Accuracy of cycle computers



Doctor J. Frink wrote:

>> So the outside circumference of an inflated tyre is
>> larger than that of a flat tyre is it?
>>
>> Curious
>
> Take bike.
>
> Deflate front tyre.
>
> Wheel descends to floor to rest on rim.
>
> Note distance r_flat from hub to floor (ie the radius of
> the circle that we would be rolling on).

Also you may notice that the centre of the tread is nearer
to the rim all the way round. (More noticeable with some
tyres than others). eg. Vittoria Open Corsa 23 is only
18mm deep when flat (on MA2 700c rim) but 23mm deep when
fully inflated.

> Pump up tyre.
>
> Wheel rises from floor as tyre inflates.
>
> Note distance r_inflated from hub to floor (ie the radius
> of the circle that we would be rolling on).
>
> You'll find that r_inflated > r_flat.
>
> The actual circumference of the tyre itself may not change

It does actually (slightly), but it is indeed the radius
that counts here.

My points about the actual circumference being variable are
purely to illustrate that it is possible to travel a shorter
distance per wheel revolution than the circumference of the
tread centre when the tyre is fully inflated (without the
tyre slipping). Something that the other doctor doesn't seem
to accept.

~PB
 
On Thu, 17 Jun 2004 03:44:34 +0100, Pete Biggs wrote:
> My points about the actual circumference being variable
> are purely to illustrate that it is possible to travel a
> shorter distance per wheel revolution than the
> circumference of the tread centre when the tyre is fully
> inflated (without the tyre slipping). Something that the
> other doctor doesn't seem to accept.

Can't see why not. My commute is 19.8 and a bit miles with
rock hard (MTB slick) tyresi at 65 psi, and 19.9 and a most
miles when the tyres are ready to reinflate at 40-odd.
Nearly 0.2 miles in 20, or 1%. It's much larger than the
average daily variation of about 0.05 miles.

--
Trevor Barton
 
Pete Biggs:
>Those who still can't get their head round the concept should look at and measure a flat or very soft racing tyre* from the side.

I don't have a problem with the concept - I know the truth of it from experience.

What I can't get my head around is where the extra tyre circumference disappears to. This problem had never even occurred to me until I read this thread.

If you have a belted slick tyre, the circumference won't reduce through tyre compression, will it? The compression will be lateral rather than longitudinal, right?

And I don't buy the "slipping" explanation - at least, I don't understand what is meant by it, but maybe slipping doesn't mean what I think it means...

If someone could explain it to me, I'd be most grateful.

d.
 
davek wrote:
> Pete Biggs:
>> Those who still can't get their head round the concept
>> should look at and measure a flat or very soft racing
>> tyre* from the side.
>
> I don't have a problem with the concept - I know the truth
> of it from experience.
>
> What I can't get my head around is where the extra tyre
> circumference disappears to. This problem had never even
> occurred to me until I read this thread.

Actually, it's starting to do my head in now :)

> If you have a belted slick tyre, the circumference won't
> reduce through tyre compression, will it? The compression
> will be lateral rather than longitudinal, right?

Blimey, I'm having a light bulb moment! :) .............

I think it's both lateral and longitudinal. A tyre is
basically round in cross section so it has an infinite
number of inifinitely narrow circumferences. Which one are
you thinking of? Circumference at the middle decreases as
circumference towards the edge increases. Not magic,
material is not disappearing, the shape is simply changing.
One dimension can decrease as another increases. The *mean*
circumference might remain the same but that doesn't mean
all of it has to.

The circumference of the middle of the tyre will reduce when
the air pressure is lowered so I would have thought the same
thing happens through compression, but I'm not sure it's
anything more than accademic anyway because distance
travelled per wheel revolution is shortened by the changing
shape of the tyre. It's calculated from the radius or the
*effective* circumference, not the actual circumference of
the whole tyre.

~PB
 
Pete Biggs:
>Blimey, I'm having a light bulb moment! :)

I could do with one of those.

d.
 
A couple of thoughts:

I did a boring stretch of road in france once and to pass the time started measuring the km markers against my computer, they matched almost perfectly. (A very good road surface and quite a high load so not much weaving about going on, computer calibrated with me and the load on board)

When using a GPS in the car it is noticeable that the GPS speed is lower than the speedo when on hills (or at least the difference is greater on the hills)

Regards

Steve D
 
"davek" <[email protected]> wrote in message
news:[email protected]...
> Pete Biggs:
> >Those who still can't get their head round the concept
> >should look at and measure a flat or very soft racing
> >tyre* from the side.
>
> I don't have a problem with the concept - I know the truth
> of it from experience.
>
> What I can't get my head around is where the extra tyre
> circumference disappears to. This problem had never even
> occurred to me until I read this thread.
>

...

First up, having actually done the experiment suggested
by Dr Flink I concede that Dr Flink, and everybody else
is correct.

The wheel certainly does travel a shorter distance with the
tyre completely deflated.

The reasoning with the tyre slightly deflated is still a
bit tricky

If we assume a radius of 33cm then the circumference of a
fully inflated tyre is 33cm x 2 x 3.14 = approx 207cm

This is measured along the centre line of the tread on the
raised band if there is one.

However when you let down the tyre and move the bike - its
fairly obvious - but only when you think about it, that the
effective radius and thus circumference now only consists of
the rim and the squashed tyre at the bottom.

If we assume the height of the tyre is 2.5cm this gives a
circumference for the rim of 30.5 x 2 x 3.14 = approx 191 cm

If we assume a flat tyre is 0.5cm high this gives a radius
of 31 at the bottom and a circumference of 31 x 2 x 3.14 =
approx 194.6

So that when you move the bike forward one revolution in a
dead straight line it only covers 194.6 cm. However given
that the whole of the tyre has also completed one revolution
where did the missing 12.5 cm go from the middle of the
tread? Because they must have gone around as well.

Well the point is that when the tyre is fully inflated, the
centre line 207cm goes around parallel to the rim. In a dead
straight line. However when the tyre is deflated the centre
line follows a snaking pattern from side to side. Basically
it makes a snaking path along the ground weaving from side
to side which is still 207cm in length but only covers 194.6
when measured straight ahead. The shape of the tyre forces
it to do this.

I think.

Curious

...
 
"Just zis Guy, you know?" <[email protected]> wrote in message
news:[email protected]...
> On Tue, 15 Jun 2004 18:45:00 +0100, "Dr Curious"
> <[email protected]> wrote in message <2j8qvtFumvvvU1@uni-
> berlin.de>:
>
> >So the idea of letting air out of the tyres in order to
> >increase traction in unfavourable conditions is totally
> >wrong then?
>
> Different effect. The idea there is to increase the width
> (for bike tyres) of the contact patch.

Point taken.

Curious.
 
Dr Curious wrote:
> Well the point is that when the tyre is fully inflated,
> the centre line 207cm goes around parallel to the rim. In
> a dead straight line. However when the tyre is deflated
> the centre line follows a snaking pattern from side to
> side. Basically it makes a snaking path along the ground
> weaving from side to side which is still 207cm in length
> but only covers 194.6 when measured straight ahead. The
> shape of the tyre forces it to do this.
>
> I think.

I can only imagine the snaking is from your solid raised
band not being able to deform properly*. Not all tyres do
that. What about tyres without a raised band? Mine don't
weave from side to side when flat.

I'd be interested in your view on my other replies. How do
you explain the actual centre-line circumference of my Open
Corsa tyre being smaller when air is let out, even in its
static unridden state?

Think of a tyre made of tissue paper. What would its centre-
line circumference be when uninflated? And would the whole
thing be wrinkled or would it just be flat and flopped out
sideways? I think the latter.

* which incidentally hinders normal performance when
inflated.

~PB
 
"Pete Biggs" <ppear{remove_fruit}@biggs.tc> wrote in message
news:[email protected]...
> Dr Curious wrote:
> > Well the point is that when the tyre is fully
> > inflated, the centre line 207cm goes around parallel
> > to the rim. In a dead straight line. However when the
> > tyre is deflated the centre line follows a snaking
> > pattern from side to side. Basically it makes a
> > snaking path along the ground weaving from side to
> > side which is still 207cm in length but only covers
> > 194.6 when measured straight ahead. The shape of the
> > tyre forces it to do this.
> >
> > I think.
>
> I can only imagine the snaking is from your solid raised
> band not being able to deform properly*. Not all tyres do
> that. What about tyres without a raised band?

The raised band was only put in to help visualise the thing.

> Mine don't weave from side to side when flat.

What exactly is stopping them ?

I think you need to answer that. Because I think the actual
shape of the tire will cause it to twist and weave to some
extent to compensate.

You probably don't need a lot of weaving about.
Unfortunately I've no string to hand but all that's
necessary is to cut two lengths of string of 207cm and
194cm, stretch the 195cm one tight at both ends and see how
much weaving about you need in the 207 one for it meet the
194cm. one at both ends.

...

>
> I'd be interested in your view on my other replies. How do
> you explain the actual centre-line circumference of my
> Open Corsa tyre being smaller when air is let out, even in
> its static unridden state?

...

Unless the tyre is actually elastic like an inner tube,
rather than simply flexible, then what you are claiming
appears to defy the laws of physics. That's all I can say
about that.

...

>
> Think of a tyre made of tissue paper. What would its centre-
> line circumference be when uninflated? And would the whole
> thing be wrinkled or would it just be flat and flopped out
> sideways? I think the latter.

...

I'm sorry you've lost me with that one. Basically you could
cut away the rest of the paper tyre, and just have the thin
circular strip of paper which forms the centre line. Now so
long as you measure along the strip
i.e. equivalent to parallel to the centre line - then the
length will always be the same.

So this is equivalent to measuring the centre line on the
tire when the centre line is running parallel with the rim.
However when the tire's deflated there's nothing to keep the
centre line running parallel with the rim any more. It can
and will move from side to side.

At least so I think.

Curious

>
> * which incidentally hinders normal performance when
> inflated.
>
> ~PB
 
"Pete Biggs" <ppear{remove_fruit}@biggs.tc> wrote in message
news:[email protected]...
> Dr Curious wrote:
> > Well the point is that when the tyre is fully
> > inflated, the centre line 207cm goes around parallel
> > to the rim. In a dead straight line. However when the
> > tyre is deflated the centre line follows a snaking
> > pattern from side to side. Basically it makes a
> > snaking path along the ground weaving from side to
> > side which is still 207cm in length but only covers
> > 194.6 when measured straight ahead. The shape of the
> > tyre forces it to do this.
> >
> > I think.
>
> I can only imagine the snaking is from your solid raised
> band not being able to deform properly*. Not all tyres do
> that. What about tyres without a raised band?

The raised band was only put in to help visualise the thing.

> Mine don't weave from side to side when flat.

What exactly is stopping them ?

I think you need to answer that. Because I think the actual
shape of the tire will cause it to twist and weave to some
extent to compensate.

You probably don't need a lot of weaving about.
Unfortunately I've no string to hand but all that's
necessary is to cut two lengths of string of 207cm and
194cm, stretch the 195cm one tight at both ends and see how
much weaving about you need in the 207 one for it meet the
194cm. one at both ends.

____________

Having found some string and actually tried this, this
sloution is obviously wrong as the amount of weaving about
required is phenomenal.

Its going to be necessary to attach string to tires in some
way in order to sort this out.

...

>
> I'd be interested in your view on my other replies. How do
> you explain the actual centre-line circumference of my
> Open Corsa tyre being smaller when air is let out, even in
> its static unridden state?

...

Unless the tyre is actually elastic like an inner tube,
rather than simply flexible, then what you are claiming
appears to defy the laws of physics.

And I'll now go along with that. Strange forces are clearly
in operation here

Curiouser and curiouser
 
Dr Curious wrote:

>> Mine don't weave from side to side when flat.
>
> What exactly is stopping them ?

Nothing is making them. They don't need to weave.

> I think you need to answer that. Because I think the
> actual shape of the tire will cause it to twist and weave
> to some extent to compensate.

The tyre appears just to spread out sideways as weight is
applied and goes as flat as a pancake.

> You probably don't need a lot of weaving about.
> Unfortunately I've no string to hand but all that's
> necessary is to cut two lengths of string of 207cm and
> 194cm, stretch the 195cm one tight at both ends and see
> how much weaving about you need in the 207 one for it meet
> the 194cm. one at both ends.

Sorry I don't follow that. Will be difficult to stop string
sliding off sideways if you mean tie it around the tyre.

>> I'd be interested in your view on my other replies. How
>> do you explain the actual centre-line circumference of my
>> Open Corsa tyre being smaller when air is let out, even
>> in its static unridden state?
> ... Unless the tyre is actually elastic like an inner
> tube, rather than simply flexible, then what you are
> claiming appears to defy the laws of physics. That's all I
> can say about that.

Please think about it some more, thinking specifically
about circles rather than linear bands. Perhaps playing
with a tyre of the type I mentioned would help. Either this
could make you understand, if I am right, or enable you to
more clearly correct me if I am wrong, for which I would be
most grateful.

Maybe you need to think again about the laws of physics (or
maybe I do).

With the tyre I mentioned, if I manually press in the
sidewalls, the tread centre raises. It's doesn't seem to be
stretching, just moving as the shape changes. The centre
circumference does get larger when inflated. Is elasticity
your only explanation? The tyre starts to get "higher"
(above rim) before I could imagine the canvas stretching (if
it ever does to any relevant extent).

~PB
 
Pete Biggs:
> Please think about it some more, thinking specifically
> about circles rather than linear bands.

But lots of tyres have linear bands ("belts") that aren't
elastic, so won't stretch or compress in the way they would
need to to make your explanation work. And if the tyre is
slick, there is no tread to absorb the compression either.
Or am I misunderstanding something somewhere along the line?

I really do feel like I'm out of my depth here, but I'm
completely fascinated.

d.
 
"Pete Biggs" <ppear{remove_fruit}@biggs.tc> wrote in message
news:[email protected]...
> Dr Curious wrote:
>
> >> Mine don't weave from side to side when flat.
> >
> > What exactly is stopping them ?
>
> Nothing is making them. They don't need to weave.
>
> > I think you need to answer that. Because I think the
> > actual shape of the tire will cause it to twist and
> > weave to some extent to compensate.
>
> The tyre appears just to spread out sideways as weight is
> applied and
goes
> as flat as a pancake.
>
> > You probably don't need a lot of weaving about.
> > Unfortunately I've no string to hand but all that's
> > necessary is to cut two lengths of string of 207cm and
> > 194cm, stretch the 195cm one tight at both ends and see
> > how much weaving about you need in the 207 one for it
> > meet the 194cm. one at both ends.
>
> Sorry I don't follow that. Will be difficult to stop
> string sliding off sideways if you mean tie it around
> the tyre.
>
> >> I'd be interested in your view on my other replies. How
> >> do you explain the actual centre-line circumference of
> >> my Open Corsa tyre being smaller when air is let out,
> >> even in its static unridden state?
> > ... Unless the tyre is actually elastic like an inner
> > tube, rather than simply flexible, then what you are
> > claiming appears to defy the laws of physics. That's all
> > I can say about that.
>
> Please think about it some more, thinking specifically
> about circles rather than linear bands. Perhaps playing
> with a tyre of the type I mentioned would help. Either
> this could make you understand, if I am right, or enable
> you to more clearly correct me if I am wrong, for which
I
> would be most grateful.
>
> Maybe you need to think again about the laws of physics
> (or maybe I do).
>
> With the tyre I mentioned, if I manually press in the
> sidewalls, the
tread
> centre raises. It's doesn't seem to be stretching, just
> moving as the shape changes. The centre circumference does
> get larger when inflated. Is elasticity your only
> explanation? The tyre starts to get "higher" (above rim)
> before I could imagine the canvas stretching (if it ever
> does to any relevant extent).
>
> ~PB
>

____________

Years ago I bought a few Nutrak, IIRR, folding tyres. About
22cm wide. These things folded more or less flat across.
According to most of what I've been arguing so far, I now
realise that these tyres were a physical impossibility. As
according to any normal way of thinking the circumferance
around the middle of the tread must be longer than the
circumferance around the bead. Which should thus prevent
them from lying flat. Or being folded into a flat shaped
package. ISTR they were folded over three or four times.
I've a strong suspicion that this was all explained in one
of those Open University maths programmes that used to be
broadcast on Saturday mornings.

Apparently the effect of load on "rollout distance" is well
recognised and quantified....

"Typically this distance, for a 700-28 tire at 120 lbs
pressure, can be as much as 30 mm shorter under load than
rolling the unloaded wheel for one revolution."

http://draco.acs.uci.edu/rbfaq/FAQ/8b.24.html

But that still leaves these awkward questions.

Curious
 
After closer inspection, I've noticed my tyres are more
elastic than I previously realised. I can stretch the
material with a firm tug and can actually see the label
getting bigger as the tyre is inflated.

However, I doubt the flat tyre is stretching when I gently
squeeze the sidewalls to make the tyre taller. Surely the
circumference would be greater if I could do that all the
way round somehow?

There will also be some contortions or wrinkling at the
bottom of the tyre when weight is applied as the curved
section tries to conform to the flat road, but that must
only account for a tiny area/length of material.

~PB
 
"Dr Curious" <[email protected]> writes:

>"Pete Biggs" <ppear{remove_fruit}@biggs.tc> wrote in
>message news:[email protected]...
>> Dr Curious wrote:

>> > Well the point is that when the tyre is fully inflated,
>> > the centre line 207cm goes around parallel to the rim.
>> > In a dead straight line. However when the tyre is
>> > deflated the centre line follows a snaking pattern from
>> > side to side. Basically it makes a snaking path along
>> > the ground weaving from side to side which is still
>> > 207cm in length but only covers 194.6 when measured
>> > straight ahead. The shape of the tyre forces it to do
>> > this.

>> I can only imagine the snaking is from your solid raised
>> band not being able to deform properly*. Not all tyres do
>> that. What about tyres without a raised band?

>You probably don't need a lot of weaving about.
>Unfortunately I've no string to hand but all that's
>necessary is to cut two lengths of string of 207cm and
>194cm, stretch the 195cm one tight at both ends and see how
>much weaving about you need in the 207 one for it meet the
>194cm. one at both ends.

>Unless the tyre is actually elastic like an inner tube,
>rather than simply flexible, then what you are claiming
>appears to defy the laws of physics. That's all I can say
>about that.

I think after a great deal of head-banging cogitation, our
scientist is finally staggering towards the conclusion! See
that word in that last paragraph? Elastic? He wonders if the
tyre is elastic. Well, it's made of rubber. Is that going to
be a big enough clue?

Don't hold your breath folks!

--
Chris Malcolm [email protected] +44 (0)131 651 3445 DoD #205
IPAB, Informatics, JCMB, King's Buildings, Edinburgh, EH9 3JZ, UK
[http://www.dai.ed.ac.uk/homes/cam/]
 
On Thu, 17 Jun 2004 20:27:17 +0100, Dr Curious wrote:

snip

> tire
>

snip

>
> tyre

snip

Dr Curious is a curious person, isn't he? How many people
are inconsistent in their spelling of a simple word such
as 'tyre'?

Is he pretending to be something he's not?

--
Michael MacClancy Random putdown - "I've had a perfectly
wonderful evening. But this wasn't
it." -Groucho Marx www.macclancy.demon.co.uk
www.macclancy.co.uk
 
"Chris Malcolm" <[email protected]> wrote in message
news:[email protected]...
> "Dr Curious" <[email protected]> writes:
>
> >"Pete Biggs" <ppear{remove_fruit}@biggs.tc> wrote in
> >message news:[email protected]...
> >> Dr Curious wrote:
>
> >> > Well the point is that when the tyre is fully
> >> > inflated, the centre line 207cm goes around parallel
> >> > to the rim. In a dead straight line. However when the
> >> > tyre is deflated the centre line follows a snaking
> >> > pattern from side to side. Basically it makes a
> >> > snaking path along the ground weaving from side to
> >> > side which is still 207cm in length but only covers
> >> > 194.6 when measured straight ahead. The shape of the
> >> > tyre forces it to do this.
>
> >> I can only imagine the snaking is from your solid
> >> raised band not being able to deform properly*. Not all
> >> tyres do that. What about tyres without a raised band?
>
> >You probably don't need a lot of weaving about.
> >Unfortunately I've no string to hand but all that's
> >necessary is to cut two lengths of string of 207cm and
> >194cm, stretch the 195cm one tight at both ends and see
> >how much weaving about you need in the 207 one for it
> >meet the 194cm. one at both ends.
>
> >Unless the tyre is actually elastic like an inner tube,
> >rather than simply flexible, then what you are claiming
> >appears to defy the laws of physics. That's all I can say
> >about that.
>
> I think after a great deal of head-banging cogitation, our
> scientist is finally staggering towards the conclusion!
> See that word in that last paragraph? Elastic? He wonders
> if the tyre is elastic. Well, it's made of rubber. Is that
> going to be a big enough clue?
>
> Don't hold your breath folks!

I don't know Chris, he's starting to catch some pretty big
fish if he's got you on the hook...!

Of course, saying that in this thread means he's got
me too....
--
"You are the most stupid asshole I have yet encountered on
this newsgroup. Congratulations. That is no small
achievement as there are many other stupid assholes on this
newsgroup. But they can't hold a candle to you."
- Ed Dolan in alt.rec.bicycles.recumbent
 
"Chris Malcolm" <[email protected]> wrote in message
news:[email protected]...
> "Dr Curious" <[email protected]> writes:
>
>
> >Unless the tyre is actually elastic like an inner tube,
> >rather than simply flexible, then what you are claiming
> >appears to defy the laws of physics. That's all I can say
> >about that.
>

> I think after a great deal of head-banging cogitation, our
> scientist is finally staggering towards the conclusion!
> See that word in that last paragraph? Elastic? He wonders
> if the tyre is elastic. Well, it's made of rubber. Is that
> going to be a big enough clue?

...

It would be indeed a big enough clue, were it the case that
tyres consisted solely of rubber. However they don't. You
appear to be overlooking the the fact that most tyres are
moulded around at least two layers of woven fabric casing.
Are you claiming that the twin ply casings used in bicycle
tyres are designed to be elastic* too?

And even if that were the case, it's one thing to be able to
throw around technical buzz words like "elastic". Its quite
another thing to be able to explain precisely how this
purported elasticity might help in explaining quite where
where this "missing" tread goes.

However given the confident manner in which you've addressed
the NG on this topic Mr Malcolm, I'm sure you'll be able to
explain what you mean in simple readily understandable
terms, and thus finally put the less enlightened among us
out of our misery.

>
> Don't hold your breath folks!
>

No indeed Mr Malcolm!

No indeed.

Curious

*And in any case there's elasticity and elasticity. Steel
railway track is elastic to the extent that it expands in
warm weather.

___________________________________________________________-
____________

> --
> Chris Malcolm [email protected] +44 (0)131 651 3445
> DoD #205 IPAB, Informatics, JCMB, King's Buildings,
> Edinburgh, EH9 3JZ, UK
> [http://www.dai.ed.ac.uk/homes/cam/]
 
"Michael MacClancy" <[email protected]> wrote in message
news:eek:[email protected]...
> On Thu, 17 Jun 2004 20:27:17 +0100, Dr Curious wrote:
>
> snip
>
> > tire
> >
>
> snip
>
> >
> > tyre
>
> snip
>
> Dr Curious is a curious person, isn't he? How many people
> are inconsistent in their spelling of a simple word such
> as 'tyre'?

hint: that might depend on how many results they expected to
get when Googling on tyre/tire related topics.

Would it not?

>
> Is he pretending to be something he's not?

If I formerly claimed to be somebody who spelt in a
consistent way, then I suppose such a remark might
make sense.

"Might" being the operative word here, I think

Curious

>
> --
> Michael MacClancy Random putdown - "I've had a perfectly
> wonderful evening. But this
wasn't
> it." -Groucho Marx www.macclancy.demon.co.uk
> www.macclancy.co.uk
 
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