Re: wheel circumference


"Tegger" <invalid@xxxxxxxxxxx> wrote in message
news:Xns9CBA63CD94FA9tegger@xxxxxxxxxxxxxxxx
"C. E. White" <cewhite3remove@xxxxxxxxxxxxxx> wrote in
news:IdednamjeIFc2G_XnZ2dnUVZ_gKdnZ2d@xxxxxxxxxxxxx:




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.



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. If all four tires were the same, then the speed of
the tires would also be the same and the ABS would function properly.

The tire size _can_ influence the speed display on the speedometer, but as
has been shown in several charts, if the stock size is changed for an
aftermarket size that is properly equivelent, the influence over the
speedometer is insiginficant, and if the new tire is a few percent larger,
the actual effect on the speedometer is desirable -- it makes the speedo
display the proper speed instead of display a lower speed than one is
actually travelling.

My car came with a 205/50x16, the car before it came with a 225/55x15, I put
a set of 225/45x17s on both of these cars, and all three tires are
functional equivelents for one another. (I had a car that had the 15s on it,
I put on the 17s, and a lady in a double-know pants suit and blue hair
slammed into me. I bought the car with the 16 on it, and move the 17s to
it). The affect on the speedo was that the 17's changed the error at 80mph
from almost 5mph to just under 2mph. Foremrly, the speedo would read 80 when
doing 75-ish, now it reads 80 when doing 78-ish. But since all four tires
are the same size, and functional equivelents, the ABS system still works
right.

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.)

If he wants to go down an inch on the rims he's using, then he would go up
5% on the aspect ratio (the center figure in 190 / 55 x 15) to keep the same
tread width. If he wanted to keep the same rims but get a larger sidewall,
then he's have to get a thinner tire.

On second thought, I'd have to run the numbers, I'm not sure he can get more
sidewall on the same rim because if the width got less but the aspect ratio
went up, the result would be the same sidewall -- 190 x .55 and 180 x .6 are
different by 3.5mm, hardly a useful difference for any practical reason. He
would get 10mm less rubber on the ground and virtually no change in the
sidewall.





.

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