Speed limits, tolerance + 10% ?
Donwill
Am I allowed to go 10% above legal speed limit ? e.g. in 70mph dual
carriageway am I allowed to do 77mph legally?
Don
Mark Goodge
typed:
>Sorry if it's been asked before, I wanted to know if the above is true.
>Am I allowed to go 10% above legal speed limit ? e.g. in 70mph dual
>carriageway am I allowed to do 77mph legally?
No. You can only legally go up to the limit. ACPO guidelines suggest a
margin of tolerance of 3mph + 10% (eg, 80mph in a 70mph limit or 36mph in a
30mph limit) which normally needs to be exceeded before a prosecution is
deemed appropriate, but it is only a guideline and you can't rely on it
always being followed. In particular, if you were driving like a loon at
71mph, the police may well choose to prosecute you for the absolute offence
of speeding rather than the harder-to-prove offence of dangerous driving.
Mark
--
Blog: http://mark.goodge.co.uk
Stuff: http://www.good-stuff.co.uk
Donwill
comprehensive reply, I don't need a reply from this group unless someone
is particularly interested.
Cheers
Don
TTman
"Donwill" <Donwill...@invalid.invalid> wrote in message
news:8pvib3...@mid.individual.net...
speedometer, to enable a successful prosecution.
If your speedo reads 10% slow, and you do 77mph in a 70, you're actually
doing.... wait for it....well over 80.
A bit historic really.... in the old days, spedo accuracy was in that order.
Today, it's more like 1%..
Colin Jackson
"Mark Goodge" <use...@listmail.good-stuff.co.uk> wrote in message
news:9mclj6psl51kf134a...@news.markshouse.net...
Not so Mark,
I have just been done for travelling at 36mph on a main road (A22 if you
ask)
ACPO states that 35 mph is the top limit.
Colin
Nightjar <"cpb"@
No. It is always an offence to exceed the speed limit. However, the
Association of Chief Police Officers (ACPO) have issued guidelines that
police officers may exercise discretion about whether to take
enforcement action if the speed is *less than* the limit + 10% + 2mph or
less than 25mph in a 20mph limit.
Once the speed of the limit +10% +2mph (or 25mph in a 20 limit) has been
reached, the guidelines recommend that, as a minimum, a fixed penalty
notice be issued. They also set a speed at which a summons would be
considered the minimum action required - the fixed penalty speed +50% of
the limit up to 40mph limit or the limit +26mph for higher limits.
The guidelines make it clear that Police Officers have complete
discretion to issue a fixed penalty notice or a summons at lower speeds
than recommended.
A google search on
Speed Enforcement Guidelines - ACPO
will bring up a link to a word document giving the guidelines in full.
Colin Bignell
Steve Firth
> No. You can only legally go up to the limit. ACPO guidelines suggest a
> margin of tolerance of 3mph + 10% (eg, 80mph in a 70mph limit or 36mph in a
> 30mph limit)
I'd quibble with that slightly, 10% + 2mph is the guidance I remember.
> which normally needs to be exceeded before a prosecution is
> deemed appropriate, but it is only a guideline and you can't rely on it
> always being followed. In particular, if you were driving like a loon at
> 71mph, the police may well choose to prosecute you for the absolute offence
> of speeding rather than the harder-to-prove offence of dangerous driving.
It's also worth noting that the ACPO guidance is just that, guidance and
some Chief Constables have gone against ACPO guidance. The one that
sticks in my head is Cleveland where the CC made a decision to enforce
limits strictly. I was working up there at the time and can recall the
tales of woe in the staff canteen.
What was amusing was that the police only performed strict enforcement
in nice middle class suburbs. The rougher estates where speeding was a
problem were ignored because the polic were scared of the violent
approaches made to the operators of speed camera vans.
Cleveland is also the only area that I have worked in where I can recall
the police using "stealth" camera vans. Plain white with the speed
camera being located on a very short black tripod that was located some
distance from the van (about 30 metres in most cases).
Mark Goodge
>
>"Donwill" <Donwill...@invalid.invalid> wrote in message
>news:8pvib3...@mid.individual.net...
>> Sorry if it's been asked before, I wanted to know if the above is true. Am
>> I allowed to go 10% above legal speed limit ? e.g. in 70mph dual
>> carriageway am I allowed to do 77mph legally?
>> Don
>No, the 10% thing is to remove any doubt as to the accuracy of a vehicle's
>speedometer, to enable a successful prosecution.
>If your speedo reads 10% slow, and you do 77mph in a 70, you're actually
>doing.... wait for it....well over 80.
Speedos aren't allowed to read slow. Any speedo that does is illegal. They
are permitted to overread (ie, give a speed figure which is higher than
reality) by up to 10% (eg, it could say that you're doing 77mph when you're
only doing 70mph), but they are not permitted to err at all in the other
direction.
Peter Parry
wrote:
>No, the 10% thing is to remove any doubt as to the accuracy of a vehicle's
>speedometer, to enable a successful prosecution.
>If your speedo reads 10% slow, and you do 77mph in a 70, you're actually
>doing.... wait for it....well over 80.
>A bit historic really.... in the old days, spedo accuracy was in that order.
>Today, it's more like 1%..
Speedometer accuracy is the subject of world wide regulation - United
Nations Economic Commission for Europe (UNECE) Regulation 39. The
main purpose of the UNECE regulations (set by UNECE Working Party 29 -
the World Forum for Harmonization of Vehicle Regulations ) is to ease
trade in motor vehicles by agreeing uniform type approval standards.
The UK transposes these regulations in to UK law by the Road Vehicles
(Construction and Use) Regulations 1986 which allows the use of
speedometers that meet the requirements of EC Community Directive
75/443(97/39) or ECE Regulation 39.
The accuracy requirements to be applied at the time of vehicle type
approval are that the indicated speed must not be more than the true
speed plus 10 per cent plus 4 km/h and that the indicated speed must
never be less than the true speed.
0 =/< V1-V2 =/< v"/10 + 4km/h where V1 is the indicated sped and V2
the true speed.
http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CONSLEG:1975L0443:19970725:en:PDF
Producing a speedometer which reads +0 to +10% over speed but never
lower than the true speed is a bit difficult so in production, to meet
UNECE conformity audits, indicated speed is increased to 110 percent
plus 6 km/h for cars, buses, trucks and similar vehicles. The
production tolerance used is 0 to +5% plus 10km/h, this ensures that
no production unit should misread a lower speed than the actual speed.
Due to a historic error (not altering 10km/h to 4 km/hr when the EU
Directive changed) the UK Construction and Use regulations for cars
require for all actual speeds between 25 mph and 70 mph the
indicated speed must not exceed 110% of the actual speed, plus 6.25
mph. For a vehicle meeting UK regulations, at 50MPH the speedometer
may show anything between 50 and 61MPH.
In practice therefore almost all normal speedometers over read. No
matter what type of sensor is used changing the size of the tyres or
the tyre pressure will of course alter the speedometer reading.
Nobby Anderson
> Due to a historic error (not altering 10km/h to 4 km/hr when the EU
> Directive changed) the UK Construction and Use regulations for cars
> require for all actual speeds between 25 mph and 70 mph the
> indicated speed must not exceed 110% of the actual speed, plus 6.25
> mph. For a vehicle meeting UK regulations, at 50MPH the speedometer
> may show anything between 50 and 61MPH.
Comparinjhg my satnav speed indication with the speedo reading of the
three cars I've used it in, they have all indicated about 33mph on the
speedo at a satnav speed of 30mph, and 77 at an satnav speed of 70,
pretty consistently.
Using the same satnav unit in Austalia in two different hire cars
the satnav speed was indentical to the speed indicated on the speedos,
well withing the limits of that accuracy you can read a speedo by,
probably about +/- 1kmh. I'm guessing that the difference is down to
different regulations in the two countries. I'm also assuming that
the satnav is the more accurate, but nevertheless it was the same unit
in all 5 cars so I guess the differences between here and Australia
are real.
Nobby
Roland Perry
16:10:02 on Sat, 22 Jan 2011, Nobby Anderson <no...@invalid.invalid>
remarked:
>Comparinjhg my satnav speed indication with the speedo reading of the
>three cars I've used it in, they have all indicated about 33mph on the
>speedo at a satnav speed of 30mph, and 77 at an satnav speed of 70,
>pretty consistently.
>
>Using the same satnav unit in Austalia in two different hire cars
>the satnav speed was indentical to the speed indicated on the speedos,
>well withing the limits of that accuracy you can read a speedo by,
>probably about +/- 1kmh. I'm guessing that the difference is down to
>different regulations in the two countries.
I'd be sceptical that car specifications are that localised (rather than
global), and Satnavs will always be true. But I can agree with you that
in the UK speedos seem to be about 10% optimistic. In my car the typical
56mph-limited-HGV appears to be doing a little over 60mph.
--
Roland Perry
Nobby Anderson
These were all modern cars and I bet the speedo reading is a tweak in software,
and also the Australian cars were Australian-specific models, one Holden and
a Ford model that's only available in Australia. I *think* satnav speeds
should be pretty accurate because of the way the speed is calculated from
the satellite signal (someone who I believe once explained it to me why) but
I wouldn't swear in court that that's correct (just to re-introduce some legal
content!).
Nobby
jj
"Roland Perry" <rol...@perry.co.uk> wrote in message
news:m7HA8UPI...@perry.co.uk...
.........and Satnavs will always be true.........
Wouldn't be so sure about that. Just replaced an old PDA based TomTom with a
new all in one TomTom unit. The old one would show 70mph at a speedo reading
of 77mph, the new one shows 70mph at a speedo reading of 74mph ! Same car
just a newer satnav, both tested on a motorway at constant speed for several
miles. They can't both be right, but also neither should be wrong !
Nightjar <"cpb"@
....
> Comparinjhg my satnav speed indication with the speedo reading of the
> three cars I've used it in, they have all indicated about 33mph on the
> speedo at a satnav speed of 30mph, and 77 at an satnav speed of 70,
> pretty consistently.
>
> Using the same satnav unit in Austalia in two different hire cars
> the satnav speed was indentical to the speed indicated on the speedos,
> well withing the limits of that accuracy you can read a speedo by,
> probably about +/- 1kmh. I'm guessing that the difference is down to
> different regulations in the two countries. I'm also assuming that
> the satnav is the more accurate, but nevertheless it was the same unit
> in all 5 cars so I guess the differences between here and Australia
> are real.
The accuracy of GPS for speed measurement is variable. You will get the
most accurate reading when travelling in a straight line on a level road
at a constant speed. As you move away from any of those ideal
conditions, the accuracy will reduce and the unit will tend to over read.
Colin Bignell
Roger
GPS device (not permanently built-in but portable e.g. Tom Tom) which
not only indicates the vehicle's speed but also shows the speed limit
for the current stretch of road and is set up to warn visually and
audibly if the legal speed limit was exceeded, would this be likely to
be an acceptable defence should police charge the driver for the
offence of having a defective (non-operating) speedometer? Is anyone
aware of any precedent? (there is a reference to a driver in
Australia successfully defending a speeding prosecution where the GPS
log showed he was not exceeding the speed limit)
P.S. this is a real scenario as 'a friend's'* speedo packed up two
days ago and it will take at least 10-14 days for the vehicle
manufacturer's authorised parts dept. to get the required part (gear
box transponder?) and I would like to continue to use the vehicle
meanwhile. This would cause difficulty in offering either the defence
that the speedometer became defective during the journey being
undertaken*, or steps have been taken to have the defect remedied by
replacement or repair with all reasonable expedition
Of course the 'friend' wouldn't be daft enough to post this himself
on public forum. It would of course be a complete coincidence if the
same thing happened to this poster subsequently . . .
John Doe
adamele...@hotmail.co.uk
> P.S. this is a real scenario as 'a friend's'* speedo packed up two
> days ago and it will take at least 10-14 days for the vehicle
> manufacturer's authorised parts dept. to get the required part (gear
Perhaps your friend should ask the garage for a loan vehicle for the
intervening period?
> ... and I would like to continue to use the vehicle
> meanwhile.
PS Are you insured to drive your friend's car.
Owain
Roger
Bother! How did that 'I' creep into my - , I mean my friend's, post -
no, I mean my post about my 'friend' - blimey, this spade's getting
heavy . . .
Roger
> On Jan 22, 8:50 pm, Roger wrote:
>
> > P.S. this is a real scenario as 'a friend's'* speedo packed up two
> > days ago and it will take at least 10-14 days for the vehicle
> > manufacturer's authorised parts dept. to get the required part (gear
> > box transponder?) ...
>
> Perhaps your friend should ask the garage for a loan vehicle for the
> intervening period?
It's not a 'normal' vehicle but rather specialised so a similar loan
vehicle wouldn't be available from the garage (don't want to give too
much away in case of 'coincidental' consequences'!).
> > ... and I would like to continue to use the vehicle
> > meanwhile.
>
> PS Are you insured to drive your friend's car.
Ah, yes, good point (wink, wink) I'd better check . . .
> Owain
R. Mark Clayton
"Nightjar <"cpb"@ @giganews.com>" <"insertmysurnamehere> wrote in message
news:DoSdnVTA2-fPVKfQ...@giganews.com...
> On 22/01/2011 08:25, Donwill wrote:
>> Sorry if it's been asked before, I wanted to know if the above is true.
>> Am I allowed to go 10% above legal speed limit ? e.g. in 70mph dual
>> carriageway am I allowed to do 77mph legally?
>
> No. It is always an offence to exceed the speed limit. However, the
> Association of Chief Police Officers (ACPO) have issued guidelines that
> police officers may exercise discretion about whether to take enforcement
> action if the speed is *less than* the limit + 10% + 2mph or less than
> 25mph in a 20mph limit.
Mostly because someone accused of such a minor infraction could calim doubt
about the police reading and / or that there was doubt that they knew they
were speeding. In the past the courts allowed a 10% margin for error except
[oddly] for 70mph.
sid
I thought the guideline was 10%+2?
In practice, over here they are quite generous (on dual carriageways at
any rate) and I was unofficially told after being stopped for speeding
in temporary roadworks, that they go by 15mph over the limit. Generally
that means at 83mph they won't bother, or in my case 53 coming into a
40, but at 55 they ticket. I suspect in GB they are not quite so
forgiving, so the 10%+2 rule is what I go by even when I am over here.
sid
HGVs are supposed to be limited to 80kph, but there is an allowance made
of 10kph, so in practice most companies get the limiters set to 90kph
(which is 56mph).
Geoff Berrow
<no...@invalid.invalid> wrote:
>Comparinjhg my satnav speed indication with the speedo reading of the
>three cars I've used it in, they have all indicated about 33mph on the
>speedo at a satnav speed of 30mph, and 77 at an satnav speed of 70,
>pretty consistently.
That's also been my observation in my Alfa Romeo 156 but Dawn's Ford
Ka is much more accurate
--
Geoff Berrow (Put thecat out to email)
It's only Usenet, no one dies.
My opinions, not the committee's, mine.
Simple RFDs www.4theweb.co.uk/rfdmaker
Neil Williams
<cojack6ATbti...@giganews.com> wrote:
> ACPO states that 35 mph is the top limit.
10% + 2, I believe, which fits your example.
Neil
--
Neil Williams, Milton Keynes, UK
Neil Williams
<no...@invalid.invalid> wrote:
Beware that some sat navs may calculate speed on the basis of 2D
movement, so could under-read on a slope.
Nightjar <"cpb"@
> If the vehicle's built-in speedometer is faulty but the driver uses a
> GPS device (not permanently built-in but portable e.g. Tom Tom) which
> not only indicates the vehicle's speed but also shows the speed limit
> for the current stretch of road and is set up to warn visually and
> audibly if the legal speed limit was exceeded, would this be likely to
> be an acceptable defence should police charge the driver for the
I doubt it, as having a working speedometer with defined error limits is
a requirement under the Construction and Use Regulations and a GPS unit
is not a calibrated measuring device.
Colin Bignell
Nightjar <"cpb"@
> "Nightjar<"cpb"@ @giganews.com>"<"insertmysurnamehere> wrote in message
> news:DoSdnVTA2-fPVKfQ...@giganews.com...
>> On 22/01/2011 08:25, Donwill wrote:
>>> Sorry if it's been asked before, I wanted to know if the above is true.
>>> Am I allowed to go 10% above legal speed limit ? e.g. in 70mph dual
>>> carriageway am I allowed to do 77mph legally?
>>
>> No. It is always an offence to exceed the speed limit. However, the
>> Association of Chief Police Officers (ACPO) have issued guidelines that
>> police officers may exercise discretion about whether to take enforcement
>> action if the speed is *less than* the limit + 10% + 2mph or less than
>> 25mph in a 20mph limit.
>
> Mostly because someone accused of such a minor infraction could calim doubt
> about the police reading and / or that there was doubt that they knew they
> were speeding. In the past the courts allowed a 10% margin for error except
> [oddly] for 70mph.
The ACPO guidelines are in line with Road Research Laboratory
recommendations, dating back to 1962, that speed limits should not be
treated as a way to keep traffic at or below the limit, but as a way to
significantly reduce the number of vehicles that grossly exceed the set
limit.
Colin Bignell
Steve Walker
> If the vehicle's built-in speedometer is faulty but the driver uses
> a GPS device (not permanently built-in but portable e.g. Tom Tom)
> which not only indicates the vehicle's speed but also shows the
> speed limit for the current stretch of road and is set up to warn
> visually and audibly if the legal speed limit was exceeded, would
> this be likely to be an acceptable defence should police charge the
> driver for the offence of having a defective (non-operating)
> speedometer?
The GPS unit won't be type-approved, I would imagine.
But pragmatically, under what circumstances does your friend anticipate the
Police finding out about his broken speedometer?
Roger
Agreed it's very highly improbable, particularly since the vehicle
involved is neither a car nor a goods vehicle and is of a type that
very rarely attracts attention from traffic police, particularly since
it is unlikely to be being driven above the speed limit anyway . . .
however, in the unlikely event of being stopped and then being
accompanied by an officer whilst actually driving the vehicle, since
only then would it be detected that the speedo was faulty, would it be
best to refer (politely and not in a clever-dick way!) to the use of
the GPS at that point or reserve this as a defence in the event of
being reported?
steve robinson
> Sorry if it's been asked before, I wanted to know if the above is
> true. Am I allowed to go 10% above legal speed limit ? e.g. in
> 70mph dual carriageway am I allowed to do 77mph legally? Don
Theoretically no you have commited an offence , however most Police
forces allow a tolerence because vehicles speedometers can misread
and at present there is no requirement to have them periodically
calibrated .
A car with new tyres will travel at a slightly higher speed than a
car with worn tyres at the same given reading on the speedometer as
would cars with under or over inflated tyres fully loaded or unladen
vehicles will also show a variation
Nobby Anderson
> A car with [...]
> with under or over inflated tyres fully loaded or unladen
> vehicles will also show a variation
This confuses me. All modern tyres have a steel belt in them
which can't expand or contract if the tyre is under or over inflated,
so the length of the belt must be constant and therefore the
circumference of the tyre and hence the the distance the tyre
travels must be constant and independant of inflation or load.
Surely?
Nobby
Nightjar <"cpb"@
What is important is not the tyre circumference, but the rolling radius
- the distance between the centre of the axle and the road - which does
vary according to inflation, load and even speed. The last is because
the tyre has to flex to accommodate the difference between the rolling
circumferece and the actual circumference, which heats the tyre, which
increases the pressure, which changes the rolling radius and the faster
you travel, the greater the effect.
Colin Bignell
David Hearn
The moment 'he' goes into a tunnel, he'll lose his ability to know what
speed he's doing, so I would imagine GPS only speedos wouldn't be
sufficient to meet the regulations.
D
Nobby Anderson
> On 23/01/2011 23:40, Nobby Anderson wrote:
>> steve robinson<st...@colevalleyinteriors.co.uk> wrote:
>>> A car with [...]
>>> with under or over inflated tyres fully loaded or unladen
>>> vehicles will also show a variation
>>
>>
>> This confuses me. All modern tyres have a steel belt in them
>> which can't expand or contract if the tyre is under or over inflated,
>> so the length of the belt must be constant and therefore the
>> circumference of the tyre and hence the the distance the tyre
>> travels must be constant and independant of inflation or load.
>>
>> Surely?
>
> What is important is not the tyre circumference, but the rolling radius
> - the distance between the centre of the axle and the road - which does
> vary according to inflation, load and even speed.
Yes, that makes sense but that surely means that the tyre has to scrub
along the road surface, because for every revolution there is more tyre
circumference than there is distance travelled. For every 1cm the
rolling diameter decreases, the rolling circumference decreases by 3.14cm
but the length of rubber in each revolution must stay the same because of
the steel belt, so the tyre must slip 3cm.
Hmmm, maybe not, if it flexes upwards in a mirror image hump where it
contacts the road rather than being flat on the road. Yes, that might
work and then the forward speed would be the forward component of the
tyre tangential speed at the ends of the contact patch rather than the
tangential speed at the centre of the patch. Ok, that sounds plausible.
Anyway, it's not really ulm though, sorry.
Nobby
Robert Coates
Doubt it. I think it's just chance what you've found.
My previous car would read 77mph when my gps read 70.
When I swopped that car, new car reads 70mph when the satnav reads 70.
Same satnav different cars in the UK.
My wife's car reads 73mph at 70.
The only thing about Oz is they have no ACPO type guidlines.
They take the view that the speed limit is exactly that not a "target"
one with tolerance as we do. Hence a mate of mine got done for doing
51kph in a 50 limit.
They also do things like "double points on bank holidays".
Makes the ACPO guidlines look quite generous!
David Hearn
Exactly, it's the old "it's only flat at the bottom" joke. It doesn't
matter the distance between the axle and the top of the tyre, it's the
distance between the axle and the road. On a flat tyre this is quite
different. Whilst the circumference may remain the same due to the
steel belt - the axle will be closer to the ground at the bottom, like a
rotating oval. Thus making the effective circumference (or rolling
radius) less, giving rise to (I think) an over reading of the speed from
the speedo (which uses the rotation speed, with a known
radius/circumference, to calculate the speed). Inflation and load
differences will give the same effect (after all, a flat tyre is just
one which is grossly underinflated).
D
Roland Perry
08:31:24 on Mon, 24 Jan 2011, Nobby Anderson <no...@invalid.invalid>
remarked:
>> What is important is not the tyre circumference, but the rolling radius
>> - the distance between the centre of the axle and the road - which does
>> vary according to inflation, load and even speed.
>
>Yes, that makes sense but that surely means that the tyre has to scrub
>along the road surface, because for every revolution there is more tyre
>circumference than there is distance travelled. For every 1cm the
>rolling diameter decreases, the rolling circumference decreases by 3.14cm
>but the length of rubber in each revolution must stay the same because of
>the steel belt, so the tyre must slip 3cm.
The physics of a car with four reasonably similar tyres, all four of
them scrubbing by the same amount, defies my internal logic. When the
tyre rotates once, the distance travelled will be the [unchanged]
circumference; end of.
And anyone who has driven a classic Landrover with front-rear
diff-locked, across dry tarmac, will confirm the enormous forces
involved, even with ostensibly equally inflated tyres.
On the other hand, many cars these days have spacesaving "spare" tyres
with a smaller radius, which would affect the reading when on a driven
wheel (although I understand that some manufacturers instruct that they
are not put on driving wheels - but probably more to do with traction
control confusion than speed indication).
--
Roland Perry
Adrian
were saying:
> On the other hand, many cars these days have spacesaving "spare" tyres
> with a smaller radius, which would affect the reading when on a driven
> wheel (although I understand that some manufacturers instruct that they
> are not put on driving wheels - but probably more to do with traction
> control confusion than speed indication).
Unlikely, since traction control works by comparing rotational speeds
amongst all wheels - with a certain amount of latitude, else going round
bends might become entertaining.
It's far more likely to be down to the ubiquity of FWD and the widespread
"better tyres on the front" advice. (And this _really_ isn't the group
for that debate to break out again...)
Nightjar <"cpb"@
> Nightjar<"cpb"@<"insertmysurnamehere>"@giganews.com> wrote:
>> On 23/01/2011 23:40, Nobby Anderson wrote:
>>> steve robinson<st...@colevalleyinteriors.co.uk> wrote:
>>>> A car with [...]
>>>> with under or over inflated tyres fully loaded or unladen
>>>> vehicles will also show a variation
>>>
>>>
>>> This confuses me. All modern tyres have a steel belt in them
>>> which can't expand or contract if the tyre is under or over inflated,
>>> so the length of the belt must be constant and therefore the
>>> circumference of the tyre and hence the the distance the tyre
>>> travels must be constant and independant of inflation or load.
>>>
>>> Surely?
>>
>> What is important is not the tyre circumference, but the rolling radius
>> - the distance between the centre of the axle and the road - which does
>> vary according to inflation, load and even speed.
>
> Yes, that makes sense but that surely means that the tyre has to scrub
> along the road surface, because for every revolution there is more tyre
No. The difference is taken up in creating the footprint - the bit of
tyre that is in contact with the road. As the rolling radius decreases,
the footprint gets longer and vice versa.
Colin Bignell
Roland Perry
2011, Adrian <tooma...@gmail.com> remarked:
>> On the other hand, many cars these days have spacesaving "spare" tyres
>> with a smaller radius, which would affect the reading when on a driven
>> wheel (although I understand that some manufacturers instruct that they
>> are not put on driving wheels - but probably more to do with traction
>> control confusion than speed indication).
>
>Unlikely, since traction control works by comparing rotational speeds
>amongst all wheels - with a certain amount of latitude, else going round
>bends might become entertaining.
Apparently some cars can tell the difference - because measurements from
the other three wheels can detect a corner, I suspect. But I agree,
there are other forums to discuss that aspect. I was interested in
whether there was a legal reason (to do with preserving the speedo
reading).
--
Roland Perry
Adrian
were saying:
>>> On the other hand, many cars these days have spacesaving "spare" tyres
>>> with a smaller radius, which would affect the reading when on a driven
>>> wheel (although I understand that some manufacturers instruct that
>>> they are not put on driving wheels - but probably more to do with
>>> traction control confusion than speed indication).
>>Unlikely, since traction control works by comparing rotational speeds
>>amongst all wheels - with a certain amount of latitude, else going round
>>bends might become entertaining.
> Apparently some cars can tell the difference - because measurements from
> the other three wheels can detect a corner, I suspect. But I agree,
> there are other forums to discuss that aspect. I was interested in
> whether there was a legal reason (to do with preserving the speedo
> reading).
Again, unlikely - since not all cars take the speedo signal from the
driven wheels. I can think of at least one FWD car which uses the signal
from a rear wheel's ABS sensor.
Roland Perry
on Mon, 24 Jan 2011, "Nightjar <\"cpb\"@"
<"insertmysurnamehere>"@giganews.com> remarked:
>> Yes, that makes sense but that surely means that the tyre has to scrub
>> along the road surface, because for every revolution there is more tyre
>> circumference than there is distance travelled...
>
>No. The difference is taken up in creating the footprint - the bit of
>tyre that is in contact with the road. As the rolling radius decreases,
>the footprint gets longer and vice versa.
Even if the footprint spreads sideways, the centre line of the footprint
is the circumference of the tyre. If the difference is enough to change
speedo readings significantly (let's say 1mph or 2%) that's about two
inches, which is a huge amount of scrub per rotation.
--
Roland Perry
Roland Perry
2011, Adrian <tooma...@gmail.com> remarked:
It's not going to be nearly as unlikely when the speed is derived from
the rotation of the driven wheels, which I think is the normal case
>I can think of at least one FWD car which uses the signal
>from a rear wheel's ABS sensor.
And not the gearbox, which is traditional.
--
Roland Perry
Adrian
were saying:
>>> I was interested in whether there was a legal reason (to do with
>>> preserving the speedo reading).
>>Again, unlikely - since not all cars take the speedo signal from the
>>driven wheels.
> It's not going to be nearly as unlikely when the speed is derived from
> the rotation of the driven wheels, which I think is the normal case
It's not the "normal case" which determines whether reasoning is derived
from legal stipulations, but the edge cases.
"The exception which proves the rule".
GB
>
> In practice therefore almost all normal speedometers over read. No
> matter what type of sensor is used changing the size of the tyres or
> the tyre pressure will of course alter the speedometer reading.
Curiously, I don't think that altering the tyre pressure (within reason)
will have any effect on the speed reading. The circumference of the tyre
stays the same whatever the pressure as the tyres are not stretchy in that
direction - they contain steel cords and so on to stop them stretching like
that.
--
Murphy's ultimate law is that if something that could go wrong doesn't,
it turns out that it would have been better if it had gone wrong.
Roland Perry
2011, Adrian <tooma...@gmail.com> remarked:
>>>> I was interested in whether there was a legal reason (to do with
>>>> preserving the speedo reading).
>
>>>Again, unlikely - since not all cars take the speedo signal from the
>>>driven wheels.
>
>> It's not going to be nearly as unlikely when the speed is derived from
>> the rotation of the driven wheels, which I think is the normal case
>
>It's not the "normal case" which determines whether reasoning is derived
>from legal stipulations, but the edge cases.
I don't understand that at all. If fitting a smaller wheel as a spare
will make your car's speedo read incorrectly if you fit to a particular
wheel, why would the law say that was OK, and more to the point why
wouldn't the manufacturer try to advise against it?
>"The exception which proves the rule".
Are you suggesting that there's a law which says drivers are exempt from
speedo offences when a spare tyre is fitted, and that this exception
proves that there's a more general rule about the accuracy of speedos (I
didn't think we disagreed about the existence of such a general rule).
--
Roland Perry
Adrian
were saying:
>>>>> I was interested in whether there was a legal reason (to do with
>>>>> preserving the speedo reading).
>>>>Again, unlikely - since not all cars take the speedo signal from the
>>>>driven wheels.
>>> It's not going to be nearly as unlikely when the speed is derived from
>>> the rotation of the driven wheels, which I think is the normal case
>>It's not the "normal case" which determines whether reasoning is derived
>>from legal stipulations, but the edge cases.
> I don't understand that at all. If fitting a smaller wheel as a spare
> will make your car's speedo read incorrectly if you fit to a particular
> wheel, why would the law say that was OK, and more to the point why
> wouldn't the manufacturer try to advise against it?
>>"The exception which proves the rule".
> Are you suggesting that there's a law which says drivers are exempt from
> speedo offences when a spare tyre is fitted, and that this exception
> proves that there's a more general rule about the accuracy of speedos (I
> didn't think we disagreed about the existence of such a general rule).
Umm, no.
I'm pointing out that it's very unlikely there's any "legal reason" to do
with speedos why space-saver spares (always strictly stipulated as for
temporary use only, don't forget) have to be fitted to non-driven wheels.
If nothing else, it'd make space-savers illegal on anything all-wheel
drive.
Roland Perry
2011, Adrian <tooma...@gmail.com> remarked:
>I'm pointing out that it's very unlikely there's any "legal reason" to do
>with speedos why space-saver spares (always strictly stipulated as for
>temporary use only, don't forget) have to be fitted to non-driven wheels.
Only because it means you then have a speedo which may be reading
outside the required accuracy range.
>If nothing else, it'd make space-savers illegal on anything all-wheel
>drive.
That rather depends on the technology of the all-wheel drive. As you
pointed out, some vehicles will take a speed indication off a specific
wheel, and in that case all you'd have to do was make sure that wheel
had a normal tyre fitted.
Where the wheels are driven through a series of differentials, I suspect
that having just one wheel an incorrect size will have one third of the
effect on the RPM of the gearbox output, compared to the situation where
a 2WD car has one "wrong size" driven wheel.
--
Roland Perry
Man at B&Q
> In message <k8Cdnd7Gc8GKpaDQnZ2dnUVZ8mWdn...@brightview.co.uk>, at
> 08:31:24 on Mon, 24 Jan 2011, Nobby Anderson <no...@invalid.invalid>
> remarked:
>
> >> What is important is not the tyre circumference, but the rolling radius
> >> - the distance between the centre of the axle and the road - which does
> >> vary according to inflation, load and even speed.
>
> >Yes, that makes sense but that surely means that the tyre has to scrub
> >along the road surface, because for every revolution there is more tyre
> >circumference than there is distance travelled. For every 1cm the
> >rolling diameter decreases, the rolling circumference decreases by 3.14cm
> >but the length of rubber in each revolution must stay the same because of
> >the steel belt, so the tyre must slip 3cm.
>
> The physics of a car with four reasonably similar tyres, all four of
> them scrubbing by the same amount, defies my internal logic. When the
> tyre rotates once, the distance travelled will be the [unchanged]
> circumference; end of.
The distance travelled when the wheel rotates once will be pi*r*r
where r is the reduced radius measured between the axle and the road.
MBQ
pensive hamster
Yes, Nightjar and David are correct I think - speed is proportional to
the rolling radius (the distance between the centre of the axle and
the road) times the speed of rotation of the wheel.
Consider the slightly more extreme example of a vehicle with tank
tracks. The speed of such a vehicle would be proportional to the
rolling radius (the radius of the wheel plus the thickness of the
track) times the speed of rotation of the wheel, and the overall
circumference of the whole track would be irrelevant to that
calculation. That is just a more extreme version of a car with very
deflated tyres, where the tyre would not be round in profile, so the
circumferance times the speed of rotation of the wheel would not give
an accurate value for the speed of the vehicle in that case.
Car tyres commonly have a.rim diameter of 16 or 17 inches, plus the
thickness of the tyre. Some models have an option of larger wheels, 18
or 19 inches. Apparently they are more bling. Is there a legal
requirement to have the speedo recalcalibrated if larger than standard
wheels are fitted? Strikes me there ought to be. (Trying to bring this
back on-topic here)
martin
That would of course be 2*pi*r
Roland Perry
<488ac058-4513-40fe...@v31g2000pri.googlegroups.com>, at
16:20:02 on Mon, 24 Jan 2011, Man at B&Q <manat...@hotmail.com>
remarked:
>> The physics of a car with four reasonably similar tyres, all four of
>> them scrubbing by the same amount, defies my internal logic. When the
>> tyre rotates once, the distance travelled will be the [unchanged]
>> circumference; end of.
>
>The distance travelled when the wheel rotates once will be pi*r*r
pi*2r
>where r is the reduced radius measured between the axle and the road.
But the rolling radius isn't constant (especially on a flatter tyre).
The static radius can be used to measure the length of the rubber
"around" the tyre. I don't think that will change by a quarter of an
inch, let alone the two inches needed to make a noticeable difference in
the speedo reading.
--
Roland Perry
Roland Perry
<51ec0ad5-248d-408e...@u32g2000yqe.googlegroups.com>, at
17:55:02 on Mon, 24 Jan 2011, pensive hamster
<pensive...@hotmail.co.uk> remarked:
>Car tyres commonly have a.rim diameter of 16 or 17 inches, plus the
>thickness of the tyre. Some models have an option of larger wheels, 18
>or 19 inches. Apparently they are more bling. Is there a legal
>requirement to have the speedo recalcalibrated if larger than standard
>wheels are fitted? Strikes me there ought to be. (Trying to bring this
>back on-topic here)
I have a feeling that most insurance companies would regard this as a
"modification" they'd like to hear about (because it says something
about the personality of the driver rather than the grip of the tyres).
It might well be that a very much larger wheel will also make the speedo
illegally under-read (one inch extra on an 18" wheel is approximately
5%, which is the typical margin by which a speedo would over-read as
delivered from the factory).
--
Roland Perry
Nightjar <"cpb"@
This is not the forum for discussing tyre mechanics and they are, in any
case, quite complex. However, the steel plys are not on the outside of
the tyre. There is a layer of rubber between them and the road and that
compresses as it passes through the contact area, effectively reducing
the circumference of the outer layer.
Colin Bignell
Roland Perry
on Tue, 25 Jan 2011, "Nightjar <\"cpb\"@"
>>>> Yes, that makes sense but that surely means that the tyre has to scrub
>>>> along the road surface, because for every revolution there is more tyre
>>>> circumference than there is distance travelled...
>>>
>>> No. The difference is taken up in creating the footprint - the bit of
>>> tyre that is in contact with the road. As the rolling radius
>>> decreases, the footprint gets longer and vice versa.
>>
>> Even if the footprint spreads sideways, the centre line of the footprint
>> is the circumference of the tyre. If the difference is enough to change
>> speedo readings significantly (let's say 1mph or 2%) that's about two
>> inches, which is a huge amount of scrub per rotation.
>
>This is not the forum for discussing tyre mechanics and they are, in
>any case, quite complex. However, the steel plys are not on the outside
>of the tyre. There is a layer of rubber between them and the road and
>that compresses as it passes through the contact area, effectively
>reducing the circumference of the outer layer.
I don't want to discuss the tyre mechanics in detail, but there's an
important "sniff test" involved here.
For the speedo reading to vary enough to matter in a legal context (and
I've chosen 2%, or 1mpg at 50mph) the effective circumference of the
tyre would have to alter by about two inches. And two inches seems an
awful lot to "stretch" or "scrub" a car tyre per revolution, given the
strength of the rubber let alone the steel (the latter being about half
an inch from the road surface).
Could you say what percentage difference you think a tyre at (say) 2/3
full inflation would make to the speedo reading, and how much closer to
the road surface you would estimate the "axle" is. Then we can decide if
it's within the expected tolerance the law expects for our speedo.
--
Roland Perry
Geoff Berrow
wrote:
>Even if the footprint spreads sideways, the centre line of the footprint
>is the circumference of the tyre. If the difference is enough to change
>speedo readings significantly (let's say 1mph or 2%) that's about two
>inches, which is a huge amount of scrub per rotation.
If my back of an envelope calculations are correct you could see a 2%
variation between a new tyre and a worn tyre.
--
Geoff Berrow (Put thecat out to email)
It's only Usenet, no one dies.
My opinions, not the committee's, mine.
Simple RFDs www.4theweb.co.uk/rfdmaker
pensive hamster
> In message <X6OdnQji-b5FhKPQnZ2dnUVZ8ludn...@giganews.com>, at 00:35:02
Consider a wheel with a rim diameter of 16 inches, plus a tyre
thickness of say 3 inches (when fully inflated, fitted to the car, and
measured at the point where it bears against the road surface) So the
rolling radius (the distance between the centre of the axle and the
road) is 8+3 = 11 inches. (Normal tyre)
Imagine also that this same tyre is then partially deflated, so that
it is squished down to 2 inches thickness. The rolling radius would
then be 8+2 = 10 inches. (Deflated tyre)
Circumference = 2 x pi x radius. In the above two cases, this works
out at:
Normal tyre circumference = 69.124 inches
Deflated tyre circumference = 62.84 inches
60 mph = 1 mile per minute.
1 mile = 1760 x 36 inches = 63360 inches
The normal tyre must rotate at 63360 divided by 69.124 (its
circumference) rpm (revolutions per minute) to travel at 60 mph. This
works out at 917 rpm.
If the deflated tyre also rotates at 917 rpm (giving the same speedo
reading), then it would travel forward at 917 x 62.84 (its
circumference) = 57624 inches per minute. This is 9% less than 63360
(which is 1 mile in inches).
So if the speedo is accurate for a normal tyre, it over-reads by 9%
when the tyre is deflated by 1". Conversely, if you were to replace
the 16 inch wheels with 18 inchers, the speedo would under-read by
approximately 9% at 60 mph.
That's assuming my above arithmetic is correct, which is quite a big
assumption.
Roland Perry
<0810641d-d621-4072...@i17g2000vbq.googlegroups.com>, at
11:00:06 on Tue, 25 Jan 2011, pensive hamster
<pensive...@hotmail.co.uk> remarked:
I'll give you the benefit of the doubt over the arithmetic, although 1"
deflation (out of 3") seems a bit extreme.
The problem is that, irrespective of where the rolling centre of the
wheel is, the circumference of the tyre stays the same. And one rotation
of the wheel results in exactly (no more, no less) than moving along the
road by that distance. Which is unchanged if the tyre is deflated.
Think of it like a caterpillar track, if that helps. Irrespective of the
position of the 'pulleys' around which the track rotates, for every
rotation of the track the vehicle moves a constant distance forward.
--
Roland Perry
Roland Perry
Tue, 25 Jan 2011, Geoff Berrow <blth...@ckdog.co.uk> remarked:
>>Even if the footprint spreads sideways, the centre line of the footprint
>>is the circumference of the tyre. If the difference is enough to change
>>speedo readings significantly (let's say 1mph or 2%) that's about two
>>inches, which is a huge amount of scrub per rotation.
>
>If my back of an envelope calculations are correct you could see a 2%
>variation between a new tyre and a worn tyre.
Thanks for doing that. So a regular tyre of say 30cm radius loses 0.6cm
thickness of rubber as it wears, sounds about right.
--
Roland Perry
ARWadsworth
The profile of the tyre is altered along with the size of the wheel. See
http://www.kouki.co.uk/utilities/visual-tyre-size-calculator
and compare for a 185/55/15 to a 215/30/18. The difference to the speedo
between the two is smaller than the difference between having say 8mm of
tread and 2mm of tread on the tyre.
I would expect a manufacturer to sell low profile alloys with suitable tyres
to keep the speedo within this sort of tolerance.
A quick look at the manual for my van suggests that I can use 14 to 17 inch
wheels as long as I use the correct profile tyres.
--
Adam
Roland Perry
Tue, 25 Jan 2011, ARWadsworth <adamwa...@blueyonder.co.uk> remarked:
>>> Car tyres commonly have a.rim diameter of 16 or 17 inches, plus the
>>> thickness of the tyre. Some models have an option of larger wheels,
>>> 18 or 19 inches. Apparently they are more bling. Is there a legal
>>> requirement to have the speedo recalcalibrated if larger than
>>> standard wheels are fitted? Strikes me there ought to be. (Trying to
>>> bring this back on-topic here)
>>
>> I have a feeling that most insurance companies would regard this as a
>> "modification" they'd like to hear about (because it says something
>> about the personality of the driver rather than the grip of the
>> tyres). It might well be that a very much larger wheel will also make
>> the speedo illegally under-read (one inch extra on an 18" wheel is
>> approximately 5%, which is the typical margin by which a speedo would
>> over-read as delivered from the factory).
>
>The profile of the tyre is altered along with the size of the wheel. See
>http://www.kouki.co.uk/utilities/visual-tyre-size-calculator
>and compare for a 185/55/15 to a 215/30/18. The difference to the speedo
>between the two is smaller than the difference between having say 8mm of
>tread and 2mm of tread on the tyre.
>
>I would expect a manufacturer to sell low profile alloys with suitable tyres
>to keep the speedo within this sort of tolerance.
>
>A quick look at the manual for my van suggests that I can use 14 to 17 inch
>wheels as long as I use the correct profile tyres.
OK, so the "wheel size" thing is all a red herring. I had wondered how
people got the larger wheel/tyres to fit in the wheel arches, but you
have explained how (most of the time) this is managed.
(I did once have a car where the slightly larger wheel/tyre combination
kept rubbing on the bolts holding the front mudflaps on, which was a
long term irritation that I never found a good solution for.)
--
Roland Perry
pensive hamster
> 11:00:06 on Tue, 25 Jan 2011, pensive hamster
As regards the caterpillar track, the 'pulleys' rotate faster than the
track. And you cannot calculate the circumference of the track from
the radius of the pulleys.
In the case of the car tyre, the tyre is not perfectly circular,
because it is flattened slightly where it meets the road. Therefore
its circumference is somewhat analogous to the circumference of the
much less circular caterpillar track. And the 'rolling radius' of the
tyre is somewhat analogous to the radius of the 'pulley' of the
caterpillar track.
Also, while the circumference of the tyre stays the same, its shape
does not. If you consider a particular point on the circumference of
the tyre, as well as rotating, it moves in and out relative to the
centre of the wheel (because the tyre is flattened slightly where it
meets the road).Looking at an offside wheel, it rotates clockwise as
the car moves forward. Between about 4 o'clock and 5 o'clock, your
chosen point will be moving horizontally backwards relative to the
centre of the wheel, as it is compressed due to the road pressure.
And again between about 7 o'clock and 8 o'clock, your chosen point
will be moving horizontally backwards relative to the centre of the
wheel, as it expands to its full radius after having been flattened by
the road pressure. So every rotation, your chosen point moves a little
distance horizontally backwards relative to the centre of the wheel.
These little backward distances add up to account for the difference
between the non-circular measured circumferance of the tyre, and the
circular rolling circumferance, or the distance your chosen point
moves along the road surface per revolution. It is not necessary to
posit tyre scrub to account for the difference.
(It is also true that your chosen point moves forward horizontally
relative to the centre of the wheel as it rotates between 9 o'clock
and 3 o'clock, but this is exactly cancelled out by its moving
horizontally backwards relative to the centre of the wheel as it
rotates between 3 o'clock and 9 o'clock)
I think the above is correct, as far as I can work out. I am sure
there must be a clearer way of explaining it, but I can't think of one
at the moment. Or I could just be talking nonsense.
Would it be appropriate to allocate this matter to the Fast Track ...?
Geoff Berrow
wrote:
>Think of it like a caterpillar track, if that helps. Irrespective of the
>position of the 'pulleys' around which the track rotates, for every
>rotation of the track the vehicle moves a constant distance forward.
They are not the same. Each revolution of the wheel takes the entire
circumference of the tyre with it. That's not the same with track
laying vehicles.
Roland Perry
Tue, 25 Jan 2011, Geoff Berrow <blth...@ckdog.co.uk> remarked:
>>Think of it like a caterpillar track, if that helps. Irrespective of the
>>position of the 'pulleys' around which the track rotates, for every
>>rotation of the track the vehicle moves a constant distance forward.
>
>They are not the same. Each revolution of the wheel takes the entire
>circumference of the tyre with it. That's not the same with track
>laying vehicles.
Each revolution of the track propels the vehicle the same distance
forward. Just as every revolution of the tyre does. In both cases,
there's no slippage at the point of contact with the road.
Ignore the wheels (/pulleys etc) as they are clearly confusing the
situation (apart from the fact that a car wheel rotates exactly once for
each circumference of the tyre, and there's a similar fixed constant
between the pulleys and the caterpillar track).
--
Roland Perry
Roland Perry
<ce7fe72c-6ae5-431c...@i22g2000prd.googlegroups.com>, at
13:25:02 on Tue, 25 Jan 2011, pensive hamster
<pensive...@hotmail.co.uk> remarked:
>As regards the caterpillar track, the 'pulleys' rotate faster than the
>track.
The ones I'm thinking of have teeth, and rotate exactly "with" the
track. If you attached a speedo detector to any of the pulleys, it would
give an accurate indication of the speed of the track (and as the track
is in contact with the ground, that's the same as the speed of the
vehicle).
>And you cannot calculate the circumference of the track from
>the radius of the pulleys.
But I'm not trying to!
--
Roland Perry
Geoff Berrow
wrote:
>>They are not the same. Each revolution of the wheel takes the entire
>>circumference of the tyre with it. That's not the same with track
>>laying vehicles.
>
>Each revolution of the track propels the vehicle the same distance
>forward. Just as every revolution of the tyre does. In both cases,
>there's no slippage at the point of contact with the road.
That's true for inflated tyres.
>
>Ignore the wheels (/pulleys etc) as they are clearly confusing the
>situation (apart from the fact that a car wheel rotates exactly once for
>each circumference of the tyre, and there's a similar fixed constant
>between the pulleys and the caterpillar track).
You can't ignore it. To be analogous the entire track would have to
go around the driven wheel each rotation. That clearly doesn't
happen.
Driving on a flat tyre is like driving with your brakes on. That
rolling resistance has to come from somewhere.
m...@privacy.net
pensive hamster <pensive...@hotmail.co.uk> wrote:
> Car tyres commonly have a.rim diameter of 16 or 17 inches, plus the
> thickness of the tyre. Some models have an option of larger wheels, 18
> or 19 inches. Apparently they are more bling. Is there a legal
> requirement to have the speedo recalcalibrated if larger than standard
> wheels are fitted? Strikes me there ought to be. (Trying to bring this
> back on-topic here)
Larger wheels are generally used with lower profile tyres, the overall
diameter of the tyre remaining constant.
--
BD
Change lycos to yahoo to reply
Nightjar <"cpb"@
That is virtually impossible to give a generic answer to, as it depends
not only upon the size of the tyre, but things like the aspect ratio and
the load but, even then, neither the hanging radius nor the rolling
radius will accurately predict the distance travelled per revolution.
The text book answer is that a properly inflated 'average' radial ply
tyre will cover about 97-98% of the length of the hanging circumference
per revolution, while the rolling circumference may be as low as 94% of
the hanging circumference. For anything more specific, it would be
necessary to consult the manufacturer's specification for an individual
make, model and size of tyre and that is unlikely to include data about
improperly inflated tyres.
Colin Bignell
Roland Perry
>>forward. Just as every revolution of the tyre does. In both cases,
>>there's no slippage at the point of contact with the road.
>
>That's true for inflated tyres.
Tyres that are below the recommended pressure are still inflated. Is
there some opposite effect if the tyre is over-inflated? (ie rather than
causing drag on the car, it propels it?) That offends my sniff test
again.
--
Roland Perry