tire wear and all wheel drive

[Edward Hayes]

Calculations look right on. My 2000 Forester AT has a "continuously variable
transfer clutch pack. That is where the slip must occur. The manual has a
viscous coupling locking center differential so the silicon fluid in the
viscous coupling must absorb the difference. Eddie

 

[Bayard Webb]

: >
: > Time for some math (someone double check this)
: >
: > 1 mile is 63360"
: > the circumfrance of a 215/60/16 is approx 82.3"
: > with that, it requires that wheel to turn 770 times to go a mile
: >
: > if you increase the circumfrance by 1/4 inch (.25) this wheel will
: > require 767.5
: > turns to go a mile, which is a 2.5 turn delta.
: >
: > At 60 MPH, the larger wheel will turn 2.5 turns less than the smaller
: > wheel every minute.
: >
: > Where is this difference obsorbed and will it cause damage? Is it
: > different for Autos Vs Manual?

The circumference of the tire is irrelevant. Before you blast me, please
consider the following. Remember the old saying "It's only flat on the
bottom"? Well I think that applies here. If a tire is underinflated and the
axle sits closer to the ground by, for example 1/4", then the effective
rollout of the tire has been affected as if the diameter was reduced by
1/2". The length of the rubber that makes up the tread is unchanged, this
is true, but since we are no longer talking about a circle, we have to
consider the *effective* circumference. I think the effective circumference
is

2*pi*(the distance from the center of rotation to the road)

Think of the extreme case: your tire is dead flat and you are riding on the
rim. It doesn't matter how much extra rubber you have flapping around, you
are riding on a shortened effective circumference.

Bayard

 

[kfarndog]

No kidding. Then you get tire ruined and have to throw the spare into the
rotation, you know that spare has a lot less wear than the others, but
there is no way you are going to buy three new tires. I mention this
because I had to do it recently. If a Soob can't tolerate that much
difference I don't know how impressed I am am with that engineering. I 'd
be interested to hear some field experience.
-keith

 

[D H]

No kidding. Then you get tire ruined and have to throw the spare into the
rotation, you know that spare has a lot less wear than the others, but
there is no way you are going to buy three new tires. I mention this
because I had to do it recently. If a Soob can't tolerate that much
difference I don't know how impressed I am am with that engineering. I 'd
be interested to hear some field experience.
-keith

Well, I was in that situation twice already. Once, I used the spare
because the mileage on the others was pretty low, and I put the brand new
tire I had to purchase onto the spare rim. Still measured about 1/4"
difference circumference, if I recall correctly. No problems as a result
_that I know of_!
The second time, I had a tire belt problem, so I had to get another tire
under warranty (glad it was still available even though no longer being
manufactured.) I think I measured several tires on both my and my wife's
Foresters, and chose those closest together for mine and for hers, maybe
mixing one tire from the opposite car. We had to use the new one because at
this point our spares did not match the new tires we bought when we replaced
the factory tires with another type Yokohama tire. So far, we have not had
any problems _that we know of_!
My judgement is that the 1/4" is WAY too ridiculously tight a tolerance
to abide by, and you just have to do your best to get close. If mine were
above 1/2" difference, I think I would probably opt to buy another set and
just try to recoup something out of these selling as used or something. (Or
more likely, let them sit and rot somewhere, I'm afraid.)

 

[Gilles Gour]

Underinflation should not cause any problems. The circumference of the
tire remains the same independent of low air pressure, so the number
of wheel revolutions per mile remains the same.

I made an experiment with a bicycle : front wheel with 60 psi and rear
with 30
psi. After just a few revolutions in a straight line, the mark had I
made on the rear tire where it touched the ground had crept noticeably
from the mark I had made on the front tire.

 

[John Varela]

I made an experiment with a bicycle : front wheel with 60 psi and rear
with 30
psi. After just a few revolutions in a straight line, the mark had I
made on the rear tire where it touched the ground had crept noticeably
from the mark I had made on the front tire.

Radial tires have a circumferential steel belt so inflation won't change the
circumference of a radial as much as it will a cross-ply tire, which I assume
is what you have on your bike.

 

[John Varela]

The circumference of the tire is irrelevant. Before you blast me, please
consider the following. Remember the old saying "It's only flat on the
bottom"? Well I think that applies here. If a tire is underinflated and the
axle sits closer to the ground by, for example 1/4", then the effective
rollout of the tire has been affected as if the diameter was reduced by
1/2".

Consider the point on the tire's circumference directly below the axle. Give
the wheel and tire one full revolution. The same point on the tire will again
be under the axle. How far forward will the axle have moved? It will have
moved a distance equal to the circumference of the tire.

If you don't believe that, consider a "wheel" that is square or rectangular
and made of wood. Give it one revolution, thump, thump, thump, thump. How
far has it moved? You can run this test with one of your kid's toy blocks, or
a piece of 2x4 will do. It doesn't matter what the "radius" of the wooden
block is; the distance moved will be the sum of the lengths of the sides of
the block.

As I said in another note, if the tire is rotated at high speed the tire will
stretch and deform, and differently inflated tires will have different
*effective* circumferences, or so I am led to believe.

 

[Bayard Webb]

: On Sat, 16 Aug 2003 02:07:17 UTC, "Bayard Webb"
:
: > The circumference of the tire is irrelevant. Before you blast me,
please
: > consider the following. Remember the old saying "It's only flat on the
: > bottom"? Well I think that applies here. If a tire is underinflated and
the
: > axle sits closer to the ground by, for example 1/4", then the effective
: > rollout of the tire has been affected as if the diameter was reduced by
: > 1/2".
:
: Consider the point on the tire's circumference directly below the axle.
Give
: the wheel and tire one full revolution. The same point on the tire will
again
: be under the axle. How far forward will the axle have moved? It will
have
: moved a distance equal to the circumference of the tire.

Consider a flat tire, riding on the rim. Drive so that the wheel makes one
full revolution. How far will you travel? a distance equal to the
circumference of the rim. The rest of the tire just flops around.
:
: If you don't believe that, consider a "wheel" that is square or
rectangular
: and made of wood. Give it one revolution, thump, thump, thump, thump.
How
: far has it moved? You can run this test with one of your kid's toy
blocks, or
: a piece of 2x4 will do. It doesn't matter what the "radius" of the
wooden
: block is; the distance moved will be the sum of the lengths of the sides
of
: the block.

Now take your block of wood and wrap it in soft foam so that the outer
surface is round. Press down on the block as you perform your experiment.
After one revolution you will still have moved the sum of the four sides
even though you have built up the circumference with padding. The fact that
you are compressing the bottom as it hits the ground shortens the total
distance traveled. This is analogous to running an underinflated tire.
:
: As I said in another note, if the tire is rotated at high speed the tire
will
: stretch and deform, and differently inflated tires will have different
: *effective* circumferences, or so I am led to believe.

Agreed that there are other effects, and that the behavior will be
different at different speeds. Also agreed that with an overinflated tire
the effective circumference is the same as the real circumference since the
weight of the car is not deforming the tire at the contact patch.
:
: --
: John Varela

 

[Lance B]

I still stand by my original statement that a tires circumference does
not change with tire pressure (or not enough to make a big difference
to the circumference). I do agree that the diameter decreases with
less inflation of the tire and this is what obviuosly causes a
difference to the tires rpm. Tire circumference and tire diameter are
two completely different things when pressure differences are brought
into the equation.

 

[John Varela]

Underinflation should not cause any problems. The circumference of the
tire remains the same independent of low air pressure, so the number
of wheel revolutions per mile remains the same.

Here's something that should settle some of these raging arguments.

At dmses.dot.gov/docimages/pdf68/135285_web.pdf is a US Department of
Transportation report on tests of tire pressure monitoring systems (TPMS).
It's a 161 page .pdf document; if you don't want to download it, Google's HTML
conversion is at http://tinyurl.com/ke1a

This report says:

"As was mentioned in the preceding section, TPMS can be divided into two main
categories, Wheel-Speed Based - WSB (Indirect) and Pressure-Sensor Based - PSB
(Direct). WSB systems infer tire pressures by using the vehicle's ABS
hardware, specifically the wheel speed sensors, to measure tire-to-tire
differences in tire rotational velocities that indicate that a tire is at a
different pressure from the others."

The report also says that WSB sensors don't work very well:

"Four WSB systems installed as original equipment on 2000 or2001 vehicles were
evaluated. Testing showed that all of the WSB systems would warn of a single
tire being significantly underinflated (50 percent low) on a winding road
course. However, on the 7.5 mile oval test track, which requires little
turning input, only three of the four systems could detect tire pressure as
low as 14 psi. The WSB systems also did not warn of two tires equally
underinflated on the same side of the vehicle or on the same axle. Three of
four systems warned of two underinflated tires in diagonal positions. All of
the systems warned of three tires equally underinflated. None of the four WSB
systems were able to warn of all four tires equally underinflated. The
observed warning capabilities of these systems corresponded to the theoretical
limitations of the sensing algorithm documented in this report."