RPMs Tire Size conundrum

[Yeller]

Have you ever tried to compress steel? Dig into how tires are constructed.

 

[Chiclassics]

Whats your point @Yeller Are you saying that no matter what air is in the tire the Rotation Per Mile is same?

 

[Broncobowsher]

Early TPMS systems used wheel speed sensors from the ABS system to tell a low tire because that one would turn faster. They only ran that for a few years before everyone switched over to in wheel radio based systems. The wheel speed system could (usually) tell if one tire was low. but was confused with a new tire and a worn tire, or all the tires went down by the same amount. So the amount of air in a tire absolutely influences revolutions per mile. Not a huge amount, but it does. A few decades back there was even a magazine article where they measured distance traveled per revolution of a tire at vastly different tire pressures. It was noticable. Steel belts in a tire do not make it immune from this. They flex, as does the rubber, and generate heat in the process.

 

[Yeller]

Whats your point @Yeller Are you saying that no matter what air is in the tire the Rotation Per Mile is same?

More or less. There’s rubber flexing affecting it but it’s a minor amount. No worries test is coming

 

[Chiclassics]

Well 10% with around 36-37 PSI tire pressure which is considered street, for off-road might be more at 20-25PSI

 

[Yeller]

@Yeller won’t say it, but I will.

The rotation per mile on a radial tire within reasonable inflation ranges from 10 to 50 psi are largely the same.

There are some small effects from elastic deformation of the sidewall, (so it’s not the “same” but the tire tread belts act more like a hula-hoop hung by springs. It certainly does not behave linear with air pressure, and not linear with diameter. (Or distance from ground to axle center.)

You can convince yourself at the extreme. If you run the tire with zero pressure, it should have infinite revolutions per mile.

There was a study posted on YouTube years ago by the guys with Hummer H1’s and CTIS. They found empirically that changes in tire inflation pressure did not have the expected change in revs per mile. (It did have SOME, but it wasn’t anywhere near what it should have been.)

Thanks @jamesroney i was going to say it but was waiting till I could prove my point

@Chiclassics 20-25 psi is street pressure for my test mule and 8-12psi is trail pressure. I have enough GPS verified tracks to know I’m right but don’t have that all important empirical data, with pictures, which is coming.

 

[Madgyver]

Too much research. Just drive and enjoy your ride...

 

[Chiclassics]

@jamesroney at zero pressure the rotations per mile would not be infinite but what it would take for the wheel size.

@jamesroney @Yeller I will take your own points to argue. It may touch your ego but I am doing it for growth of knowledge so don't hate me.

Why are you reducing your PSI for for trails in first place? to get more contact with ground meaning you are agreeing your tire is spreading wider to get more grip, which in turn reduces overall circumference. Meaning basically you are going from lets say 33 10.5 tire to 31 11.5 temporarily by bulging because at any point the only constant in a "Rubber" tire is the overall volume - not diameter, not radius and hence certainly not circumference so a big no to same rotations per mile. The rotation per miles change needs to be considerate enough (10% or more ) in the same way reducing tire pressure to work on trails....because they are two sides of same coin. I can write a whole paragraph on radial tires and how ford changed suspension when they are introduced but i dont want to upset @Madgyver and will move on....

I would to see that youtube video though...if you do find it. So I could bust their myth haha

 

[Madgyver]

Why are you reducing your PSI for for trails in first place?
to get more contact with ground meaning you are agreeing your tire is spreading wider to get more grip, which in turn reduces overall circumference.

I can answer this one.
Traction. Gearing change because of tire size from low tire pressure won't make much difference. It's a sacrifice for more traction.
Try it and you'll know and feel the difference. Experience can only teach you this one.

10PSI didn't help here.. soft sand is brutal in this area. He didn't want to go lower and risk popping the bead.. Had to get pulled out.
FYI, These are 37" tires.

 

[EPB72]

@DirtDonk I didnt take into consideration of 200RPMs C4 adds due to non-locking aspect. I think my #s kind of make sense now. Awesome!

Being a math geek I can tell @AZ73 is right. A tire under load especially when not at full air pressure loses to hold the formula of 2*pi*r for circumference. Essentially its not a perfect circle any more. But a circle of radius measured from wheel center to ground. So effectively lets say my 33 tires results in a circumference of 103.62 (2*pi*16.5) inches ideally, under load lets say my overall tire height is 31- i wouldnt even say thats effective diameter because top part of wheel might be 16, bottom half can be 15(due to the squish), so effectively 30. So the effective circumference becomes 2*pi*15 = 94.2 that is 90% of original circumference. So in simple terms a 10% reduction in distance travelled hence the speed. Meaning revolutions per mile will CHANGE when air is let out from tire effectively making it a smaller tire. I bet someone to test the theory though.

YES Ive done this on a simular discussion a year ago or so,, I was dealing with a BMW with indirect TPMS system and since I had my scope out I ran some #s,, don't remember exact but I recall the same tire when pressure dropped 10 psi about.. the wheel speed changed about 16 millisecond per revolution there abouts So small but will trigger a light on the dash .... so as a tire wears so does the revolutions per mile change

nothing new about airing down

 

[toddz69]

Why are you reducing your PSI for for trails in first place?

Traction, flotation and a better ride.

Todd Z.

 

[DirtDonk]

I've never had a traction issue (because I haven't taxed the traction I had all that much!), but I have always lowered my pressures for the softer ride.
Not much mind you, but to somewhere between 12-15. Not enough for the most extreme of us, but still safe enough to go for awhile when you get back out to pavement before airing back up.
For me it's not really the hard trails either. Anyone every taken the road to the lake at the Sierra Trek event? Or the road out of the Rubicon to Tahoe? Washboard roads are the worst part of wheeling/camping. So a softer ride is a blessing!
A 35-40 psi setting is at the higher end of the scale, and over what I bet most of us are running. It's probably good for heavier duty tires, or heavier rigs, carrying a heavier load. But lower pressure is very common. I would guess that most of us here with most common tires are running in the 25-30 range for the front, and 20-25 for the rear. Well below most of the sidewall max ratings.
But tires are different, and so are Broncos. So tire pressures can be different as well. Tire wear is my usual goal, so I base my final settings on how that works out for each vehicle.

 

[Chiclassics]

Moving on...

h2<h1 that for me explains it all. Just so it might give you little more insight here what I can leave you with.

I AGREE that the TIRE circumference itselt doesn't change however the effective contact point that is touching the ground is rotating at a different radius than tires ideal radius. This results in the deflated tire needing to do little more rotations to cover a mile than an inflated one.

 

[El Kabong]

I guess the points I'm hearing say either that the radius at the flat point establishes the distance travelled per rotation, or the circumference stays the same as a full round tire, even if it is flat on the bottom & spread out fore & aft.

Thinking of an extreme example, here's a bulldozer track.

Looking at the track, I wonder if the flat bottom of the tire is somewhat similar. The circumference travels the same distance per revolution, even though it's not in the shape of a circle. The radius becomes useless in calculating the out of round shape, while the circumference remains constant. If the track were in a perfect circle, you could calculate its circumference from the radius, but it would still be the same distance as when it's in its odd shape.

 

[AZ73]

Exactly! When the tread acts as an incompressible belt, then it behaves exactly like a track.

This looks good in theory, but it's just not true. The actual rotations per mile published by the tire manufacturer already takes into account the "flat" part of the tire, which is a shorter distance TRAVELED. It seems logical that, because the "outside" circumference is a constant due to the inability of the belt to change length, that Jame's drawing is correct, but it's not. He's showing you the TIRE, and not the distance on the ROAD. He's describing the TIRE circumference, not the ROLLING circumference. The loaded ROLLING circumference is, depending on the size of a tire, is 1-4% less than the TIRE circumference unloaded. There's a lot going on here but I'll try and make it simple: The STEEL BELTS on the INSIDE Of the tire don't change their circumference, it's the TREAD and SIDEWALLS, which are flexible rubber that do. It gets squeezed into the channels between the treads and out the sides. That's why it's flatter on the bottom. There's a reason why people who race shave the treads on their tires - to remove all the flex of the tread.

Picture 1 is a tire unloaded, inner circle is the steel belts and the outer circle is the tread.
Picture 2 is a tire loaded. Inner circle is the steel belts and the outer circle is the tread.
The TRAVEL distance, the straight line between A and B is shorter than the dotted curved distance. The steel belts don't change their distance. Just like the wheel doesn't change it's distance. But the steel belts and wheel aren't at the outside circumference of the tire. The rubber tread is.
Picture 3 is the detail of this. The straight line between A and B is shorter distance than the curved line between A and B.
So where does the difference between the straight line and the curved line on the tread go? It gets squeezed down and out making the circumference of the rubber on the outside of the tire 1-4% shorter depending on the tire size. On a BFG K/O 2 the difference is about 30 revs per mile.

The comparison to the tank tread is apples to oranges because the tank tread is solid, like the wheel the tire is mounted to. An apples to apples comparison would be if the tire did what it looks like in picture 4. That way the distance between the straight line and the curved line is made up by the outside of the tire bulging out towards the front and back making them equal, instead of out the sides and between the treads. The circumference of the tire would match the distance traveled in picture 4.

In an extreme example, look at the flat tire. Do you think it travels the same distance as an inflated tire if the owner kept driving it? The steel belt doesn't change its length so it should. Nope. The outside diameter of the rubber is squished out the sides. What happens to the length of the steel belt? It's pushed up into the wheel. Take a look at the flat spot of a flat tire that's sitting around. It's not perfectly flat on the bottom. It's all distorted to make up the distance between the flat spot and the circumference of the steel belts. I couldn't find a picture but the diagram for runflat tires shows it.

 

[rjrobin2002]

I had a c4 with 35s and 4.56 gears and loved it. I'm sure the tires actually measured about 33.

It cruised about 60mph right in the power band.

I never had a tach, but I loved its pep.

 

[Chiclassics]

@Yeller @jamesroney @El Kabong what did you get in math in your college ? JK. But ya as @AZ73 stated meticulously the tire effectively acts as smaller tire when deflated. lets move on...

 

[Yeller]

I was going to drive 10 miles at street pressure and turn around and drive the same 10 miles at 8 psi and compare the difference in the odometer. I got lazy and just enjoyed driving the bronco. But I did let someone else do the work and found their test. I’ve never said their wasn’t a difference, just not much of one. You be the judge. BTW this has been a fun debate.

And yes I failed algebra, barely passed geometry, but can build you a race car out of my head and see why a suspension works or doesn’t by looking at it before watching it.

 

[Yeller]

Thanks for the kind words @jamesroney. I forgot to mention I turned down a full ride scholarship to the brand new at the time GM sponsored automotive prototype engineering program at the university of Arkansas. Decided that it wasn’t for me when I was on tour and visited with the professor and he told me I was wrong, a vehicle could not drive up or across a 100% slope.. I told him he needed some practical experience lol. I guess that was my payback for laughing at his “extreme” angle test slope that was at 60% slope, the steepest a vehicle could traverse

 

[Chiclassics]

@jamesroney appreciate the detailed explanation. Glad to hear your background and I love a good discussion. It is missing lately as people move towards digital and EVs and pampered with someone else fixing things for them.

Back to the discussion, I dont think anyone is saying its linear relationship. That 12.5% directly impacts the distance travelled. Axle height is a good linear indicator of distance travelled per rotation. Doesnt matter what tire pattern you are seeing, the squishing of side belts reduces the radius hence "effective" tire size..

Btw I still respect all flat earth believers , because they truly believe in it