Tire air pressure

[petersonj]

Charts can be convenient, but won't help much if you are using equipment that uses tires that you didn't purchase, or the neighbor wants to know how much air to put in his trailer tires that he knows nothing about.

I use a simple formula to calculate what air pressure I want in a particular tire for a particular load. Every tire has a load limit (in pounds and kilograms) that it can carry at full inflation pressure as a single tire or a dual tire. The load (in lbs) and the air pressure (in psi) are the two numbers that interest me.

For example (I will use round numbers for the purpose of explanation):

A tire states that it can carry 3,200 lbs at 80 psi (single tire).

* 3,200 lbs / 80 psi = 40 lbs per 1 psi for one tire. One axle (two tires) will be 80 lbs per 1 psi.

So, if one axle can carry 80 lbs per 1 psi, then 800 lbs can be carried by 10 psi, or 3,200 lbs can be carried by 40 psi, and so on.

So, in the above example, if one knows the approximate weight carried on each axle, it is very easy to get very close to the needed inflation pressure.

If the above tire were to be used on a tandem axle trailer, then two axles would equal four tires, which would equate to 160 lbs per 1 psi for the tandem axle set.

What I like about this method is that you only have to figure out the lbs / 1 psi just once. Because that number is a constant, just write down that number for the truck axles or the tandem axles. You will always recalculate tire pressure using that number any time axle loaded weight changes.

- John

 

[AH64ID]

Charts can be convenient, but won't help much if you are using equipment that uses tires that you didn't purchase, or the neighbor wants to know how much air to put in his trailer tires that he knows nothing about.

I use a simple formula to calculate what air pressure I want in a particular tire for a particular load. Every tire has a load limit (in pounds and kilograms) that it can carry at full inflation pressure as a single tire or a dual tire. The load (in lbs) and the air pressure (in psi) are the two numbers that interest me.

For example (I will use round numbers for the purpose of explanation):

A tire states that it can carry 3,200 lbs at 80 psi (single tire).

* 3,200 lbs / 80 psi = 40 lbs per 1 psi for one tire. One axle (two tires) will be 80 lbs per 1 psi.

So, if one axle can carry 80 lbs per 1 psi, then 800 lbs can be carried by 10 psi, or 3,200 lbs can be carried by 40 psi, and so on.

So, in the above example, if one knows the approximate weight carried on each axle, it is very easy to get very close to the needed inflation pressure.

If the above tire were to be used on a tandem axle trailer, then two axles would equal four tires, which would equate to 160 lbs per 1 psi for the tandem axle set.

What I like about this method is that you only have to figure out the lbs / 1 psi just once. Because that number is a constant, just write down that number for the truck axles or the tandem axles. You will always recalculate tire pressure using that number any time axle loaded weight changes.

- John

This method defiantly works and what I did before charts.

If you have enough data for the math method you have enough data for the charts.

The math method also calls for more psi per lb than the charts, which isn’t bad, it’s just more.

Example, to carry 2680lbs on my LT275/70r18’s the charts want 50psi. The math method says 59 psi.

 

[Grumpy8811]

lets face it, older load range system A B C D E has been superceded by the load index rating, where the load index is numerical and corresponds to the tire being able to support a given weight ( aka all load index 120 tires will carry the same weight regardless of their size) whereas the older alphabetic weight system limited itself to one size tire with a C rating just meant it wouldn't carry as much weight as the same size tire in a D or E load range as a way to compare tires.

Load Index is a more precise way of saying how much weight a tire can carry instead of quantifying the weight rating by tire size.

six of one, half dozen of the other.

 

[johnmyster]

lets face it, older load range system A B C D E has been superceded by the load index rating, where the load index is numerical and corresponds to the tire being able to support a given weight ( aka all load index 120 tires will carry the same weight regardless of their size) whereas the older alphabetic weight system limited itself to one size tire with a C rating just meant it wouldn't carry as much weight as the same size tire in a D or E load range as a way to compare tires.

Load Index is a more precise way of saying how much weight a tire can carry instead of quantifying the weight rating by tire size.

six of one, half dozen of the other.

The entertaining conversation is when someone wants to talk about plys. Not to mention that actual ply count went by the wayside many years ago, replaced by ply rating, and then load range, etc.

I do still maintain that load inflation tables reflect a minimum cold inflation value for safely carrying a given load without overheating the tire. Semantics. Any value above that is literally above or over the required value, but I don't know that I'd call it overinflated, especially if that higher value has some mental energy behind it to compensate for high speed, cold weather, handling dynamics, side to side weight imbalance, keeping tire temperatures in check, or the inconvenience of regularly refilling a slow leaker.

 

[Grumpy8811]

The entertaining conversation is when someone wants to talk about plys. Not to mention that actual ply count went by the wayside many years ago, replaced by ply rating, and then load range, etc.

I do still maintain that load inflation tables reflect a minimum cold inflation value for safely carrying a given load without overheating the tire. Semantics. Any value above that is literally above or over the required value, but I don't know that I'd call it overinflated, especially if that higher value has some mental energy behind it to compensate for high speed, cold weather, handling dynamics, side to side weight imbalance, keeping tire temperatures in check, or the inconvenience of regularly refilling a slow leaker.

exactly... which leads me to pointing out that some people can't see the forest because of the trees and have so much technical information available to them without any corresponding understanding of the variables that I have seen people on different forums who have a tire pressure monitoring which may be accurate to 1/10ths of a psi go to the lengths of adjusting the air pressure in their tires on almost a daily basis, simply because of too much information without comprehension of normal behavior of a gas in a variable temperature's.

 

[User113]

exactly... which leads me to pointing out that some people can't see the forest because of the trees and have so much technical information available to them without any corresponding understanding of the variables that I have seen people on different forums who have a tire pressure monitoring which may be accurate to 1/10ths of a psi go to the lengths of adjusting the air pressure in their tires on almost a daily basis, simply because of too much information without comprehension of normal behavior of a gas in a variable temperature's.

So, that's another pet peeve of mine. What are display makers thinking? More digits is better? Just because an air pressure sensor might have a display that reads out in increments of 1/10th of a PSI (resolution) does not mean it's ACCURATE to 1/10th of a PSI; it just has electronics that can sense tiny variations in an electrical signal that may or may not even be related to the parameter it is trying to measure. And trust me, there are a lot of spurious electrical signals flying around the wiring in any vehicle. Even IF the parameter at the least significant digit level is actually fluctuating by more than the resolution shows, the overall ACCURACY of the display is probably only within ± 2-3% of the current reading*, and even that depends on the integrity of the associated analog sensor that's been used and abused for who knows how long. Digital gauges need to be "filtered" and the signal averaged over some period of time and/or some number of samples.

Take, for example, the "digital" Engine Coolant Temperature (ECT) display on a SMARTY Touch (and probably the MM3, too); it reads the PID signal from the ECM and then it displays it with a resolution of 1/100°F as if that actually means something! Assume accuracy of ± 1 %, an ECT of 200°F can't be trusted to within ± 2°F so why does the display show resolutions beyond the decimal point? But yet, it does. Not only that, but the two "least significant digits" are fluctuating so fast you can't even read them. As you can see the Smarty also displays other PIDs with the same ridiculous resolution of 5 digits, like TRANS-ST, ECM-V, STC-SV, and so on.

Pisses me off. Sorry. Thanks for listening.

*the BestReviews website of tire gauges states "On high-quality gauges, the degree of inaccuracy can be plus or minus 1% to 5%, which can make a big difference if you run high-pressure tires." Car and Driver states "Look for gauges known for their precision. For reputable models, the manufacturer will usually list a spec—commonly a percentage or psi delta—indicating the accuracy of the unit" which conflates precision with accuracy. even then, they only included the accuracy of one of the reviewed gauges as being ± 1 %.

 

[AH64ID]

So, that's another pet peeve of mine. What are display makers thinking? More digits is better? Just because an air pressure sensor might have a display that reads out in increments of 1/10th of a PSI (resolution) does not mean it's ACCURATE to 1/10th of a PSI; it just has electronics that can sense tiny variations in an electrical signal that may or may not even be related to the parameter it is trying to measure. And trust me, there are a lot of spurious electrical signals flying around the wiring in any vehicle. Even IF the parameter at the least significant digit level is actually fluctuating by more than the resolution shows, the overall ACCURACY of the display is probably only within ± 2-3% of the current reading*, and even that depends on the integrity of the associated analog sensor that's been used and abused for who knows how long. Digital gauges need to be "filtered" and the signal averaged over some period of time and/or some number of samples.

Take, for example, the "digital" Engine Coolant Temperature (ECT) display on a SMARTY Touch (and probably the MM3, too); it reads the PID signal from the ECM and then it displays it with a resolution of 1/100°F as if that actually means something! Assume accuracy of ± 1 %, an ECT of 200°F can't be trusted to within ± 2°F so why does the display show resolutions beyond the decimal point? But yet, it does. Not only that, but the two "least significant digits" are fluctuating so fast you can't even read them. As you can see the Smarty also displays other PIDs with the same ridiculous resolution of 5 digits, like TRANS-ST, ECM-V, STC-SV, and so on.

Pisses me off. Sorry. Thanks for listening.

View attachment 143074

*the BestReviews website of tire gauges states "On high-quality gauges, the degree of inaccuracy can be plus or minus 1% to 5%, which can make a big difference if you run high-pressure tires." Car and Driver states "Look for gauges known for their precision. For reputable models, the manufacturer will usually list a spec—commonly a percentage or psi delta—indicating the accuracy of the unit" which conflates precision with accuracy. even then, they only included the accuracy of one of the reviewed gauges as being ± 1 %.

I don’t disagree on the uselessness of decimals on many of the OBDII PIDs but you can slow the refresh rate down on the Touch/MM3. When I was beta testing the Touch for MADS, pre-release, the refresh rate was one of my comments to them and they made the refresh rate selectable. What good is a gauge you can’t read because it refreshes so fast??

I though you could also select the number of digits, but its been a while since I’ve used a Touch/MM3.

 

[User113]

I don’t disagree on the uselessness of decimals on many of the OBDII PIDs but you can slow the refresh rate down on the Touch/MM3. When I was beta testing the Touch for MADS, pre-release, the refresh rate was one of my comments to them and they made the refresh rate selectable. What good is a gauge you can’t read because it refreshes so fast??

I though you could also select the number of digits, but its been a while since I’ve used a Touch/MM3.

Thanks for listening, makes me feel validated.

The pic I posted above was "stolen" from a YouTube video at around 1:18. What's so amazing is the truck is not even running (RPMs = 0) and still the ECT decimals are flickering to beat the band, just like what happens in my truck.

Also, my sincerest apologies for hijacking the thread.

 

[petersonj]

Sorry. Thanks for listening.

I'm glad someone else sees it the same way I do! And since you were on a roll, and since your tachometer is showing in the photo - what's with making the tickmarks divisible by numbers that nobody would ever use? I've played with different gauge displays on the Smarty Touch and it's that way on all of their analog gauges, no matter which scale you choose. Is this new math? Who thinks this stuff up?

I think this was all starting back when I bought my new truck. The first two things that I noticed when the truck was new were the speedometer (whole numbers every 10 mph, but each tickmark represents 2.5 mph), and the tachometer (whole number every 1,000 rpm, but each tickmark represents 125 rpm) Who thought of that?

- John

 

[AH64ID]

Somewhere I have multiple photos of my Touch and MM3, but I don’t think it’s on my phone. I need to find them and see what was displayed.

I’m currently running a CTS3, but have never have the fondness for it like I did the Touch or MM3.

 

[Grumpy8811]

So, that's another pet peeve of mine. What are display makers thinking? More digits is better? Just because an air pressure sensor might have a display that reads out in increments of 1/10th of a PSI (resolution) does not mean it's ACCURATE to 1/10th of a PSI; it just has electronics that can sense tiny variations in an electrical signal that may or may not even be related to the parameter it is trying to measure. And trust me, there are a lot of spurious electrical signals flying around the wiring in any vehicle. QUOTE

that is the reason I dont even use the display's in my Tradesman.. I don't need to know I have 20 pounds of boost or the transmission is 176 degrees..
I just turn the display off, never mind using an aftermarket display and hanging it off the side pillar. figuring sometimes you are better off not knowing
than spending time thinking about it. That's why they put warning lights.

I remember one of my friends getting a Electra Glide that was his first bike with an oil pressure gage and he would ask everybody what their oil pressure was..
it was a near constant thing.. hey, what oil pressure do you have" and finally one of our other friends just flat out told him "you've had about ten of these bikes before that didn't have a gage and you were not concerned about oil pressure so why don't you cover that gage with some tape and quit buggin people about it."

 

[Grumpy8811]

I'm glad someone else sees it the same way I do! And since you were on a roll, and since your tachometer is showing in the photo - what's with making the tickmarks divisible by numbers that nobody would ever use? I've played with different gauge displays on the Smarty Touch and it's that way on all of their analog gauges, no matter which scale you choose. Is this new math? Who thinks this stuff up?

I think this was all starting back when I bought my new truck. The first two things that I noticed when the truck was new were the speedometer (whole numbers every 10 mph, but each tickmark represents 2.5 mph), and the tachometer (whole number every 1,000 rpm, but each tickmark represents 125 rpm) Who thought of that?

- John

yeah, it does make you wonder why? in the old days they used to design that gage stuff ( except for the speedo and tach) where the centerpoint in the gage was what would be normal.. ak it is in the green.. so a quick glance at the gage if the needle was more or less in the center area you knew everything was OK.. but with the advent of computer and data they have the possibility to tell you hundreds of things you probably shouldn't care about.. and sometimes they do.

I still just wait for an idiot light. it is a lot simpler.

 

[User113]

I remember hearing, many decades ago, that a USAF study was once commissioned (your tax dollars at work) to examine cockpit gauges and supposedly discovered that for many intelligence categories the optimum display for a pilot was a circular gauge about 2 inches in diameter, needle axis in the center and with the "happy" zone where the needle was pointing straight up. And, I'm sure, there are probably thousands of similar studies done in every environment conceivable, by car makers, gauge manufacturers, foot-press operators, etc. As Grumpy and John point out, it's really a shame that so much effort has already been put into optimizing the conveyance of information and then that knowledge is quickly discarded for something that looks flashier, but is far inferior. (Like, WHY does an automotive gauge need to have user selectable "skins"? This ain't a teenybopper's MP3 player!)

Also, as I stated above, that is not my video or photo that I linked to, it's just one of several that I was able to find that actually showed what I was griping about. I'm hoping that the "fair use" doctrine covers me for that. Nevertheless, I'm sure whoever it is that posted it would love a few likes if y'all feel so inclined.

 

[Diesel85]

I remember hearing, many decades ago, that a USAF study was once commissioned (your tax dollars at work) to examine cockpit gauges and supposedly discovered that for many intelligence categories the optimum display for a pilot was a circular gauge about 2 inches in diameter, needle axis in the center and with the "happy" zone where the needle was pointing straight up. And, I'm sure, there are probably thousands of similar studies done in every environment conceivable, by car makers, gauge manufacturers, foot-press operators, etc. As Grumpy and John point out, it's really a shame that so much effort has already been put into optimizing the conveyance of information and then that knowledge is quickly discarded for something that looks flashier, but is far inferior. (Like, WHY does an automotive gauge need to have user selectable "skins"? This ain't a teenybopper's MP3 player!)

Also, as I stated above, that is not my video or photo that I linked to, it's just one of several that I was able to find that actually showed what I was griping about. I'm hoping that the "fair use" doctrine covers me for that. Nevertheless, I'm sure whoever it is that posted it would love a few likes if y'all feel so inclined.

To add to this, supposedly this is why Ford was a huge follower of the needle straight up. To basically make people worry less. Especially for the oil pressure on many of their vehicles. Along with once the coolant gets up to temperature. The coolant one on most Fords is fascinating because once it's in the middle, it takes a good overheat for it to suddenly "spring" to the red zone. Like an on/off switch. Fascinating.

 

[Jim W]

I remember hearing, many decades ago, that a USAF study was once commissioned (your tax dollars at work) to examine cockpit gauges and supposedly discovered that for many intelligence categories the optimum display for a pilot was a circular gauge about 2 inches in diameter, needle axis in the center and with the "happy" zone where the needle was pointing straight up. And, I'm sure, there are probably thousands of similar studies done in every environment conceivable, by car makers, gauge manufacturers, foot-press operators, etc. As Grumpy and John point out, it's really a shame that so much effort has already been put into optimizing the conveyance of information and then that knowledge is quickly discarded for something that looks flashier, but is far inferior. (Like, WHY does an automotive gauge need to have user selectable "skins"? This ain't a teenybopper's MP3 player!)

Also, as I stated above, that is not my video or photo that I linked to, it's just one of several that I was able to find that actually showed what I was griping about. I'm hoping that the "fair use" doctrine covers me for that. Nevertheless, I'm sure whoever it is that posted it would love a few likes if y'all feel so inclined.

You asked why this has changed?

My reason since I was in design many, many moons ago, a new generation of designers and marketing people are making these decisions today. And they have to make their mark to prove they are worth the cost of employment. Also, they for the most of them all grow up playing on the Video platforms of the day and not doing real world day to day work. Nor for the most part they never took apart a Lawn Mower engine or any other mechanical device in their young life's.

When I was working just before retirement most of the new engineers and vehicle designers didn't even know which end of a screwdriver to use to fasten with.

 

[AH64ID]

Funny thing about aviation gauges, the only round gauges in the helo’s I fly are altitude and airspeed. Everything else is a vertical colored scale.

To add to this, supposedly this is why Ford was a huge follower of the needle straight up. To basically make people worry less. Especially for the oil pressure on many of their vehicles. Along with once the coolant gets up to temperature. The coolant one on most Fords is fascinating because once it's in the middle, it takes a good overheat for it to suddenly "spring" to the red zone. Like an on/off switch. Fascinating.

Your Ram is no different on coolant temp and our oil pressure/temp aren’t even real.

 

[Grumpy8811]

You asked why this has changed?

My reason since I was in design many, many moons ago, a new generation of designers and marketing people are making these decisions today. And they have to make their mark to prove they are worth the cost of employment. Also, they for the most of them all grow up playing on the Video platforms of the day and not doing real world day to day work. Nor for the most part they never took apart a Lawn Mower engine or any other mechanical device in their young life's.

When I was working just before retirement most of the new engineers and vehicle designers didn't even know which end of a screwdriver to use to fasten with.

there is some validity to that... my Brother is an mechanical engineer nearing retirement age at a well known driveline manufacturer , so he has been around the block to put it mildly. He said to me that sometimes the problem at companies is the old guys who tried the same idea 30 years ago and discovered its shortcoming's might not be around to tell the newer breed why not or what happened when we tried that. In effect a continuous process of re inventing the wheel has been instituted by the lack of experience at more senior levels who say NO..

 

[Grumpy8811]

Funny thing about aviation gauges, the only round gauges in the helo’s I fly are altitude and airspeed. Everything else is a vertical colored scale.



Your Ram is no different on coolant temp and our oil pressure/temp aren’t even real.

yep, Coolant and Trans temps are actual readings.. oil pressure and temp are computer calculations on Ram. Explains why I turn off the display on my Tradesman.
If the light isn't on I am good to go.