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From: Tegger on 8 Nov 2009 09:43
The reply I made below was to a thread (started by Badgolferman) that's
getting old and might get ignored, so I'm starting a new one with that
reply.

My test showed that I was more right than wrong. See below...


---------------------

Tegger wrote:


> "C. E. White" <cewhite3(a)mindspring.com> wrote in news:hd18tv$dr6$1
> @news.eternal-september.org:
>
>
>
> I am sure you are greatly over emphasizing the contribution of the
> loaded radius on the rolling diamter of tires, but don't know of any
> other effective arguements. Maybe an experiment would convince you. If
> you have the time, measure the loaded radius, mark the tire, move the
> car for 100 revolutions of the tire, and then measure the distance
> moved...You will find that it moved a significantly greater distance
> than 2 x pi x loaded radius x 100.
>


I must be nuts, because I actually went out and tested your theory (and
mine). I hope you do me the grace of actually reading this, because I
did perform the test instead of being Usenet-snarky and telling you to
go and do the test yourself.

After making a gauge with corrugated cardboard, I discovered that the
unloaded diameter of both front and rear tires is 23.25" dead-on.

23.25" x 3.14159 = 73.04" unloaded circumference.

Tire pressures at time of test were all the same, 31 lbs (checked hot).

On a flat, newly paved industrial parking lot, I marked the tires (and
the lot) with chalk. Leaning out the window, I then slowly rolled the
car so that the mark on the left tire described ten revolutions, coming
down to the very bottom again. That covered almost 60 feet (100
revolutions was not practical for me.)

I did this four times each, for the front and then for the rear tires
(both sides). The results were very consistent.

The results?
Actual distance covered for the fronts: 704.5"
Actual distance covered for the rears: 708.5"

Now, how about the "loaded" radius?
For the front left, it's 11.75", which gives a circumference of 67.54"
However, that tire actually covered 70.45" per rev, not 67.54". But at
the same time it was not covering 73.04" either.

For the rear left, the loaded radius is 11.0625". This gives a
circumference of 69.51". That tire actually rolled 70.85" in the test.

Let's summarize, as percent reduction from unloaded to loaded:
Front hypothetical rolling circum based on loaded radius: 7.5% less
Front actual rolling circum based on test result: 3.55% less
Rear hypothetical rolling circum based on loaded radius: 4.8% less
Rear actual rolling circum based on test result: 3% less

Looks like the fronts split the difference between unloaded and loaded,
and the rears were affected pretty close to what I theorized.




--
Tegger

From: Jeff Strickland on 8 Nov 2009 10:50

"Tegger" <invalid(a)invalid.inv> wrote in message
news:Xns9CBD62C887A5Ftegger(a)208.90.168.18...
> The reply I made below was to a thread (started by Badgolferman) that's
> getting old and might get ignored, so I'm starting a new one with that
> reply.
>
> My test showed that I was more right than wrong. See below...
>
>
> ---------------------
>
> Tegger wrote:
>
>
>> "C. E. White" <cewhite3(a)mindspring.com> wrote in news:hd18tv$dr6$1
>> @news.eternal-september.org:
>>
>>
>>
>> I am sure you are greatly over emphasizing the contribution of the
>> loaded radius on the rolling diamter of tires, but don't know of any
>> other effective arguements. Maybe an experiment would convince you. If
>> you have the time, measure the loaded radius, mark the tire, move the
>> car for 100 revolutions of the tire, and then measure the distance
>> moved...You will find that it moved a significantly greater distance
>> than 2 x pi x loaded radius x 100.
>>
>
>
> I must be nuts, because I actually went out and tested your theory (and
> mine). I hope you do me the grace of actually reading this, because I
> did perform the test instead of being Usenet-snarky and telling you to
> go and do the test yourself.
>
> After making a gauge with corrugated cardboard, I discovered that the
> unloaded diameter of both front and rear tires is 23.25" dead-on.
>
> 23.25" x 3.14159 = 73.04" unloaded circumference.
>
> Tire pressures at time of test were all the same, 31 lbs (checked hot).
>
> On a flat, newly paved industrial parking lot, I marked the tires (and
> the lot) with chalk. Leaning out the window, I then slowly rolled the
> car so that the mark on the left tire described ten revolutions, coming
> down to the very bottom again. That covered almost 60 feet (100
> revolutions was not practical for me.)
>
> I did this four times each, for the front and then for the rear tires
> (both sides). The results were very consistent.
>
> The results?
> Actual distance covered for the fronts: 704.5"
> Actual distance covered for the rears: 708.5"
>
> Now, how about the "loaded" radius?
> For the front left, it's 11.75", which gives a circumference of 67.54"
> However, that tire actually covered 70.45" per rev, not 67.54". But at
> the same time it was not covering 73.04" either.
>
> For the rear left, the loaded radius is 11.0625". This gives a
> circumference of 69.51". That tire actually rolled 70.85" in the test.
>
> Let's summarize, as percent reduction from unloaded to loaded:
> Front hypothetical rolling circum based on loaded radius: 7.5% less
> Front actual rolling circum based on test result: 3.55% less
> Rear hypothetical rolling circum based on loaded radius: 4.8% less
> Rear actual rolling circum based on test result: 3% less
>
> Looks like the fronts split the difference between unloaded and loaded,
> and the rears were affected pretty close to what I theorized.
>


There is a flaw in your measurements. If the UNLOADED diameter is 23.25, the
radius is 11.625. The LOADED radius must be less than 11.625.

Any difference you note would be precisely the kind of thing that the Low
Pressure Monitor would look for to determine over time that a tire was in
fact low. The same results could happen in the short term as a result of
turning. Whatever happens relative to the loaded and unloaded radius of an
individual tire would not affect the speedometer, the ABS or the Traction
Control.

So, if your goal was to say that a narrow sidewall tire might not be
detected by a low pressure monitor, I think we all agree that this is
probably true. OR, since the question was really about replacing a narrow
sidewall tire with a large sidewall tire, then the pressure monitor might
give off false positives (indicate low pressure when the pressure was
proper), then that is also true -- and more likely to be true than the
former condition. But the speedo would not be adversely -- does not HAVE to
be adversely -- impacted by changing the tires if the tire selection results
in an outside diameter that equates to the tires that came on the car as
factory fitment.

Using the Camry example from the previoius thread, if the base model car is
fitted with a 195/70x14 and the premium trim (XLE ?) gets a 215/45x17, then
BadGolfer can put the base model tire package on his XLE, and the speedo
won't change in any significant way. I'd be inclined to put the XLE tire
package on my base model Camry, but that's just me, I suppose.







From: Tegger on 8 Nov 2009 13:58
"Jeff Strickland" <crwlrjeff(a)yahoo.com> wrote in
news:hd6pcq$4f2$1(a)news.eternal-september.org:

>
> "Tegger" <invalid(a)invalid.inv> wrote in message
> news:Xns9CBD62C887A5Ftegger(a)208.90.168.18...

>>
>> The results?
>> Actual distance covered for the fronts: 704.5"
>> Actual distance covered for the rears: 708.5"
>>
>> Now, how about the "loaded" radius?
>> For the front left, it's 11.75", which gives a circumference of
>> 67.54" However, that tire actually covered 70.45" per rev, not
>> 67.54". But at the same time it was not covering 73.04" either.
>>
>> For the rear left, the loaded radius is 11.0625". This gives a
>> circumference of 69.51". That tire actually rolled 70.85" in the
>> test.
>>
>
>
> There is a flaw in your measurements. If the UNLOADED diameter is
> 23.25, the radius is 11.625. The LOADED radius must be less than
> 11.625.



I made an unfortunate typo: That "loaded" radius on the front was
TEN point seven-five, not ELEVEN. This would have been obvious if you'd
worked my circumferences back to the radius.

Check my post again, this time correcting that typo, and you'll find I am
correct.




--
Tegger
From: Tegger on 8 Nov 2009 14:42
A repost of my original test results, with the one typo corrected
(11.75" corrected to 10.75")

----------------------------

After making a gauge with corrugated cardboard, I discovered that the
unloaded diameter of both front and rear tires is 23.25" dead-on.

23.25" x 3.14159 = 73.04" unloaded circumference.

Tire pressures at time of test were all the same, 31 lbs (checked hot).

On a flat, newly paved industrial parking lot, I marked the tires (and
the lot) with chalk. Leaning out the window, I then slowly rolled the
car so that the mark on the left tire described ten revolutions, coming
down to the very bottom again. That covered almost 60 feet (100
revolutions was not practical for me.)

I did this four times each, for the front and then for the rear tires
(both sides). The results were very consistent.

The results?
Actual distance covered for the fronts: 704.5"
Actual distance covered for the rears: 708.5"

Now, how about the "loaded" radius?
For the front left, it's 10.75", which gives a circumference of 67.54"
However, that tire actually covered 70.45" per rev, not 67.54". But at
the same time it was not covering 73.04" either.

For the rear left, the loaded radius is 11.0625". This gives a
circumference of 69.51". That tire actually rolled 70.85" in the test.
(Note that's eleven and one-sixteenth, not eleven and five-eighths.)

Let's summarize, as percent reduction from unloaded to loaded:
Front hypothetical rolling circum based on loaded radius: 7.5% less
Front actual rolling circum based on test result: 3.55% less
Rear hypothetical rolling circum based on loaded radius: 4.8% less
Rear actual rolling circum based on test result: 3% less

Looks like the fronts split the difference between unloaded and loaded,
and the rears were affected pretty close to what I theorized.




--
Tegger

From: C. E. White on 8 Nov 2009 15:55
As I tried to point out before, the rolling radius is more influenced by the
diameter of the steel belts in the tire, not the actual outside diameter.
What brand and tire size were you testing? If we know that we probably can
find the manufacturers revolutions per mile specification. If you check
these, they are not a function of the outside tire diameter times pi. They
are different than that.

For instance....

From the chart I posted in the previous string...

Revolutions
Overall Per
Tire Size Diameter Mile
245/40ZR17 24.7" 841
225/45ZR17 24.8" 837
255/35ZR18 25.2" 824
225/40ZR18 25.3" 821
265/35ZR18 25.3" 819
255/40ZR18 25.9" 770
285/35ZR18 25.9" 804
255/45ZR17 26" 798
255/45ZR17 26" 798
255/40ZR18 26" 800
285/35ZR18 26" 800
225/40ZR19 26.1" 797
265/40ZR18 26.6" 782
245/45ZR18 26.8" 780
245/45ZR18 26.8" 780
245/45ZR18 26.8" 775
255/45ZR18 26.9" 771
285/40ZR18 26.9" 771
235/50ZR18 27.2" 762

Take the 285/40ZR18 as an example:

Outside diameter = 26.9" This implies an outside circumfrence of
26.9*pi=84.51". This equates to 749.74 revs per mile versus the
manufacturers specification of 771 (this impies a rolling diameter of
26.16") . Clearly, the outside diameter of the tire is not the cheif
determining factor in the rolling diameter. Again, think tank tread, not
hard wheel!

So if you know the size and type of tire you are experimenting with, we
could find the revs per mile specification and compare that to you
experimentally determined numbers.

On other thing you might try. Reduce the air pressure in your tires by 20%
and rerun the test. The loaded diamter will change. There may be a slight
difference in the results, but I'll wager it will be less than the
difference in loaded diameter.

Ed

"Tegger" <invalid(a)invalid.inv> wrote in message
news:Xns9CBD95793DAF4tegger(a)208.90.168.18...
>A repost of my original test results, with the one typo corrected
> (11.75" corrected to 10.75")
>
> ----------------------------
>
> After making a gauge with corrugated cardboard, I discovered that the
> unloaded diameter of both front and rear tires is 23.25" dead-on.
>
> 23.25" x 3.14159 = 73.04" unloaded circumference.
>
> Tire pressures at time of test were all the same, 31 lbs (checked hot).
>
> On a flat, newly paved industrial parking lot, I marked the tires (and
> the lot) with chalk. Leaning out the window, I then slowly rolled the
> car so that the mark on the left tire described ten revolutions, coming
> down to the very bottom again. That covered almost 60 feet (100
> revolutions was not practical for me.)
>
> I did this four times each, for the front and then for the rear tires
> (both sides). The results were very consistent.
>
> The results?
> Actual distance covered for the fronts: 704.5"
> Actual distance covered for the rears: 708.5"
>
> Now, how about the "loaded" radius?
> For the front left, it's 10.75", which gives a circumference of 67.54"
> However, that tire actually covered 70.45" per rev, not 67.54". But at
> the same time it was not covering 73.04" either.
>
> For the rear left, the loaded radius is 11.0625". This gives a
> circumference of 69.51". That tire actually rolled 70.85" in the test.
> (Note that's eleven and one-sixteenth, not eleven and five-eighths.)
>
> Let's summarize, as percent reduction from unloaded to loaded:
> Front hypothetical rolling circum based on loaded radius: 7.5% less
> Front actual rolling circum based on test result: 3.55% less
> Rear hypothetical rolling circum based on loaded radius: 4.8% less
> Rear actual rolling circum based on test result: 3% less
>
> Looks like the fronts split the difference between unloaded and loaded,
> and the rears were affected pretty close to what I theorized.
>
>
>
>
> --
> Tegger
>

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