Accuracy of cycle computers



Gawnsoft <[email protected]> wrote:

<Snipped Text>
> >Presumably because a larger contact area produces greater
> >friction.
>
> I'm assuming because there's more work being put into
> bulging the tyre and compressing the air inside. So more
> gets lost as heat and noise.

You were doing well until here.

Friction is entirely independent of surface area. What you
have here is traction. Certainly you get more heat in the
side walls of a tyre that is underinflated though - it's
one of the biggest causes of blowouts in car tyres in fact.
All that flexing causes a lot of heat, try it with an
elastic band, stretch it fully a few times it should get a
little warmer.

--
Andy Hewitt ** FAF#1, (Ex-OSOS#5) - FJ1200 ABS Honda
Concerto 16v: Windows free zone (Mac G5 Dual Processor)
http://www.thehewitts.plus.com - now online
 
"davek" <[email protected]> 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.

On a crossply the circumference does disappear.

Look at a wire mesh kitchen sieve and press it against a
flat surface. It accommodates this by changing the crossover
angle of the wires around the flattened area. At some point
it stops and the centre bulges inwards.

YMMV. I found two sieves in my kitchen and one gave me
about 1 sq in before collapsing, the other went up to
about 20 sq in.

It would seem that a folding tyre probably does change its
area between flat and inflated states by similar changes
of angle, not by elastic changes (other than in the
covering rubber).
 
On Sat, 19 Jun 2004 10:27:24 +0100, [email protected] (Andy
Hewitt) wrote (more or less):

>Gawnsoft
><[email protected]> wrote:
>
><Snipped Text>
>> >Presumably because a larger contact area produces
>> >greater friction.
>>
>> I'm assuming because there's more work being put into
>> bulging the tyre and compressing the air inside. So more
>> gets lost as heat and noise.
>
>You were doing well until here.
>
>Friction is entirely independent of surface area. What you
>have here is traction. Certainly you get more heat in the
>side walls of a tyre that is underinflated though - it's
>one of the biggest causes of blowouts in car tyres in fact.
>All that flexing causes a lot of heat, try it with an
>elastic band, stretch it fully a few times it should get a
>little warmer.

Which is what I just said... to refute the quote about
friction.

Perhaps I should have been more clear. Maybe "I'm instead
assuming..."?

--
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links (harvested from comp.lang.smalltalk)
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Gawnsoft <[email protected]> wrote:

<Snipped Text>
> >Friction is entirely independent of surface area. What
> >you have here is traction. Certainly you get more heat in
> >the side walls of a tyre that is underinflated though -
> >it's one of the biggest causes of blowouts in car tyres
> >in fact. All that flexing causes a lot of heat, try it
> >with an elastic band, stretch it fully a few times it
> >should get a little warmer.
>
> Which is what I just said... to refute the quote about
> friction.
>
> Perhaps I should have been more clear. Maybe "I'm instead
> assuming..."?

Quite so. I thought after all that good technical stuff,
you'd gone and lost it with a simple basic rule. :)

--
Andy Hewitt ** FAF#1, (Ex-OSOS#5) - FJ1200 ABS Honda
Concerto 16v: Windows free zone (Mac G5 Dual Processor)
http://www.thehewitts.plus.com - now online
 
On Sat, 19 Jun 2004 10:27:24 +0100, [email protected] (Andy
Hewitt) wrote in message
<1gfmi36.23veaj16ejgjjN%[email protected]>:

>Friction is entirely independent of surface area.

Are you entirely sure about that? Although that's the theory
for rigid objects, where you have a conformant object (such
as a car tyre) I can see that the area of the contact patch
could easily affect friction. Several wibbles suggest the
same, but the InterWeb is notoriously unreliable.

Guy
--
May contain traces of irony. Contents liable to settle after posting.
http://www.chapmancentral.co.uk

88% of helmet statistics are made up, 65% of them at Washington University
 
Just zis Guy, you know? <[email protected]> wrote:

> On Sat, 19 Jun 2004 10:27:24 +0100,
> [email protected] (Andy Hewitt) wrote in message
> <1gfmi36.23veaj16ejgjjN%[email protected]>:
>
> >Friction is entirely independent of surface area.
>
> Are you entirely sure about that? Although that's the
> theory for rigid objects, where you have a conformant
> object (such as a car tyre) I can see that the area of the
> contact patch could easily affect friction. Several
> wibbles suggest the same, but the InterWeb is notoriously
> unreliable.

Yes, I'm sure. What you have with a tyre on tarmac is
tractive resistance, and is overcome by tractive effort [1].
This only works up to the point you skid, at which point you
then rely solely on friction, and then it makes no
difference how big your tyres are, you are completely at the
mercy of the mass of the vehicle and gravity, and it'll take
just as long to stop. Indeed, at the point skidding occurs
you can actually accelerate.

Of course there are factors that affect how much friction
there is. The coefficient of friction for the tarmac and
tyre are different for a start, and will indeed be different
again for various tyre materials and road surface materials.

I've just actually checked back on my notes from my accident
reconstruction course, and it confirms this. Indeed a CE for
tarmac and tyre is about 0.75 [2]. The difference made for
the footprint of the tyre was so small as to not be useful
in calculations.

And of course there's also momentum to take into account. As
much as the mass is working with gravity to hold you on the
tarmac, the same mass is working with your velocity to keep
pushing you in your given direction of motion - of course on
cornering this will be centrifugal force.

[1] Copied almost straight from my college notes.
[2] a wet surface is only 0.4.

--
Andy Hewitt ** FAF#1, (Ex-OSOS#5) - FJ1200 ABS Honda
Concerto 16v: Windows free zone (Mac G5 Dual Processor)
http://www.thehewitts.plus.com - now online
 
On Sat, 19 Jun 2004 19:50:30 +0100, [email protected] (Andy
Hewitt) wrote in message
<1gfn75d.8ghpx3zscz2oN%[email protected]>:

>Yes, I'm sure. What you have with a tyre on tarmac is
>tractive resistance

That's just a ter for the coefficient of rolling friction,
though, innit?

>This only works up to the point you skid, at which point
>you then rely solely on friction, and then it makes no
>difference how big your tyres are

But the "independent of area" theory rests on the idea that
any two objects only contact at three points; if one of them
is flexible I am not convinced that is the case.

The skid is the switch from static to dynamic friction,
dynamic being much less. Would a car skid further if the
tyres were inflated more? I don't know because I haven't
tried it. The current car has such grippy tyres that I can't
even get the ABS to come on :)

There is no reason I can see why static friction (albeit
static friction at a moving point on the surface of the
tyre) should vary with area and dynamic friction not.

Guy
--
May contain traces of irony. Contents liable to settle after posting.
http://www.chapmancentral.co.uk

88% of helmet statistics are made up, 65% of them at Washington University
 
Just zis Guy, you know? <[email protected]> wrote:

> On Sat, 19 Jun 2004 19:50:30 +0100,
> [email protected] (Andy Hewitt) wrote in message
> <1gfn75d.8ghpx3zscz2oN%[email protected]>:
>
> >Yes, I'm sure. What you have with a tyre on tarmac is
> >tractive resistance
>
> That's just a ter for the coefficient of rolling friction,
> though, innit?

It's what we used in the formulas - tr.

> >This only works up to the point you skid, at which point
> >you then rely solely on friction, and then it makes no
> >difference how big your tyres are
>
> But the "independent of area" theory rests on the idea
> that any two objects only contact at three points; if one
> of them is flexible I am not convinced that is the case.

Maybe, perhaps the calculations we used were making some
assumptions, but the term 'friction is independent of
surface area' was used almost weekly during our accident
reconstruction subject.

> The skid is the switch from static to dynamic friction,
> dynamic being much less. Would a car skid further if the
> tyres were inflated more? I don't know because I haven't
> tried it. The current car has such grippy tyres that I
> can't even get the ABS to come on :)

Quite possibly, tyre inflation was indeed one of the factors
included in the (tr) part of the formula.

However, an underinflated tyre could have an equally low
contact area, as the centre will fold in.

> There is no reason I can see why static friction (albeit
> static friction at a moving point on the surface of the
> tyre) should vary with area and dynamic friction not.

I must admit, we were normally only concerned about the
dynamic calculations at the time, as that's the only bit you
can measure after an accident - assuming that a vehicle did
actually skid of course.

In all, perhaps the only way to understand this is to
actually go out and do some experiments under controlled
conditions.

--
Andy Hewitt ** FAF#1, (Ex-OSOS#5) - FJ1200 ABS Honda
Concerto 16v: Windows free zone (Mac G5 Dual Processor)
http://www.thehewitts.plus.com - now online
 
"Andy Hewitt" <[email protected]> wrote in message
news:1gfn98j.1hqxesl10msayfN%[email protected]...
> Just zis Guy, you know? <[email protected]>
> wrote:
>
> > On Sat, 19 Jun 2004 19:50:30 +0100,
> > [email protected] (Andy Hewitt) wrote in message
> > <1gfn75d.8ghpx3zscz2oN%[email protected]>:
> >
> > >Yes, I'm sure. What you have with a tyre on tarmac is
> > >tractive resistance
> >
> > That's just a ter for the coefficient of rolling
> > friction, though, innit?

Yes.

> It's what we used in the formulas - tr.
>
> > >This only works up to the point you skid, at which
> > >point you then rely solely on friction, and then it
> > >makes no difference how big your tyres are

Sliding friction.

> > But the "independent of area" theory rests on the
> > idea that any two objects only contact at three
> > points; if one of them is flexible I am not convinced
> > that is the case.

Nope, it's not a theory. More a definition of coefficient of
friction. See below.

> Maybe, perhaps the calculations we used were making some
> assumptions, but the term 'friction is independent of
> surface area' was used almost weekly during our accident
> reconstruction subject.

The frictional force is proportional to the normal force per
unit area, with the constant of proportionality being the
coefficient of friction. Usually, if one increases the area
of the contact patch, the pressure (force per unit area)
experienced in the contact patch reduces inversely. The
total frictional force contributed by the contact patch
therefore remains the same.

The assumptions are vulnerable to the contact patch becoming
severely distorted.
--
"Since you must keep improving, a $5 bike offers a lot more
opportunities to improve it, and can be improved cheaper.
It's expensive to improve on a $2000 bike." - Rick Onanian
in rec.bicycles.tech
 
On Sat, 19 Jun 2004 22:00:35 +0200, "Mark South"
<[email protected]> wrote in message
<[email protected]>:

>The frictional force is proportional to the normal force
>per unit area, with the constant of proportionality being
>the coefficient of friction.

That will do for me.

Guy
--
May contain traces of irony. Contents liable to settle after posting.
http://www.chapmancentral.co.uk

88% of helmet statistics are made up, 65% of them at Washington University
 
Just zis Guy, you know? <[email protected]> wrote:

> On Sat, 19 Jun 2004 22:00:35 +0200, "Mark South"
> <[email protected]> wrote in message
> <[email protected]>:
>
> >The frictional force is proportional to the normal force
> >per unit area, with the constant of proportionality being
> >the coefficient of friction.
>
> That will do for me.

Aye, I found this one as well.

http://www.pa.uky.edu/~phy211/Friction_book.html

It seems that with a flexible surface all your calculations
do indeed go entirely out of the window.

--
Andy Hewitt ** FAF#1, (Ex-OSOS#5) - FJ1200 ABS Honda
Concerto 16v: Windows free zone (Mac G5 Dual Processor)
http://www.thehewitts.plus.com - now online
 
Ian G Batten <[email protected]> wrote in message news:<[email protected]>...
> In article
> <[email protected]>, sean
> <[email protected]> wrote:
> > millennium map web page) I go there frequently to have a
> > row. I used a Timex gps device to measure the 2K and
> > found it true within a couple of bike lengths. So it
> > would be a very good place to do some
>
> Two bike lengths is below the precision of the GPS anyway,
> for pretty well anything other than survey grade (which
> just uses the doppler shift of the SVs). Even differential
> or WAAS won't get to that accuracy.
>
> ian

I had to go and look WAAS up in the dictionary man....

I don't know if you were saying that gps's are not supposed
to be that accurate or not so today I did some 'tests' from
the start line to the 2k finish line. I cycled back and
forth six times each time resetting the gps timex and
keeping to the same begining and ending points.

1st run = 1.98k timex reading. 2nd run = 1.99k 3rd run =
1.98k 4th run = 1.98k 5th run = 1.98k 6th run = 1.99k

Each run I stopped immediately the timex registered 2k and
looked back. Sometimes I was four or five bike lengths past
the 'finish' but once I was just over two.

In my original post I was trying to say that because the 2k
is a flat, straight and presumably accurately measured 2k it
would be a good place to test and/or calibrate any cycle
computer. Sean.
 
FranklynMint wrote:
> Greetings fellow cyclists I have a mid-price cycle
> computer, which is set up according to instructions (double-
> checked and triple checked).

Did you put in their number or did you take your own wheel
measurements and convert that to a number, specific to your
bike when ridden by you.
 
> Pete Biggs: 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.

It doesn't go anywhere. As the tyre comes into contact with
the road, the road presses against it and compresses it. As
the tyre comes away from the road. the road stops pressing
against it and it expands again.

Thus, no slippage and the effective radius of the tyre on
the road is that measured where the tyre is in contact with
the road, not some other arbitrary radius that the tyre
assumes when it is not in contact with a surface.

(QEFD - Must remember that one.)

Vince.
 
Originally posted by Taywood
FranklynMint wrote:
> Greetings fellow cyclists I have a mid-price cycle
> computer, which is set up according to instructions (double-
> checked and triple checked).

Did you put in their number or did you take your own wheel
measurements and convert that to a number, specific to your
bike when ridden by you.

Ha Ha! Originally put in their number, then took advice from peeps in this thread (thank you). Moved the sensor closer to centre, thinking, OK, it was originally attached with double sided sticky, I've taken this off in moving it, I'll get some more later. Attached it as tight as I could, went out for ride, and sesnsor slipped, leading to knackerd spokes, sensor, pride (mind you, I didn't fall off!). Moral of story: never think "It'll do for now". It won't! Lesson learned!
 
Vince wrote:
>> Pete Biggs: 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.

I didn't write that. Mind your accreditations please.

~PB
 
"Pete Biggs" <ppear{remove_fruit}@biggs.tc> wrote:
> Vince wrote:
> >> Pete Biggs: 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.
>
> I didn't write that.

Well somebody wrote it. I didn't make it up.

V.
 
In news:[email protected],
Vince <[email protected]> typed:
> "Pete Biggs" <ppear{remove_fruit}@biggs.tc> wrote:
>> Vince wrote:
>>>> *davek* wrote: 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.
>>
>> I didn't write that.
>
> Well somebody wrote it. I didn't make it up.

davek did. It's your responsibility not to make that
mistake.

A
 
Vince:
> Well somebody wrote it. I didn't make it up.

Wake up at the back!

'Twas me. And thanks to Pete and others I had already been
disillusioned. But thanks for your input anyway.

d.
 
[email protected] (Chris Malcolm) writes:

>Ian G Batten <[email protected]> writes:

>>In article <[email protected]>,
>>FranklynMint <[email protected]> wrote:
>>> Greetings fellow cyclists I have a mid-price cycle
>>> computer, which is set up according to instructions (double-
>>> checked and triple checked). On a long ride, recently
>>> (recommended by a friend, who said it was about

>>> But looking at a map later, I thought something was
>>> amiss. I reckon that the computer is about 1 mile out
>>> every 15.

>>Over ten miles this morning mine agreed to within about
>>0.05 miles (~100 yards) with the GPS box I had clipped to
>>the handlebars. Given they're measuring different things
>>in different ways, given I was running the GPS at a slow
>>update rate and given the fact that the computer had
>>recorded 100 yards _more_ I'd say that's about bang on.
>>The more is reassuring, as the GPS box will be measuring a
>>succession of short chords to the route.

>I tested mine over several 3 mile journeys by comparing it
>to GPS distance readings, and got a similar result -- error
>always less than 1%.

Recalling this thread, I checked out my new urban cycle
commute distance today. Bike computer says I travelled 4.59
miles. GPS says I travelled 4.60 miles.

--
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/]