Those new speed cameras

drofluf

Thanks @Sporky @Kittyfrisk and @BillDL ;

Kittyfrisk

^ I really do hate it that this place never updates a thread while you are reading or typing in it 
@Sporky quick draw strikes again 

BillDL

Here's how the topic of speedometer accuracy was debated in the House of Lords in 2001:

https://hansard.parliament.uk/lords/2001-03-12/debates/e08bd310-b9ce-4441-86ac-7e04348d8bdb/SpeedometerAccuracy

and here's the relevant EEC Directive quoted that refers to manufacturing and testing tolerances:

https://eur-lex.europa.eu/legal-content/en/ALL/?uri=CELEX:31975L0443

(note the air temperatures, etc)

Dominic

Haven't had time to read but a few weeks ago an automotive design engineer was explaining to be about how the amount of wear on a tire makes a change to the way a speedo reads 
apparently 4mm of tread on a brand new tyre causes circumference difference significant enough to be accountable for at least 6% speedo error
I'm not good at physics  so I zoned out a bit and didn't really listen

BillDL

Some of the commercial "trunk drivers" at my work have preferences for specific vehicles while hauling trailers for 9 hours.  It doesn't take them long to realise that new tyres on a vehicle after the tachograph (and connected speedo) was calibrated allows them to go faster than the speedo says they are going.  This is realised quite easily from the length of time it would normally take them to do their 4 hrs 30 min drive.  They can thus avoid tachograph infringements, but they still need to watch out for speed traps.

Kittyfrisk

Dominic said:

Haven't had time to read but a few weeks ago an automotive design engineer was explaining to be about how the amount of wear on a tire makes a change to the way a speedo reads 
apparently 4mm of tread on a brand new tyre causes circumference difference significant enough to be accountable for at least 6% speedo error
I'm not good at physics  so I zoned out a bit and didn't really listen

As does altering wheel rim sizes from OE, or fitting low profile or oversize tyres.
Every alteration has a knock-on effect, even if you aren't intending it or aware of it.
Hours of fun available here & other sites https://www.wheel-size.com/calc/

BillDL

@Kittyfrisk I am going to nominate you as my counsel (and "precog") in the event that I receive a speeding "ticket" and decide to plead Not Guilty.

I just had two new front tyres fitted and my wee car still can't pull the skin off a bowl of custard.

Sporky

Kittyfrisk said:

@Sporky quick draw strikes again 

Pew-pew-pew! 

darthed1981

springhead said:

I’ve been caught twice and done the driver awareness course. They said 10%+2 is a myth, or at least if it ever was a thing it isn’t now. I was doing 36 in a 30 the first time and 34 the second time. Someone on the course was doing 32. 

It did once exist, but not as a rule as in "you can't be nicked for anything less than this" which people took it for.  Speeding is an absolute offense, as I said in my post, you are committing a crime a 1mph over the speed limit and can be nicked for it.

The ACPO guidelines for speed enforcement had a table in them indicating the thresholds at which a fixed penalty notice was the recommended minimum sanction, and when a summons was the suitable response.  These happened to be set at + 10% + 2mph for a fpn.

This was when cameras were everywhere before the funding from government got turned off under David Cameron.

Now, the guidelines are defunct, and a police officer can nick you for committing any crime - including driving 1mph over the speed limit, but then they always could, the ACPO guidelines were just guidelines.

They still however mostly allow you a bit of an allowance, even if it's only 2mph for calibration reasons though that would be very unusual.

Safest way to avoid a ticket remains... don't speed.

Side note, the old rules also included a lot of the popular myths about visibility etc. - now in Northampton I regularly see a speed camera than the local coppers use that is completely unmarked, just a grey van, and they do ANPR as well.

m_c

Dominic said:

Haven't had time to read but a few weeks ago an automotive design engineer was explaining to be about how the amount of wear on a tire makes a change to the way a speedo reads 
apparently 4mm of tread on a brand new tyre causes circumference difference significant enough to be accountable for at least 6% speedo error
I'm not good at physics  so I zoned out a bit and didn't really listen

It doesn't make as big a difference as you'd think.
Lets use a fairly middle sized 235/60R16 tyre as an example (size made up)
16" = 406.4mm
60% of 235 = 141mm
So at 'standard' size, overall diameter is 688.4mm.
8mm is only 1.16% of a variation.

Even taking something like an old fashioned 165/80R13, the overall diameter is 594.2, and 8mm variation is 1.35%

However that is only the theoretical size, and tyre pressures have a far bigger effect on rolling size.

crosstownvamp

m_c said:
Lets use a fairly middle sized 235/60R16 tyre as an example (size made up)
16" = 406.4mm
60% of 235 = 141mm
So at 'standard' size, overall diameter is 688.4mm.
8mm is only 1.16% of a variation.

Even taking something like an old fashioned 165/80R13, the overall diameter is 594.2, and 8mm variation is 1.35%

However that is only the theoretical size, and tyre pressures have a far bigger effect on rolling size.

You need to be looking at the circumference and from that you get distance travelled per revolution. Maybe you are? Working not shown clearly!! And should be mentioned it's maths not physics!

Sporky

crosstownvamp said:

You need to be looking at the circumference and from that you get distance travelled per revolution. Maybe you are? Working not shown clearly!! And should be mentioned it's maths not physics!

As the circumference is the diameter times pi, it makes no difference. The percentage change in the diameter is the same as the percentage change in circumference. 

m_c

crosstownvamp said:

m_c said:
Lets use a fairly middle sized 235/60R16 tyre as an example (size made up)
16" = 406.4mm
60% of 235 = 141mm
So at 'standard' size, overall diameter is 688.4mm.
8mm is only 1.16% of a variation.

Even taking something like an old fashioned 165/80R13, the overall diameter is 594.2, and 8mm variation is 1.35%

However that is only the theoretical size, and tyre pressures have a far bigger effect on rolling size.

You need to be looking at the circumference and from that you get distance travelled per revolution. Maybe you are? Working not shown clearly!! And should be mentioned it's maths not physics!

It doesn't make any difference to the percentage whether you do the calculation on the diameter, or circumference.

crosstownvamp

m_c said:

It doesn't make any difference to the percentage whether you do the calculation on the diameter, or circumference.

Oh.. bother.

p90fool

There's not much in it even in the real world. 
I have a bike which does 62mph at 4,000 rpm in top on a new tyre, which drops to 60mph with a worn out tyre.
So it's noticeable but rarely licence threatening. 

Sporky

For 4mm of wear to make a 6% difference you'd need to have 13.5cm diameter wheels (including tyres) . If my sums are right. I think my sack truck has bigger wheels than that. 

BillDL

Sporky said:

 If my sums are right. I think my sack truck has bigger wheels than that. 

You must have some size of sack !!

skunkwerx

The way I try and view it, is cameras arent a trap or hidden. The road limits are the absolute limits. Stick to them and dont worry about cameras. 

BUT as drivers we all know it aint that easy. So much going on, shitty road signs or none at all. 

Sporky

BillDL said:

Sporky said:

 If my sums are right. I think my sack truck has bigger wheels than that. 

You must have some size of sack !!

I just measured, and the hubs are 12.5cm diameter, the tyres are about 28cm diameter. 

I'm sure you can all calculate the sack size from that! 

mrkb

Sporky said:

BillDL said:

Sporky said:

 If my sums are right. I think my sack truck has bigger wheels than that. 

You must have some size of sack !!

I just measured, and the hubs are 12.5cm diameter, the tyres are about 28cm diameter. 

I'm sure you can all calculate the sack size from that! 

Sporky earlier today