Tire pressure question

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^^^^^ Spent a few years in the business myself^^^^^

I run 40PSI in my front Skinnies 26x6.00 R15LT's and 10 PSI in the rear 275/60/15's unless I want to smoke the rears a bit easily, then air up to 32 PSI.

Home aligned with the string method, Keep an eye on the wear and make adjustments as needed! This really is not rocket science. JMO!
 
MoparLeo said:
Actually RustyRatRod I speak from over 40 years retail experience working for tire manufacturers .
You never "wing it" on tire pressure. Number #1 cause of tire failure. I have been involved with 2 major tire recalls in 1978 and again in 2000 in which millions of tires were replaced primarily because the Vehicle manufacturer recommend their "own pressures" based on comfort, not load and most people never give the air in their tires a second thought until someone tells them their tire is low or they get a flat.
We found that the average tire pressure when people came in to replace their tires was 18 lbs.
The law is now to install the vehicle placard pressure. The customer may change that but it is never recommended or done "for the customer"
We are supposed to be the experts. Why would you endangerer the safety of a customer and open the company you work for open to liability if there ever was a problem. The first thing their lawyer will say is, But my client is not an expert...
What if you did oil changes and the customer told you how much oil he wanted. Would you put in a quart more or a quart less because "that is what the customer wanted?" or would you use your own common sense and educate him on why he shouldn't do that.
The customer isn't always right...
Go back to post # 15
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Op is using a far larger tire on a much bigger rim than stock, on a modified car. The "factory recommended inflation" is WORTHLESS.
I've also had a "tire professional" tell me he wouldnt/couldn't put my new tires on where I wanted them. He lost that $500 sale, and any future sales, and I have more than ten cars.
Edit: sorry about the underlining, it wasn't intended. I just wanted to accentuate the "I" and screwed up
 
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MoparLeo said:
Actually RustyRatRod I speak from over 40 years retail experience working for tire manufacturers .
You never "wing it" on tire pressure. Number #1 cause of tire failure. I have been involved with 2 major tire recalls in 1978 and again in 2000 in which millions of tires were replaced primarily because the Vehicle manufacturer recommend their "own pressures" based on comfort, not load and most people never give the air in their tires a second thought until someone tells them their tire is low or they get a flat.
We found that the average tire pressure when people came in to replace their tires was 18 lbs.
The law is now to install the vehicle placard pressure. The customer may change that but it is never recommended or done "for the customer"
We are supposed to be the experts. Why would you endangerer the safety of a customer and open the company you work for open to liability if there ever was a problem. The first thing their lawyer will say is, But my client is not an expert...
What if you did oil changes and the customer told you how much oil he wanted. Would you put in a quart more or a quart less because "that is what the customer wanted?" or would you use your own common sense and educate him on why he shouldn't do that.
The customer isn't always right...
Go back to post # 15
Click to expand...
Well good. I'm glad you have experience. I would never slight you in the least though. No one has the same experience so we should all respect that. Not everyone has to worry about what a lawyer would do. Thank you, carry on.
 
MoparLeo said:
A lot of funny answers. Since the tire is supporting weight and a tire is just an air container with a specific volume of air capacity the pressure is always dependent on load.
Find out what the axle weight of the vehicle is first.
Using water and chalk and anything other than a good quality air gauge is just too funny. And I would never admit to "being in the business" and giving such unprofessional advice about pressures. I have to assume you were talking about a car with the correct, factory sized tire on it.
The pressure on the side of the tire is Not a recommendation. It is the maximum safe tire pressure for that particular tire.
Look up a load and inflation chart to find the load capacity of the tires you are thinking of using. Now look up the factory tire that came on that car and look up the load capacity of that tire. The tire you want to use should have at least the same capacity at the same pressure as what the car came with. Armed with the actual axle weight you can see what the safe limits are for the amount air pressure used.
Do a little reading to educate yourself a little more on tires. Not complete but has some good info for the average guy.
Tire Load And Inflation Chart at Tire Rack
Click to expand...

Registered to this forum to react.
I am yust a hobbyist " pigheaded Dutch self declared Tire-pressure specialist" .

But the quoted answer is the best to my opinion.
Remark is that the pressure/ load capacity lists found in US, often are made with the formula used before 2006, and give to high loadcapacity for the pressure.
In 2006 US stepped over, only for P tires, to the official calculation in Europe used for decades, for all kind of car tires.
I got hold of this calculation end 2007 and system, and went running with it.
I can calculate the needed pressure for you, but need real axleweights and max speed you use, and won't go over for even a minute.
Especially determining the weights is the most tricky part in it all.

But here a dangerous estimation.
Because light car with oversized tires, mayby even 20 psi can do.

Would like further contact with MopartLeo, maybe we can make each other wiser
Have made multilingual pressure calculators, and studied side subjects like Nitrogen filling, rimmwidth for section width and H/W division, and maxspeed / max load relation, and more.
 
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Thanks for the responses, I had no idea that a tire pressure question would evoke such emotion. I agree that tire pressure is important. I am sure that one would never find tires of the vehicles that I am associated to 'under inflated'.
My question is driven by the fact that this car with these tires will be driven hard in performance venues like drag races and autocross events. The car is far from stock. That is why I found the thought that I should be referencing the 1968 OE stock sized tires and 1968 OE load capacity amusing.
I feel much the same way concerning using the calculations mentioned in the above post, as mentioned due to the vehicle weight vs tire capacity with modern OS tires. To me it is obvious that to just carry they 'load' not much air pressure will be needed. And that would not be a good situation for handling.
I am wanting the air pressure required to flatten the tire tread to the road surface and to properly stiffen the side wall to minimize heat build up and minimize side wall tire flex in sharp turns. Doing the chalk test as described in post #2 to me is a direct route to the flatten the tire tread to the road surface half of the question.
I can get the other half answered with some track time.
thanks
 
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RustyRatRod said:
it all depends on what the owner WANTS
Click to expand...
Like. Everybody has their own ways,thoughts and likes and can do anything they want. If you over inflate,Good for you,if you under inflate and that is what you want,Good for you.
I think we beat this subject to death enough and is now starting to be a pissin' contest.
Play nice boys...
 
Darter6 said:
Like. Everybody has their own ways,thoughts and likes and can do anything they want. If you over inflate,Good for you,if you under inflate and that is what you want,Good for you.
I think we beat this subject to death enough and is now starting to be a pissin' contest.
Play nice boys...
Click to expand...
Exactly.
 
Okay! Now that we know what you want to do with your car.....
For autocross, I would start a couple pounds short of max, (51?), then maybe 48psi. Then play with pressures, depending
on over or under steer.
For drags, fronts at 51, rears, I would have to experiment, probably start around 25, and play around from there (run as much pressure as you can that will hook)
For street, your original first post number is fine.
Just my opinion....
 
Well, it only took 34 posts for the OP to tell us, as Paul Harvey used to say "the rest of the story."
If you would have given us that specific info in the beginning, you would have gotten more specific answers based on your application.
Obviously tires used on a closed course do not have to worry about potholes, dips, poor road surfaces, debris etc... And you are not concerned with comfort or tire wear but performance.
You still must know your axle weight and the chosen tires load capacity. The traction characteristics of the chosen tire, speed rating/ load index.
Start with that as a baseline.
You would be using higher cold inflation pressures than for typical passenger cars and proper rim width and suspension mods would also effect preferred air pressure as the tire is a part of the suspension system and things such as wheel/tire weight, spring rates, shock absorber types etc.. all have a combined effect on preferred tire pressures.
Nitrogen is often used because of its larger molecule size which translates to less pressure loss through the liner and that it also displaces the moisture inside the tire retarding the corrosion/oxidation common on alloy wheels and other benefits. Less pressure variation caused by the heat generated in the tire due to flexing when rolling. Higher pressure also help to reduce the slip angles of the tires when cornering helping to maintain stability.
We could go on but the point is, Safety first.
If you are going racing of any type. It is a lot more complex than you think.
This might give you a little more appreciation for the racing teams crew chiefs and technicians. They must have a working knowledge of physics, metallurgy, and many other
disciplines.
Street cars are not race cars . Street cars are designed as a compromise between performance, longevity and comfort. Race cars have short component life, constant maintenance. Something that is unacceptable to the typical motorist.
As mention in the following text, the rim width is also very important. Rule of thumb isn't always accurate. Modern tires have different designed rim widths based on whether the tires are designed with low rolling resistance/increased fuel economy in mind or just the typical dimension formulas used for years.
On radial tires the tread width is typically narrower than the section width of the tire. The old wide tread bias ply tires had a flatter, wider tread contact patch to increase traction.
Radial tires because of the sidewall and tread area working more independently from each other maintains tread contact in corners much better than the old bias ply and needs less tread area to maintain traction .



Wheel/Rim Size Calculator - Shows acceptable rim width range for the tyre size
 
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I am almost afraid to post this but have not seen any consideration regarding rim bead width to tire tread width. The decision to mount correct tire size to available rim comes first. I was always taught tire width should equal bead width as a point of reference. Yes you can deviate from this and by how much I don't know but have seen too wide of tire on too narrow of a rim. Is it possible to correct for proper wear by adjusting pressure in this situation?
 
jawbone said:
I am almost afraid to post this but have not seen any consideration regarding rim bead width to tire tread width. The decision to mount correct tire size to available rim comes first. I was always taught tire width should equal bead width as a point of reference. Yes you can deviate from this and by how much I don't know but have seen too wide of tire on too narrow of a rim. Is it possible to correct for proper wear by adjusting pressure in this situation?
Click to expand...
I actually have both situations for one car.
I have a 315/60x15 drag radials, recommended for a 8.5-11 rim on a twelve. I have seen them on a fourteen. They work best on a wider rim (11.5 tread width)
The street tires I have for the same car, are too wide for the rim. They have 15+inches of tread, on a twelve inch rim. They are run fairly soft, under 20 lbs, to avoid wearing out the center. They will still have plenty of excess load rating, it's a light car.
 
33IMP said:
I actually have both situations for one car.
I have a 315/60x15 drag radials, recommended for a 8.5-11 rim on a twelve. I have seen them on a fourteen. They work best on a wider rim (11.5 tread width)
The street tires I have for the same car, are too wide for the rim. They have 15+inches of tread, on a twelve inch rim. They are run fairly soft, under 20 lbs, to avoid wearing out the center. They will still have plenty of excess load rating, it's a light car.
Click to expand...
Thanks for your response. That's exactly what I'm talking about. This must be taken into consideration as well as tire pressure. As everyone has said, tire pressure varies car to car depending on application. No set rules.
 
Exactly. Race cars are not street cars. Low pressures spell doom in cornering where it might be desirable in straight line acceleration. Higher pressures give the tires stiffer slip angles which are more desired in cornering...
 
my68barracuda said:
car is a 68 Barracuda
tires front 215/60/16 on 7 inch wide wheels
tires rear 225/60/16 on 8 inch wide wheels
the car is somewhat light weight, fiberglass hood and front bumper, aluminum front bumper brackets, light weight last year built slant. Carpet and interior trim is present, but no back seat, no under carpet sound deadening material.

The side wall of the tires says that max load capacity is at 51 psi.

Does a cold tire pressure on 38 psi seem about right?
The car rides and drives fine.
Thought I would ask to see what other folks run for tire pressure with non stock sized tires.
Click to expand...
An infrared temp gun is inexpensive and quick. Go for a drive of say 5 miles. Pull over and take the tread temps on the inside, center and outside. You are looking for even temps across the tire. Positive camber on the front wheels will have the outer warmer and the inner cooler. The center should be about the average of those two readings. Negative camber like used on road racing or slalom carswill be opposite with the inside warmer and the outside cooler. For stock street you should probably have slight positive camber with minor temp variance.
Too high pressure will heat the center and too low will heat both edges.
 
MoparLeo said:
Exactly. Race cars are not street cars. Low pressures spell doom in cornering where it might be desirable in straight line acceleration. Higher pressures give the tires stiffer slip angles which are more desired in cornering...
Click to expand...
Another consideration for road racing is what happens if you have an off track event. With soft tires and sliding sideways, if you hit a bump or hole you could break the bead loose and lose all your tire pressure. The rim digs into the dirt and over you go.
Hard tires do not flex and heat up as much.
 
my68barracuda said:
car is a 68 Barracuda
tires front 215/60/16 on 7 inch wide wheels
tires rear 225/60/16 on 8 inch wide wheels
the car is somewhat light weight, fiberglass hood and front bumper, aluminum front bumper brackets, light weight last year built slant. Carpet and interior trim is present, but no back seat, no under carpet sound deadening material.

The side wall of the tires says that max load capacity is at 51 psi.

Does a cold tire pressure on 38 psi seem about right?
The car rides and drives fine.
Thought I would ask to see what other folks run for tire pressure with non stock sized tires.
Click to expand...
I always start with "below max", depending on application (example: towing or maxing out weight would be higher psi) I use the coin or other measuring instrument method. Check the depth between the treads and adjust psi according.
 
jawbone said:
I am almost afraid to post this but have not seen any consideration regarding rim bead width to tire tread width. The decision to mount correct tire size to available rim comes first. I was always taught tire width should equal bead width as a point of reference. Yes you can deviate from this and by how much I don't know but have seen too wide of tire on too narrow of a rim. Is it possible to correct for proper wear by adjusting pressure in this situation?
Click to expand...
I was waiting for a post as yours. It's more than the PSI. What kind of tire or type and rim type is a "BIG" factor. Example: I was always told, not to install a radial tire on a 60 era rim. Is it because of the steel rim or maybe the bead? Max air pressures of the poly tires are 32 PSI and Radials are 35 and higher. All mags that I have seen do except radials.
 
my68barracuda said:
car is a 68 Barracuda
tires front 215/60/16 on 7 inch wide wheels
tires rear 225/60/16 on 8 inch wide wheels
the car is somewhat light weight, fiberglass hood and front bumper, aluminum front bumper brackets, light weight last year built slant. Carpet and interior trim is present, but no back seat, no under carpet sound deadening material.

The side wall of the tires says that max load capacity is at 51 psi.

Does a cold tire pressure on 38 psi seem about right?
The car rides and drives fine.
Thought I would ask to see what other folks run for tire pressure with non stock sized tires.
Click to expand...
I would look at the original factory inflation pressure recommendation. Now over time tire construction and the size descriptions have changed. In '68 bias ply tires were the norm and imperial measurements used or a letter for width and a number for aspect ratio. Then they moved to bias belted tires to combat some of the bias tire squirm that cause.d wear. On these tires the second in row of ribs from each side wore more than the rest of the tread.
Then when radial ply construction was introduced on imported cars in the late '60s, the benefits became obvious. Longer tread life, better adhesion, better handling. Along with this came the metric size descriptor pretty much standard now.
Keep in mind that the max load tire pressure is what the tire carcass is capable of holding, not what the vehicle requires with that tire. A tire proffessional should be able to made an educated guess of the required pressure to run. Chalk across the tread as described elsewhere here or a temperature gun will be the best bet to fine tune from there. If the tire guy looks at the sidewall and tells you to run the max embossed on the sidewall, run. They are no proffessional. Another point on new vehicles, the engineers generally do a fine job of determining all the proper operating conditions. In todays world of Corporate Average Fuel Economy, running the tires hard with higher pressure lowers rolling resistance and increases the fuel mileage slightly. This gets them brownie points with the Feds 'n Heads, but the tires are most likely going to wear the centers. Get a tire wear guage and check the wear pattern monthly. You need to measure the center and both edges. Make sure not to measure on one of the treadwear indicators, a slightly raised point in the bottom of the grooves. Using this if you notice a different depth showing up you can adjust your pressures to keep the wear even.
 
Car manufacturer recommendations or aftermarket new tire recommendations? Original car manufacturer recommendations were skinny bias ply tires on narrow rims.
 
For a "51lb max" tire
38-41lbs cold.
Just a hair of center wear, but best handling and mileage without hurting the tires/wearing them abnormally.

35 is where discount tire sets them..and they're noisy and side wall flex on turns, too soft for me.
 
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