Tire pressure??

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quote:
Originally posted by blupupher:
If your really anal about it, you should find out you vehicles actual curb weight (with pass and cargo) and find the front to rear split, then find your Max pressure/weight on the tire and figure it from there.
So if you had a 4000 lbs car split 50/50 front to rear weight distribution and you tire has a max pressure of 35 psi/1000 lbs rating, then you would be at 35 psi per tire. a 3000 lb car would be 26.25 psi
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That would only work if the vehicle remained still.

The dynamic loads require higher pressures on the steering tires and permit lower pressures on the rear tires.

I run much higher inflation pressures on the front due to my driving style, or lack of style.

For example, I run 38psi on the front of a FWD sedan. I arrived at that by measuring the tread wear at 1,000 mile intervals and adjusting the pressure to get even wear.
 
quote:
Originally posted by medic:
I typically run my rear tires in the 20-25 psi range and have been doing this for over 100,000 miles. I just have to remember to increase the pressure if I am going to haul anything.
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This being a truck you can do that? If I do that on my 1/2 ton pickup, common wisdom says I will compromise the tires and could end up with an Explorer type blowout rollover, no?
 
quote:
Originally posted by TallPaul:

quote:
Originally posted by medic:
I typically run my rear tires in the 20-25 psi range and have been doing this for over 100,000 miles. I just have to remember to increase the pressure if I am going to haul anything.
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This being a truck you can do that? If I do that on my 1/2 ton pickup, common wisdom says I will compromise the tires and could end up with an Explorer type blowout rollover, no?
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As of yet, no blowouts. I increased the tire size from LT235/70-16 to LT265/75-16. Upon doing this, I had to decide what pressure is proper for these new tires. For determining the proper pressure, I used the concept that a radial tire is supposed be flat on the road (the entire tread coming in contact with the road). I started with a full tank of gas and 35 psi. I dropped the psi 5 lbs at a time until I acheived the desired effect, which ended up being 35 psi front, 20 psi rear.

If, for some reason, I loaded the truck up with weight or towed a heavy trailer and didn't increase the tire pressure, I would expect tire failure. When I am going to load the truck up, I increase the rear tires to 50 psi.

As a rule, I see many trucks with the center of the rear tires worn down and the outside tread untouched. I used to drive 30-40k miles per year. I needed to acheive maximum tire life.

Here's a picture of my truck - do these tires look underpressurized to you? There is a bulge at the bottom of the tires, but radials are supposed to look like this.
-
 
You make a good case. Just never heard of it before. It seems pretty apparent that it takes less air pressure to hold up less weight. It would be interesting to see what the pressure is hot, immediately after running it a long time, comparing hot increase on the front vs the back. If they don't increase in pressure proportionately much more than the fronts, that would support your procedure.

I suppose we've never heard of this because anyone in a position to be held responsible (auto dealer, tire shop, etc.) would not want to be liable for someone who fails to pump them up for the load situation.

What method did you use to determine when the tire was flat on the road?

Also, nice truck!
 
''As a rule, I see many trucks with the center of the rear tires worn down and the outside tread untouched.'' Goes to show how little they need a truck. If pressure relates to load, why not less pressure in the rear for empty pickups and FWD?
 
quote:
What method did you use to determine when the tire was flat on the road?
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There have been a few different methods discussed here. I sprayed half the driveway with water and then drove through it. The wet tire would leave a footprint on the dry part, allowing me to see how much of the tread was coming into contact with the road.

Another method is to draw a line across the tread with chalk. Then drive a short distance and see if any chalk remains on the tire.

quote:
Also, nice truck!
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Thanks. Many people don't believe that its 8 years old with 140k miles.

quote:
Goes to show how little they need a truck
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I agree, and I am not going to get into this discussion. As Americans, we don't NEED many of the things we posess.

quote:
If pressure relates to load, why not less pressure in the rear for empty pickups and FWD?
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You lost me here. This is exactly what I do.

everyday, unloaded driving:
Front: 35 psi
Rear: 20 psi

loaded driving or towing:
Front: 35 psi
Rear: 50 psi
 
My feeling is that most factory psi recommendations are for ride comfort. My cars I run 35 all around, my truck and van get 35 front and 30 rear (unless I know I'm going to load it or tow). That gets me the best tire wear, I keep an eye on that.
 
A point of interest...When it gets real cold--below zero cold, tires tend to lose pressure quickly. The rubber inner-liner becomes more porous as temperature drops.

Correct pressure involves a lot of things, most have been covered in this thread. All tires have a maximum load and a maximum pressure. On trucks, the maximum load the tire CAN carry can be much less than what the tire IS carrying. That being the case, the pressure can be greatly reduced when the load is light and/or the speed is slow. When driving slowly on un-improved roads, reducing pressure to 20-25psi greatly improves traction and ride.

On the highway at high speed, inflate to or near maximum permitted levels for best handeling, best mileage, and lowest tire temperature. Heat is your tires biggest enemy...
 
..."The rubber inner-liner becomes more porous as temperature drops."

Refering to tubeless tires, more likely is that the bead is much stiffer and leaks when hitting potholes are much more likely. And potholes can also be called tirestops at the mall, curbs, manholes covers, etc; ie anything that causes a sharp and highly localized stress that, when cold, transfers to the bead or tire-wheel interface.

"broken" beads are quite common in cold weather, even the "cold" we get here in Houston. Where it is a pleasant if muggy 73F....
 
Whoa, guys!!

There's a lot of misinformation here and I guess it's time for me to stop sitting on the sidelines.

Not to pick on medic, but he has supplied enough numbers to illustrate the point I want to make.

1) The footprint that gives best wear is not the same as the footprint that gives best durability. Optimizing for wear results in a lower inflation pressure, which is the wrong way for durability.

2) Inflation pressure and tire size = load carrying capacity. So the tire loads have to be considered when you start trying to change tire size. When inflation pressures are specified by vehicle manufacturers, the actual loading of the tire is just the one of the considerations, albeit the most important one. Most vehicle manufacturers have some reserve capacity built into the specification. (An old professor of mine used to say "Overdesign - Underutilize!". Good advice.) If one is going to change tire size, you have to maintain the reserve capacity the vehicle manufacturer was using.

3) There are quite a few factors beyond load and inflation pressure that affect the footprint. One of the largest ones is rim width. I'll bet medic has a Ford F-150 with P235/70R16's which came with 7" wheels - the design rim wdth. (Please note the P, not LT) However an LT265/75R16 has a design rim width of 7 1/2. The narrower width forces the tread to "arch" a little bit, which changes the footprint in the wrong direction - toward lower load capacity.

4) Vehicle manufacturers spend a lot of effort to test the handling of their vehicles so they do predictable things, especially in emergency situations. Changing the front to rear inflation balance changes this, and it's going towards oversteer if the rear tires use lower inflation pressures than in the front - not a particularly predictable situation.

5) Tires are held onto the rims with both a mechanical action (sfaety humps) as well as the inflation pressure. By far the greatest force is the inflation pressure. For this reason I always recommend that 25 psi be the lowest pressure used in a P metric tire in order to prevent the bead from dislodging during emergency manuevers. (Point of reference - The government's preferred test for rollover characertistics of SUV's is a "Fishook test", which sometimes results in the tire coming off the rim - so tubes are part of the test specification.)

I guess what I am trying to say is:

PLEASE don't underinflate your tires just to get good treadwear.
 
quote:
Originally posted by CapriRacer:

1) The footprint that gives best wear is not the same as the footprint that gives best durability. Optimizing for wear results in a lower inflation pressure, which is the wrong way for durability.
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Not necessarily.

In the case of my FWD sedan, optimum wear on the front is at 38 psi. That is also a high durability inflation pressure.

quote:

4) Vehicle manufacturers spend a lot of effort to test the handling of their vehicles so they do predictable things, especially in emergency situations. Changing the front to rear inflation balance changes this, and it's going towards oversteer if the rear tires use lower inflation pressures than in the front - not a particularly predictable situation.
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Of course, since the average vehicle was designed with understeer (exceptions exist - Porsche 911 comes to mind), that means you're moving to neutral handling. Unless you're a blue-haired lady from Iowa, that's good.
 
Just to put a few of you at ease regarding the max labeled inflation pressure, I have a short story...
Many years ago in high school, a friend of mine said his 89 Honda Accord was riding hard. Turns out he took his car to a gas station and it had one of those old-timey pumps that use the shop air, with the dial regulator. The thing is supposed to stop dinging when the dialed in pressure was reached. These are often inaccurate/broken/non-functional. So he dialed it in and inflated until it stopped. I drove the car and it rode like a rock. I got out my brookstone digital pressure gauge and checked all 4 of his tires. They ranged from around 65psi to a high of 90-ish. Nothing exploded. I safely let air out of all of them and I am certain he experienced no tire-related failures for the rest of the life of the car (had like 220k at the time anyway).
This doesn't mean it's safe to grossly overinflate. It just means that you shouldn't have a small heart attack if a tire is 5psi over the "max." In fact, I'd run mine daily at 5psi over max (or 10) if that gave me the handling/ride characteristics I wanted.

The max is probably just the highest pressure a manufacturer would accept liability for a non-puncture-related blowout. Along those lines, saying a tire will burst when inflated over this max is akin to saying a car's engine will self-destruct when the powertrain warranty expires (Ford products notwithstanding
grin.gif
).

[ January 04, 2005, 12:12 PM: Message edited by: kevm14 ]
 
Okay, while I realize you must have much more knowledge about tires than I do (which I think is obvoiusly needed here), I will ask some questions of a few of your statements.

quote:
1) The footprint that gives best wear is not the same as the footprint that gives best durability. Optimizing for wear results in a lower inflation pressure, which is the wrong way for durability.
Click to expand...
This one seems to be contradicting itself. What is the difference between maximum tread life and durability?

durable: Capable of withstanding wear and tear or decay

quote:
2) Inflation pressure and tire size = load carrying capacity. So the tire loads have to be considered when you start trying to change tire size. When inflation pressures are specified by vehicle manufacturers, the actual loading of the tire is just the one of the considerations, albeit the most important one. Most vehicle manufacturers have some reserve capacity built into the specification. (An old professor of mine used to say "Overdesign - Underutilize!". Good advice.) If one is going to change tire size, you have to maintain the reserve capacity the vehicle manufacturer was using.
Click to expand...
Okay, I agree with how this starts in regards to tire size, inflation pressures and load range of the tire. However, "Overdesign - Underutilize" means one thing to me, and it should mean the same to any engineer - wasteful. Why should I overinflate my tires everyday just so they're ready when I do place a heavy load on them?

quote:
3) There are quite a few factors beyond load and inflation pressure that affect the footprint. One of the largest ones is rim width. I'll bet medic has a Ford F-150 with P235/70R16's which came with 7" wheels - the design rim wdth. (Please note the P, not LT) However an LT265/75R16 has a design rim width of 7 1/2. The narrower width forces the tread to "arch" a little bit, which changes the footprint in the wrong direction - toward lower load capacity.
Click to expand...
Never did I advocate anybody changing the tire size recommended by the vehicle manufacturer. In fact I recommended agaist it, stating that engineers designed these specs, and most of us know nothing about engineering. Therefore, we should follow the specs provided by the vehicle manufacturer. Do as I say, not as I do kinda thing
wink.gif


quote:
5) Tires are held onto the rims with both a mechanical action (sfaety humps) as well as the inflation pressure. By far the greatest force is the inflation pressure. For this reason I always recommend that 25 psi be the lowest pressure used in a P metric tire in order to prevent the bead from dislodging during emergency manuevers. (Point of reference - The government's preferred test for rollover characertistics of SUV's is a "Fishook test", which sometimes results in the tire coming off the rim - so tubes are part of the test specification.)
Click to expand...
As I stated, I have been running my rear tires at this low PSI for over 100k miles. In that time I have had to make some emergency manuvers and was quite impressed that I didn't roll the truck over during one of these events. If the tires didn't pop off the bead with the force of a sudden directional change at 75 mph, I doubt I have anything to worry about. Please keep in mind that this was not a high speed lane change, it was an evasive manuver.


So, once again, with passenger cars, I recommend following the vehicle manufacturers recommendations. With pickups, I maintain that, unless carrying a load, the specified rear tire pressure is too high. This pressure owuld include that reserve that you are talking about. In certain circustances, this overpressurization can cause the rear tires to bounce and lose contact with the road. In your words, not a particularly predictable situation.

Here is a decent chart regarding tire size, load, & inflation. Unfortunatly, it only includes a few different size tires. http://www.michelintruck.com/michelintruck/pdf/rv_load_inflation.pdf
 
Medic, I doubt that 20 psi is going to increase the life of your tire. If anything I would think it would decrease it because of excessive heat buildup. Feel the tire in the summer after a long drive and they will be much hotter with 20 psi than with 32 psi. My F150 recommends 29 psi in the front and 32 in the rear, but I believe they may have increased those pressures on newer trucks because of the Firestone deal. I usually run 32 - 35 psi in both front and rear and have good tire wear.
 
My intent here is not to "flame" anyone, but to try to add to folks' knowledge. So here goes:


"This one seems to be contradicting itself. What is the difference between maximum tread life and durability

durable: Capable of withstanding wear and tear or decay"

When I discuss durability, I'm thinking in terms of structural failure. This primarily deals with increased probabilities of failure. Just so you get a feel for things, tire structural failure rates are in the fractions of a percent - meaning that the average person might be doing something that increases his risk 100 times and not ever experience a structual tire failure. And as we all know, tire failures can have some horrific results.

"Okay, I agree with how this starts in regards to tire size, inflation pressures and load range of the tire. However, "Overdesign - Underutilize" means one thing to me, and it should mean the same to any engineer - wasteful. Why should I overinflate my tires everyday just so they're ready when I do place a heavy load on them?"

If the science part of product design is completely understood and no short cuts are taken, then, yes, overdesign would be wasteful. However, most of the science is only understood enough to make a useful product, and it is common for "rules of thumb" to be used in product design. This is as true for tires as it is for any other vehicle component. What I am trying to say is that it is common (and good engineering practice) to specify something that is MORE capable than the minimum calculated, in order to account for those unforeseen things, thereby reducing the chances of a failure.

"As I stated, I have been running my rear tires at this low PSI for over 100k miles. In that time I have had to make some emergency manuvers and was quite impressed that I didn't roll the truck over during one of these events. If the tires didn't pop off the bead with the force of a sudden directional change at 75 mph, I doubt I have anything to worry about. Please keep in mind that this was not a high speed lane change, it was an evasive manuver."

Admittedly the "fishook" is an extreme test, but I found it rather disturbing that tires were being unseated during the test. This has just strengthened my resolve on the issue of low inflation pressure. I would hate for someone to be injured in that 1 in 100 situation where low inflation pressure resulted in the tire unseating.


"So, once again, with passenger cars, I recommend following the vehicle manufacturers recommendations. With pickups, I maintain that, unless carrying a load, the specified rear tire pressure is too high. This pressure would include that reserve that you are talking about. In certain circustances, this overpressurization can cause the rear tires to bounce and lose contact with the road. In your words, not a particularly predictable situation."

I agree, but I worry more about folks who "forget" to increase the pressure when they load the truck. I'm also of the opinion that if your truck is bouncing, you're either driving too fast for the road conditions or need new shocks.

"Here is a decent chart regarding tire size, load, & inflation. Unfortunatly, it only includes a few different size tires."

The chart sited in the standard load table. The natural tendancy is for folks to use this table as is. But there are some things that aren't in the table. For example, increasing vehicle speed increases the heat stress on a tire, so more inflation pressure should be used as speeds increase.

My biggest fear when we discuss using low inflation pressures is that folks without common sense will be left with the impression that they can use any pressure they want. Using low inflation pressures increases the risk of failure, and I just want everyone to know that.
 
quote:
Originally posted by tmorris1:
Medic, I doubt that 20 psi is going to increase the life of your tire. If anything I would think it would decrease it because of excessive heat buildup.
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Increasing the tire pressure decreases the heat buildup.

The two risks of higher inflation pressures are poor wear - the crown may be worn before the edges of the tread, and an increasing probability of a structural failure (e.g., split sidewall etc) if something like a pothole is hit.
 
Well I think Medic's point is good, but I did not want to take it too far so I put my rear tires (F150 unloaded, but for 4 sandbags) at 30 psi and the fronts I left at 33.

IIRC it seems that the center wear / edge wear was a much more prevalent problem in the olden days of bias ply tires and that modern radials don't generally have that problem.
 
quote:
Originally posted by Whimsey:
I have a tire pump that uses my cig lighter for power
Whimsey
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that's a neat gadget.
Where do you buy such a pump, and how much does it cost?
any particular brands?
Does it have a good, accurate gauge, too?
If not too expensive I might get one, too...I'm lazy about maintaining tire pressure.

Thanks.
-----
My '97 T-bird (v/8) weighs 3,995 lbs. the recommended pressure is 30 psi for 225/60/16. I always thought this is pretty low for such a heavy car. I wonder if should get it up to about 32-33 psi...wonder what other v8 T-bird owners run.
The tire(Yoko avs dbS2) has max. pressure at 35 psi (I "think")

[ January 05, 2005, 10:12 PM: Message edited by: 97tbird ]
 
quote:
Originally posted by 97tbird:

My '97 T-bird (v/8) weighs 3,995 lbs. the recommended pressure is 30 psi for 225/60/16. I always thought this is pretty low for such a heavy car. I wonder if should get it up to about 32-33 psi...wonder what other v8 T-bird owners run.
The tire(Yoko avs dbS2) has max. pressure at 35 psi (I "think")
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You should have no problems at all running 5 psi over the recommended pressure. With radial tires, wear should still be even enough.
 
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