Please explain wheel size differences.

There have been several threads lately about wheel sizes, but I'm a
bit in the dark as to what difference wheel diameter makes. I seem to
remember hearing that changing wheel diameter doesn't necessarily
change spedometer accuracy. If that's the case, then the tire
thickness (rim to road) must be less with larger diameter wheels.
Then it would seem to follow that there would be less tire distortion
when cornering, all other things being equal. Am I on the right
track? Are there other advantages/disadvantages? Should I just go
for a ride and not think about it?

"Should I just go for a ride and not think about it?"
Yes! Life is too short to worry about the minutiae!

Short sidewalls must be very stiff in comparison to a normal height
sidewall.
Remember that the sidewall has a part in improving the ride as well as
effecting handling.
To me, the current crop of large rim diameters and narrow sidewalls are more
for appearance than any other useful reason. We watches a poor fool try to
autocross a car with the narrow sidewall tires and large rims. The rims
ground in sharp corners (sparks and all that)
If the tire does not absorb shock, the suspension and shocks/struts are the
next line of defense.

"Dave Smith" > wrote in message
...
> There have been several threads lately about wheel sizes, but I'm a
> bit in the dark as to what difference wheel diameter makes. I seem to
> remember hearing that changing wheel diameter doesn't necessarily
> change spedometer accuracy. If that's the case, then the tire
> thickness (rim to road) must be less with larger diameter wheels.
> Then it would seem to follow that there would be less tire distortion
> when cornering, all other things being equal. Am I on the right
> track? Are there other advantages/disadvantages? Should I just go
> for a ride and not think about it?
>


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In article >,
Dave Smith > wrote:

> I seem to
> remember hearing that changing wheel diameter doesn't necessarily
> change spedometer accuracy. If that's the case, then the tire
> thickness (rim to road) must be less with larger diameter wheels.

Correct. All the common Miata sizes have about the same overall tire
diameter: 185/60-14, 195/50-15, etc.--only the size of the hole is
different See the tire calculator in the miata.net Garage, which lets
you compare revs/mile for any two tire sizes.

> Then it would seem to follow that there would be less tire distortion
> when cornering, all other things being equal.

That's one theory. In reality, the inflation pressure, brand, and model
of tire make much more difference to grip than the profile (aspect
ratio). Extremely low ratios (less than 40 or 50) do not protect the
wheels as well from pothole impact damage. In general, larger-diameter
wheels also weigh more, and so do their tires (despite the bigger hole),
which is detrimental to both ride and handling. The sweet spot seems to
be 195/50-15, which also offers a greater choice of tire options than
14" sizes.

--
Lanny Chambers, St. Louis, USA
'94C
the alignment page:
http://www.hummingbirds.net/alignment.html

In article >,
"chuckk" > wrote:

> "Should I just go for a ride and not think about it?"
> Yes! Life is too short to worry about the minutiae!
>
> Short sidewalls must be very stiff in comparison to a normal height
> sidewall.
> Remember that the sidewall has a part in improving the ride as well as
> effecting handling.
> To me, the current crop of large rim diameters and narrow sidewalls are more
> for appearance than any other useful reason. We watches a poor fool try to
> autocross a car with the narrow sidewall tires and large rims. The rims
> ground in sharp corners (sparks and all that)
> If the tire does not absorb shock, the suspension and shocks/struts are the
> next line of defense.

I suggest you pay attention to the aspect ratios typical of racing cars
when not limited by regulation...

>
> "Dave Smith" > wrote in message
> ...
> > There have been several threads lately about wheel sizes, but I'm a
> > bit in the dark as to what difference wheel diameter makes. I seem to
> > remember hearing that changing wheel diameter doesn't necessarily
> > change spedometer accuracy. If that's the case, then the tire
> > thickness (rim to road) must be less with larger diameter wheels.
> > Then it would seem to follow that there would be less tire distortion
> > when cornering, all other things being equal. Am I on the right
> > track? Are there other advantages/disadvantages? Should I just go
> > for a ride and not think about it?
> >
>
>
> ---
> Outgoing mail is AVG certified Virus Free.
> Checked by AVG anti-virus system (http://www.grisoft.com).
> Version: 6.0.782 / Virus Database: 528 - Release Date: 10/22/2004

--
Alan Baker
Vancouver, British Columbia
"If you raise the ceiling 4 feet, move the fireplace from that wall
to that wall, you'll still only get the full stereophonic effect
if you sit in the bottom of that cupboard."

In article >,
"Dustin" > wrote:

> I am not disagreeing, but why do corvettes and vipers run such large tires?

Same reason as Ferrari: the factory thinks the owners expect bling-bling
wheels. And they're right--few buyers of these cars know or care about
handling. They bought image, not performance.

FWIW, the first thing a savvy Corvette owner does is lose the heavy
run-flat tires.

--
Lanny Chambers, St. Louis, USA
'94C
the alignment page:
http://www.hummingbirds.net/alignment.html

In article >, Lanny Chambers > wrote:
>In article >,
> "Dustin" > wrote:
>
>> I am not disagreeing, but why do corvettes and vipers run such large tires?
>
>Same reason as Ferrari: the factory thinks the owners expect bling-bling
>wheels. And they're right--few buyers of these cars know or care about
>handling. They bought image, not performance.

There are a couple of legit reasons for bigger wheels:
Allows bigger brakes (which is sometimes, though extremely rarely
the case).
Allows shorter sidewalls for a given wheel OD which, up to a
point, will give a more responsive ride. Though likely with
increased harshness.

Now almost everyone mentions the increased weight. Yes, the wheel
weight will increase, but the tire weight will decrease! [Given we
stay at the same tread width and OD]. And tires weigh as much as
wheels. So, the overall weight doesn't really have to change
much.

But I agree most of it is just about style. Big wheels are in.
My Miata has the stock 16", I'd be as, or more, happy with 15's.

>FWIW, the first thing a savvy Corvette owner does is lose the heavy
>run-flat tires.

I'm with you on that! Damn this move to run flats and no spares!
What does Porsche do now, give you a can of fix a flat?

(Dave) wrote:

>In article >, Lanny Chambers > wrote:
>>In article >,
>> "Dustin" > wrote:
>>
>>> I am not disagreeing, but why do corvettes and vipers run such large tires?
>>
>>Same reason as Ferrari: the factory thinks the owners expect bling-bling
>>wheels. And they're right--few buyers of these cars know or care about
>>handling. They bought image, not performance.
>
>There are a couple of legit reasons for bigger wheels:
>Allows bigger brakes (which is sometimes, though extremely rarely
>the case).
>Allows shorter sidewalls for a given wheel OD which, up to a
>point, will give a more responsive ride. Though likely with
>increased harshness.
>
>Now almost everyone mentions the increased weight. Yes, the wheel
>weight will increase, but the tire weight will decrease! [Given we
>stay at the same tread width and OD].

A long time ago, Richard Decker posted data showing that this is
typically not true. I don't have them anymore, but if I look
at the web site of our favorite T1s's, it lists
205/55R15 88V weight: 18.3 outer diameter: 23.9
205/40ZR17 84WRD weight: 18.5 outer diameter: 23.5

Moreover, it is somewhat misleading, since the 18.5 lb R17 tire
is the lightest available. With a 14" wheel, you have the
option of going down to a 15 lb 195/45R14 77V. In addition to
having a much lighter wheel. If you are in a situation where
keeping your tires on the road is a consideration, you are out
of luck with a 17" wheel.

Leon

> And tires weigh as much as
>wheels. So, the overall weight doesn't really have to change
>much.
>
>But I agree most of it is just about style. Big wheels are in.
>My Miata has the stock 16", I'd be as, or more, happy with 15's.
>
>>FWIW, the first thing a savvy Corvette owner does is lose the heavy
>>run-flat tires.
>
>I'm with you on that! Damn this move to run flats and no spares!
>What does Porsche do now, give you a can of fix a flat?

--
Leon van Dommelen Bozo, the White 96 Sebring Miata .)
http://www.dommelen.net/miata
EXIT THE INTERSTATES (Jamie Jensen)

"Lanny Chambers" > wrote in message
...
> In article >,
> "Dustin" > wrote:
>
>> And they're right--few buyers of these cars know or care about
> handling. They bought image, not performance.

Yet the Corvette continues to outperform the Miata's handling despite our
lovely car's nimbleness. Could we possibly be over-exaggerating the wheel
issue here?

In article >, (Leon van Dommelen) wrote:
(Dave) wrote:

>>Now almost everyone mentions the increased weight. Yes, the wheel
>>weight will increase, but the tire weight will decrease! [Given we
>>stay at the same tread width and OD].

>A long time ago, Richard Decker posted data showing that this is
>typically not true. I don't have them anymore, but if I look
>at the web site of our favorite T1s's, it lists
>205/55R15 88V weight: 18.3 outer diameter: 23.9
>205/40ZR17 84WRD weight: 18.5 outer diameter: 23.5

Damn you for bringing data to the table! 'Course it is only one
data point ... exactly one more than I had :-)

Seems strange though. I'd expect a shorter sidewall would require
less thickness for a given spring rate and strength, thus lighter.
Maybe something else is happening. Anyway, most go to wider
wheels and tires which certainly does add more weight. But even
with all that, I think the rotating mass arguments generally used
on the net are exaggerated. And maybe the unsprung mass issues?

Be that as it may, I'm still not a fan of big wheels & tires.
There simply isn't much benefit. There are some drawbacks
(whether minor or not). And they cost a ton more.

(Dave) wrote:

>In article >, (Leon van Dommelen) wrote:
(Dave) wrote:
>
>>>Now almost everyone mentions the increased weight. Yes, the wheel
>>>weight will increase, but the tire weight will decrease! [Given we
>>>stay at the same tread width and OD].
>
>>A long time ago, Richard Decker posted data showing that this is
>>typically not true. I don't have them anymore, but if I look
>>at the web site of our favorite T1s's, it lists
>>205/55R15 88V weight: 18.3 outer diameter: 23.9
>>205/40ZR17 84WRD weight: 18.5 outer diameter: 23.5
>
>Damn you for bringing data to the table! 'Course it is only one
>data point ... exactly one more than I had :-)
>
>Seems strange though. I'd expect a shorter sidewall would require
>less thickness for a given spring rate and strength, thus lighter.

I would guess it may have to do with design trade-offs making
the sidewall conform to a flat contact patch in a short distance.
Fatigue jumps to mind.

> Maybe something else is happening. Anyway, most go to wider
>wheels and tires which certainly does add more weight. But even
>with all that, I think the rotating mass arguments generally used
>on the net are exaggerated.

The ones I have seen are. The rotation adds a bit to the effective
mass, but nowhere as dramatic as some claim.

> And maybe the unsprung mass issues?

That is a real issue to me. While your wheel is off the ground,
its traction performance is quite poor. It also reduces driving
comfort.

Leon

>Be that as it may, I'm still not a fan of big wheels & tires.
>There simply isn't much benefit. There are some drawbacks
>(whether minor or not). And they cost a ton more.

--
Leon van Dommelen Bozo, the White 96 Sebring Miata .)
http://www.dommelen.net/miata
EXIT THE INTERSTATES (Jamie Jensen)