On Wednesday, July 19, 2017 at 2:06:41 PM UTC-4, Bob F wrote:
On 7/19/2017 9:53 AM, Mad Roger wrote:
On Wed, 19 Jul 2017 12:17:36 -0400,
Frank wrote:

I found it interesting and first sentence says it, "They look cool."

I agree that people do things for looks alone, but what is strange is that
there doesn't seem to be a single beneficial performance impact of an
overall larger diameter "tire-and-wheel assembly".

I wasn't expecting huge performance gains, but I would have expected at
least one or two benefits - and not all negatives based on the two articles
noted.

If the diameter of the wheel and tire assembly increases by one inch
overall due to the one-inch increase in rim size - and assuming everything
else is kept equal in materials and aspect ratio and tread width - then the
two articles stated...
+ The engine delivers ~25% less driving force to the wheel contact patch

That's crazy! Where on earth did that come from. 1" diameter increase
would only cause that change if you started with 4" O.D. tires.

Yeah, that number seems suspect to me too.



Low profile tires compensate for the wheel diameter increase, so no
increase in tire diameter occurs, and no torque loss should occur.

+ Which results in a decrease in acceleration

+ And which decrease in fuel economy (presumably at all times)
It should increase economy. Just like driving in a high gear does.

My understanding is that low profile tires are being used because they
have lower rolling friction - the rubber is flexing less, so there is
less loss to heating the tire.

I think that's true. It's not clear to me anymore what is being
discussed here. Is it the hypothetical and unusual case where
the same profile and width tire is use, just larger diameter?
Or is the most common case, where with a larger wheel, you go to
a different profile tire, so the overall rolling diameter stays
about the same?



+ And unsprung weight goes up by a few pounds

The main astounding number is the fact the torque felt at the wheels is
astoundingly less for a single inch in overall diameter change.

Yes, it is astounding. In fact, I would suggest it is unbelievable.


+1




Presumably that torque loss happens at all speeds (why would it not?) so
that denies us the one intuitive performance advantage of highway MPG.


And I don't believe less torque translates into lower fuel economy either.
Yes, it will accelerate slower with a larger diameter wheel, but that
just means you have less torque applied over a longer period. It's
not torque, it's energy that you need to look at.