No Such Thing As A Dumb Question

[Wild horse 75]

Nope. This is where the facts destroy the assumptions.

Everybody always thought that the distance traveled by a tire was proportional to it's rolling radius. So if you measure from the ground to your axle center...on a 35 inch tire...it is 17 inches. If you deflate your tires by 2 inches, to a rolling radius of 15 inches, The revolutions per mile are UNCHANGED.

Once you think about it...you will say: "Wow...that James is a genius." But I'm really not. And in this thread, I'm not allowed to point out anything dumb...

So here's the deal. Back in the early 1980's the Military developed CTIS (Central Tire Inflation System) on the HMMWV. They ran with 37 inch Goodyear Bias Ply tires. The tire inflation could be adjusted in the cab. In order to keep speed with other vehicle in a convoy the operator would adjust his indicated speed to offset his tire inflation. In 1990, a new radial tire was deployed during desert storm. Operators were shocked that they could maintain convoy speed and speedometer reading REGARDLESS of air pressure. This has been proven on video. Changing the rolling radius of a radial tire does NOT change the distance it travels per revolution. There was a youtube of a a test track and a video camera counting rotations verus travelling a set distance. It's very enlightening.

The reason is because your concept of the movement dynamics of a radial tire is flawed. If you model the tire as a hula-hoop, with springs extending from the rim to a center hub...you can see that one rotation of the hula hoop travels the same distance no matter how low the center hub sits. And that's what happens with a radial tire. The carcass is basically a huge chunk of in-compressible belts under the tread...with an inner bead riding inside of it. All of the squirming happens in the sidewall.

I admit that it took me a while to get my head around this...So enjoy!

This is interesting. I’ve never heard it before. Not questioning it but why does it only apply to radial tires and not bias ply? I’ve always been under the assumption of what you said in your post so I’m wondering if it was old teachings from the bias ply days that were still in place when I was in school.

 

[jamesroney]

This is interesting. I’ve never heard it before. Not questioning it but why does it only apply to radial tires and not bias ply? I’ve always been under the assumption of what you said in your post so I’m wondering if it was old teachings from the bias ply days that were still in place when I was in school.

Bias ply tires have treds that can squirm on the carcass...and the carcass is somewhat pliable. So it's like your bicycle tire when it is completely flat. Radial construction can have lots of lateral movement, but almost no ability for circumferential shifting.

Question it all you want. I don't learn anything by being right all of the time. I LIKE to be questioned, challenged, judged, and convicted. If I'm standing naked in a parade I would certainly wouldn't want you to tell me how nice my clothes look.

Questioning, confirmation, experimentation, challenging, arguing and assertion are very important. Dissent is essential. Without it, you end up with Boys in the Girls room. And we'd all drive a Prius.

 

[Wild horse 75]

Bias ply tires have treds that can squirm on the carcass...and the carcass is somewhat pliable. So it's like your bicycle tire when it is completely flat. Radial construction can have lots of lateral movement, but almost no ability for circumferential shifting.

Question it all you want. I don't learn anything by being right all of the time. I LIKE to be questioned, challenged, judged, and convicted. If I'm standing naked in a parade I would certainly wouldn't want you to tell me how nice my clothes look.

Questioning, confirmation, experimentation, challenging, arguing and assertion are very important. Dissent is essential. Without it, you end up with Boys in the Girls room. And we'd all drive a Prius.

Thanks. What I wasn’t questioning was the reasoning you posted. I believe what you wrote to be true. It made sense I just didn’t understand why it changed between radial and bias ply. When I was in school bias plus we’re done but not long gone. And school text books don’t get updated very frequently. So when we learned about ratios the testing you quoted hadn’t happened yet so the text would’ve been what was known at the time.

 

[jamesroney]

Thanks. What I wasn’t questioning was the reasoning you posted. I believe what you wrote to be true. It made sense I just didn’t understand why it changed between radial and bias ply. When I was in school bias plus we’re done but not long gone. And school text books don’t get updated very frequently. So when we learned about ratios the testing you quoted hadn’t happened yet so the text would’ve been what was known at the time.

I don't think anyone actually KNEW until it was discovered empirically. It's not something that would be measured often, and all of the testing to develop the theory would have been on bias ply tires...which are more like balloons than incompressible belts.

I used to inflate my rear tires until both sides had exactly the same radius from axle center to the pavement...so I wouldn't wear out my limited slip clutches. Then I bought a set of Humvee beadlocks, and a full case detroit. I didn't want to have my Detroit constantly indexing...and it always wanted to push me around under acceleration. I thought I could adjust it with tire pressure. Turns out I couldn't. But an internet search with tire pressure, HUMMWV, and rolling radius got me to the guys with their CTIS on video. Then I put in an ARB.

On a Side note, when you see the dead carcass of a blown out Dually trailer tire on the highway, it is not the result of a tire losing pressure, and overloading the other tire. The tire that blows is always the one that lost pressure. The overloaded, full pressure tire just keeps going. The underinflated tire loses sidewall and generates heat. Then it generates more heat. Then it sucks in the tube. Then it melts, and disintegrates, and throws the belt. If it were the overloaded tire that failed...there would be a truck parked next to the dead carcass. But the truck and trailer just keep going...

 

[Wild horse 75]

I don't think anyone actually KNEW until it was discovered empirically. It's not something that would be measured often, and all of the testing to develop the theory would have been on bias ply tires...which are more like balloons than incompressible belts.

I used to inflate my rear tires until both sides had exactly the same radius from axle center to the pavement...so I wouldn't wear out my limited slip clutches. Then I bought a set of Humvee beadlocks, and a full case detroit. I didn't want to have my Detroit constantly indexing...and it always wanted to push me around under acceleration. I thought I could adjust it with tire pressure. Turns out I couldn't. But an internet search with tire pressure, HUMMWV, and rolling radius got me to the guys with their CTIS on video. Then I put in an ARB.

On a Side note, when you see the dead carcass of a blown out Dually trailer tire on the highway, it is not the result of a tire losing pressure, and overloading the other tire. The tire that blows is always the one that lost pressure. The overloaded, full pressure tire just keeps going. The underinflated tire loses sidewall and generates heat. Then it generates more heat. Then it sucks in the tube. Then it melts, and disintegrates, and throws the belt. If it were the overloaded tire that failed...there would be a truck parked next to the dead carcass. But the truck and trailer just keep going...

I guess that’s why companies like Subaru don’t care too much about inflation of their tires but heaven forbid you’re more than 10% difference in wear. I’ve always thought like you did. Make sure those tires are damn close to prevent wearing diff clutches. I knew about the trailer thing. It’s always heat and that comes from friction which is only cause when a tire is low on air. Same reason you end up with rubber marbles inside a tire if it’s been driven on flat for any length of time. That can really mess with you trying to balance a tire if you didn’t know it had been plugged.

 

[ksagis]

The reason is because your concept of the movement dynamics of a radial tire is flawed. If you model the tire as a hula-hoop, with springs extending from the rim to a center hub...you can see that one rotation of the hula hoop travels the same distance no matter how low the center hub sits.

James, I think I follow the overall argument that you're presenting, but struggling to reconcile what I think the rationale being presented is with grade school trig.

i.e. s = r times omega, where s is the radial arc, r is the radius, and omega is the included angle.

For one inch out of the mile traveled, if r goes down (per your hula hoop analogy above,) then omega goes up. If omega goes up, then there has to be more rotations per mile if the radius goes down.

Do we agree that for a lower tire pressure the center of the hub will go down? For simplicity, let's assume we have a slowly rotating tire such that centrifugal forces aren't in play.

 

[ba123]

I didn't find any videos, but didn't look for long. I think I get it and I think it's because (1), its's a steel belted tire, and (2), the tire can't slip on the wheel. So, if position A is marked on the tire tread when full, that position will always be that spot in relation to the wheel. and the circumference never changes because of that.

I think maybe the only thing that really changes is how much of the tire touches the ground (the size of the flat spot) and it just kinda moves more like a tank track instead of a tire.

 

[ksagis]

I didn't find any videos, but didn't look for long. I think I get it and I think it's because (1), its's a steel belted tire, and (2), the tire can't slip on the wheel. So, if position A is marked on the tire tread when full, that position will always be that spot in relation to the wheel. and the circumference never changes because of that.

I think maybe the only thing that really changes is how much of the tire touches the ground (the size of the flat spot) and it just kinda moves more like a tank track instead of a tire.

Videos are interesting to me, but I'm always going to start with first principles and my counter to James above (after all, he said he like to debate!)

 

[jamesroney]

James, I think I follow the overall argument that you're presenting, but struggling to reconcile what I think the rationale being presented is with grade school trig.

i.e. s = r times omega, where s is the radial arc, r is the radius, and omega is the included angle.

For one inch out of the mile traveled, if r goes down (per your hula hoop analogy above,) then omega goes up. If omega goes up, then there has to be more rotations per mile if the radius goes down.

Do we agree that for a lower tire pressure the center of the hub will go down? For simplicity, let's assume we have a slowly rotating tire such that centrifugal forces aren't in play.

No, we are not in agreement. For a lower tire pressure, the center of the wheel hub will certainly go down. But the does not equate to the center of rotation. S is not equal to r x w. Because you are using the wrong r. The distance from the hub to the ground is not r. It turns out that r is 1/2 of D. And D is constrained to be the be the circumference/ pi.

I like the model presented by @ba123 1 rotation of the hula hoop is always equal to 1 rotation of the hub. Doesn’t matter where the hub is located spatially.

 

[jamesroney]

I didn't find any videos, but didn't look for long. I think I get it and I think it's because (1), its's a steel belted tire, and (2), the tire can't slip on the wheel. So, if position A is marked on the tire tread when full, that position will always be that spot in relation to the wheel. and the circumference never changes because of that.

I think maybe the only thing that really changes is how much of the tire touches the ground (the size of the flat spot) and it just kinda moves more like a tank track instead of a tire.

You got it.

 

[CabinCruiser]

For people with long legs, does switching the gas and break pedals from stock to any aftermarket brand (lokar probably the most common for EBs) result in any more right knee extension while driving?

 

[ba123]

For people with long legs, does switching the gas and break pedals from stock to any aftermarket brand (lokar probably the most common for EBs) result in any more right knee extension while driving?

I would think the lokar one does the way it mounts and how it moves.

Here's my lokar pedal.

I can take other pics if needed but it's on page 4 of my build thread.

 

[Wild horse 75]

Is that not the factory pedal with simply the Lokar pedal pad on it?

nevermind you were talking about you through floor gas pedal. Carry on.

 

[jamesroney]

For people with long legs, does switching the gas and break pedals from stock to any aftermarket brand (lokar probably the most common for EBs) result in any more right knee extension while driving?

Maybe, but switching the bucket seat mounts for the one's with 3 inches more legroom certainly does. I'm 6'-3" and appreciate the extra legroom.

 

[ksagis]

No, we are not in agreement. For a lower tire pressure, the center of the wheel hub will certainly go down. But the does not equate to the center of rotation. S is not equal to r x w. Because you are using the wrong r. The distance from the hub to the ground is not r. It turns out that r is 1/2 of D. And D is constrained to be the be the circumference/ pi.

I like the model presented by @ba123 1 rotation of the hula hoop is always equal to 1 rotation of the hub. Doesn’t matter where the hub is located spatially.

I still disagree, remember my use case is for a slowly rotating tire. In a contrived example to illustrate my point:

• Take a very compliant tire with no air in it
• I assert it would be sitting on the wheel
• The center of rotation is effectively half the diameter of the wheel since the tire sidewall is no longer contributing to height
• Circumference now is a meaningless term since its not a circle anymore
• We can now debate what is the perimeter of this configuration. My engineering intuition tells me its less that the inflated circumference (any opinions out there?) If that is the case, seems to me that previously stated rationale breaks down.
• I don’t think there is “Conservation of Circumferential Length” in play in my example!

Standing by for a rebuttal!

 

[gnpenning]

I might have got the question wrong

 

[Speedrdr]

I think you forgot a trailing zero and an extension of the exponent…or maybe not!

Randy

 

[ksagis]

I might have got the question wrong


View attachment 915225

Best post ever! Well done sir

 

[jamesroney]

I still disagree, remember my use case is for a slowly rotating tire. In a contrived example to illustrate my point:

• Take a very compliant tire with no air in it
• I assert it would be sitting on the wheel
• The center of rotation is effectively half the diameter of the wheel since the tire sidewall is no longer contributing to height
• Circumference now is a meaningless term since its not a circle anymore
• We can now debate what is the perimeter of this configuration. My engineering intuition tells me its less that the inflated circumference (any opinions out there?) If that is the case, seems to me that previously stated rationale breaks down.
• I don’t think there is “Conservation of Circumferential Length” in play in my example!

Standing by for a rebuttal!

I've been block sanding and welding holes. Sorry for the delay.

Let's take your example.

Compliant tire with no air in it. -this will be the most critical part of your assumption...
I assert it would be sitting on the wheel. - well, no...not if your first assumption is correct. it will be hanging in space, either from a boundary layer of air, or from elastic compressive forces of the squished sidewall. So it will certainly be near the wheel, but it won't be mechanically coupled to the wheel.
The center of rotation is half the diameter of the wheel, since the sidewall is no longer contributing to height. - you seen to be stuck on the height thing. But there is no center of rotation as it pertains to the tread belt. Maybe it would be more clear if you imagine a Morse type steel timing chain, or a solid rubber belt of any kind. If you mark a line on the belt, and then push the belt along the surface until the line comes back, the belt will have travelled a distance equal to the length of the belt or 1 perimeter. If the belt is round, then that distance is 1 circumference. Circumference is never meaningless, because we only used circumference to simplify the math. The figure of merit was always perimeter. As soon as the belt is no longer perfectly round, the math gets really hard. Belt length is easy when it is flat and when it is a circle. Anything in the middle is complex.

We can debate the complexity of the deformed shape all you want. Given that "my engineering intuition" is a oxymoron, I'm going to translate that to "my engineering assessment" Then I'll make some kind of wisecrack about public school. There's probably some Engineer from Purdue lurking out there too. The fact remains that my original presumption was that the radial ply steel belts were in-compressible in the tangential direction. Thus creating a perfectly inelastic compressive member. I believe that I used "hula-hoop." But a stiff leather belt will do the trick.

So My base assumption in the hypothesis is that you have a FIXED length perimeter. It is convenient to assume that it is always circular, but that is irrlelvant to the model. But yes, a circular ring of fixed circumference, connected to a floating hub via a "compliant tire." 1 revolution of the ring is equal to one revolution of the hub, and the hub always coincides with a line perpindicular to the point of tangency. For every revolution of the ring, the hub rotates 1 revolution, and translates the exact same distance as the ring perimeter.

I think if your model is fine. Just assume the perimeter is conserved, and you will be fine.

 

[DirtDonk]

Can we just test any of this with speedometers, GPS or otherwise, and an accurate tachometer?
Just drive it around a specific area at a verified speed and see if the RPMs change?
Or see if the reading on the speedometer changes at a given rpm with changes to air pressure?
Seems like if the measuring devices are reasonably accurate, we would see a difference, or not.