Ken Wilshire wrote:


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

Okay, so having looked at those minutes, specifically
http://www.merton.gov.uk/democratic_...eports/146.pdf

I see the following with regard to that policy:

2.1 It is the Council¢s policy to improve road safety by reducing
vehicular speeds and volume on borough roads. The key objective of these
proposals are to convert existing traffic calmed roads within the
proposed areas into 20mph zones / 20mph speed limits. The majority of
roads have traffic calming measures in place, therefore minimum
changes are required for the introduction of these measures.

2.2 20mph zones / 20mph speed limits are dedicated areas where improving
safety and maintaining the quality of life for local residents takes
precedence over the general objective to ease traffic flows.

2.3 For a 20mph zone, traffic calming features in the form of road
humps; speed cushions; road closures; one way systems; pedestrian refuge
islands and road narrowing are required to achieve a legal and
self-enforceable zone. A 20mph speed limit, however, does not require
any form of traffic calming features as part of the legal process. Signs
and road markings are mandatory within zones and limits.


So it's not "anti-car, not common sense" - it's "pro-resident". Indeed,
if the above is correct (and surely there's some liability if it isn't)
then although you can see 20 signs sprouting, they are only sprouting in
areas where there's already traffic calming in place. So I can't see
that any high-speed routes are being lost to the 20 zones.

Indeed, referring to para 2.3 - if I had the choice between driving
along a 30 with lumps in the road, and a 20 that was plain, I'd go for
the latter, and I assume most drivers would. I resent speed bumps much
more than I resent low speed limits.

--
http://gallery120232.fotopic.net/p9638813.html
(08 103 and 08 920 at Birmingham New Street, 1979)

"Adrian" wrote in message

"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.

Yes, I'm sure that's correct. However, the large wheel/low profile tyre
combo rolling radius is less likely to be affected by air pressure
variability.

"Tim Woodall" wrote in message
e.uk

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.

What about the sports cars which have retractable rear spoilers that pop
up at, say, ~75mph?

On Sat, 31 Jul 2010 19:55:31 +0100,
Recliner wrote:
"Tim Woodall" wrote in message
e.uk

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.

What about the sports cars which have retractable rear spoilers that pop
up at, say, ~75mph?


No idea. But if they do generate appreciable down force then they'll
also cause significant drag and you'll see that in the fuel economy.

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/

On Sat, 31 Jul 2010 19:55:31 +0100, "Recliner"
wrote:
"Tim Woodall" wrote in message
. me.uk

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.

What about the sports cars which have retractable rear spoilers that pop
up at, say, ~75mph?


Some of them pop up at much lower speeds - I think the Porsche 911 and
Audi TT spoilers pop up at 40 mph.

"Tim Woodall" wrote in message
e.uk
On Sat, 31 Jul 2010 19:55:31 +0100,
Recliner wrote:
"Tim Woodall" wrote in message
e.uk

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.

What about the sports cars which have retractable rear spoilers that
pop up at, say, ~75mph?


No idea. But if they do generate appreciable down force then they'll
also cause significant drag and you'll see that in the fuel economy.

Yup, but that's probably not something that owners of cars with
automatic rear spoilers (eg, Porsches) worry about too much.

"Bruce" wrote in message

On Sat, 31 Jul 2010 19:55:31 +0100, "Recliner"
wrote:
"Tim Woodall" wrote in message
e.uk

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.

What about the sports cars which have retractable rear spoilers that
pop up at, say, ~75mph?


Some of them pop up at much lower speeds - I think the Porsche 911 and
Audi TT spoilers pop up at 40 mph.

I think it's 75 mph by default with both of those cars, but may be lower
in other cars. It can also come up at lower speeds to cool a hot engine
in the case of the Porsche. Of course, they can usually be deployed
manually, which is especially useful in countries with speed limits 75
mph, like the UK!

In message , Bruce
writes
Some of them pop up at much lower speeds - I think the Porsche 911 and
Audi TT spoilers pop up at 40 mph.
I remember reading about this. In Germany, they deploy at the proper
speed of 90 mph, but in the UK where the speed limit is 70 mph they come
up at 50 mph to allow the owner to show off, it's a sales gimmick.
--
Clive

On Sat, 31 Jul 2010 19:55:31 +0100, Recliner wrote
"Tim Woodall" wrote in message
e.uk

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.

What about the sports cars which have retractable rear spoilers that pop
up at, say, ~75mph?

In many cases they have an appreciable effect at speeds not often reached in
the UK.

In order to give them some 'pose value' they are recalibrated to rise at
speeds more likely to be achieved in the UK, otherwise the driver might not
feel they're getting value for money/the required pose effect.

Of course, there is an argument that says a properly designed car shouldn't
need 'add-on' wings/spoilers etc fixed or otherwise.

On Sun, 1 Aug 2010 15:01:31 +0100, Stimpy
wrote:

Of course, there is an argument that says a properly designed car shouldn't
need 'add-on' wings/spoilers etc fixed or otherwise.


Many cars suffer from increased lift at higher speeds. The spoiler
provides increased downforce to counter this.

The Audi TT (first model) suffered a significant increase in rear lift
at higher speeds. In some markets, but not the UK, the automatic rear
spoiler was standard equipment. In the UK, which has a 70 mph limit,
it was an optional extra, presumably on the basis that it was not
actually needed at 70 mph.

There was some discussion in the motoring press about whether TTs sold
in the UK should all be so fitted, in case they were taken to (for
example) Germany and driven at much higher speeds - some stretches of
German autobahn have no speed limit for cars in good weather.

IIRC Audi UK accepted the suggestion. Subsequently all TTs sold in
the UK had the spoiler fitted as standard.