The Donut In The Trunk

[CapriRacer]

Like this (in green):

And the tread profile would be the distance between the top of the tread down and the top of the belt (or cap plies).

[RidingOnRailz]

Originally Posted by CapriRacer

Like this (in green):

And the tread profile would be the distance between the top
of the tread down and the top of the belt (or cap plies).

And what is the significance of that in maintaining even contact patch in wider, lower profile tires?

[CapriRacer]

Originally Posted by RidingOnRailz

And what is the significance of that in maintaining even contact patch in wider, lower profile tires?

The lower the tread radius, the more pressure in the middle of the contact patch. This has implications for treadwear and grip.

[RidingOnRailz]

Originally Posted by CapriRacer

The lower the tread radius, the more pressure in
the middle of the contact patch. This has implications for
treadwear and grip.

Ok, first things first: On a higher tread radius tire, is the tread section rounder(your green line), and on a lower tread radius tire, is it flatter/straighter?

Because what you said, above, goes completely counter to my gut. Remember Barry, I think with my belly(gut instinct), not that thing upstairs.

[CapriRacer]

So first, I am pleased that when I changed that photo, that this website picked that up. This version is sooooo much easier to deal with.

Originally Posted by RidingOnRailz

Ok, first things first: On a higher tread radius tire, is the tread section rounder(your green line), and on a lower tread radius tire, is it flatter/straighter? .....

You have it exactly backwards. I am puzzled that you don't know that a small radius results in a smaller circle, and that a larger radius results in a larger circle - and that if I am measuring an arc (part of a circle) a larger radius results in a flatter arc.

Originally Posted by RidingOnRailz

...... Because what you said, above, goes completely counter to my gut. Remember Barry, I think with my belly(gut instinct), not that thing upstairs.

Does your gut feel better, now?

[RidingOnRailz]

Originally Posted by CapriRacer

So first, I am pleased that when I changed that photo, that this website picked that up.
This version is sooooo much easier to deal with.



You have it exactly backwards. I am puzzled that you don't know that a small radius results
in a smaller circle, and that a larger radius results in a larger circle - and that if I am measuring
an arc (part of a circle) a larger radius results in a flatter arc.



Does your gut feel better, now?

You're "puzzled"?

Barry:
.
.
.

I can't even multiply or divide! Let alone understand basic radii theory.

I had to get a deferment - for lack of better term - to be allowed to attend university and get a degree, my secondary math grades were so poor. Goes back to issues in early childhood.

In subjects I could could care less about: social studies, history, English, I averaged A-PLUS. And I'm not proud of that.

So please break it down, this radius thing, for me.

[CapriRacer]

Well, the smaller the tread radius, the more pressure along the centerline of the footprint. That means there is an optimal tread radius where the footprint pressure is such that it promotes even tread wear and max grip.

[RidingOnRailz]

Originally Posted by CapriRacer

Well, the smaller the tread radius, the more pressure along the centerline of the footprint. That means there is
an optimal tread radius where the footprint pressure is such that it promotes even tread wear and max grip.

Can this tread radius be engineered into a wider tire so that, at a given pressure, that footprint pressure against the gound is equally distributed?

[CapriRacer]

Originally Posted by RidingOnRailz

Can this tread radius be engineered into a wider tire so that, at a given pressure, that footprint pressure against the ground is equally distributed?

This question seems to say that you think there is something wrong with wide tires that might be fixed by applying "tread radius". Would you please explain?

[RidingOnRailz]

Originally Posted by CapriRacer

This question seems to say that you think there is something
wrong with wide tires that might be fixed by applying "tread radius". Would you please explain?

"Lay a straight edge across the face of the tread of a
tire - shoulder to shoulder. You will notice that the tread
is not flat - it's a curve. You can measure the radius of
that curve. That curve affects the distributio of the pressure
in the footprint - and that, in turn, affects wear and grip."


Your words - not mine. The onus is on you to explain how that curve, the tread radius curve that you highlighted in reply #17 I believe, can be adjusted in manufacturing to affect load distrib. across a wider tires' contact patch.

Now that I've thought about the basic concepts, and finally grasp, them, I'll continue to wait patiently for your explanation.

[CapriRacer]

The reason I asked the question is that I am confused about why you would think wide tires and narrow tires would be different when it comes to tread radius. Perhaps if I answer your question, it can serve as a basis so you can answer mine.

Let’s start with an assumption that in the contact patch, a flat belt is needed to get an even pressure distribution. That means that the tread thickness is the same shoulder to shoulder. Remember this is an assumption, which we will revisit later.

If I measure the tread radius when the tire is completely unloaded (commonly called Freestanding), I can get a value – and I can translate that back to the radius in the mold, because I know the relationship between freestanding and molded tire shapes.
But what if my initial assumption about the belt needing to be parallel to the road surface is wrong? I can add or subtract rubber as needed. And I don’t even have to change the tread depth. I can change what we tire engineers call the undertread – the rubber between the bottom of the grooves and the top of the belt. The net result would be I’d get a different freestanding tread radius.

What if it takes different freestanding tread radii for different tire sizes? Well it doesn’t matter, I just do the same thing for each size.
In the old days, that was time consuming, because you’d have to build tires. But today, we can do that in virtual space – lots easier and faster. And, of course, once its been done for one tire line, the same thing can be applied to different tire lines.

So back to the question I asked, why do you think there is a difference between wide tires and narrow tires when it comes to tread radius?

[RidingOnRailz]

Originally Posted by CapriRacer

The reason I asked the question is that I am confused about why you would think wide tires and narrow tires would be different when it comes to tread radius. Perhaps if I answer your question, it can serve as a basis so you can answer mine.

Let’s start with an assumption that in the contact patch, a flat belt is needed to get an even pressure distribution. That means that the tread thickness is the same shoulder to shoulder. Remember this is an assumption, which we will revisit later.

If I measure the tread radius when the tire is completely unloaded (commonly called Freestanding), I can get a value – and I can translate that back to the radius in the mold, because I know the relationship between freestanding and molded tire shapes.
But what if my initial assumption about the belt needing to be parallel to the road surface is wrong? I can add or subtract rubber as needed. And I don’t even have to change the tread depth. I can change what we tire engineers call the undertread – the rubber between the bottom of the grooves and the top of the belt. The net result would be I’d get a different freestanding tread radius.

What if it takes different freestanding tread radii for different tire sizes? Well it doesn’t matter, I just do the same thing for each size.
In the old days, that was time consuming, because you’d have to build tires. But today, we can do that in virtual space – lots easier and faster. And, of course, once its been done for one tire line, the same thing can be applied to different tire lines.

So back to the question I asked, why do you think there is a difference between wide tires and narrow tires when it comes to tread radius?

Before I related it to tread radius, let's discuss the relative integrity of, IE, the 195-75R14 off my 1981 Buick and the 225-50R17 off my 2010 Honda.

The Century's tire consists of a narrower action surface(tread) spanning two taller sidewalls.

The Accord's tire consistes of a significantly wider tread, spanning two shorter sidewalls.

Going by my gut again, Capri, with the wider tread, something - either the tread's construction itself, its free-wheeling shape, or, what fills the tire to maintain the tread's shape integrity and thus equal force across its contact patch - must be changed from what worked for that narrower Buick tire.

Please tell me what that is. If not a change in the tread radius(what you illustrated with the green line), then something else. Just tell me what that is, or what combination of properties.

[CapriRacer]

Originally Posted by RidingOnRailz

Before I related it to tread radius, let's discuss the relative integrity of, IE, the 195-75R14 off my 1981 Buick and the 225-50R17 off my 2010 Honda.

The Century's tire consists of a narrower action surface(tread) spanning two taller sidewalls.

The Accord's tire consistes of a significantly wider tread, spanning two shorter sidewalls.

Going by my gut again, Capri, with the wider tread, something - either the tread's construction itself, its free-wheeling shape, or, what fills the tire to maintain the tread's shape integrity and thus equal force across its contact patch - must be changed from what worked for that narrower Buick tire.

Please tell me what that is. If not a change in the tread radius(what you illustrated with the green line), then something else. Just tell me what that is, or what combination of properties.

I'm a little unclear what you are asking. Could you reword the question a bit?

[RidingOnRailz]

Originally Posted by CapriRacer

I'm a little unclear what you are asking.
Could you reword the question a bit?

Alright, in laymans terms: Is the tread on a wider tire with shorter sidewalls “floppier” more prone to flexing than the tread on a narrow tire with taller sides?

That’s why I arrived at the conclusion that a 45- or 50-series tire, on the same car in place of 60-or higher series tire required higher air pressure to maintain the same rigidity across the patch.

[CapriRacer]

Thanks for the clarification. A part of me wondered if you were asking me to compare a 40 year old tire to a 10 year old tires - and that was going to be difficult! Im glad its current tires.

OK, on to the question. What's different? Actually not much.

First, the steel belt consists of 2 layers of rubber coated steel wire, with the wires running parallel to each other in each layer. If you were to look through the 2 layers, it would look like a cross hatch pattern.

But those are the same to each tire, the only difference is that the wider tire has wider belts - and within each tire, the belts are the same underneath the treadface, except for the ends - meaning the strength of the belts are the same throughout either tire except at the ends - no difference.

But there is a difference in that the wider tire has wider belts so each individual wire wraps further around the circumference of the tire. I don't think that makes a difference in how the tire behaves, but it is a difference, but probably in the wrong direction for your theory.

So let's look at your assertion that lower profile tires get more pressure. According to Tire Guides, 2010 Honda Accord. It came with P215/60R16 94H at 30 psi and P225/50R17 93V at 32 psi. If I do the math, the load carrying capacity of each is the same - which would explain why there's a difference in specified pressure. But the Accord also came with P235/45R16 94V at 34 psi. That seems to be the odd man out, but I note that Tire Guides says this was only for the EX-L Coupe V-6, and that for the 2011 and 2012 model year, that same entry is listed as 32 psi. That's doubly odd. Can't tell what is going on there.

Maybe we could get some insight if we looked at the Civic. In 2010 it came with P195/65R15 89 S &H at 32 psi, P205/55R16 89H at 32 psi, and P215/45R17 87V at 32 psi. Huh?

What about the Accord in 2013? 205/55R16 95H at 32/32, 215/55R17 94V at 33/33, 235/45R18at 33/32. That doesn't seem to follow any pattern at all!

So I don't think your theory holds up.