Criminals on TV

On Thu, 16 Aug 2007 03:08:15 -0700, CoyoteBoy
wrote:

On 14 Aug, 10:24, (Steve Firth) wrote:
Most cyclists grossly over estimate their speed, few have speedometers
and none of thsoe are calibrated.

Every cyclist I know has a speedo, even the kids as its a gadget. As
for calibration - mines a GPS so it would struggle not to be

Fascinating.

How is it "calibrated" then?

DG

In article , Roger
Merriman says...
Steve Firth wrote:

Mark T
pleasegivegenerously@warmail*turn_up_the_heat_to_ reply*.com.invalid
wrote:

Just watched the Vid on youtube, and I spotted four
criminals crossing double white lines to pass him.

This is not illegal, you could do with learning your Highway Code. See
rule 108.

ITYM not necessarily illegal.

The hair-splitting debate is over there --

The cyclist could have been travelling at 10mph or less. This is as
unlikely, as even most trundlies potter about at more than that.

Most cyclists grossly over estimate their speed, few have speedometers
and none of thsoe are calibrated.

true but same could be said of cars, trucks etc, particully in the urban
speed range.

HARDLY. A truck speedo has to be checked for accuracy every two years
at a proper DFT approved test center. A test is carried out on a DFT
certified rolling road to ensure that the speed, at several set limits,
indicated is accurate to within 2% and that the distance measured is
also accurate to that same percentage. The speedo head and speedo
sender unit are all checked to ensure the tamperproof seals are intact
and the numbers on the seals correspond. A lorry speedo is not allowed
to exceed a 2% overread or it is defective. If any work is done on the
lorry which means the tamperproof seals are broken, the lorry must go
to a tachograph test center and undergo the test again before being
sealed. Until that test is done, it is not allowed to undergo
commercial work. The seals carry a full paperwork trail that can go
back to the individual issuing them.


--
Conor

If the facts don't fit the theory, change the facts. -- Albert Einstein

On 16 Aug, 11:34, Ace wrote:

Sure, but only if you also accept that it's the same level of
faithleap that calculations you type into your electronic calculator
will give a correct result, or indeed that when I type the letter 'T'
on my keyboard it will display that way to the rest of the world in my
posting.


Not entirely. Since fitting a camera to the handlebars I have realised
just how much I am constantly turning the bars to keep in a straight
line on bumpy roads. This means speed over the ground actual
forward velocity.

Secondly the way wheel rotation is detected in crude and
inconsistent. When the magnet passes the sensor you don't get
a simple pulse. What happens in reality is when the magnet
approaches the sensor the reed switch closes, the contacts
bounce several times, the switch stays closed for a finite part
of the wheel rotation and then open again with another bounce
or two. Also when you hit a bump you unload the spokes
opposite the point of contact which changes the distance
between magnet and sensor, so the point at which the switch
closes will be different every time.

Paul George wrote:
On 16 Aug, 11:34, Ace wrote:

Sure, but only if you also accept that it's the same level of
faithleap that calculations you type into your electronic calculator
will give a correct result, or indeed that when I type the letter 'T'
on my keyboard it will display that way to the rest of the world in
my posting.


Not entirely. Since fitting a camera to the handlebars I have realised
just how much I am constantly turning the bars to keep in a straight
line on bumpy roads. This means speed over the ground actual
forward velocity.

Secondly the way wheel rotation is detected in crude and
inconsistent. When the magnet passes the sensor you don't get
a simple pulse. What happens in reality is when the magnet
approaches the sensor the reed switch closes, the contacts
bounce several times, the switch stays closed for a finite part
of the wheel rotation and then open again with another bounce
or two. Also when you hit a bump you unload the spokes
opposite the point of contact which changes the distance
between magnet and sensor, so the point at which the switch
closes will be different every time.

All of which make what percentage difference to the shown/recorded mph
compared to the actual exactly?

On 16 Aug, 11:57, "Brimstone" wrote:

All of which make what percentage difference to the shown/recorded mph
compared to the actual exactly?


No idea and without seeing the software source code I couldn't
possibly say.

Within +/- 1mph is a safe bet, but it is not a calibrated device.

Paul George wrote:

Not entirely. Since fitting a camera to the handlebars I have realised
just how much I am constantly turning the bars to keep in a straight
line on bumpy roads. This means speed over the ground actual
forward velocity.

Turning the bars plus or minus ten degrees (which is quite a lot for
just riding in a straight line) will introduce a speed error of less than 1%


Secondly the way wheel rotation is detected in crude and
inconsistent. When the magnet passes the sensor you don't get
a simple pulse. What happens in reality is when the magnet
approaches the sensor the reed switch closes, the contacts
bounce several times, the switch stays closed for a finite part
of the wheel rotation and then open again with another bounce
or two.

Switch bounce is well understood and easily dealt with - most
electronics would not work properly otherwise and every time you tried
to click something with your computer mouse the computer would register
a double click.

Also when you hit a bump you unload the spokes
opposite the point of contact which changes the distance
between magnet and sensor, so the point at which the switch
closes will be different every time.


A negligible effect since the spokes are still under significant tension
even when "unloaded"

Tony

Conor wrote:
Please explain - why is it not calibrated.

Until it is tested on a government approved testing apparatus,
typically at a tachograph testing centre, that has been certified in
its accuracy and a calibration certificate awarded to the speed
indicating device, it's not calibrated.


Is the wrong answer. I agree it wouldn't meet the requirement of
Section 97 of The Transport Act 1968 but since a bicycle is not a goods
vehicle, a public service vehicle or a motor vehicle capable of carrying
more than 12 passengers it is as relevant to whether the bicycle
speedometer is calibrated or not as the Civil Aviation Authority
regulations on air speed indicators are to your tachograph being
calibrated or not.

Tony

On 16 Aug, 11:49, Derek Geldard wrote:
On Thu, 16 Aug 2007 03:08:15 -0700, CoyoteBoy
wrote:

On 14 Aug, 10:24, (Steve Firth) wrote:
Most cyclists grossly over estimate their speed, few have speedometers
and none of thsoe are calibrated.

Every cyclist I know has a speedo, even the kids as its a gadget. As
for calibration - mines a GPS so it would struggle not to be

Fascinating.

How is it "calibrated" then?

DG


GPS recievers do not need calibrating as such, they work by
calculating positions relative to up to 12 (on mine) satellites and
from the precise phase difference between the signals from each it
triangulates the position. Although that position maybe absolutely
inaccurate by a few meters, once the signals have been aquired and
assuming not too many are lost during operation, the relative
positional accuracy can be high as a few centimeters (Although they
wont display it they use that level in the calculations). They then
use complex filtering algorithms to ensure that velocity is accurately
calculated, although it still struggles under significant acceleration
as they are tailored for constant-motion accuracy. They find vertical
motion a little more tricky, but the VAST differences involved between
satellites and the short wavelength used mean the margin of error is
TINY.

In article , Ace says...
On Thu, 16 Aug 2007 11:45:55 +0100, Conor
wrote:

In article , Mortimer says...
"Steve Firth" wrote in message
.. .
Martin Dann wrote:

You measure the distance travelled by a bike for one wheel
rotation, whilst the bike is loaded. You then put that
number into the speedometer. The speedometer is then
calibrated.

Umm not it's not.

Please explain - why is it not calibrated.

Until it is tested on a government approved testing apparatus,
typically at a tachograph testing centre, that has been certified in
its accuracy and a calibration certificate awarded to the speed
indicating device, it's not calibrated.

Rubbish. You're using a "police car speedometer" value of calibrated,
which has no bearing whatsoever on the use of the term with respect to
cycle computers.

Actually, no. Construction and Use legislation definition of
calibration. All vehicles have to have such approval although mass
manufacturers submit a sample of a model and get type approval.

And it's not me trying to argue that bicycle speedos are calibrated.

--
Conor

If the facts don't fit the theory, change the facts. -- Albert Einstein

In article , Tony Raven
says...
Conor wrote:
Please explain - why is it not calibrated.

Until it is tested on a government approved testing apparatus,
typically at a tachograph testing centre, that has been certified in
its accuracy and a calibration certificate awarded to the speed
indicating device, it's not calibrated.


Is the wrong answer. I agree it wouldn't meet the requirement of
Section 97 of The Transport Act 1968 but since a bicycle is not a goods
vehicle, a public service vehicle or a motor vehicle capable of carrying
more than 12 passengers it is as relevant to whether the bicycle
speedometer is calibrated or not as the Civil Aviation Authority
regulations on air speed indicators are to your tachograph being
calibrated or not.

Cars have to meet the same standard although the manufacturers submit a
sample and gain type approval.


--
Conor

If the facts don't fit the theory, change the facts. -- Albert Einstein