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Tire Pressure, again

TJ1978

TJ1978

I have MANY questions
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Just finished browsing all the “tire pressure” threads and the only constant is everyone runs different PSI..

For a78’ blazer with 350/350/203
3/4 ton axles
37x12.50x17 pro comp AT tires
17x9 pro comp wheels..
No bead locks
What pressure front and rear?

I just took it out at 40psi it felt stiff, I guess. Went down to 35 felt better.
Most of the driving will be street with forays into the AZ desert.

Some pics after a romp in the dirt then pavement home

38E51D37-0F3D-41D6-9678-545035DBB57E.jpeg

Rear
0C56FEE0-6A6D-4096-9F79-D4306BBA50C2.jpeg

Front
34B85C42-266F-482A-BC5D-040FD551160A.jpeg
 
Looks way over filled to me, you shouldn't have that dirt at the sides. I do 32-33 for long distance roads
 
My k30 crew cab long bed on 35s is at 28 psi. Good even foot print.

Go to a level spot with good blacktop or concrete. Rub chalk all over a portion of your tread from side to side. Do a front and a rear.

Roll forward a little bit and look at the chalk. It should be affected evenly across the whole surface.
Untouched in the center, add air.
Center bare but edges untouched, remove air. Once it's good, check pressure and use that.
 
Everyone is running different pressure because we all have different weight above the tires.
Youll want a footprint of the whole tread width. Very easy to do the chalk method to see how its changing.
You want the most air pressure possible but with a full width contact patch.

If you add a thousand pounds or something, youll need to bump the pressure up to accommodate.
 
Yes... this was the first drive with these tires in the pictures. I have already let them out to about 35. Will keep letting out air until it’s even.

I will get it my kids chalk and have a go with it.
 
Please post up the max pressure and max weight numbers on the sidewall of these tires. Also any information about the weight of the rig (like stock weight plus mods and stuff onboard). There are scrap yards and gravel yards out there that can give you axle weights whenever you want. Some keep the scales on 24/7 with a display. Some will let you go on the scale for free during business hours, but only if you don't want a print-out. That set of numbers lets you get pretty close and real numbers for your rig are better than reading 1000 posts online about other people's setups. You didn't seem to want my input in your other thread, but remember that a modified rig requires some level of real science from the owner. Isn't it silly to spend big money on new tires and not treat them right? If you can't get actual numbers, just adjust and chalk a dozen times until things look pretty good and that will be totally good enough to get a good ride and good life out of your tire investment.

The new wheels are beautiful. I always liked that style.
 
Per our specs page https://ck5.com/forums/resources/k5-blazer-specifications-1969-1979.9/ your rig was only like 4000lbs stock. It doesn't look like you've added a lot of weight to it, other than axles. So with some upgrades, some sound deadening and other stuff, let's call that 800lbs. These trucks are actually more weight balanced front to rear than you might expect, with the short wheelbase, 4x4 drivetrain in the middle, hardtop and heavy tailgate. With an average weight per tire of 1200lbs, you're at 34% of the weight rating, or 17psi. So don't be afraid to keep letting air out in your experiments.
 
Blue85 said:
Per our specs page https://ck5.com/forums/resources/k5-blazer-specifications-1969-1979.9/ your rig was only like 4000lbs stock. It doesn't look like you've added a lot of weight to it, other than axles. So with some upgrades, some sound deadening and other stuff, let's call that 800lbs. These trucks are actually more weight balanced front to rear than you might expect, with the short wheelbase, 4x4 drivetrain in the middle, hardtop and heavy tailgate. With an average weight per tire of 1200lbs, you're at 34% of the weight rating, or 17psi. So don't be afraid to keep letting air out in your experiments.
Click to expand...
Thanks. The math isn’t my strong suite. I wasn’t trying to ignore your advice, I just didn’t now how to make the numbers you gave into an equation to make sense..
Hahahahah
Interesting, 30 may even be high. I will keep messing with it.
 
While the math is sound, running tires below 50% of max rating will do damage to the life expectancy of the tires, esp if driven at high speeds and high high heat. The lower the tire pressure the higher the heat generated. Life is a compromise. I do it on my ford, tires are 10 ply + and my dune buggy < because = Toy. The suburban no, but it is closer to weight rating limit of tires.
 
Wes Harden said:
While the math is sound, running tires below 50% of max rating will do damage to the life expectancy of the tires, esp if driven at high speeds and high high heat. The lower the tire pressure the higher the heat generated. Life is a compromise. I do it on my ford, tires are 10 ply + and my dune buggy < because = Toy. The suburban no, but it is closer to weight rating limit of tires.
Click to expand...
So if the max is 50psi running below or at 25psi for street/hwy could potentially cause damage to the tire?
 
Yes, esp in the AZ summer if not spring and fall too. Plus I don't think you will like the handling if aired down much below 30psi. Lot of side wall and D rated is not as stiff as E, D's ride better then E's for our purposes.

oh and 1 word to illustrate low air pressure. "Explorer"
 
Wes Harden said:
Yes, esp in the AZ summer if not spring and fall too. Plus I don't think you will like the handling if aired down much below 30psi. Lot of side wall and D rated is not as stiff as E, D's ride better then E's for our purposes.

oh and 1 word to illustrate low air pressure. "Explorer"
Click to expand...
I’m at 35 now. I may go 35ish up front as they seemed to wear the dirt off more evenly than the rear at that psi and go with 30-32 in the rear... then tweak it...
Either way, I have an air compressor so it’s really not an issue messin around with it.

Thanks for all the input..
 
Wes Harden said:
While the math is sound, running tires below 50% of max rating will do damage to the life expectancy of the tires, esp if driven at high speeds and high high heat. The lower the tire pressure the higher the heat generated. Life is a compromise. I do it on my ford, tires are 10 ply + and my dune buggy < because = Toy. The suburban no, but it is closer to weight rating limit of tires.
Click to expand...
It's a major revelation that a majority of vehicles should not run the majority of tires available in the marketplace, so please expound. Obviously we would only choose tires with load ratings close to the weight we run our vehicles at. Of course nobody should move much from the stock tire size for their vehicle.
 
Just saying if you run your tire to low you will experience tire failure. Example Ford Explorer the tires were 44psi max at max load, Ford spec was 28. High speed, low pressure equals high heat, which normally results in tread separation.
 
Went to about 32psi front and rear. Dirt wore off more evenly or at least good enough. At 35psi it was still a bit too high especially in the rear. But having a damn near empty gas tank may have impacted that a bit.
 
Wes Harden said:
Just saying if you run your tire to low you will experience tire failure. Example Ford Explorer the tires were 44psi max at max load, Ford spec was 28. High speed, low pressure equals high heat, which normally results in tread separation.
Click to expand...
I agree that if you find some P235/75R15 ATX, ATX II, and Wilderness AT Firestone tires from the late 90's you should keep them at high pressure. Or better yet scrap them.
 
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