wheel circumference

#11

"C. E. White" <cewhite3 (AT) mindspring (DOT) com> wrote

Quote:

"Tegger" <invalid (AT) invalid (DOT) inv> wrote in message
news:Xns9CBA63CD94FA9tegger (AT) 208 (DOT) 90.168.18...
"C. E. White" <cewhite3remove (AT) mindspring (DOT) com> wrote in
news:IdednamjeIFc2G_XnZ2dnUVZ_gKdnZ2d (AT) earthlink (DOT) com:

While agree that there may be a small difference (a very small
difference) in rolling diameter for tires of the same overall diameter
but with different inside (wheel) diameters, I don't entirely buy you
explaination of why.

For sure you are right about how the ABS based low tire pressure
sensors work, but they take miles of driving to detect a very
significant difference in tire pressure. BUT.....

Modern radial tires are not like hard wheels, they are like tank
treads. The rolling diameter is mostly based on the diamter of the
steel belt in the tire as long as the tires are properly inflated.

I think you need to go do some actual observation and measurement. Go
outside and measure those distances on your own tires.

I'll use my own car's front tires as an example for illustration:
My tire size is 195/60-14. That gives me a nominal diameter of 23.21".

An actual (as best as I can eyeball) diametrical measurement reveals 23",
when measured across the unloaded portion of the tire from front-to-back.

That means the unloaded radius is 11.5".

If I measure from dead-center of the hub to the road (the loaded, or
"working" radius) however, I get 10.625".

That's 7/8" difference, or about 8% less than the unloaded radius.

Now...

Consider my REAR tires. The car has 61-39 front/rear weight distribution.
Same tire, same pressure, much lighter loading. The working radius here I
measure at 11.0625". The rear tires thus have a 4% larger working radius
than the fronts.

It is impossible to have a contact patch on the road unless the tire
develops a "flat spot" where the tire contacts the road. This has nothing
to do with the steel belt or anything else. Even a hard-rubber towmotor
tire has a flat-spot and a contact patch. The center of the contact patch
is the end point for that "working" radius line.

If you have bigger wheels and shorter, stiffer tire sidewalls within the
same unloaded diameter, the loaded distortion will be less, which means
the
wheel/tire assembly will have a larger working circumference. How much
more? Possibly up to four or five percent, possibly as low as one or two
percent. Depends. But there WILL be a difference.

I tried really hard to get you to see this differently. Go back and rear
my prior post. Tires are not hard wheels. The revolutions per mile is not
directly related to the loaded radius of the tire in the manner you are
suggesting. It is likely a factor, but a very minor one....

Think! Car tires are not hard wheels like forklift tires.

How can what you are saying be true? Look at it a different way...every
time the tire goes through a revolution, there is apporximately a one to
one relationship between the tire and the road surface (every part of the
tire contact the road). One revolution of the tire will move the car
forward by the circumfrence of the tire. If it doesn't you have to have
slip (which will be true if you spin the tires, but is a negligible factor
if you are cruising straight ahead on a level road at a moderate sped).
The fact that the tire flattens out at the road surface doesn't change
this. Of course rubber is flexible and can stretch/shrink, which is why
the really important circumfrence is the circumfrence of the steel belts
inside the tire. The treads area can stretch and flex (think about how
tank treads move) but the steel belt stretchs only by tiny amounts. Unless
the tire is so underinflated that the tire assumes a concave shape at the
road surface, the effective rolling radius is not significantly dependednt
on the distance from the road to the wheels center axis. Like most real
world things, there are other factors that come into play, so I don't
doubt that there is some effect on revolutions per mile related to tires
of the same outside diameter with different inner diameters (i.e., wheel
size), but the I don't think the loaded radius is the prime reason.

One more thing to think about. If you are right, where does the extra tire
go? You are suggesting that one revolution of the tire will result inthe
car moving forward by a distance equal to the loaded radius of the tire
times 2 times pi. But the actual circumfrence of the tire is the unloaded
diameter of the tire times pi. Since 2 times the loaded radius of the tire
is less than the unloded diameter of the tire times pi, this implies
something is happening witht he "extra" circumfrecne in your senario.
Where is it going? You might convince me that the rubber portion is
stretching and shrinking to accomodate your theory, but what is happenign
with the steel belt in the tire?

One last analogy - think conveyor belt.....

Did you look at the chart I included with the prior note?

Ed

When ONE tire is different than the others, it has to go a different speed
to travel the same distance. It has to. The differential is able to handle
the difference, so the car does not pull itself apart -- gross exageration,
I know. But the speed sensors on each wheel will see the different speed of
the different tire -- it's different because it's a different size or
because the air pressure is lower, or higher I suppose. In any case, the
speed sensors see the different size and treat it as a low pressure warning,
reason to activate the ABS system, or the Traction Control, depending on
other factors that the computer monitors.

You are right, though. You posted a chart that shows a variety of tire sizes
and lists equivelents that result by changing the various specs of the
tires. If all tires are the same size (staggered tires -- different front
and rear present complications) and the proper equivelent for the factory
size, then the pressure monitor that is speed-based, the ABS and the
Traction Control would all work properly. Staggered tires present
complications because they are already slightly different sizes from front
to rear, and altering the balance might be problematic. But if the tires
remained equivelent from left to right, then the variations from front ot
rear should remain a constant, and the systems should still work properly.
I'm not saying there will be a problem for staggered tires setups, I'm only
saying there is an added consideration -- which I've never dealt with
before.

#12

"C. E. White" <cewhite3 (AT) mindspring (DOT) com> wrote in news:hcur0a$o5e$1
@news.eternal-september.org:

Quote:

How can what you are saying be true? Look at it a different
way...every time the tire goes through a revolution, there is
apporximately a one to one relationship between the tire and the road
surface (every part of the tire contact the road). One revolution of
the tire will move the car forward by the circumfrence of the tire.

The /working/ circumference, which changes with all sorts of factors,
pressure, weight, sidewall height.

As I said before, ABS-based low-pressure warning systems
_could not work_ unless this was true.

Quote:

If
it doesn't you have to have slip (which will be true if you spin the
tires, but is a negligible factor if you are cruising straight ahead
on a level road at a moderate sped).

Not so. It's called "scrub" and is the reason tires wear even in a
straight-ahead, steady-state position.

Quote:

The fact that the tire flattens
out at the road surface doesn't change this.

But that makes /all/ the difference.

Quote:

Of course rubber is
flexible and can stretch/shrink, which is why the really important
circumfrence is the circumfrence of the steel belts inside the tire.
The treads area can stretch and flex (think about how tank treads
move) but the steel belt stretchs only by tiny amounts. Unless the
tire is so underinflated that the tire assumes a concave shape at the
road surface, the effective rolling radius is not significantly
dependednt on the distance from the road to the wheels center axis.

Then how can an ABS-based warning system work if it cannot depend on
changes in working radius? And monitoring changes in rolling
circumference due to changed working radius _IS_ how such systems
operate.

Quote:

Like most real world things, there are other factors that come into
play, so I don't doubt that there is some effect on revolutions per
mile related to tires of the same outside diameter with different
inner diameters (i.e., wheel size), but the I don't think the loaded
radius is the prime reason.

One more thing to think about. If you are right, where does the extra
tire go?

Where did that 1/4" go? I don't know. I suspect there's a minuscule
stretch or displacement around the unloaded portion of the tire. But in
order to confirm that, I'd need to jack the wheel off the ground and
take some really careful measurements both loaded and unloaded.

My contact patch is about 4.25" front-to-back (about 45deg of rotation).
On my tire, a line traced from front-to-back on that flat spot is about
a quarter-inch shorter than the curve would be if the tire was not
flattened by the load.

If that 1/4" were distributed evenly around the unloaded portion of my
tire, each of the 315 degrees of unloaded tire would have to displace or
stretch about .0008", which is plausible. Plus some of the distortion
would be compressed into the flat spot, so the .0008 might be an
overestimate.

Quote:

result inthe car moving forward by a distance equal to the loaded
radius of the tire times 2 times pi. But the actual circumfrence of
the tire is the unloaded diameter of the tire times pi. Since 2 times
the loaded radius of the tire is less than the unloded diameter of the
tire times pi, this implies something is happening witht he "extra"
circumfrecne in your senario. Where is it going? You might convince me
that the rubber portion is stretching and shrinking to accomodate your
theory, but what is happenign with the steel belt in the tire?

Well, I have one guess: The steel belt is a woven assembly with the
wires on a bias. It's possible the weave distorts under compression and
extension, much like rope, cloth or window screen can.

Quote:

One last analogy - think conveyor belt.....

Did you look at the chart I included with the prior note?

I did, but the chart misses the point.

My trump card is ABS-based low-pressure warning systems. I know for a
fact that they work by sensing a low-pressure tire spinning faster than
one with higher pressure, which can ONLY happen if the working radius
(and circumference) can shrink and grow on an otherwise inextensible
tire.

--
Tegger

#13

Jeff Strickland wrote:

Quote:

The OP wants a larger sidewall for some reason, not a smaller one. (I
don't get the logic he's using, but it's not my decision.)

I don't want anything changed on _my_ car. I was just wondering if
there is a difference between tires with thin sidewalls and those with
very wide sidewalls when it comes to odometer/speedometer readings.
And I'm not talking about one inch differences, more like 45s vs. 85s.

After you guys work out all this technical minutia let me know what the
consensus is.

#14

"badgolferman" <REMOVETHISbadgolferman (AT) gmail (DOT) com> wrote in
news:xn0ghaisbeui4k004 (AT) reader (DOT) albasani.net:

Quote:

Jeff Strickland wrote:

The OP wants a larger sidewall for some reason, not a smaller one. (I
don't get the logic he's using, but it's not my decision.)

I don't want anything changed on _my_ car. I was just wondering if
there is a difference between tires with thin sidewalls and those with
very wide sidewalls when it comes to odometer/speedometer readings.
And I'm not talking about one inch differences, more like 45s vs. 85s.

After you guys work out all this technical minutia let me know what the
consensus is.

You'll never get "consensus".

The fact is:
Given a specific sidewall height and overall tire diameter, if you change
ONLY the sidewall height (bigger wheel), you WILL change the rolling
circumference.

If you go from a 60 to a 50, you probably won't notice the difference. If
you go from an 80 to a 45, there would be a big difference.

--
Tegger

#15

"Jeff Strickland" <crwlrjeff (AT) yahoo (DOT) com> wrote in
news:hcv2ad$r3o$1 (AT) news (DOT) eternal-september.org:

Quote:

None of that matters to the ABS if all tires on the ground are the
same size, even if the tires are not the size that is specified for
the car. The ABS/Low Tire Pressure systems look at differences in the
rotational speed of one tire vs. the others.

That's right! And it can ONLY do that if the rolling circumference is
DIFFERENT between the two wheels being monitored.

If the wheels are traveling the same path on the same car but their
rotational speeds are different, then one of them is presenting a smaller
circumference to the road and has to spin faster to cover the same ground
as the tire with the larger circumference.

And if both wheel/tire assemblies being monitored are identical except for
their tire pressure, then their rolling radii MUST be different, otherwise
they'd spin at the same rotational speed.

The lower-pressure tire's sidewalls create the smaller circumference by
compressing more, which is what causes the smaller circumference.

Back to the original subject: A taller sidewall (i.e.: 80) will distort
more under load than a short one (i.e.: 45), and will cause a larger
reduction in rolling radius from unloaded to loaded.

It cannot be any other way.

--
Tegger

#16

Tegger, 11/5/2009,4:03:51 PM, wrote:

Quote:

"badgolferman" <REMOVETHISbadgolferman (AT) gmail (DOT) com> wrote in
news:xn0ghaisbeui4k004 (AT) reader (DOT) albasani.net:

Jeff Strickland wrote:

The OP wants a larger sidewall for some reason, not a smaller
one. (I don't get the logic he's using, but it's not my decision.)

I don't want anything changed on my car. I was just wondering if
there is a difference between tires with thin sidewalls and those
with very wide sidewalls when it comes to odometer/speedometer
readings. And I'm not talking about one inch differences, more
like 45s vs. 85s.

After you guys work out all this technical minutia let me know what
the consensus is.

You'll never get "consensus".

The fact is:
Given a specific sidewall height and overall tire diameter, if you
change ONLY the sidewall height (bigger wheel), you WILL change the
rolling circumference.

If you go from a 60 to a 50, you probably won't notice the
difference. If you go from an 80 to a 45, there would be a big
difference.

So will the wheel with the 80 tire rotate more or less than the wheel
with the 45? Which one will show more than the correct speed and which
one will show less?

--
"If you don't read the newspapers you are uninformed; if you do read
the newspapers you are misinformed." ~ Mark Twain

#17

"badgolferman" <REMOVETHISbadgolferman (AT) gmail (DOT) com> wrote

Quote:

Jeff Strickland wrote:

The OP wants a larger sidewall for some reason, not a smaller one. (I
don't get the logic he's using, but it's not my decision.)

I don't want anything changed on _my_ car. I was just wondering if
there is a difference between tires with thin sidewalls and those with
very wide sidewalls when it comes to odometer/speedometer readings.
And I'm not talking about one inch differences, more like 45s vs. 85s.

After you guys work out all this technical minutia let me know what the
consensus is.

The short answer is, no, there is no difference.

The long asnwer is that the overall diameter has to be the same for the
short answer to be true. For example, my truck has a 265/75x16. I could go
to a 265/35x20 and the tire should be the same overall diameter, therefore
no affect on the speedo.

#18

"badgolferman" <REMOVETHISbadgolferman (AT) gmail (DOT) com> wrote in
news:xn0ghau681ojn70000 (AT) news (DOT) albasani.net:

Quote:

Tegger, 11/5/2009,4:03:51 PM, wrote:

"badgolferman" <REMOVETHISbadgolferman (AT) gmail (DOT) com> wrote in
news:xn0ghaisbeui4k004 (AT) reader (DOT) albasani.net:

Jeff Strickland wrote:

The OP wants a larger sidewall for some reason, not a smaller
one. (I don't get the logic he's using, but it's not my
decision.)

I don't want anything changed on my car. I was just wondering if
there is a difference between tires with thin sidewalls and those
with very wide sidewalls when it comes to odometer/speedometer
readings. And I'm not talking about one inch differences, more
like 45s vs. 85s.

After you guys work out all this technical minutia let me know what
the consensus is.

You'll never get "consensus".

The fact is:
Given a specific sidewall height and overall tire diameter, if you
change ONLY the sidewall height (bigger wheel), you WILL change the
rolling circumference.

If you go from a 60 to a 50, you probably won't notice the
difference. If you go from an 80 to a 45, there would be a big
difference.

So will the wheel with the 80 tire rotate more or less than the wheel
with the 45? Which one will show more than the correct speed and
which one will show less?

The OEM wheel, tire size, and speed rating will be closest to the
manufacturer's intended speedometer calibration.

When it comes to aftermarket, I wish I could pin that question down for
you, but I can't. The answer depends on the actual rolling circumference
that each tire describes when loaded the way your car will load it.

One maker's 225/45-16 won't necessarily measure the same actual unloaded
diameter as another maker's 225/45-16. And those tire-shop tire-size
equivalency charts are fraught with uncertainty on account of that.

But: Given two differently-seriesed tires with identical unloaded
circumferences, the one with the shorter sidewall will have the larger
rolling circumference and the lower speedometer reading.

The upshot is that the difference will likely not be enough for anybody
to notice unless he's very specifically looking for that difference.
Even the cops recognize that, which is partly why they give you 5mph
grace (or more depending on the jurisdiction) before whacking you.

--
Tegger

#19

"Jeff Strickland" <crwlrjeff (AT) yahoo (DOT) com> wrote in
news:hd044q$sph$1 (AT) news (DOT) eternal-september.org:

Quote:

"badgolferman" <REMOVETHISbadgolferman (AT) gmail (DOT) com> wrote in message
news:xn0ghaisbeui4k004 (AT) reader (DOT) albasani.net...
Jeff Strickland wrote:

The OP wants a larger sidewall for some reason, not a smaller one. (I
don't get the logic he's using, but it's not my decision.)

I don't want anything changed on _my_ car. I was just wondering if
there is a difference between tires with thin sidewalls and those
with very wide sidewalls when it comes to odometer/speedometer
readings. And I'm not talking about one inch differences, more like
45s vs. 85s.

After you guys work out all this technical minutia let me know what
the consensus is.

The short answer is, no, there is no difference.

The long asnwer is that the overall diameter has to be the same for
the short answer to be true. For example, my truck has a 265/75x16. I
could go to a 265/35x20 and the tire should be the same overall
diameter, therefore no affect on the speedo.

Supposing the two sizes have identical unloaded diameters, the 35 will have
the larger rolling circumference and the slower speedometer reading.

--
Tegger

#20

"Tegger" <invalid (AT) invalid (DOT) inv> wrote

Quote:

"Jeff Strickland" <crwlrjeff (AT) yahoo (DOT) com> wrote in
news:hd044q$sph$1 (AT) news (DOT) eternal-september.org:

"badgolferman" <REMOVETHISbadgolferman (AT) gmail (DOT) com> wrote in message
news:xn0ghaisbeui4k004 (AT) reader (DOT) albasani.net...
Jeff Strickland wrote:

The OP wants a larger sidewall for some reason, not a smaller one. (I
don't get the logic he's using, but it's not my decision.)

I don't want anything changed on _my_ car. I was just wondering if
there is a difference between tires with thin sidewalls and those
with very wide sidewalls when it comes to odometer/speedometer
readings. And I'm not talking about one inch differences, more like
45s vs. 85s.

After you guys work out all this technical minutia let me know what
the consensus is.

The short answer is, no, there is no difference.

The long asnwer is that the overall diameter has to be the same for
the short answer to be true. For example, my truck has a 265/75x16. I
could go to a 265/35x20 and the tire should be the same overall
diameter, therefore no affect on the speedo.

Supposing the two sizes have identical unloaded diameters, the 35 will
have
the larger rolling circumference and the slower speedometer reading.

NOT true. Well, not entirely true.

For every inch increase in the rim, the aspect ratio is decreased 5%. The
result is a overall diameter, or radius, or circumference (depending on the
specy you prefer) will be virtually identical. The difference -- if done on
all four corners -- is insignificant. There will be a small change to the
speedometer -- a change measured in single digit percentages.

The speedometer would be more sensitive to the circumference, but since
circumference is calculated by diameter X pi, or radius X 2 X pi, then all
are the same.

I assume proper inflation, and ignore any argument that assumes improper
inflation. Improper inflation causes a variance, and the variance is
measured by the speed sensors, not the speedometer.