Tire pressure for a hobby truck. Looking for advice.

[Vladislav]

The question - what minimum pressure can I put in the tires to not loose them off the rims?

The wheels are 11R24.5 tubless Alcoas. The truck is tandem R-model tractor.

It's far from a showroom condition now but I like to drive it from time to time. Going a good road it rides smooth and I feel nice in it. It's long enough, has Neway airride on Canadian spread and air suspended cab with air seat. Although when catching some washboard parts of the road wich I meet plenty of it shackes as hell and flips its hood if I don't slow down fast enough. Once I had to pass about 20km of dirt road and it took me no less than an hour to. I know some load could help but I'd like to keep it as a tractor with only 5th wheel on it.

Being not a truck driver I absolutely have no idea on how deep I can deflate the tires. Currently I ride both fronts and rears with nearly 80 pci / 5.5 bar as the previous owner recommended. The fronts look deformed under the weight but the rears show contact trace in the middle of the tire threads only, just don't touch the pavement with thread shoulders.

So maybe loose the nippels? Or at least deflate the inside ones? Any suggestions from guru's?

[Back In Black Pulling]

Been running mine around for couple years with 25psi in the rear on capped tires. But afraid to go much over 40 since i ran them for so long at low pressure.

but always keep front at full pressure

[41chevy]

I would dump some air pressure from the rears a few PSI at a time, until you get a full or almost full contact patch on the road. Do all 8 not just the insides. Low inflation on the inner 4 and normal on the outer 4 will cause you grief in the long run.   Paul

[kscarbel2]

Vlad, have you tried running 6 bar in the front, and 4 bar in the rear?

[Maddog13407]

I run about 80 in the front of my B and 40psi in the back recaps......

[Timmyb]

I think the same as a lot of guys here. Run the steers at 80 or 90 psi. The rears at 40 or 50 psi. I don't think you will have any trouble. Lots of guys here let the pressures down when hitting the dirt, doesn't hurt any. 

Please let us know if it makes a difference in the way it rides. 

[Vladislav]

Thank you for the suggestions gentlemen.

 I see I may drop pressure in rears well and it makes hopes to me. Forgot to mention I drive new tyres all around, not recaps. They are Chinese Aeolus though.

Why do you think it's one bad idea to deflate the inners deeper than outers? I would keep the outers stronger to not loose the wall off the rim in a steep turn. A common conditions would just let the outers run and the inners to revolve with about no load. 4 normally pressurized tyres (nearly 4 bar / 55pci) should carry the truck with no overload in them. 

One more idea is to differ the pressure in the airbags making the rear rear axle hold the chassis only. The FR would require another level valve and could be set for a couple inch higer. Meaning that you achieve something like too long wheelbase single axle truck. Sure no load is planned.

Edited May 10, 2016 by Vladislav

[Timmyb]

In my (limited) experience I have noticed that with unequal pressure from inner to outer tyres will come premature tyre wear. Albeit this was observed under working conditions. 

[41chevy]

How much air pressure is in the bags will effect your ride as much as tire pressure will. 

Low air pressure on an inner or outer tire will generate more heat and wear on those tires because they are not if full road contact and will have a tendency to "skip or skid" as opposed to a full contact roll. 

Start with lowering tire pressure  equally on all 8 drives. Don't over think the solution. I would do tire pressure first, than maybe the air bag pressure.

I would put a hand air pressure regulator valve in with a gauge to lower the air to the bags so that way lowering the pressure but still keeping the ride height. (Kind of like a lift axle set up)

 Paul

Edited May 10, 2016 by 41chevy

[Freightrain]

I run about 70# all the way around on my truck.  Seems to do fine.  The road are bad enough around here, but I don't travel long distance on dirt roads either.

[Retro Racer]

G'day Vlad,

On my short wheelbase F model with 44,000lb camel back suspension, I was advised to run between 30-45psi. I recently drove right across Australia 3,400 km's like this. Same on good roads it was fine, but as the roads became worse I let the tyres down in 10 psi steps every day. The best ride was at 30 psi cold, measured in the morning before driving. All 8 tyres on the drive are 11r22.5 Aeolus also. 30 psi on the rears made the tyres contact patch look similar to the fronts at 90psi.

The tyres did not get hot, even on warmer days of 25°C. 

I hope this helps.

All the best

Roland

Sydney Australia

[Vladislav]

21 minutes ago, Retro Racer said:

G'day Vlad,

On my short wheelbase F model with 44,000lb camel back suspension, I was advised to run between 30-45psi. I recently drove right across Australia 3,400 km's like this. Same on good roads it was fine, but as the roads became worse I let the tyres down in 10 psi steps every day. The best ride was at 30 psi cold, measured in the morning before driving. All 8 tyres on the drive are 11r22.5 Aeolus also. 30 psi on the rears made the tyres contact patch look similar to the fronts at 90psi.

The tyres did not get hot, even on warmer days of 25°C. 

I hope this helps.

All the best

Roland

Sydney Australia

G'day mate!

Quite an interesting point. Did you do 30 pci with load? Or with an empty trailor? Sounds impressive anyway. I think your camelback was too pleased initiating horse riding Thank you for sharing experience.

[Vladislav]

Ok, the gerneral line is clean.

Thinking on pressure "uneventy" I work out the idea of progressive suspension flexibility. The same way as multiple leaf spring works. When load (or road hit) is light generally main leaf deforms. And spring reacts smooth. When you inctease the load every next leaf goes into work. I just suppose if you deflate for example outers to the level they support the truck normally and then get inners to lower pressure it seems to me (in theory) that inners would just go with about no load. So no way to overheat them. As for uneven wear I agree it will became at some point. But what I'm planning is 2-3 thousand km a year. So I think brand new set of tyres will wear off for a decade.

Deflating airbags seems as a story of the same kind. I would deflate the central axle to about zero. But what bothers me is possible wobbling and jumping of whole the axle having no load on it excepting own weight. Shocks might calm it down though.

Anyway I'm going to start with 35-40 pci in rears all over (maybe 35 inners wityh 40 outers???) and see the difference. Too probably my highly complicated theories will get needless after some practicing.

[41chevy]

 Running an underinflated tire in a dual assembly shifts its share of the load to its mate, which then becomes overloaded,stressed and eventually fails.

In tests conducted by Bridgestone Americas a few years ago, it was found that tires with mismatched air pressures transferred the loads from the underinflated tires to the properly inflated tires.

When the pressure in the underinflated tire was reduced to 5 psi (.3 bar) from 95 psi (6.5bar), the good tire ended up carrying 8,300 pounds when its maximum allowable load was 5,070 pounds! 

Testing has shown that even a 5 psi difference creates a 5/16-inch( 8.3mm) difference in tire circumference in truck tire, an inflation mismatch of greater than 5 psi will result in two tires of a dual assembly being significantly different in circumference. of up to or beyond 1/2 inch (13mm)

Because they are bolted together, dual tires have to cover the same amount of road in a single revolution. So the large tire drags the smaller one. In a single mile, a 5/16-inch difference causes the smaller tire to be scuffed 13 feet (3.96 meters) per revolution.

Edited May 11, 2016 by 41chevy

[Mike]

Vlad,

Paul is on top of the subject. I'll put my money on what he said.

[fxfymn]

Not to hijack the thread, but this really points out why tire pressure monitoring systems are a worthwhile investment, especially for a trailer with duals. Being able to read the tire pressure with the press of a button makes it far easier to keep the proper pressure in all tires along with the warning the system gives you if a tire loses pressure.

[Tbear]

Very interesting read. I always new tire pressure was important ,but I didn't realize tire circumference was so greatly effected with so little difference in airpresser .thanks for the info.

[Brocky]

This is very interesting.. Did not know about the circumference difference also. This must be why racers pay such a close watch? to balance grip against speed?

[41chevy]

2 hours ago, Brocky said:

This is very interesting.. Did not know about the circumference difference also. This must be why racers pay such a close watch? to balance grip against speed?

Air pressure affects the tire contact patch and diameter. Heat from driving effect the tire diameter, contact patch (width) and wear. The slicks on my Mustang go from 11.5 wide and 27 inches tall warm at the end of my pass they are 9 1/2 to 10 wide and 30+ tall from the heat and air pressure . 

Run two same tires on with a 5 psi difference cold you have a 5/16 difference in height and a contact patch on the lower psi tire will the outer edge of the tread. When the tires run the heat generated causes the tire diameter to grow. The low psi tire diameter will grow 1/4 inch mostly from centrifugal force not heat. The tire with the higher psi will grow 1/4 to 3/8 inches. Final tire disparity will be 7/16 inches diameter between the high and low inflated tires and a temperature difference of 100 degrees F or  more.

[Vladislav]

Ok, now I totally surrender. 

The different circumference issue makes a lot of sence.

Paul, thanks for finding that Bridgestone test bulletin.

The only thing I haven't figure out so far is that radial tyres have their cord layed over circumference. So being spinned fast they don't deform along their thread area. Saying another words they don't became a bubble form as bias ones do. So getting greater tyre circumference means increase of breaker cord length. It seems strange to me. Although heat can make many things different. And I have no intend to doubt Bridgestone engeneer's skill.

Checked out the pressure I had today. Found out 4 bar / 60 pci in the rears all over. Dropped them down to 3 bar/45 pci so far. Going to make a road test. Will do the other day. Currently I have my polished outer wheels removed for winter storage. Hope to put them on in a couple of days to get on the road. 

[41chevy]

1 hour ago, Vladislav said:

Ok, now I totally surrender. 

The different circumference issue makes a lot of sence.

Paul, thanks for finding that Bridgestone test bulletin.

The only thing I haven't figure out so far is that radial tyres have their cord layed over circumference. So being spinned fast they don't deform along their thread area. Saying another words they don't became a bubble form as bias ones do.

The way cording is layered on a radial flexes like a Chinese Finger puzzle, which is way a radial tire will hold on corners better than a Bias ply 

The diameter according to Bridgestone, the tire and casing  becomes elliptical through side wall flex, low PSI/ BAR and inertia. I tried to post their High Speed video of the tire changes but the site won't let me transfer or even link it. 

Edited May 11, 2016 by 41chevy

[Vladislav]

I would agree a radial tyre becomes elliptical but ellipse doesn't mean longer circumference. It used to have the same overall length as a circle it became from.

Not to continue to argue, just like to figure out the physical matter.

[41chevy]

40 minutes ago, Vladislav said:

I would agree a radial tyre becomes elliptical but ellipse doesn't mean longer circumference. It used to have the same overall length as a circle it became from.

Not to continue to argue, just like to figure out the physical matter.

The ellipse forms to the unloaded side, towards the top of the casing away from the highway. 

[Retro Racer]

G'day mate!

Quite an interesting point. Did you do 30 pci with load? Or with an empty trailor? Sounds impressive anyway. I think your camelback was too pleased initiating horse riding[emoji4] Thank you for sharing experience.

G'day Vlad,

It had this tipper body on with 2 spare tyres in it only.

So I estimate the total load was

I did the same as you. Day 1, 90psi to 60 psi which didn't produce much of an improvement. Day 2, 45psi with a noticeable improvement, but 30psi was the best so far. This weekend I'm going to test 25psi on the way to the workshop.

Oh yeah, I'm a rodeo rider now with the camelback lol.

All the best

Roland

Sydney Australia

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[Lmackattack]

When we were sled pulling we dropped the air down to 20 psi for better traction. they never gave out on us with all the abuse.

 

I was running my RS700L around the same just to reduce the bounce.