much "side-wall-bulge" for on-road use and would increase my fuel consumption too. (And I think the extra bulge would hasten the deterioration/cracking of the sidewalls.)

 

[DieselFumes]

Nice Chart- I am sure they have their engineering reasons for the difference in front vs rear- I will try it out...

I too go by road feel, look etc, don't really do it with my HJ47 as I see no need or funny wear, but I will air the tires up to where I feel they need be and on a flat even surface, draw a 2-3" line across the tread with white chalk and roll the truck about 20 ft- you will be able to see what part of the tire is hitting the surface and adjust from there. Works GREAT on modified street cars (with a seat feel test also) and large tire trucks run on the street with different load ranges and sizes than factory. ie. a Jeep I had lightened up greatly and threw a high HP motor in with 38"x15.50 swampers- the chalk test put those tires around 24 psi street cold in order to wear the chalk off evenly and that was RIGHT on as I went through the centers of some other 38" tires I had on there at 34 psi... too heavy duty of a tire for the vehicle and it didn't need that much sidewall until I got in the rocks. I would also air those down to 12 psi offroad with no beadlocks and never had a prob.

 

[bigbrowndog]

Cheers, Those tables were scanned from the owners manual at the top of the picture.
My opinion only. The sand pressures are probably kept high to compensate from patches of hard to soft sand and back to hard or vica versa being transversed [big white knuckle:whoops: swervs LOL]. Also the problem when people air down on a beach then drive down the highway to the nearest servo to air up again [this happens here regularly and when the rollthey blame every one else rather than themselves] The cross ply on a split rim has a nice long footprint for when you drive in mud that the tire can brake through and for sharp rocky or scrub but the radial is better on most other conditions. Never seen snow so I can't comment on those conditions. Also the higher pressures tend to look after the sidewalls better by running cooler. I think we need to allow for the advance in tire manufacturing technology since the early 1960'S when the Owners Manual was printed. Do they still retread car tires in Kiwi Land or N. America? I can't remember last time I saw a car retread or light truck.
I also run 34psi in the front and back of my 80 with 31.5x15 Bridgestones.The 12.5x15 Buckshots on my Hj75 have a maximum pressure of 250kpa or 35lb at max load 1,000Kg so it makes one wonder. The difference in ride between the 80 and 75 is night and day with the same tire pressures of course. Just pick the lower pressure which is comfortable with out bagging out the tire or deforming tread contact as pointed out by Lostmarbels.

 

[bigbrowndog]

I would not air them down to much or you may end up pinching the inner tube also being 10 ply they need sufficient air pressure so the side walls dont start to seperate or delaminate I would keep them at 50 psi for the road they will ride stiff but that is what you get with a heavy constructed tire

Good advise, I do not believe a 10 ply was designed to be run at low pressure. This rigid design sidewall is purposely built for load carrying efficiency. They would be good sidewalls for the bush tracks.

 

[herbs]

Bias tyres on split rims go pretty good, only real problem I find is that it takes a few kilometers for the 'flat spot' to disappear after the vehicle has been sitting for a while. Only noticeable on bitumen.

 

[bigbrowndog]

One other point for us to kick around.
I am thinking a radial tire bead seems to grip better on the wheel rim than the other tire at lower pressure. Also if you spin a radial a bit on the rim at lower pressure it does not rip the valve out like a tubed tire. What do you blokes think?

 

[lostmarbles]

Good advise, I do not believe a 10 ply was designed to be run at low pressure. This rigid design sidewall is purposely built for load carrying efficiency. They would be good sidewalls for the bush tracks.

Agreed Bazza. (ie. Crossply/bias tyres weren't designed for low pressures ...but when run at low pressures in mud/sand/snow they perform better than radials .... And more plys tend to give them extra advantage here ..as well as resisting side-wall punctures better .)

The way I look at it is ....A stiff sidewall makes a tyre resist "deformation" which makes the "bulge" act more like a canoe paddle in mud/softsand.

But having said this, I think 10-plys are likely to be so stiff that they will heat up fast when run at low pressures. So I think they wouldn't be able to be run far/long like this before the rubber starts to deteriorate/separate.

Which of course is a long-winded way of saying exactly what you have said.

Bias tyres on split rims go pretty good, only real problem I find is that it takes a few kilometers for the 'flat spot' to disappear after the vehicle has been sitting for a while. Only noticeable on bitumen.

The flat-spots on bias/crossply tyres don't worry me as much as the shortened tyre-life (compared to radials) Herbs.

One other point for us to kick around.
I am thinking a radial tire bead seems to grip better on the wheel rim than the other tire at lower pressure. Also if you spin a radial a bit on the rim at lower pressure it does not rip the valve out like a tubed tire. What do you blokes think?

I thought this thread was confined to spit-rims Bazza (which of course must have tubes regardless of what type of tyre is in use).. In which case ANY tyre that loses its grip on the bead will tend to rip the valve out.

Best way to guard against this problem is to run rusty slit-rims. (Rust grips the rubber tyre beads really well!)

(Or run a B-diesel like me.... in which case you're struggling to find the power to make them lose grip )

 

[bigbrowndog]

Agreed Bazza. (ie. Crossply/bias tyres weren't designed for low pressures ...but when run at low pressures in mud/sand/snow they perform better than radials .... And more plys tend to give them extra advantage here ..as well as resisting side-wall punctures better .)

The way I look at it is ....A stiff sidewall makes a tyre resist "deformation" which makes the "bulge" act more like a canoe paddle in mud/softsand.

But having said this, I think 10-plys are likely to be so stiff that they will heat up fast when run at low pressures. So I think they wouldn't be able to be run far/long like this before the rubber starts to deteriorate/separate.

Which of course is a long-winded way of saying exactly what you have said.



The flat-spots on bias/crossply tyres don't worry me as much as the shortened tyre-life (compared to radials) Herbs.



I thought this thread was confined to spit-rims Bazza (which of course must have tubes regardless of what type of tyre is in use).. In which case ANY tyre that loses its grip on the bead will tend to rip the valve out.

Best way to guard against this problem is to run rusty slit-rims. (Rust grips the rubber tyre beads really well!)

(Or run a B-diesel like me.... in which case you're struggling to find the power to make them lose grip )

:
LOL Us old boys loose concentration quick, just widening the scope was what I meant to type.
My six splits with 95% rubber have been sitting under the Lychee Tree for quite a while now. Doesn that say something ?

 

[herbs]

But when you can get bias tyres hell cheap, they're worth the punt, especially when the radials don't seem to last that much longer - unless you're running 100% highway tread.

 

[bender3581]

Do they still retread car tires in Kiwi Land or N. America? I can't remember last time I saw a car retread or light truck.

They sure make them around these parts


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