"Graeme" wrote in message

In message
"Recliner" wrote:

"Bruce" wrote in message

On Sat, 31 Jul 2010 11:52:24 +0100, "Recliner"
wrote:

"Adrian" wrote in message

Bruce gurgled happily, sounding much like
they were saying:

The margin (maybe not so much nowadays) is necessary to allow
for tyre wear (and IIRC tyre type on some vehicles) as well as
the capabilities of a mechanical speedo; the normal consequence
of tyre wear is that the indicated speed will be progressively
too high so to avoid underindication the average speedo will
probably already be over-reading from new.

The legal requirement is that a speedometer measures road speed
with a tolerance of +10%, -0%.

Actually, it probably isn't.

It's difficult to be sure, since the Construction & Use regs
aren't on the web. The nearest that is simple to find is the
requirements for the IVA test - which are definitely nowhere near
as simple as that. There's a table of allowable readings against
accurate speed.

0 under-read is true, though.

Mind you, I'd love to know what sort of tyres are being used to
affect calibration by 10% as they wear... Something like a total
of 6mm variation due to tread wear on a typical overall tyre
radius of about 320mm?

Wouldn't tyre pressure have a much bigger effect?


Indeed so, and that forms the basis of tyre pressure monitoring in
many modern cars - when the rotational speed (RPM) of one wheel
exceeds that of the others by more than a predetermined percentage,
an alarm sounds and a warning light illuminates.

I repeat that the legal requirement is that a speedometer measures
road speed with a tolerance of +10%, -0%. I am told by a friend who
is a car designer that this is stated in the Construction & Use
Regulations. He says it is very out of date because speedometers
can now be made to much tighter tolerances.

Yes, whenever I've compared my speedo reading in modern cars with an
accurate GPS, the speedo is very close to 5% over. I suspect that
manufacturers always aimed at this, but modern electronic devices
deliver it accurately. Given that any car with ABS presumably can
measure wheels speeds, it's annoying that most car manufacturers
charge so much for the tyre pressure monitoring option.

Oddly enough, however, when I set the digital cruise control, it
doesn't quite agree with the more accurate analogue speedo. In other
words, if I want to do exactly 50mph through motorway road works, a
speedo reading slightly above 50mph is OK, but I need to set the
digital cruise control to something like 55mph to achieve this
precisely (if the traffic is heavy, I don't bother, and just let the
adaptive cruise control keep my speed the same as the cars I'm
following).

When I reach the end of the road works, (if necessary) I change the
target speed on the cruise control to, say, 77 mph to ensure that I
do exactly 70 mph. Again, the ACC slows me down if needed.



Interesting numbers, how old is your car? My two year old Laguna
measures a lot closer to the limits than that, 73mph indicated gives
just about 70 on the Sat Nav.

Yes, your car's speedo is the normal ~5% over, just like mine and most
other modern cars. Does it also have a digital adapative cruise control
(ACC) where you can pre-set an exact target speed, adjustable in 1 mph
increments? That's where mine is about 10% over. I don't think ACC is
standard on any cars, and is a fairly expensive option when offered.

On Sat, 31 Jul 2010 13:18:05 +0100, "Recliner"
wrote:
"Bruce" wrote in message

On Sat, 31 Jul 2010 12:36:57 +0100, "Recliner"
wrote:
Oddly enough, however, when I set the digital cruise control, it
doesn't quite agree with the more accurate analogue speedo. In other
words, if I want to do exactly 50mph through motorway road works, a
speedo reading slightly above 50mph is OK, but I need to set the
digital cruise control to something like 55mph to achieve this
precisely (if the traffic is heavy, I don't bother, and just let the
adaptive cruise control keep my speed the same as the cars I'm
following).

When I reach the end of the road works, (if necessary) I change the
target speed on the cruise control to, say, 77 mph to ensure that I
do exactly 70 mph. Again, the ACC slows me down if needed.


That sounds more like 10% over rather than 5%. Please explain?

Yes, the adaptive cruise control needs to be set approximately 10% over
the targeted true speed for some reason. The analogue speedo then shows
a speed about 5% over the true speed. It was a bit disconcerting at
first, but I soon got used to it, and haven't bothered to raise it with
the garage as I doubt that they'd know what I was talking about.


I'm not sure I know either, but as long as it works ...

There's a real problem with garage staff being way behind the
technology curve. It's a bit like the NHS where, all too often, the
only way to get proper treatment is to learn about your condition and
research it yourself.

My partner's Octavia has had chronic gearbox/clutch problems since we
bought it last April. After a new gearbox and five further visits,
the problem still wasn't solved. Following a few Google searches and
asking questions on a Skoda forum, it took a change of dealer to one
that services Skoda, VW and Audi (and therefore has a greater range of
experience of these mechanically identical brands) to identify and
quickly solve a problem with the dual mass flywheel.

The most senior mechanic at the previous dealer has achieved "master
Technician" status, which is apparently the highest level of Skoda's
technical qualifications, yet he didn't even know what a dual mass
flywheel was, nor how it could go wrong (and did).

Now I have learned about the vagaries of dual mass flywheels, that
Octavia is the last manual gearbox car we will ever buy. ;-)

On Sat, 31 Jul 2010 13:14:04 +0100, john wright
wrote:

Its quite possible for things like speed cameras to cost a lot but also
bring in a lot of money. So motorists who claim them to be a cash cow
may be right, but people who say they will save lots of money by
abolishing them may also be right.


They can both be right because the money to install them came from a
central government budget, so they didn't cost local authorities
anything significant at the outset, but the fines were given to local
authorities to spend.

In message , Graeme
writes
nteresting numbers, how old is your car? My two year old Laguna measures a
lot closer to the limits than that, 73mph indicated gives just about 70 on
the Sat Nav.
Between SWMBO and me, we have two cars, both with sat-navs. I've tried
both together in both cars and the are in agreement, one a tomtom the
other a Garmin. At 70mph on both sat-navs the 52 Nissan Almera is
indicating 77mph and the 09 Toyota Auris is indicating 72mph.
--
Clive

In message
"Recliner" wrote:

"Graeme" wrote in message

In message
"Recliner" wrote:

"Bruce" wrote in message

On Sat, 31 Jul 2010 11:52:24 +0100, "Recliner"
wrote:

"Adrian" wrote in message

Bruce gurgled happily, sounding much like
they were saying:

The margin (maybe not so much nowadays) is necessary to allow
for tyre wear (and IIRC tyre type on some vehicles) as well as
the capabilities of a mechanical speedo; the normal consequence
of tyre wear is that the indicated speed will be progressively
too high so to avoid underindication the average speedo will
probably already be over-reading from new.

The legal requirement is that a speedometer measures road speed
with a tolerance of +10%, -0%.

Actually, it probably isn't.

It's difficult to be sure, since the Construction & Use regs
aren't on the web. The nearest that is simple to find is the
requirements for the IVA test - which are definitely nowhere near
as simple as that. There's a table of allowable readings against
accurate speed.

0 under-read is true, though.

Mind you, I'd love to know what sort of tyres are being used to
affect calibration by 10% as they wear... Something like a total
of 6mm variation due to tread wear on a typical overall tyre
radius of about 320mm?

Wouldn't tyre pressure have a much bigger effect?


Indeed so, and that forms the basis of tyre pressure monitoring in
many modern cars - when the rotational speed (RPM) of one wheel
exceeds that of the others by more than a predetermined percentage,
an alarm sounds and a warning light illuminates.

I repeat that the legal requirement is that a speedometer measures
road speed with a tolerance of +10%, -0%. I am told by a friend who
is a car designer that this is stated in the Construction & Use
Regulations. He says it is very out of date because speedometers
can now be made to much tighter tolerances.

Yes, whenever I've compared my speedo reading in modern cars with an
accurate GPS, the speedo is very close to 5% over. I suspect that
manufacturers always aimed at this, but modern electronic devices
deliver it accurately. Given that any car with ABS presumably can
measure wheels speeds, it's annoying that most car manufacturers
charge so much for the tyre pressure monitoring option.

Oddly enough, however, when I set the digital cruise control, it
doesn't quite agree with the more accurate analogue speedo. In other
words, if I want to do exactly 50mph through motorway road works, a
speedo reading slightly above 50mph is OK, but I need to set the
digital cruise control to something like 55mph to achieve this
precisely (if the traffic is heavy, I don't bother, and just let the
adaptive cruise control keep my speed the same as the cars I'm
following).

When I reach the end of the road works, (if necessary) I change the
target speed on the cruise control to, say, 77 mph to ensure that I
do exactly 70 mph. Again, the ACC slows me down if needed.



Interesting numbers, how old is your car? My two year old Laguna
measures a lot closer to the limits than that, 73mph indicated gives
just about 70 on the Sat Nav.

Yes, your car's speedo is the normal ~5% over, just like mine and most
other modern cars. Does it also have a digital adapative cruise control
(ACC) where you can pre-set an exact target speed, adjustable in 1 mph
increments? That's where mine is about 10% over. I don't think ACC is
standard on any cars, and is a fairly expensive option when offered.



The cruise control appears to be less than 1mph lower than the speedo, may
just be parallax error because, being tall, I sit a long way back from the
instrument panel and am looking slightly below the axis.

--
Graeme Wall

This address not read, substitute trains for rail
Transport Miscellany at www.greywall.demon.co.uk/rail
Photo galleries at http://graeme-wall.fotopic.net/

On Fri, 30 Jul 2010 12:24:43 +0100,
Chris Tolley wrote:

The thinking distance is merely the speed in mph expressed in feet. The
stopping distance is merely the speed in mph squared and divided by 20,
then expressed in feet.

vmph v ft thinking + v*v/20 stopping

20mph --- 20 ft thinking + 20*20/20 stopping ---- 40ft
30mph --- 30 ft thinking + 30*30/20 stopping ---- 75ft
40mph --- 40 ft thinking + 40*40/20 stopping ---- 120ft
..
70mph 70 70*70/20 ---- 315ft


Why is this surprising? This is just thinking time = 2/3 second and
CoF=2/3 and then rounded to easy numbers.

If you wanted it in mph - metres the formula would be

x mph - 3x/10 thinking distance and 3x^2/200 stopping distance

20mph - 6m thinking and 6m stopping - 12m overall
30mph - 9m thinking and 13.5m stopping - 22.5m overall
40mph - 12m thinking and 24m stopping - 36m overall
....
70mph - 21m thinking and 73.5m stopping - 94.5m overall


If you want it in kph - metres the formula would be

x kph - x/5 thinking distance and 6*x^2/1000

20kph - 4m thinking and 2.4m stopping
30kph - 6m thinking and 5.4m stopping
40kph - 8m thinking and 9.6m stopping
....
110kph - 22m thinking and 72.6m stopping



As for your Ford Anglia allegation, the Highway Code predates Ford
Anglias by several decades. The same figures were included in the 1946
HC, and may have been in versions before that; I can't be bothered to
look them up.

But you only need to pick a reasonable value for thinking time and CoF
and the rest is basic physics.

Thinking times from about .7s to about 1.5s is reasonable depending on
what you really mean by "alert driver" and CoF between about .5 and .8
for rubber on dry asphalt for a typical road-legal car at road-legal
speeds.

Obviously, this breaks down for high performance cars at very high
speeds. I doubt that any road-legal car generates signficant down force
at speeds much below about 100mph due to the dire effect it has on fuel
consumption.

Tim.


--
God said, "div D = rho, div B = 0, curl E = - @B/@t, curl H = J + @D/@t,"
and there was light.

http://www.woodall.me.uk/

Modern downward tinkering of speed limits is practically all about
anti-car, not common sense, cf ever increasing swathes of 20 mph
zones, etc.

Cite?

I was with you until that. Some 20mph zones are excessive (the
ludicrous one on the approach to Ambleside was one example but it's
now mostly been increased to 30, and most people did 30 anyway), but
many or most of the ones on estates are justified.

That said, the better approach on newer residential estates is to
design the road layout with curves and natural chicanes (on-street
parking) so the natural speed is 20mph or below, then it doesn't
matter if the limit is the default 30. This is done to great effect
on many Milton Keynes estates, especially newer ones.

Neil

Chris / Neil

I was specifically thinking about the London Borough of Merton which
for the last three years has been going through the borough converting
whole swathes of streets (not enclosed estates) into 20 mph zones
(from 10 to 20 roads at a time!). The SW19 and SW20 postcode areas.
Minutes of the Street Management Advisory Committee meetings can be
found he http://www.merton.gov.uk/council/com...tee&com_id=221

Ken

"Graeme" wrote in message
...

Interesting numbers, how old is your car? My two year old Laguna measures
a
lot closer to the limits than that, 73mph indicated gives just about 70 on
the Sat Nav.

It's probably less to do with the age of the car and a lot more to do with
what wheels/tyres are fitted vs the range of options for the car. My car
(also two years old) has options for 17", 18" or 19" wheels. I have the the
17" wheels and the speedo reads about 10% over actual speed. If I were to
fit 18 or 19" wheels the speedo would still read more than actual speed, but
by a lot less as the rolling radius of wheel/tyre combination gets larger.
The car it replaced was 10 years old and it had (for all intents and
purposes) an identical arrangement.

--
DAS

"David A Stocks" gurgled happily, sounding much like
they were saying:

My car (also two years old) has options for 17", 18" or 19" wheels. I
have the the 17" wheels and the speedo reads about 10% over actual
speed. If I were to fit 18 or 19" wheels the speedo would still read
more than actual speed, but by a lot less as the rolling radius of
wheel/tyre combination gets larger.

I strongly suspect you'll find that there's very little difference in
rolling radius between the standard-fit tyre sizes. As the rim diameter
goes up, so the tyre sidewall profile comes down, keeping the overall
size around the same.

If there is a marked difference between, then cars with the different
tyre sizes either factory or dealer-fitted should have the speedo and odo
recalibrated.

Adrian gurgled happily, sounding much like they
were saying:

If there is a marked difference between, then cars with the different
tyre sizes either factory or dealer-fitted should have the speedo and
odo recalibrated.

bad form, etc
....which is merely an option in the dealer's software on modern cars.