No Such Thing As A Dumb Question

[Wild horse 75]

6'-2" here and I'm thinking I might install these.

https://www.agasooutdoor.com/parts-and-gear/p/driver-seat-extension-brackets

Yeah I’ve looked at those too. Will probably get those or make my own regardless. I’m 6’-2” as well.

 

[ba123]

6'-2" here and I'm thinking I might install these.

https://www.agasooutdoor.com/parts-and-gear/p/driver-seat-extension-brackets

Wait until I finish mounting my seats and I’ll tell you how far back they go, but I think they’ll go so far back that they’ll get stopped by the wheel well or at least close.

 

[ssray]

The radial tire issue has been an interesting topic to follow. I’m not an engineer, but enjoy physics and discussions such as this. Circumference not changing did make sense to me when thinking about the design of a radial tire. While any material is elastic I just couldn’t imagine that construction varying much In a steel belted radial. Temperature will affect materials, but that goes out the window as the tire would expand from heat if under inflated . The tank analogy is good as you know the length of the track does mot change due to,the drive chain that it’s based on. One rotation of the drive sprocket will move the track the distance of the circumference of the track around that sprocket. Think of the tire that way and when the pressure is lowered it’s like you have a couple of bogey wheels on the ground inside the tire at the front and rear of the of the contact patch. The lower the pressure the farther apart they move the more the tire squats, but the tread still travels the same distance per rotation.

But then the indirect TPMS comes up and confusion reigns and everything goes out the window again. Thinking about that for a while I wondered if something could be going on in the contact patch of a flattened tire like a wrinkle or wrinkles in the tread in that area that shrink the circumference of the tire. I actually have doubts about that but it’s all I could come up with.

 

[jamesroney]

The radial tire issue has been an interesting topic to follow. I’m not an engineer, but enjoy physics and discussions such as this. Circumference not changing did make sense to me when thinking about the design of a radial tire. While any material is elastic I just couldn’t imagine that construction varying much In a steel belted radial. Temperature will affect materials, but that goes out the window as the tire would expand from heat if under inflated . The tank analogy is good as you know the length of the track does mot change due to,the drive chain that it’s based on. One rotation of the drive sprocket will move the track the distance of the circumference of the track around that sprocket. Think of the tire that way and when the pressure is lowered it’s like you have a couple of bogey wheels on the ground inside the tire at the front and rear of the of the contact patch. The lower the pressure the farther apart they move the more the tire squats, but the tread still travels the same distance per rotation.

But then the indirect TPMS comes up and confusion reigns and everything goes out the window again. Thinking about that for a while I wondered if something could be going on in the contact patch of a flattened tire like a wrinkle or wrinkles in the tread in that area that shrink the circumference of the tire. I actually have doubts about that but it’s all I could come up with.

Well, the one good thing about arguing with people is that you get to learn a lot. I like the analogy of tank track, or hula hoop or link belt or silent timing chain. The indirect TPMS is annoying, and I am not convinced that it actually measures pressure. I have a hunch that it is only looking for small deviations in pulse count relative the other tires. So I don't think it actually does Tire Pressure Monitoring. I think it is a "you have one tire doing something funny" monitor.

But @DirtDonk has made the challenge to prove my assertions...but it is profoundly easier for me to just scream louder and force someone else to disprove my assertion. Years in Engineering Management has provided me with a couple of arguments in my toolbox for occasions just like this. My presumption in any debate is that we should begin with this. "Let's assume that I'm right."

(@ksagis might not appreciate this presumption...)

But there is something that I can do relatively easily. A few years ago, my 2007 JKUR had the infamous christmas tree dash. Part of the troubleshooting is to use a diagnostic tool on the OBD2 port to stream live data. One of the available data streams was a 4 channel live monitor of each individual wheel speed sensor. I used it in live stream mode to see the actual digital input to the ECU. The problem becomes data saturation, since there is just too much data to capture and process. BUT...I thought i remembered an smoothing algorithm that gave some kind of average pulses per minute on each sensor. It should be easy enough for me to get a baseline differential of the 4 data inputs, and then just lower the pressure on 1 wheel, and run the baseline over again. I SHOULD be able to see if there is a difference, and how much difference that there is. I'm going to guess that there will be a measurable change, but I am also going to guess that the change is not linear with tire pressure, and certainly not with wheel hub height.

I just need to go find my OBD2 dongle, and my laptop, and my software key. Wait a minute...I have kids for this!!!

Stay tuned. Gonna take a week.

 

[Slowleak]

I completely agree with your first paragraph. I don’t believe anyone has claimed that indirect TPMS actually measures pressure. It simply uses the ABS sensors to detect changes in wheels speed and sets off an idiot light.
It takes a considerable distance to calibrate, or detect a low pressure situation. That makes me think it is based on a very small but consistent change in wheel speed. Literally everything I have read, related to indirect TPMS, from patent information to Ford service manuals claims that diameter does change with tire pressure. Only a minuscule change would be needed if data is accumulated over a distance. If that change actually occurs is the question, and if it does not, then what actually causes the change in wheel speed.

 

[Oldtimer]

Just spitballing here.
Indirect TPMS may not use wheel rotation?
Low pressure tire will cause vehicle to sag at one corner?
Computer may detect different rest states in suspension sensors, and assume you have a low tire?

No telling what the manufacturer has programmed into the computer.
Just ask VW about dieselgate.

 

[EPB72]

Well, the one good thing about arguing with people is that you get to learn a lot. I like the analogy of tank track, or hula hoop or link belt or silent timing chain. The indirect TPMS is annoying, and I am not convinced that it actually measures pressure. I have a hunch that it is only looking for small deviations in pulse count relative the other tires. So I don't think it actually does Tire Pressure Monitoring. I think it is a "you have one tire doing something funny" monitor.

But @DirtDonk has made the challenge to prove my assertions...but it is profoundly easier for me to just scream louder and force someone else to disprove my assertion. Years in Engineering Management has provided me with a couple of arguments in my toolbox for occasions just like this. My presumption in any debate is that we should begin with this. "Let's assume that I'm right."

(@ksagis might not appreciate this presumption...)

But there is something that I can do relatively easily. A few years ago, my 2007 JKUR had the infamous christmas tree dash. Part of the troubleshooting is to use a diagnostic tool on the OBD2 port to stream live data. One of the available data streams was a 4 channel live monitor of each individual wheel speed sensor. I used it in live stream mode to see the actual digital input to the ECU. The problem becomes data saturation, since there is just too much data to capture and process. BUT...I thought i remembered an smoothing algorithm that gave some kind of average pulses per minute on each sensor. It should be easy enough for me to get a baseline differential of the 4 data inputs, and then just lower the pressure on 1 wheel, and run the baseline over again. I SHOULD be able to see if there is a difference, and how much difference that there is. I'm going to guess that there will be a measurable change, but I am also going to guess that the change is not linear with tire pressure, and certainly not with wheel hub height.

I just need to go find my OBD2 dongle, and my laptop, and my software key. Wait a minute...I have kids for this!!!

Stay tuned. Gonna take a week.

James your absolutly correct indirect tpms dosen't measure tire pressure .... Yesterday I had a little extra time and the right vehicle was dropped off 06 BMW m3 from previous testing I knew exactly which wires to back probe..So I fired up my Pico {4 channel lab scope }. and set up 2 channels on both rear wheel speed sensors ...with pressures at 41 PSI @ 20mph I had a 2 milli second between the two ... I then let the air out of 1 tire {31 PSI} and retest ,, I now had a 23 milli second difference and after 10 min drive the tpms light illuminated... This vehicle if I were to set the tire pressures 15 PSI low and preformed the recalibration and completed it ,the light would stay off even with low tires .. If I recalibrated with 1 tire correct and 1 15 PSI low the light would remain off,,,, I f someone else recalibrated it at 51 psi as the tire side wall said max tire pressure and I reset pressure to what the door jamb sticker said 41PSI without recalibrating the the light would come on....

 

[jamesroney]

James your absolutly correct indirect tpms dosen't measure tire pressure .... Yesterday I had a little extra time and the right vehicle was dropped off 06 BMW m3 from previous testing I knew exactly which wires to back probe..So I fired up my Pico {4 channel lab scope }. and set up 2 channels on both rear wheel speed sensors ...with pressures at 41 PSI @ 20mph I had a 2 milli second between the two ... I then let the air out of 1 tire {31 PSI} and retest ,, I now had a 23 milli second difference and after 10 min drive the tpms light illuminated... This vehicle if I were to set the tire pressures 15 PSI low and preformed the recalibration and completed it ,the light would stay off even with low tires .. If I recalibrated with 1 tire correct and 1 15 PSI low the light would remain off,,,, I f someone else recalibrated it at 51 psi as the tire side wall said max tire pressure and I reset pressure to what the door jamb sticker said 41PSI without recalibrating the the light would come on....

THANKS! I love confirmation of my base presumptions!

BUT...your 23 millisecond vs 2 ms thing is intriguing. I wish that you had taken a measurement of the ground to center of wheel hub as well. But no matter. 20ms delta on a 31/41 psi (25%) pressure change makes me think that it won't be linear, but it might be proportional. So if the baseline is 41 psi, and you dropped it by 10 and got 23ms delta then if you drop it by 10 more, I wonder how much MORE delta you would see. Then lastly, I wonder what it does at lets say 10 psi. Would you see a 40 ms delta at 21 psi, and a 60ms delta at 11psi...or would it reach a point where there delta becomes asymptotic to zero?

I have no clue what the BMW algorithm is looking at, but it seems to take a long time to make a decision.

At the end of the day, it doesn't matter all that much to me...because you have successfully demonstrated that the pressure does not correlate directly to revolutions per mile. Which was my initial assertion. There is no doubt that there is SOME effect of balooning and inflation. But the fundamental notion that the distance from the wheel hub center to the earth is the major factor is de-bunked.

It's hard to be right all the time... But I'm coping.

 

[Slowleak]

Just spitballing here.
Indirect TPMS may not use wheel rotation?
Low pressure tire will cause vehicle to sag at one corner?
Computer may detect different rest states in suspension sensors, and assume you have a low tire?

No telling what the manufacturer has programmed into the computer.
Just ask VW about dieselgate.

Everything I have read states something like this that I posted earlier “From Bridgestone: “Based on the rate of revolution of each wheel, the computer can interpret the relative size of the tires on your vehicle. When a wheel starts spinning faster than expected, the computer calculates that the tire is underinflated and alert the driver accordingly.” Most state that an under inflated tire has a decreased diameter and spins faster. Ford, Honda, Bridgestone all state the same thing….

I expect that there is a very minor difference in diameter with a pressure change, but enough to be detected over a distance by the wheel speed sensors. That’s been my thought all along but then I also read and hear that a change in diameter does not occur…

 

[Wild horse 75]

But now I’m wondering is that difference due to rolling resistance rather than change in diameter/circumference. We all know lower tire pressure has more surface contact therefore it increases traction (I know to a point beyond which the inside tread lifts back off the ground). More resistance will slow a wheel down. That’s why you will notice a pull when one of your front tires is low.

 

[ksagis]

James your absolutly correct indirect tpms dosen't measure tire pressure .... Yesterday I had a little extra time and the right vehicle was dropped off 06 BMW m3 from previous testing I knew exactly which wires to back probe..So I fired up my Pico {4 channel lab scope }. and set up 2 channels on both rear wheel speed sensors ...with pressures at 41 PSI @ 20mph I had a 2 milli second between the two ... I then let the air out of 1 tire {31 PSI} and retest ,, I now had a 23 milli second difference and after 10 min drive the tpms light illuminated... This vehicle if I were to set the tire pressures 15 PSI low and preformed the recalibration and completed it ,the light would stay off even with low tires .. If I recalibrated with 1 tire correct and 1 15 PSI low the light would remain off,,,, I f someone else recalibrated it at 51 psi as the tire side wall said max tire pressure and I reset pressure to what the door jamb sticker said 41PSI without recalibrating the the light would come on....

This is awesome data and we should be able to back out some rough estimate of tire diameter change with pressure:

(yes I made some top level assumptions, but none of them should affect the final conclusion)

• 20 mph = 29.333 ft/sec
• Taking a guess of tire diameter of 26 inch = 87.86 inch / 7.330 ft diameter
• AT 20 mph, tire needs to rotate 4.002 times per sec
• Taking delta of 0.023 second and calculating ratio of 4.025 / 4.002 = 1.00574
• 1.00574 times 26 inches - minus orig diameter = 0.149" of diameter difference due to 10 psi delta

If I did that quick math correctly, for 10 psi difference, the diameter changed 0.149". Can somebody check, confirm, or refute my math?

Per the @jamesroney comment on why so long to make a decision, I think they likely have tuned the algorithm response time to avoid false positives due to use cases like driving around in a parking lot and differences in wheel speed due to turning (inner versus outer wheel), although they could likely look at steering wheel position to ignore that use case.

Thanks to @Slowleak and @EPB72 for bringing good data to the banter.

At the end of the day, it doesn't matter all that much to me...because you have successfully demonstrated that the pressure does not correlate directly to revolutions per mile. Which was my initial assertion. There is no doubt that there is SOME effect of balooning and inflation. But the fundamental notion that the distance from the wheel hub center to the earth is the major factor is de-bunked.

Not sure I understand this comment, I thought your original statement and position was when the military changed their tire type, that changing pressure was no effective in changing vehicle speed for convoy operations. If I understood that position correctly, I think the data conclusively refutes that.

We don’t have data on if the tire diameter changes what happens tp hub height, but my guess is first order, the center of wheel hub to ground is also changing. Anybody have a dial indicator handy to reference the front hub and take out 10 psi?

 

[Slowleak]

We don’t have data on if the tire diameter changes what happens tp hub height, but my guess is first order, the center of wheel hub to ground is also changing. Anybody have a dial indicator handy to reference the front hub and take out 10 psi?

Wheel hub to ground distance will change with diameter but it is irrelevant. The tread will cover the same distance per revolution regardless of the height of the hub. If you add weight to a vehicle the tires compress and the hub height changes, but the wheel speed does not change. The vehicle does not speed up.
What does change the wheel speed is the change in rolling circumference that occurs when the diameter changes. The theory behind indirect TPMS is that low air pressure reduces the diameter and the rolling circumference changes. The tire covers less distance per revolution and the wheel spins faster. But going back to my original post, How is that possible if, as some insist, air pressure changes do not change diameter or circumference and tread length can not change? Maybe we are all now agreeing that pressure does cause a change in rolling circumference.

 

[ksagis]

Wheel hub to ground distance will change with diameter but it is irrelevant. The tread will cover the same distance per revolution regardless of the height of the hub. If you add weight to a vehicle the tires compress and the hub height changes, but the wheel speed does not change. The vehicle does not speed up.
What does change the wheel speed is the change in rolling circumference that occurs when the diameter changes. The theory behind indirect TPMS is that low air pressure reduces the diameter and the rolling circumference changes. The tire covers less distance per revolution and the wheel spins faster. But going back to my original post, How is that possible if, as some insist, air pressure changes do not change diameter or circumference and tread length can not change? Maybe we are all now agreeing that pressure does cause a change in rolling circumference.

Yes, I am saying that pressure affects circumference (and backed into an estimate of diameter change above).

And also saying that hub height would also change if pressure changes.

Agree that circumference is main driver to different wheels rotation rates.

The only reason I suggested to index of the hub to verify the calculated 0.149” math was it would be much easier to get a good datum for the dial indicator.

 

[Slowleak]

I did some quick math and a .10 decrease in height on a 28 inch tire would result in change in circumference of .31 inches and an additional 2.56 wheel revolutions/mile.
I have no idea how to find out, or figure out, how air pressure really impacts a tire circumference. I guess I’ll just have a beer….

 

[ksagis]

I did some quick math and a .10 decrease in height on a 28 inch tire would result in change in circumference of .31 inches and an additional 2.56 wheel revolutions/mile.
I have no idea how to find out, or figure out, how air pressure really impacts a tire circumference. I guess I’ll just have a beer….

I'm gonna join you in that beer, cheers!

 

[Yeller]

A tire will squirm at a lower pressure too, which accounts for some of that change as well. If it didn’t you wouldn’t notice a pull from a low tire. What it does prove is that change is small, which makes sense, how many of you have driven on a flat tire and not known? I know I have.

 

[jamesroney]

Not sure I understand this comment, I thought your original statement and position was when the military changed their tire type, that changing pressure was no effective in changing vehicle speed for convoy operations. If I understood that position correctly, I think the data conclusively refutes that.

I will clarify on Monday when I have some time. I'm working today.

But you are Correct on my original statement. But this data does not refute the original statement. I believe that it supports it fully. The data shows that the change in circumference is impacted by air pressure. But it does not show proportionality between air pressure and the circumference.

For example, if you have a 36 inch tire on an 16 inch wheel, you would have a radius of 18 and 8 inches at tire pressures of 50psi and 0 psi. Based on this, you would expect a 50 psi change in tire pressure to yield a 44% change in circumference.

Taking your number, if 10 psi gets you .149 change in diameter then 4x that would get you .596 change in diameter. But I'm looking for 10 INCHES of change in diameter.

So I absolutely concur that air pressure is proportional to the change in diameter. No argument there. But the change in diameter is vastly smaller than expected because of the "incompressible belt" problem. Which is why a 10% change in air pressure does not translate to a 10% change in speed. An extended belt is longer than a compressed belt. So if a bias ply tire belt is capable of 5% elongation and a radial can elongate 2%...Air pressure can explain that portion. I hope I'm saying it right.

Gotta run.

Gotta run.

 

[ssray]

So I absolutely concur that air pressure is proportional to the change in diameter. No argument there. But the change in diameter is vastly smaller than expected because of the "incompressible belt" problem. Which is why a 10% change in air pressure does not translate to a 10% change in speed. An extended belt is longer than a compressed belt. So if a bias ply tire belt is capable of 5% elongation and a radial can elongate 2%...Air pressure can explain that portion. I hope I'm saying it right

To put it another way I took the example of the 26” tire and figured for 60 mph. Assuming the same engine rpm the .149 smaller diameter tire would give a bit over 59.5 mph so not much change there.

As to the discussion on rolling radius or squat related to pressure that‘s interesting as well. My impression from inflating tires around the farm over the years is that the first 10 psi gets quite an increase in height, 10 to 20 yields less gain and so on. Not linear. I could be full of hot air and fooled by the change in inflation rate as the tire gains pressure. Have to try measuring a tire to see about that!

 

[DirtDonk]

That would vary by tire, and by the weight of the vehicle it’s lifting.
But you’re right in a typical scenario, that first 10 pounds makes a big difference.
In a light duty tire, going from a flat sidewall to probably half its height or more, from when inflated. The rest takes longer, but that first few pounds makes a big difference.
A big, heavy duty rated tire on a light vehicle however, probably would never even go flat at zero psi. I’ve run into that before.

 

[Slowleak]

Maybe this will help somebody figure something out. I might as well try to read Chinese hieroglyphics. At one point it says “puff up”. I understood that part.

http://wanderlodgegurus.com/database/Theory/TireGeometry.pdf