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Default Does a tyre change its CIRCUMFERENCE when underinflated?

On 23/06/2018 17:04, The Natural Philosopher wrote:
On 23/06/18 14:28, John Rumm wrote:
On 23/06/2018 06:56, The Natural Philosopher wrote:

https://www.youtube.com/watch?v=_2l5bOhHNxU

Answer. not by very much, if at all.


So the short answer to your question is "yes".

This seems to be a perpetual urban myth.


Which you have just agreed with by claiming there is a small change.


No the urban myth says that since there is say a 10% change in what
peooplle think is 'the radius', therefore the RPM will be 10% slower.


Make up your own myths if you like... The 1% seems plausible enough for me.

Can't see that being too difficult in itself - especially as you
probably have input from other sensors and know the steering angle
input Â*Â*and so can assess when you are driving straight and not under
high acceleration etc.


Why would there be monitors on steering angle?


Stability control programs, collision avoidance system, traction control
and various other systems will use it.

Type pressure monitoring will need to be more sensitive to rate of
change than absolute difference since unequal tyre wear would
otherwise be flagged.


Well teh way it aseems to work is that one wheel will overotate with
respect to its diagonalÂ* consisetntly.


Splembib!

But not by very much.


We like that.

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John.

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On 23/06/2018 17:07, The Natural Philosopher wrote:
On 23/06/18 14:50, Huge wrote:
On 2018-06-23, John Rumm wrote:
On 23/06/2018 06:56, The Natural Philosopher wrote:

https://www.youtube.com/watch?v=_2l5bOhHNxU

Answer. not by very much, if at all.

So the short answer to your question is "yes".

This seems to be a perpetual urban myth.


I just love watching Turnip arguing that a system that exists, is in
production and works is an "urban myth". What next, Flat Earth?


Oh dear. What I was arguing was that the reason the systemn works is not
what people here believe.

If you told me that essence of angel was what was in antibiotics and
they fought the demons in the pus, it doesnt mean that I disagree that
antibiotics cure infections, when I tell you you are talking ********.

But basic logic is another thing that has passed you by it seems.



Well you were the one to introduce circumference into this, a value
which is irrelevant to the relationship between the 2 things measured
(the vehicles speed and the angular velocity of the wheel) and what
relates them (the rolling radius).

The irrelevance of the circumference can be seen by looking at
caterpillar tracks. Ignoring slippage etc a 20m track goes round once
when the vehicle moves 20m. But that's true whether it is attached to
wheels with a radius of 0.5m or 0.25m. So the circumference tells you
nothing about the relationship between speed and the RPM of the wheels.
On the other hand, the (effective) radius of the wheels does.

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On 23/06/2018 18:33, Tim Streater wrote:
In article , Robin
wrote:

Well you were the one to introduce circumference into this, a value
which is irrelevant to the relationship between the 2 things measured
(the vehicles speed and the angular velocity of the wheel) and what
relates them (the rolling radius).

The irrelevance of the circumference can be seen by looking at
caterpillar tracks.Â* Ignoring slippage etc a 20m track goes round once
when the vehicle moves 20m.Â* But that's true whether it is attached to
wheels with a radius of 0.5m or 0.25m.Â* So the circumference tells you
nothing about the relationship between speed and the RPM of the
wheels. On the other hand, the (effective) radius of the wheels does.


Aren't you overlooking that, with a track, the wheel is only in contact
with a part of the track. Whereas with a flat tire, even though the
shape of the tire changes, the wheel is still in touch with the tire
all the way round (the entire tire, IOW (phew!)).

Thus TNP is right to say that, inflated or not, for one revolution of
the wheel, the whole circumference of the tire rotates once. Or, to put
it another way, any spot on the tire circumfernce will touch the road
just once.

(ignoring slippage on the road, or slippage between the tire itself and
the wheel).


Yep. But that's just another way of saying that with a tyre on a wheel
the fact that the rolling radius changes with pressure means the
"rolling circumference"[1] also changes with pressure. As TNP admitted.
So I was and am still unclear why he asked about the circumference. I
thought at first it might be because he thought that the steel
cords/belts in tyres are like caterpillar tracks. But it can't be that
when he volunteered there is an effect.

Of course I may well be missing something. But I _know_ I'm - hic -
missing the bottle downstairs, so ...


[1] TM TNP?
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GB wrote
Robin wrote


That ruling in 2001 was in the context of what could and couldn't meet a
statutory requirement in the USA for pressure monitoring in new cars that
also alerted if more than one tyre - including all 4 tyres equally - are
25% or more below pressure. It's no surprise that indirect methods
couldn't meet that requirement.


If the car has sat nav installed and available to the car's computer, as
many do now, the speed across the ground could be compared to the wheel
rotation. That could detect all 4 wheels being equally underinflated.


Maybe. gps speed isnt that accurate in the short distances.

And if you get 4 new tyres at a time, as many do, and the tyre place
underinflates all of them by the same amount, and the tyres are a
bit different profile wise to the previous set, its going to be hard
for the system to work out that they are all underinflated. And thats
the main situation where all 4 would be equally under inflated.

But, that's 2018, not 2001.



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"Chris Hogg" wrote in message
news
On Sat, 23 Jun 2018 17:04:38 +1000, "Jeff" wrote:



"Chris Hogg" wrote in message
. ..
On Sat, 23 Jun 2018 16:08:44 +1000, "Jeff" wrote:



"The Natural Philosopher" wrote in message
news https://www.youtube.com/watch?v=_2l5bOhHNxU

Answer. not by very much, if at all.

But the radius clearly does vary significantly.

Does it?


It clearly does with the distance between the axle and the ground.


I think we're at cross-purposes here. Obviously, as you say, when a
tyre is flat, the axle is nearer the ground. But is it reasonable to
regard that as the radius of the tyre?


It is the only distance that matters with the rotation speed of the wheel.
The distance between the axle and the top of the tyre is irrelevant to that.

If you simplify the shape and
call it an ellipse, then you have two radii; quite how many radii
would be needed to describe a tyre with a flat on one side, I wouldn't
like to speculate.


But those distances are irrelevant to the rotation speed of the wheel.

But the circumference, perhaps perimeter would be a
better word, won't have changed significantly. It'll just become
distorted, i.e. no longer circular.


Yes, but that isnt what determines the rotation rate of the wheel.

But that assumes there is a weight pressing down on the tyre, which I
grant you, would be the case for a loaded tyre on a vehicle. But I was
thinking of an unloaded tyre;


Its irrelevant when deciding if the tyre is under inflated.

does the circumference change between
under-inflated and fully- or even over-inflated?


Not by much at all with steel belted radials and any system
that detects under inflated tyres has to work with those.

When I were a lad in
the days when tyres had inner tubes, we used to go to our local garage
and get old tubes that were no longer serviceable, patch them where
necessary, inflate then and use them as super-sized rubber rings for
taking down to the beach (and probably getting blown out to sea!). As
they were inflated, the radius and circumference most certainly did
increase; they blew up like a balloon. But put them into a tyre and
there'd be no significant change in the circumference as they inflated
and deflated. The structure of the tyre wouldn't allow that to happen.

Likewise, I suggest that the circumference (perimeter if you prefer)
of a modern tubeless tyre doesn't change significantly as it inflates
or deflates.


Yes, but the circumference isnt what determines the rotation
rate of the tyre on a moving car.

Every full rotation of the hub must correspond to a full rotation of
the perimeter, regardless of the state of inflation of the tyre,
otherwise serious slippage will be occurring between the tyre and the
rim, which would result in friction heating and fairly rapid failure.


Yes, but the circumference doesn't determine the rotation rate of the tyre.

So as far as speedometers and odometers are concerned, state of
inflation won't make a significant difference.


That's arguable with the very small variations being discussed.

(What amounts to 'significant' as I've used it here, I'm not sure; it
depends on the pressure difference being considered between under and
fully inflated, and the elasticity of the structures within the
treaded surface of the tyre, amongst other things, but others in this
thread have mentioned figures of around 1% for the stretching of the
perimeter as the tyre is inflated. In this context, I would regard
that as not significant).


But someone must have checked that the ABS rotation rate does change
with under inflated tyres otherwise that wouldn't be used to detect
under inflated tyres.



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On Sun, 24 Jun 2018 04:55:50 +1000, cantankerous geezer Rot Speed blabbered,
again:

If the car has sat nav installed and available to the car's computer, as
many do now, the speed across the ground could be compared to the wheel
rotation. That could detect all 4 wheels being equally underinflated.


Maybe. gps speed isnt that accurate in the short distances.

And if you get 4 new tyres at a time, as many do, and the tyre place
underinflates all of them by the same amount, and the tyres are a
bit different profile wise to the previous set, its going to be hard
for the system to work out that they are all underinflated. And that¢s
the main situation where all 4 would be equally under inflated.


Darn, Mr Know-it-all has it all worked out, yet again! LOL

--
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"You really are a clueless pillock."
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"harry" wrote in message
...
On Saturday, 23 June 2018 06:56:49 UTC+1, The Natural Philosopher wrote:
https://www.youtube.com/watch?v=_2l5bOhHNxU

Answer. not by very much, if at all.

This seems to be a perpetual urban myth.

It tyre pressure sensors are using this, it has to be a very very
complicated bit of software to detect - say - less than 1% change in RPM
relative to the other wheels.


https://en.wikipedia.org/wiki/Centra...flation_system


Bit weird with the retrofitted stuff
https://www.dropbox.com/s/yj56jztegr...93fa6.jpg?dl=0
Cant see that surviving too long off road here.

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"The Natural Philosopher" wrote in message
news
On 23/06/18 08:09, Jeff wrote:


"The Natural Philosopher" wrote in message
news
On 23/06/18 07:08, Jeff wrote:


"The Natural Philosopher" wrote in message
news https://www.youtube.com/watch?v=_2l5bOhHNxU

Answer. not by very much, if at all.

But the radius clearly does vary significantly.

This seems to be a perpetual urban myth.

It can't be given that Andy and others have had a number
of warnings that have turned out to be accurate every time.


It tyre pressure sensors are using this, it has to be a very very
complicated bit of software to detect - say - less than 1% change in
RPM relative to the other wheels.

Yes, but that isn't hard to measure when its relative to other wheels.

A piece of online research
[https://one.nhtsa.gov/cars/rules/rul...prmonsys.html]
showed that relative wheel rotation was pretty crap at detecting low
tyre pressures especially in all 4 wheels or 2 wheels on the same side
(= I am going round and round in circles!)

Hence the move to in wheel sensors.

I am notr syoing it doesn't work, just that it relies on some pretty
iffy interpreation of very small differences in road speed.

Tyre tread belts do strech under inflation, but not by much. The radius
is completely irrelevant as a road weheel is not, in use, round and does
not have a 'radius.'


Of course it does at the only place that matters, between the axle and
the road.

Oh dear. Oh dear oh dear.

You just dont get it, do you?


It is you that doesnt. That distance is the only thing that matters,
it is what determines the rotation rate of the wheel, exactly the
same way the diameter of the wheel determines the rotation
rate of the wheel, but in this case that radius clearly does vary
with the pressure in the tyre. What happens with the rest of
the tyre is completely irrelevant.

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"The Natural Philosopher" wrote in message
news
On 23/06/18 09:34, Jeff wrote:


"Tim Lamb" wrote in message
...
In message , Jeff
writes


"The Natural Philosopher" wrote in message
news On 23/06/18 07:08, Jeff wrote:


"The Natural Philosopher" wrote in message
news https://www.youtube.com/watch?v=_2l5bOhHNxU

Answer. not by very much, if at all.

But the radius clearly does vary significantly.

This seems to be a perpetual urban myth.

It can't be given that Andy and others have had a number
of warnings that have turned out to be accurate every time.


It tyre pressure sensors are using this, it has to be a very very
complicated bit of software to detect - say - less than 1% change in
RPM relative to the other wheels.

Yes, but that isn't hard to measure when its relative to other
wheels.

A piece of online research
[https://one.nhtsa.gov/cars/rules/rul...prmonsys.html]
showed that relative wheel rotation was pretty crap at detecting low
tyre pressures especially in all 4 wheels or 2 wheels on the same
side (= I am going round and round in circles!)

Hence the move to in wheel sensors.

I am notr syoing it doesn't work, just that it relies on some pretty
iffy interpreation of very small differences in road speed.

Tyre tread belts do strech under inflation, but not by much. The
radius is completely irrelevant as a road weheel is not, in use,
round and does not have a 'radius.'

Of course it does at the only place that matters, between the axle and
the road.

I've been thinking that but... unless the effective circumference is
changed, the tread in contact with the road surface will not alter.
Steel bracing etc. as mentioned above.


I'm not convinced that it is the circumference that determines
the rotation rate of the wheel.


Well unless the tyre is slipping on the rim or in the road, there is no
way anyuthing else can.


And those who actually measured
it talk about the rolling radius, for a reason and must have done
the most basic tests of watching the rotation rate as the tyre
pressure varies, and see that it does vary by enough to measure.


No, the world is full of stupid people pretending to be clever.

The 'effective rolling radius' is the circumference divided by 2 PI.


Not when the wheel and tyre isnt a perfect circle and it never is
with a vehicle wheel and inflated tyre.

Trying to make a squashed dougnut into a circle doesnt really fit though.




Maybe there is some perceptible scrubbing going on.


Why should there be any significant scrubbing with say a 20%
lower pressure in the tyre ?


Excatly. And yet those that say that the radius has altered by 20% and yet
the circumferemce hasn't altered at all, have only one way to make the RPM
rise by the amount they say the radius has rteduced. Introduce tyre scrub.


No need for any tyre scrub, just the reduced radius under the axle.
That is what determines the rotation rate of the wheel.


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"The Natural Philosopher" wrote in message
news
On 23/06/18 14:28, John Rumm wrote:
On 23/06/2018 06:56, The Natural Philosopher wrote:

https://www.youtube.com/watch?v=_2l5bOhHNxU

Answer. not by very much, if at all.


So the short answer to your question is "yes".

This seems to be a perpetual urban myth.


Which you have just agreed with by claiming there is a small change.


No the urban myth says that since there is say a 10% change in what
peooplle think is 'the radius', therefore the RPM will be 10% slower.


No one said that the change will be the same percentage,
just that when the distance between the axle and the ground
changes, that the rotation rate will change and that change
can be measured.

It tyre pressure sensors are using this, it has to be a very very
complicated bit of software to detect - say - less than 1% change in RPM
relative to the other wheels.


Can't see that being too difficult in itself - especially as you probably
have input from other sensors and know the steering angle input and so
can assess when you are driving straight and not under high acceleration
etc.


Why would there be monitors on steering angle?


Type pressure monitoring will need to be more sensitive to rate of change
than absolute difference since unequal tyre wear would otherwise be
flagged.


Well teh way it aseems to work is that one wheel will overotate with
respect to its diagonal consisetntly.

But not by very much.


But enough to measure with RPM so easy to measure differentially.



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"pamela" wrote in message
...
On 17:01 23 Jun 2018, The Natural Philosopher wrote:

On 23/06/18 13:42, Chris Hogg wrote:
On Sat, 23 Jun 2018 17:04:38 +1000, "Jeff"
wrote:



"Chris Hogg" wrote in message
...
On Sat, 23 Jun 2018 16:08:44 +1000, "Jeff"
wrote:



"The Natural Philosopher" wrote in
message news https://www.youtube.com/watch?v=_2l5bOhHNxU

Answer. not by very much, if at all.

But the radius clearly does vary significantly.

Does it?

It clearly does with the distance between the axle and the
ground.

I think we're at cross-purposes here. Obviously, as you say, when
a tyre is flat, the axle is nearer the ground. But is it
reasonable to regard that as the radius of the tyre?


Not if you have any understanding of mechanics. The whole tread,
up to and including a caterpillar track, goes round once per
revolution of the track or tyre.


Surely the circumference changes because the area in contact with the
road is a chord and not an arc.


Nope, not when the circumference doesn’t stretch and it doesn’t
with steel belted radials.

The length of the chord increases when the tyre is underinflated on
account of compression of more of the former arc in contact with the
road. The circumference is reduced.


Not with steel belted radials.

Only if all the compression of the former arc occurs at the leading
edge of contact would the speed be unaffected.


The circumference is irrelevant. What determines the rotation
rate of the wheel is the distance between the axle and the road.

THAT is what determines the RPM/speed relations ship.

What happens with tyre pressures is quite clear. The tread
stretches slightly under higher pressures. How much will be a
function of the tyre construction. And this is what the sensors
rely on.

Since no wheel is circular using radius as a concept is plain
wrong. At best you can calcualate a '*radius it would be if it
were round*,' from the actual circumference.

For it to be any other way the tyre must slip on the rim or on the
road, substantially.



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On 23/06/2018 21:47, Tim Streater wrote:
snip

Dunno what "rolling circumference" means, but the circumference can't
change, as it is fixed by the loops of steel belts in the tire (may
stretch a bit but not much).

*That* is the point.


Three points:

a. you seem to be assuming the steel is perfectly inelastic. When it
usually is made up of thin steel wires woven into cords and then into
belts I'd like to know it's actual modulus;

b. I posted a link to a paper which modelled tyres and reproduced
observed pressure effects. It noted that "the belt structure strongly
influences the effective rolling radius". But it also pointed to the
effects of pressure on the deformation of the tread - ie the stuff
between the belts and road. It seems to me credible that behaves
differently with changes in pressu eg just look at the sidewall when
pressure is low;

c. I introduced the term "rolling circumference" because how iTPMS works
can be discussed perfectly well without any reference to circumference.
So ISTM it behoves those who introduce "circumference" into it to define
what they mean. The circumference of the static, unloaded tyre? Or the
distance travelled for one rotation of the wheel - which all the
evidence shows does vary with pressure?



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On Sun, 24 Jun 2018 07:12:45 +1000, cantankerous geezer Rot Speed blabbered,
again:


Not if you have any understanding of mechanics. The whole tread,
up to and including a caterpillar track, goes round once per
revolution of the track or tyre.


Surely the circumference changes because the area in contact with the
road is a chord and not an arc.


Nope,


I KNEW it! LMAO

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asshole.
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On Sun, 24 Jun 2018 06:33:28 +1000, cantankerous geezer Rot Speed blabbered,
again:


https://en.wikipedia.org/wiki/Centra...flation_system


Bit weird with the retrofitted stuff
https://www.dropbox.com/s/yj56jztegr...93fa6.jpg?dl=0
Cant see that surviving too long off road here.


Let's hope you won't survive long here, Rot! Just keep bull****ting your way
into your grave! There's nothing else left for you to do. BG

--
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"The man is a conspicuous and unashamed ignoramus."
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On Sat, 23 Jun 2018 21:47:33 +0100, Tim Streater
wrote:

snip

Dunno what "rolling circumference" means, but the circumference can't
change, as it is fixed by the loops of steel belts in the tire (may
stretch a bit but not much).


But the 'belt's' are parallel loops around the circumference of the
tyre but woven diagonally and so form a parallelogram. This
parallelogram can change it's shape slightly and therefore allow the
tyre to become bigger or smaller due to pressure or whilst carrying a
load.

*That* is the point.


You wouldn't get the point unless it was stuck in you by someone else.
;-)

Cheers, T i m



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On 23/06/18 17:17, The Natural Philosopher wrote:
On 23/06/18 17:13, Andy Burns wrote:
The Natural Philosopher wrote:

Why would there be monitors on steering angle?


For the stability control system.

But once they're a standard part of the car you can use feed the data
to other systems, such as overlaying the curves on the reversing
camera display, or knowing when you're travelling straight and
therefore the TPMS doesn't have to worry about different wheel speeds
caused by the action of a differential.

All to modern for me. Never seen steering angle sensed yet on any car
I've driven.


https://www.knowyourparts.com/techni...r-diagnostics/
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On 23/06/18 17:40, John Rumm wrote:
On 23/06/2018 17:04, The Natural Philosopher wrote:
On 23/06/18 14:28, John Rumm wrote:
On 23/06/2018 06:56, The Natural Philosopher wrote:

https://www.youtube.com/watch?v=_2l5bOhHNxU

Answer. not by very much, if at all.

So the short answer to your question is "yes".

This seems to be a perpetual urban myth.

Which you have just agreed with by claiming there is a small change.


No the urban myth says that since there is say a 10% change in what
peooplle think is 'the radius', therefore the RPM will be 10% slower.


Make up your own myths if you like... The 1% seems plausible enough for me.

Can't see that being too difficult in itself - especially as you
probably have input from other sensors and know the steering angle
input Â*Â*and so can assess when you are driving straight and not under
high acceleration etc.


Why would there be monitors on steering angle?


Stability control programs, collision avoidance system, traction control
and various other systems will use it.

Type pressure monitoring will need to be more sensitive to rate of
change than absolute difference since unequal tyre wear would
otherwise be flagged.


Well teh way it aseems to work is that one wheel will overotate with
respect to its diagonalÂ* consisetntly.


Splembib!





But not by very much.


We like that.


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On 23/06/18 17:01, The Natural Philosopher wrote:
On 23/06/18 13:42, Chris Hogg wrote:
On Sat, 23 Jun 2018 17:04:38 +1000, "Jeff" wrote:



"Chris Hogg" wrote in message
...
On Sat, 23 Jun 2018 16:08:44 +1000, "Jeff" wrote:



"The Natural Philosopher" wrote in message
news https://www.youtube.com/watch?v=_2l5bOhHNxU

Answer. not by very much, if at all.

But the radius clearly does vary significantly.

Does it?

It clearly does with the distance between the axle and the ground.


I think we're at cross-purposes here. Obviously, as you say, when a
tyre is flat, the axle is nearer the ground. But is it reasonable to
regard that as the radius of the tyre?


Not if you have any understanding of mechanics.Â* The whole tread, up to
and including a caterpillar track, goes round once per revolution of the
track or tyre.

THAT is what determines the RPM/speed relations ship.


A revolution is the movement of one object (*point* on circumference)
around a centre (hub).
Your caterpillar track is supported by wheels, each of which will do
many revolutions for one revolution of the track. The circumference of
the track is many times the circumference of each wheel.


What happens with tyre pressures is quite clear. The tread stretches
slightly under higher pressures. How much will be a function of the tyre
construction. And this is what the sensors rely on.


No.
Since no wheel is circular using radious as a concept is plain wrong. At
best you can calcualate a '*radius it would be if it were round*,' from
the actual circumference.


The use of radius is completely right.
The circumference doesn't change. The centre of the instantaneous circle
moves closer to the radius.
The tyre is a three dimensional structure and this debate is being
conducted in a two dimensional manner.


For it to be any other way the tyre must slip on the rim or on the road,
substantially.





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On Sat, 23 Jun 2018 16:51:02 +0100, The Natural Philosopher
wrote:

snip


No, the world is full of stupid people pretending to be clever.


Oh the irony!

The 'effective rolling radius' is the circumference divided by 2 PI.

Trying to make a squashed dougnut into a circle doesnt really fit though.


Quite. Try measuring the circumference of something that isn't
circular.


Maybe there is some perceptible scrubbing going on.


Why should there be any significant scrubbing with say a 20%
lower pressure in the tyre ?


Excatly.


Exactly, there isn't.

And yet those that say that the radius has altered by 20% and
yet the circumferemce hasn't altered at all, have only one way to make
the RPM rise by the amount they say the radius has rteduced.


Yup.

Introduce
tyre scrub.]


Nope.

It's like climate change all over again.
Facts dont fit the theory, so introduce 'feeedback'


Bwhahaha.

Look into how the steel wire is laid in a steel belted radial tyre and
check out 'pantographing'.

Cheers, T i m
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On Sun, 24 Jun 2018 06:43:16 +1000, "Jeff" wrote:

snip

No, the world is full of stupid people pretending to be clever.

snip

Excatly. And yet those that say that the radius has altered by 20% and yet
the circumferemce hasn't altered at all, have only one way to make the RPM
rise by the amount they say the radius has rteduced. Introduce tyre scrub.


No need for any tyre scrub, just the reduced radius under the axle.


I wonder if Turnip actually climbed out of his basement once in a
while he's see all these things in action ITRW and realise just how
stupid *he* is being?

That is what determines the rotation rate of the wheel.


Quite. I believe the regs for these tyre pressure sensors are that
they should react before the pressure has changed by no more than 25%.

Many tyres are run at that sort of reduced pressure (slow punctures,
no TPMS) and would have to be 'scrubbed away' in no time.

They aren't, so it's something else, something Turnip doesn't
understand so it doesn't exist. ;-(

Cheers, T i m



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The hight of the axle is reduced - therefore the effective radius is
reduced and therefore its effective circumference. The part of the tire not
in contact with the ground is irrelevant.

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On Sunday, 24 June 2018 16:16:29 UTC+1, DerbyBorn wrote:
The hight of the axle is reduced - therefore the effective radius is
reduced and therefore its effective circumference. The part of the tire not
in contact with the ground is irrelevant.


I DO get what TNP is saying. Consider a fully inflated tyre with a mark
on the tread area. That mark will always be at the same relative angle
to the wheel, otherwise the tyre would be sliding on the wheel rim.

The wheel rotates one revolution. Measure the distance between the
first and second points where the mark touches the road.

Now partially deflate the tyre so that the distance between the axle
and the road is much less. Repeat the measurement. The distance
along the road will be almost exactly the same as before because the
circumference of the tyre has not changed significantly due to all
the steel wire in it and the tyre is not scrubbing against the road
surface.

In both cases the wheel rotates exactly one revolution - the tyre is
not slipping on the wheel rim - and the distance traversed in that
rotation has hardly changed.

So the "effective radius" has hardly changed, even though the radial
distance from axle to road has changed considerably. It really
is the circumference (or perimeter) of the tyre that matters, not
how much the axle has dropped relative to the road.

John
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On Sun, 24 Jun 2018 09:17:03 -0700 (PDT), wrote:

On Sunday, 24 June 2018 16:16:29 UTC+1, DerbyBorn wrote:
The hight of the axle is reduced - therefore the effective radius is
reduced and therefore its effective circumference. The part of the tire not
in contact with the ground is irrelevant.


I DO get what TNP is saying. Consider a fully inflated tyre with a mark
on the tread area. That mark will always be at the same relative angle
to the wheel, otherwise the tyre would be sliding on the wheel rim.

The wheel rotates one revolution. Measure the distance between the
first and second points where the mark touches the road.

Now partially deflate the tyre so that the distance between the axle
and the road is much less. Repeat the measurement. The distance
along the road will be almost exactly the same as before because the
circumference of the tyre has not changed significantly due to all
the steel wire in it and the tyre is not scrubbing against the road
surface.

In both cases the wheel rotates exactly one revolution - the tyre is
not slipping on the wheel rim - and the distance traversed in that
rotation has hardly changed.

So the "effective radius" has hardly changed, even though the radial
distance from axle to road has changed considerably. It really
is the circumference (or perimeter) of the tyre that matters, not
how much the axle has dropped relative to the road.


True ... so, it's either witchcraft, or your assumption that the
*effective* circumference (how far the vehicles travels per
revolution) must be wrong?

Because of the way the steel wire is laid up in the tyre they tend to
cross and form a parallelogram. As the tyre deflects the shape of the
tyre changes and therefore causes an effective change in circumference
(more accurately, how far the vehicle travels per wheel revolution
(down to pantographing)).

The smaller the distance the smaller the effective circumference (or
effective rolling or load radius as it's known) and to the faster the
wheel will spin for a fixed ground speed of the vehicle.

Cheers, T i m
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On Sunday, 24 June 2018 18:16:52 UTC+1, T i m wrote:
True ... so, it's either witchcraft, or your assumption that the
*effective* circumference (how far the vehicles travels per
revolution) must be wrong?

Because of the way the steel wire is laid up in the tyre they tend to
cross and form a parallelogram. As the tyre deflects the shape of the
tyre changes and therefore causes an effective change in circumference
(more accurately, how far the vehicle travels per wheel revolution
(down to pantographing)).

The smaller the distance the smaller the effective circumference (or
effective rolling or load radius as it's known) and to the faster the
wheel will spin for a fixed ground speed of the vehicle.

Everyone seems very fixed in their ideas about this. However, a
simple experiment should be informative.

Take a spare tyre. Wrap a tape measure around the circumference
along the centre of the tread area. Measure the circumference.
Place a plank through the tyre and stand on the ends. The tyre
will deform in a similar way to a deflated tyre on a wheel.
Re-measure the circumference. The air pressure will not have changed,
so if "pantographing" has taken place the circumference will be
significantly reduced and that change will be attributable purely
to such shape factors. If on the other hand the circumference is
not significantly altered then "pantographing" is not an important
factor.

Does that experiment seem valid?

John

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wrote in message
...
On Sunday, 24 June 2018 16:16:29 UTC+1, DerbyBorn wrote:
The hight of the axle is reduced - therefore the effective radius is
reduced and therefore its effective circumference. The part of the tire
not
in contact with the ground is irrelevant.


I DO get what TNP is saying. Consider a fully inflated tyre with a mark
on the tread area. That mark will always be at the same relative angle
to the wheel, otherwise the tyre would be sliding on the wheel rim.

The wheel rotates one revolution. Measure the distance between the
first and second points where the mark touches the road.

Now partially deflate the tyre so that the distance between the axle
and the road is much less. Repeat the measurement. The distance
along the road will be almost exactly the same as before because the
circumference of the tyre has not changed significantly due to all
the steel wire in it and the tyre is not scrubbing against the road
surface.

In both cases the wheel rotates exactly one revolution - the tyre is
not slipping on the wheel rim - and the distance traversed in that
rotation has hardly changed.

So the "effective radius" has hardly changed, even though the radial
distance from axle to road has changed considerably. It really
is the circumference (or perimeter) of the tyre that matters, not
how much the axle has dropped relative to the road.


And yet if you consider a solid wheel, no tyre, it is the distance
between the axle and the road that determines the rate at which
it rotates at the same speed of the car over the ground. So when
a wheel with tyre sees a reduction in the distance between the
axle and the road due to a lower pressure in the tyre, you get
the same change in the rotation rate of the wheel which is
easy to measure with the ABS sensor on that wheel.

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On Sun, 24 Jun 2018 21:04:49 +0100, Andy Burns
wrote:

wrote:

Everyone seems very fixed in their ideas about this


I think it all comes down to the distortion of the tyre and the reduced
length of the flat chord BA vs the arc BPA that would exist if it wasn't
loaded at all.

http://the-contact-patch.com/book/road/c2020-the-contact-patch#figure-ERr

the more heavily loaded, or the less pressure it's inflated with, the
greater z will be, and the smaller the effective rolling radius, reading
through to the end of that page gives chapter and verse formulae ...

In short the flatter the tyre, the smaller the effective radius and the
faster the wheel needs to rotate to keep up, which is what the sensors
detect.


Bingo. ;-)

Pretty sure Turnip will still argue the case ... he's still arguing
the that the world is flat. ;-)

Cheers, T i m
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On Sun, 24 Jun 2018 12:32:13 -0700 (PDT), wrote:

snip

Take a spare tyre. Wrap a tape measure around the circumference
along the centre of the tread area. Measure the circumference.
Place a plank through the tyre and stand on the ends. The tyre
will deform in a similar way to a deflated tyre on a wheel.
Re-measure the circumference. The air pressure will not have changed,
so if "pantographing" has taken place the circumference will be
significantly reduced and that change will be attributable purely
to such shape factors. If on the other hand the circumference is
not significantly altered then "pantographing" is not an important
factor.

Does that experiment seem valid?


It's sorta what you do when you accurately calibrate a cycle speedo.

You place the valve at the bottom and make a mark on the ground at
that point. Then with the tyre (of the wheel supporting the speedo
sensor) and with the right pressure in that wheel and with the bike
with it's typical load (so you and luggage etc), you wheel the bike
forward one turn of said wheel (valve at the bottom again) and then
measure the distance between those two points.

Now, presumably if it didn't matter about the load on the tyre they
would tell you to just put a tape measure round the outside of the
tyre?

When I built the kitcar I used the same process to determine the revs
/ m and so the speedo gearing, same on the electric motorbike I
designed, built and raced (and got an award from the IEEE for
'Technical innovation'). ;-)

Something to so with the sophisticated speedo I designed and built ...
weg

Cheers, T i m
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On 24/06/18 22:18, Jeff wrote:
So when
a wheel with tyre sees a reduction in the distance between the
axle and the road due to a lower pressure in the tyre, you get
the same change in the rotation rate of the wheel which is
easy to measure with the ABS sensor on that wheel.


No, you don't.

a tyre and wheel is not a sun and planet reduction gear, the tyre is not
rotating relative to the wheel, ergo every revolution of the wheel one
circumference of tyre must move along the road.

The ABS sensors do not detect a massive change in radius, but a small
change in circumference due to the tread shrinking very slightly.
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"Tjoepstil" wrote in message
news
On 24/06/18 22:18, Jeff wrote:
So when
a wheel with tyre sees a reduction in the distance between the
axle and the road due to a lower pressure in the tyre, you get
the same change in the rotation rate of the wheel which is
easy to measure with the ABS sensor on that wheel.


No, you don't.


Yes you do, as you should be able to see with a rim with no tyre at all.
As the rim diameter changes, the rotation rate will obviously change.

a tyre and wheel is not a sun and planet reduction gear,


It is a rack and pinion tho.

the tyre is not
rotating relative to the wheel,


Correct.

ergo every revolution of the wheel one
circumference of tyre must move along the road.


Yes, but it is the distance between the axle and the road
that determines the rotation rate.

The ABS sensors do not detect a massive change in radius, but a small
change in circumference due to the tread shrinking very slightly.


They actually detect the substantial change in the rotation rate
the is due to the substantial change in the distance between the
axle and the road.



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On Mon, 25 Jun 2018 07:50:42 +0100, Chris Hogg wrote:

On Mon, 25 Jun 2018 15:50:06 +1000, "Jeff" wrote:



Yes, but it is the distance between the axle and the road
that determines the rotation rate.


If the length of the perimeter of the tyre is say 1.5 metres,


At what state of inflation?

and the
vehicle is travelling at 150 km/hr, that means the tyre rotates 100000
times per hour, or 1666.6 rpm,


At what state of inflation?

assuming no slippage between tyre and
road or tyre and rim.


Ok.

There's no getting around that,


See above. ;-)

and there's no
mention of state of inflation,


But there should be as that's a key variable that affects the
effective 'perimeter length. ;-)

shape of the tyre or axle to road
distance.


No, but there should be as the shape of the tyre is instrumental in
the calculation.

On a solid tyre it wouldn't be because you have removed that variable.

Do you also deny the change in tyre diameter on a dragster (and
therefore rpm / mph)?

https://www.youtube.com/watch?v=YViiNxnQTr8

What about this. Imagine some lunar rover module where the 'tyre' is
made up of many completely independent segments.

With the vehicle up on jacks you might measure the effective
circumference of the wheel as the measurement made with a tape drawn
around it.

But put it under load and with it standing on one (sprung) segment.
the RW radius of that segment at that time under that load will be
less. Imagine that reduction in radius being passed round the wheel
segment by segment and you could then see (hopefully) that the
effective circumference would be calculable from the loaded radius and
it *would* be very different from the unloaded one.

Join those segments together on the outside by something plastic, like
say thin balloon rubber and nothing really changes.

Join them with something heavier but with the similar ability to
'give' and you have a pneumatic car tyre. ;-)

Cheers, T i m

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"Chris Hogg" wrote in message
...
On Mon, 25 Jun 2018 15:50:06 +1000, "Jeff" wrote:



Yes, but it is the distance between the axle and the road
that determines the rotation rate.


If the length of the perimeter of the tyre is say 1.5 metres, and the
vehicle is travelling at 150 km/hr, that means the tyre rotates 100000
times per hour, or 1666.6 rpm, assuming no slippage between tyre and
road or tyre and rim. There's no getting around that, and there's no
mention of state of inflation, shape of the tyre or axle to road
distance.


Yes, but it is the distance between the axle and the road
that determines the rotation rate.


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In message , Jeff
writes


"Chris Hogg" wrote in message
.. .
On Mon, 25 Jun 2018 15:50:06 +1000, "Jeff" wrote:



Yes, but it is the distance between the axle and the road
that determines the rotation rate.


If the length of the perimeter of the tyre is say 1.5 metres, and the
vehicle is travelling at 150 km/hr, that means the tyre rotates 100000
times per hour, or 1666.6 rpm, assuming no slippage between tyre and
road or tyre and rim. There's no getting around that, and there's no
mention of state of inflation, shape of the tyre or axle to road
distance.


Yes, but it is the distance between the axle and the road
that determines the rotation rate.


Why?

Long time since 'O' level maths but I suspect the relationship between
the radius and the perimeter only works for a perfect circle.


--
Tim Lamb
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On Mon, 25 Jun 2018 18:47:14 +1000, "Jeff" wrote:



"Chris Hogg" wrote in message
.. .
On Mon, 25 Jun 2018 15:50:06 +1000, "Jeff" wrote:



Yes, but it is the distance between the axle and the road
that determines the rotation rate.


If the length of the perimeter of the tyre is say 1.5 metres, and the
vehicle is travelling at 150 km/hr, that means the tyre rotates 100000
times per hour, or 1666.6 rpm, assuming no slippage between tyre and
road or tyre and rim. There's no getting around that, and there's no
mention of state of inflation, shape of the tyre or axle to road
distance.


Yes, but it is the distance between the axle and the road
that determines the rotation rate.

When considering this sort of thing I first consider the real world
facts.

iTPMS systems obviously work.

I believe the regs for such systems state that they must warn the
driver of a pressure lost *before* it passes 25% of the pre-set
pressure.

This means that it must be accurately measurable so when a 32 psi tyre
drops it's pressure to 24 psi (23?), it should raise the alarm.

The instructions for calibrating cycle speedos require you to measure
the *loaded* rolling circumference of the wheel bearing the speed
sensor (when it would be easier just to put a tape round the wheel).

So, a right brainer would take these sorts of things and then try to
look for a scientific / mathematic solution as to why this is.

A left brainer would jump to a conclusion based on their lack of
understanding and then look for information to support their denial
(from other left brainers typically). ;-)

Cheers, T i m
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On 25/06/2018 10:12, T i m wrote:
snip

So, a right brainer would take these sorts of things and then try to
look for a scientific / mathematic solution as to why this is.

A left brainer would jump to a conclusion based on their lack of
understanding and then look for information to support their denial
(from other left brainers typically). ;-)


I don't think it helps to introduce another myth[1] - let alone a mirror
image of the usual one

[1] http://www.oecd.org/education/ceri/neuromyth6.htm



--
Robin
reply-to address is (intended to be) valid


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In article ,
Tjoepstil wrote:
The ABS sensors do not detect a massive change in radius, but a small
change in circumference due to the tread shrinking very slightly.


What they detect is a change in speed of the wheel.

--
*Why were the Indians here first? They had reservations.*

Dave Plowman London SW
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In article ,
Jeff wrote:


"Chris Hogg" wrote in message
...
On Mon, 25 Jun 2018 15:50:06 +1000, "Jeff" wrote:



Yes, but it is the distance between the axle and the road
that determines the rotation rate.


If the length of the perimeter of the tyre is say 1.5 metres, and the
vehicle is travelling at 150 km/hr, that means the tyre rotates 100000
times per hour, or 1666.6 rpm, assuming no slippage between tyre and
road or tyre and rim. There's no getting around that, and there's no
mention of state of inflation, shape of the tyre or axle to road
distance.


Yes, but it is the distance between the axle and the road
that determines the rotation rate.


Seems lots on here can't understand the concept of one wheel running at
different RPM from the others. Perhaps they only ever drive in a straight
line.


--
*CAN VEGETARIANS EAT ANIMAL CRACKERS?

Dave Plowman London SW
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In article ,
Huge wrote:
I'll type this slowly for the hard of thinking.


The - Bits - Of - The - Tyre - Not - In - Contact - With - The - Road -
Are - Irrelevant. The - Only - Thing - That - Matters - Is - The -
Diameter - Of - The - Circle - Whose - Radius - Is - The - Distance -
From - The - Axle - To. The - Road.


You should have used caps, Huge. For those whose physics are still at
kindergarten level.

--
*(over a sketch of the titanic) "The boat sank - get over it

Dave Plowman London SW
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Dave Plowman wrote:

Huge wrote:

The - Bits - Of - The - Tyre - Not - In - Contact - With - The - Road -
Are - Irrelevant. The - Only - Thing - That - Matters - Is - The -
Diameter - Of - The - Circle - Whose - Radius - Is - The - Distance -
From - The - Axle - To. The - Road.


You should have used caps, Huge. For those whose physics are still at
kindergarten level.


Except there isn't a single distance from the centre of the axle to all
parts of the tyre in contact with the road ... the average distance perhaps?
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"T i m" wrote in message
news
This means that it must be accurately measurable so when a 32 psi tyre
drops it's pressure to 24 psi (23?), it should raise the alarm.


I've found that it triggers the alarm at a much smaller reduction than 25%.
Our Honda developed a very slow puncture and when I checked the tyres, one
was about 0.2 bar (3 psi) lower than it should have been.

The instructions for calibrating cycle speedos require you to measure
the *loaded* rolling circumference of the wheel bearing the speed
sensor (when it would be easier just to put a tape round the wheel).


How much does the radius of a bike tyre (at the point of contact) decrease
when you sit on the bike. It's difficult to tell when I'm the one sitting on
the bike so I don't get a side-on view. I think my speedo actually
recommends measuring the (unloaded) circumference by marking a point on the
tyre that is in contact with the ground and rolling the wheel along the
ground until the point is next in contact. But I agree that *if possible*
you should try to measure the radius under load and *assume* that the whole
tyre is that radius.

I've heard is suggested that there is *significant* error between a brand
new tyre and one with a worn tread, though I'd have thought that it was
negligible.

https://www.halfordsautocentres.com/...pth-and-safety
says that a new tyre has about 8 mm tread. If you use it until the tread is
2 mm, and assuming the same pressure in both cases, then the radius has
reduced by 6 mm in a total radius of 635 (for my car's 215/65/15 tyres) so
about 1%. I wonder how much the effective radius varies for an
under-inflated tyre, assuming the trigger level for a sensor is 25% loss of
pressure.

Any sensor has to be able to distinguish between expected change in radius
due to tyre wear and unexpected due to loss of pressure.

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