RPMs Tire Size conundrum

[ntsqd]

Did reducing the air pressure in the tire also reduce the tire's circumference at the tread? If so, how did the tread shrink it's former length?

 

[ntsqd]

I hadn't really thought about it until you put it that way.

The problem with assumptions is that sometimes we make them without realizing what we've done.

 

[Chiclassics]

No one is agreeing @jamesroney .I am new to this forum not new to automobiles.

I think who ever believes in your theory need to get together and dig this to understand why rest of world thinks otherwise. There was a reason I made flat earth comment. Here it goes....

--->
The circumference of tire is not shrinking. You are thinking, that we are thinking like that and coming up with the conclusion of less distance travelled for same rotations. There lies your "assumption", no we are not thinking like that. A deflated tire with squished sides "Although has same circumference" ( we all know how Radial tires work!), needs to make more rotations to cover same distance. It is a simple math problem not a physics problem. @jamesroney You having an engineering background should understand this fundamental thing how this is derived.

What you think is that as the tire circumference didn't change , for one rotation of axle/wheel, the tire is covering same distance right? . BUT, there is another variable at play here. The distance covered by tire per one axle rotation is NOT the physical tire circumference its rotating on but the effective tire that is touching the ground which is defined by the radius from axle to ground. Which you mentioned has 12.5% reduction - This is where you LOST your argument already.

Then you may ask my full axle rotation is moving all the circumference of tire so it should move the distance as of circumference RIGHT!!!????...NO ! Because the rate at which a tire contact point is leaving the ground plays a role. Let me dumb it down so it reshapes those neural pathways that are imprinted for a long time. In a properly inflated tire its leaving the ground almost instantaneously. However, in a deflated tire(under your own defined range) its staying on ground for a little bit longer as its part of flat touch on the ground- this delay causes the loss of distance travelled. This slight delay makes the final effect. The way you can understand this is to take it to extreme. Flat tire as others mentioned. The tire circumference still stays same but the distance travelled is more closer to wheels circumference and not tire's even for radial tire.

Let debunk another "assumption" you are trying to base your argument on. It doesn't matter if its radial tire or bias ply - the only difference would the shape of their squish. The fact that you are using "Steel Reinforcement" as your point of argument itself doesn't hold water because this was never about circumference this was about "effective radius" of the tire. Btw a bias ply tire doesn't shrink circumference it simply has uniform squish like a bicycle tire .

 

[ntsqd]

How do you account for the length of the tread not changing then? If the tire is to revolve one complete revolution, and not slip on the ground, then how do you make that shorter distance and the fixed (more or less) length of the tread compute? Without the tread slipping on the ground your theory fails to describe the reality.

 

[DirtDonk]

I don't have a dog in the math game here, so can't join the hunt. But my question is, why do all the manufacturers of all-wheel-drive vehicles insist that the tires be EXACTLY the same size, right down to tread wear and air pressure variations? Is it seriously a big variable, or are they legitimately concerned over a, for example, 5% variation, causing wear on any clutches, or too much whimsy inside the computer brain?
Or are they worried that the difference is actually quite notable? I thought about the old tire-pressure monitoring done by the tone ring method too, but that was covered a few days ago and, if I remember, it was just a microscopic difference that the system was looking for.
Another thing that I think about, but again may have no bearing on anything, are the other factors in play, between the rubber and the engine.
Ignoring any slippage that might be happening, does the distance to the axle shaft, the existence of the differential gears, the existence of the transfer case (in our case, where the speedometer is getting its reading), the existence of the transmission, all before the road input reaches the engine, have any variable effect on any of your equations? Is the engine rpm changed by just a few? I think that was the gist of a previous comment.
Maybe nothing, but they're the kind of things that keeps me awake and wanting to join the conversation.

Paul

 

[StnePny]

I don't have a dog to put in this fight .... But....

I run a 351w - C4 - 33' tire - 3.90 gear

The Calculation Equation of M.P.H x Gear Ratio OVER Tire Diameter x 336 = RPM

65 mph x 3.90 =253.5 / 33 = 7.6818 x 336 = 2,581 RPM (My Tach shows about 2,800 - 2,900 rpm)

 

[Chiclassics]

How do you account for the length of the tread not changing then? If the tire is to revolve one complete revolution, and not slip on the ground, then how do you make that shorter distance and the fixed (more or less) length of the tread compute? Without the tread slipping on the ground your theory fails to describe the reality.

Good question. This is where it gets into the physics side of things...away from the math. You probably have to come with up an explanation to match my math because math doesn't lie , physics needs to match it with right assumptions. Ill give it a try to give you insight into physics at play here but there might me other physics experts that can give better explanation.

Think of it like this. Radial tires make it a different material science problem than a bias-ply tires - still same problem but at different tensions(higher tensions for radial). Because we are literally dealing with different combination of materials like rubber+ fabric vs rubber+ steel.. For the same "Math" situation.

Radial tires are designed to operate ideally for their tire size. Meaning a 30 inch radial tire is designed to operate at 15 inch radius by properly inflating the tire. So it operates as an outer steel belt wheel of 30 inches that is welded to inner 15 inch wheel-Ideally as you are thinking. When you deflate the tire so lets say your 30 inch tire diameter is 29 inches now, 15up and 14down. Mathematically it needs to operate at a 14 inch circle with a physical material of 15inch. So when the tire makes full rotation the contact point is only making 2*pi*14 = 87.92 inches instead of 2*pi*15=94.2. Now your asking where did the 6inches go. (6 7 (lol)-new trend on social with no meaning). This is where the materials come in to pic how exactly is steel layered. The steel pattern is a mesh structure that has some range of radii it can operate and absorb the difference after which it will cause binding tensions between outer layer of tire to the inner edge where it held on to wheel. This also dissipates as heat under deflated conditions. Also sometimes results in slip on ground especially if other tire is not deflated equally. So in off-road trails there is slip on the terrains when you deflate the tires at diff air pressures. Sometimes one may not notice this due to open/limited slip diffs.

 

[ntsqd]

Math and physics are intertwined, can not really separate one from the other.
FWIW I think you've managed to confuse yourself. It is a difficult thing to work your head around.
Within the limits of the elasticity of the steel belts and the rubber composite that makes up the tire the circumference (length) of the tread does not change no matter what the inflation pressure is. One tire revolution, without slippage, must be one tire's circumference worth of lateral translation (distance traveled). The radius of rotation (axle center-line distance from the ground) does not matter to the distance traveled. The tread in this regard is no different than a D8's track, but continuing that analogy much further leads you down a wrong path.

Slippage can and does occur, but not always. Most of the time it does not unless the driver does so deliberately. If it did the tire tracks in the dirt would always be smeared with the imprint of the individual tread blocks being indistinguishable as they when the driver does deliberately spin the tires.

 

[DirtDonk]

I don't have a dog to put in this fight .... But....

I run a 351w - C4 - 33' tire - 3.90 gear

The Calculation Equation of M.P.H x Gear Ratio OVER Tire Diameter x 336 = RPM

65 mph x 3.90 =253.5 / 33 = 7.6818 x 336 = 2,581 RPM (My Tach shows about 2,800 - 2,900 rpm)

Don’t forget to add the 2 to 300 RPM of torque converter slippage present in a C4.
Gets it pretty close.
Now compare that “33 inches” in your calculation, to the exact tire diameter.
Or was that the actual measured diameter of your tire?

 

[Speedrdr]

After following this thread from the beginning, I fail to see how any of this over inflation/under inflation makes any significant difference IF we are talking about a single tire size involved…and y’all can point me to the post if I’m wrong…but it seems like an exercise in futility.

 

[Chiclassics]

@ntsqd you are projecting you confusion to me , im trying to be the therapist here. Mostly acknowledged industry approach is called rolling circumference, so you have to prove your theory not me.! As few others mentioned above it is recommended to inflate or deflate both your tires at same pressure especially when rear has lockers for the very reason. Otherwise it will cause binding similar to gear binding.

The circumference of tire at different air pressures is same if tire is hanging in air, by the time its on ground under load the differences go your contact patch, which results in drag and more friction less mpg. The simple way you can prove this to go to extremes , and completely take air out and run on flat and see the distance it covers per revolution and do same with full air in. The same affect is happening between 35PSI to 10PSI when tire diameter comes down 10-12%.

Moving on now , spent enough time on this debate ...

After following this thread from the beginning, I fail to see how any of this over inflation/under inflation makes any significant difference

Na for general RPM/Tire size calcs...we are just trying to solve age old discussion people have been arguing for decades? , nothing new. Its fun. And not productive in any manner

​

 

[ntsqd]

I suffer from no confusion on the topic. Your rambling non-explanations and officious sounding non-nonsensical phrases OTOH..... Inclines me to think you might be a lawyer except that nothing that you've said effectively counters my arguments. Effectively you're dancing in circles yelling "I'm right! I'm right! I'm right!"

I need not prove anything. I doubt it could be done with such a firmly entrenched sense of correctness.

So given all of that, I regret having joined this waste of time and will now retire from giving this thread any further attention.

 

[jamesroney]

I suffer from no confusion on the topic. Your rambling non-explanations and officious sounding non-nonsensical phrases OTOH..... Inclines me to think you might be a lawyer except that nothing that you've said effectively counters my arguments. Effectively you're dancing in circles yelling "I'm right! I'm right! I'm right!"

I need not prove anything. I doubt it could be done with such a firmly entrenched sense of correctness.

So given all of that, I regret having joined this waste of time and will now retire from giving this thread any further attention.

I also deleted my contribution in this thread. Matthew 7:6

 

[Yeller]

I also deleted my contribution in this thread. Matthew 7:6

James I’m also reminded of Proverbs 23:9

 

[ssray]

James I’m also reminded of Proverbs 23:9

That can get used a lot this day and age, but I better stop there! 🫢

 

[Chiclassics]

I will take a higher ground here and don't make derogatory remarks because i respect this forum. You on other hand do not respect another's view point and jump calling someone a fool, such an easy path to take. Quoting bible doesn't make you smart. It only exposes your weakness. I still respect you.

I have given all scientific explanations possible , exactly countering your points on circumference. But it seems like you and the few others do not want to reason which makes it a futile effort. I am no lawyer but I have a command on my logic , my officious tone is only backed by my logic but not ego and in no sense to offend anyones opinion. I believe in age old discussion of topics in a logical way which is needed for innovation.

As I said moving on. Others need help.

 

[Yeller]

The sentiment that you have spoken, is mutual. If I offend you, you have my sympathy along with empathy.

I was going to make a lengthy response, but will also take a high road and just be quiet.