Big tires, low pressure, how low can I go?

[Number Twelve]

Junkyard tires with about half their original tread, 255/70/R16 ($25 each, mounted and balanced ) on a 2005 Explorer XLT (Beats the H out of $600 and up for a set of 4 tires!)
Car rated as 4469 lbs, tires rated at 2271 pounds each @ 44 P.S.I.
The, "math", pressure proportional to the actual load, shows me at about 21.6 PSI, but the front is obviously heavier than the rear,
plus, I only installed the front two seats.
Running empty, pretending it's a minivan in the rear.
If I figure 169 pounds for the missing 5 seats that makes the weight a nice round 4300 pounds.
So, if the weight distribution is 60/40 that makes the tire pressure calculate to:
25 psi front and 16.7 psi rear

This seems absurd to me.
Anybody know the limits for this application?

Yeah, I know, get the right tires.
But until then...any solid math to work with?

Right now I have them at 35 front and 30 rear, and they show too much belly out in the contact patch.
I'm afraid to let them down until the contact patch is right...or is that the ultimate way to find the right pressure?

 

[Hugh Janus]

You dont fill tires based on the load - load is static and tires pinch-flat from dynamic chsnges. If you want to run them on the beach you can probaby keep them on the rim at 15psi.

 

[Number Twelve]

You dont fill tires based on the load - load is static and tires pinch-flat from dynamic chsnges. If you want to run them on the beach you can probaby keep them on the rim at 15psi.

Do you mean I should run them at the 44 PSI printed on the side of the tire, or the 35 PSI printed on the door label for tires 2 sizes smaller, or something else?

ps, I want to run them in the city. Max speed limit is 45 to 50 around here.

 

[Number4]

Tires can come off the rim around 21 psi.

Run them at the pressure listed on the door. I always run mine 5lbs higher than door sticker (cold.)

 

[imp]

@Number Twelve "But until then...any solid math to work with?"

What kind of "math" do you seek? Difficult to view actual "footprint" impressed by a tire, but it can be done. Ya gotta transfer medium from the tire tread surface downwards carefully onto a clean, flat floor surface. Water-based paint will work, but better would be NCR (No Carbon Required) copy paper. We used rolls of it to "blueprint" head gasket load transference.

If you can figure out a way to measure the actual area of the footprint, and know the tire's air pressure, you can multiply the two and get the weight supported by the tire. Going about this another way, if you know the weight supported by the tire (fronts are different from rear, of course), you can find the area (and therefore, the size) of the footprint by dividing the weight supported by the tire by the tire air pressure, to find the footprint area. I believe in any event, the footprint will be "ovoid" shaped, an ellipse with the ends slightly flattened.

I know that the force exerted upwards by the floor is equal to the force exerted downwards by the tire. Knowing that tells us that the tire air pressure must be equal to the ground pressure exerted upwards on the tire's footprint.

My boss long ago, a Mechanical Engineer who built race cars, was known to remark that "rubber is magic"! You can squirt it around in a mold to make parts, like tires and oil seals, but often cannot predict exactly how it will react. Tires especially, do not want to follow the rules of Classic Physics, because a paved surface has zillions of tiny "indents" in it, and the rubber enters them, to act kinda like "teeth", destroying the concept of a "frictional coefficient".

Why the hell do we want to know about the tire footprint, anyways? Did I miss something in the OP? imp
EDIT: Here's a typical footprint. While the average pressure exerted on the ground is equal to the tire pressure, the distribution of that pressure over the footprint area is NOT linear, i.e., if tire pressure is, say, 40 psi., footprint pressure will be higher in the center, perhaps 50psi., and gradually lessen to the edges, where it may be much less than 40 psi. Kapich?

 

[Number Twelve]

What kind of "math" do you seek?

The math to find the proper inflation pressure that is supposed to be in my over sized, junk yard tires.

If you can figure out a way to measure the actual area of the footprint, and know the tire's air pressure, you can multiply the two and get the weight supported by the tire. Going about this another way, if you know the weight supported by the tire (fronts are different from rear, of course), you can find the area (and therefore, the size) of the footprint by dividing the weight supported by the tire by the tire air pressure, to find the footprint area. I believe in any event, the footprint will be "ovoid" shaped, an ellipse with the ends slightly flattened.

I know that the force exerted upwards by the floor is equal to the force exerted downwards by the tire. Knowing that tells us that the tire air pressure must be equal to the ground pressure exerted upwards on the tire's footprint.

Thanks for the physics lesson. Is there a pressure number in there somewhere?

Why the hell do we want to know about the tire footprint, anyways?

I don't know. You decided to elaborate on foot-prints. I asked about pressure.

QUOTE="imp, post: 3560853, member: 126284"]Here's a typical footprint. While the average pressure exerted on the ground is equal to the tire pressure, the distribution of that pressure over the footprint area is NOT linear, i.e., if tire pressure is, say, 40 psi., footprint pressure will be higher in the center, perhaps 50psi., and gradually lessen to the edges, where it may be much less than 40 psi. Kapich?[/QUOTE]

That was a wonderful explanation. Was there a P.S.I number in there (or any method to find one)?

I have seen that the outer 30% of my tire rubber doesn't have sand sticking to it when I park the car. I have googled tire websites and found their advice about having too much load range for the car. I have followed tire pressure threads on this site and then followed one to an Excel presentation which arrives at pressures even lower than what I calculate from simple proportionality. I think I'll look up some of the "tire chalking" threads because you spent all that time writing about tire prints, therefore, that must be where the answer is.

 

[Number Twelve]

Tires can come off the rim around 21 psi.

Run them at the pressure listed on the door. I always run mine 5lbs higher than door sticker (cold.)

Thanks for telling me a low limit danger zone. My instincts tell me not to obey the math because 17 PSI just can't be right. My car bounces in a frightening way with 40 PSI. I tried that for 10 miles and came home thinking, "No wonder these things have a rep for roll-over." 35 front and 30 rear is a lot less scary, but I'm looking at 30% of the rubber with no sand stuck to it when I park. I don't even know for sure that's, "bad". I don't have any sand coverage meters for my air compressor, so I think I should find a way to use PSI.

 

[imp]

@Number Twelve
"Was there a P.S.I number in there (or any method to find one)?"
Don't quite understand what you're after. You want to relate inflation necessary using a bigger size tire or one having a high load rating capacity? Looking back at your OP I think I see now. You are relating max. pressure rating of a tire to it's load rating capacity, right? And, your thinking is that if the tire is carrying less load than that rated max., you should be reducing the inflation pressure accordingly, but I don't understand why you want to do that? Way I do mine is, if I know my tire is never going to carry full-rated load, I just inflate it to whatever pressure feels good in ride quality. If you are after something else, tell me about it. IOW, the tire label tells you you CAN carry 2271 lbs. max, but it ain't telling you ya gotta have 44 psi in it to carry that much. The tire will carry 2271 lbs. safely at less than 44 psi., but it will get hotter, wear out faster, and the maker of the tire is posting the max. pressure number for purposes of safe maximum pressure regardless of load carried.

Make any sense? I ain't very often told I make sense! imp

 

[Number Twelve]

Don't quite understand what you're after.

I want my tires to wear evenly so I get the best life span because tires are expensive.

When I park, 30% of the rubber doesn't have sand sticking to it. That means both the outside edges are clean by more than an inch, and it's consistent on all the tires. I assume I don't need to run the tires at their maximum pressure because an Explorer doesn't weigh 9000 pounds...more like half of that. I don't have a sand coverage meter for my air compressor so I want to use PSI as a way to measure proper inflation.

 

[RangerX]

I get what you're after. I find the right pressure for oversize tires using the full contact patch method. Others may not agree, but it works for me.
I apply some paint onto the tread of the tires, just a 2" fat stripe from shoulder to shoulder, and drive around for some miles of your normal driving. Then check if the paint wore off fully to the edges. If not, lower the pressure and repeat as needed.
The door sticker psi is only relevant to the tire size the vehicle can with. The tire sidewall psi is a maximum, not a recommended pressure.
I did this method when I first went to large tires on my Ranger 20 years ago. It came stock with 30.5" tires. I switched to 33", then 35", then 37", and now back to 35". All sizes have been mud tires, which many people say only last 20-30,000 miles.
I've gone through countless sets of tires (I put over 300,000 on this truck so far) and consistently get 50,000 out of them. I attribute it to running at correct pressure for the tire/vehicle combination, and also rotating on a semi regular basis.
I run 27-28 front and 23-24 rear. Yeah, it's not a Gen 3 Explorer, but the principle is the same. Your pressure won't be the same as mine, but I bet it's somehwere in the 28-33 range.
The factors that matter are tire diameter and width, rim width, and vehicle weight, IMO. My psi's may be a little lower due to a narrow rim width, which pinches the tire width. If I had the recommended width rim, I could probably run a couple lbs higher.
Btw, my truck is lifted and fairly heavily modified with an actual weight of 3800 lbs, and with the 35's at a 'low' psi, and 350,000 miles on the original 4.0, I still get 17 mpg consistently.

 

[imp]

@RangerX Well, your moniker is the worst to get an "@" in front of that I've seen yet! But, before you get rankled, here's what I think of your post. BEST approach I've heard! Some real thought went into your work. And, I'd bet you read all my B.S. above before posting. real good! Real good to know there are still guys left out there trying to find out what works best, as I did 50 years ago.
Thanks! imp

 

[Number Twelve]

I apply some paint onto the tread of the tires, just a 2" fat stripe from shoulder to shoulder, and drive around for some miles of your normal driving. Then check if the paint wore off fully to the edges.

Sounds like a plan!
And a good purpose for almost-empty rattle cans, which we all have.

So...yours weighs about 3800 and mine weighs about 4300, heavy in the front.
There is almost zero resemblance between 35 inch tires and 16 inch tires ("They ain't perty and they don't smell good."), but the method seems right. I suspect I'll end up around 30/front 25/rear.

I kinda guessed this is what, "chalking the tires" is about, but chalk seems a much quicker thing to wear off the rubber...and easily subject to anomalies, like driving on streets that are humped in the middle. "Oh look, the left edges are inflated correctly but the right edges aren't. All 4 tires with the same camber problem?" LOL

I would think every back-yard grease monkey has seen the posters in tire stores showing the results of over-inflation and under-inflation, but most people don't know what to do except wait for the tires to wear badly.

 

[RangerX]

@imp old guys do things based on how much sense it makes!

@Number Twelve your rims are 16", but your tires are 30" (255/70/16 converts to 30.1").
But if that two sizes bigger like you mentioned, your target psi wil def be lower than factory psi. And yes, the chalk does wear off too fast to really show you much, paint is better. Give it a try, and post what you figure out.

 

[imp]

@RangerX WTH do you mean about "old guys", anyhow? G.D. it! imp

 

[Number Twelve]

your rims are 16", but your tires are 30"

You certainly caught me thinking about an entirely different kind of tire!
I thought you were talking about the wheel size and huge tires, almost like tractor tires.

So, being kind of old, I'll mosey about getting this done over the course of several days. I don't put 5000 miles a year on my car, so this might take a while.

 

[Number Twelve]

Owned two 2nd. Gens, only one 3rd, but know a helluva lot more about the 3rd. What does it mean?

The 3rd gen cars break a lot more?
I know rehabilitating mine was an adventure!
177,000 miles put on it by people that don't have a clue about maintenance and wouldn't do any actual work if they did know.

 

[TechGuru]

Junkyard tires with about half their original tread, 255/70/R16 ($25 each, mounted and balanced ) on a 2005 Explorer XLT (Beats the H out of $600 and up for a set of 4 tires!)

$600? LOL You can get a brand new set with a 40,000 mile warranty for $440, and if you don't have a 4x4 you can get them in pairs, maybe like one pair one month then another pair the next month (assuming you don't have a credit card to just be able to finance them out...)

I can actually get the set for $411.23 here in Texas, we don't have a environmental fee and I can opt out of the tire disposal by keeping the old ones.

 

[2Trux]

I also prefer the paint the tread method when running different size tires. I had a 69 Bronco with 33x12.5 6 ply tires, the paint method showed I needed to run them at 15psi. This seems low but I ran them at 15psi on the highway and 5psi off-road for the life of the tire and never had an issue. They were mounted on 15x8 steel wheels no beadlocks.
Had I just used the 30psi recommended I would have worn out the center and lost a lot of tread life.