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From: Jeff Strickland on 5 Nov 2009 12:44
"C. E. White" <cewhite3(a)mindspring.com> wrote in message
> "Tegger" <invalid(a)invalid.inv> wrote in message
>> "C. E. White" <cewhite3remove(a)mindspring.com> wrote in
>>> 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.
>> 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
>> 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?
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
From: Tegger on 5 Nov 2009 13:19
"C. E. White" <cewhite3(a)mindspring.com> wrote in news:hcur0a$o5e$1
> 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.
> 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.
> The fact that the tire flattens
> out at the road surface doesn't change this.
But that makes /all/ the difference.
> 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
> 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
> 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.
> 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
From: badgolferman on 5 Nov 2009 13:52
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
From: Tegger on 5 Nov 2009 16:03
"badgolferman" <REMOVETHISbadgolferman(a)gmail.com> wrote in
> 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
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.
From: Tegger on 5 Nov 2009 16:14
"Jeff Strickland" <crwlrjeff(a)yahoo.com> wrote in
> 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.