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Author Topic: Curiosity question  (Read 851 times)

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BazaJT

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Curiosity question
« on: 10 February 2018, 21:10:01 »

I've seen/heard much discussion of late about this idea of zero tolerance on speeding and the use of cruise control to make sure of not falling foul of speed cameras etc. Given that speedometers are not truly accurate where does the cruise take its cue from?Or are cruise speeds just a best guess also?Just curious really as to what tells the system you've hit the speed you've selected.Must admit I've never yet used cruise on any car I've driven with it fitted it's just another useless piece of kit I don't particularly want or need.
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Re: Curiosity question
« Reply #1 on: 10 February 2018, 21:17:19 »

As I understand it the cruise control works in conjunction with the speedo.  As the latter is not calibrated the former is also technically inaccurate. As your tyres wear down and your system ages that must have an effect, slight maybe, but still a factor that makes the speedo and cc inaccurate compared say to a police calibrated speed monitoring device.


This, to me, is why there cannot be zero tolerance with speed limits under 10% as of now ;)
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Re: Curiosity question
« Reply #2 on: 10 February 2018, 21:28:26 »

From your last statement I would suggest that you don't understand the purpose of it ::)

But for the first part of your question, all the speed data on a vehicle is provided by the ABS system. The speedometer is the item which is calibrated on police vehicles, or the tachograph/taxi meter on suitably equipped vehicles.

General production vehicles legally have to over read by no more than ten percent, but must over read.

Next time you drive on a motorway,  set your cruise control to 56mph for five minutes and count the number of trucks which pass you... It will be almost every single one, all travelling at 56mph ;)
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Re: Curiosity question
« Reply #3 on: 10 February 2018, 21:33:09 »

As I understand it the cruise control works in conjunction with the speedo.  As the latter is not calibrated the former is also technically inaccurate. As your tyres wear down and your system ages that must have an effect, slight maybe, but still a factor that makes the speedo and cc inaccurate compared say to a police calibrated speed monitoring device.


This, to me, is why there cannot be zero tolerance with speed limits under 10% as of now ;)
Actually the fact that all production speeds are legally obliged to overread, provided your speedo doesn't exceed the posted limit, you can never be caught speeding...

Except by the fluke of physics as displayed in that video posted last week where the chap who never exceeded 72mph yet got pulled for 110 because the police failed to spot the car which the radar had actually picked up.
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Re: Curiosity question
« Reply #4 on: 10 February 2018, 21:36:29 »

I downloaded a GPS speedometer app. At 70mph my car is doing 65-66. I tried it with this supposed 10%+1.5mph =78.5mph. I would need an indicated speed of 84 to be doing that.
So, theoretically, I could sail past all of the cars in lane 2, doing a good 10mph more, and still be ok.
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Re: Curiosity question
« Reply #5 on: 10 February 2018, 22:00:39 »

I downloaded a GPS speedometer app. At 70mph my car is doing 65-66. I tried it with this supposed 10%+1.5mph =78.5mph. I would need an indicated speed of 84 to be doing that.
So, theoretically, I could sail past all of the cars in lane 2, doing a good 10mph more, and still be ok.
Precisely  :y

GPS speeds are more accurate than 10%, typically 2% due to the signal delay ;)
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Re: Curiosity question
« Reply #6 on: 10 February 2018, 22:08:13 »

GPS speeds are only that accurate when you are travelling on level roads, DG; they under-declare if you are on an incline, up or down.

Ron.
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Re: Curiosity question
« Reply #7 on: 10 February 2018, 22:27:23 »

GPS speeds are only that accurate when you are travelling on level roads, DG; they under-declare if you are on an incline, up or down.

Ron.
Straight and level. Depending how often your GPS updates it's location it might make a major underestimate of your speed if you are travelling along a winding road.
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Re: Curiosity question
« Reply #8 on: 10 February 2018, 22:32:44 »

Sorry Andy - I meant to mention wiggles as well!
It just emphasises the fact that zero tolerance policing of such an arbitrary thing as a posted speed limit is a crackpot idea, fraught with hazards for the motorist and general public alike.
The only winner will be the government's stealth tax department with its greed cameras.  >:(

Ron.
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Re: Curiosity question
« Reply #9 on: 10 February 2018, 22:53:23 »

Sorry Andy - I meant to mention wiggles as well!
It just emphasises the fact that zero tolerance policing of such an arbitrary thing as a posted speed limit is a crackpot idea, fraught with hazards for the motorist and general public alike.
The only winner will be the government's stealth tax department with its greed cameras.  >:(

Ron.
Understood re the GPS specifics... Rest of your post is, respectfully, nonsense.

If you don't exceed the posted speed according to your speedo, then you will never be speeding. Simples.
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Re: Curiosity question
« Reply #10 on: 10 February 2018, 22:55:30 »

Is that "respectfully" in the "Yes, Minister" sense ot the term, DG?

Ron.
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Re: Curiosity question
« Reply #11 on: 11 February 2018, 07:11:17 »

GPS speeds are only that accurate when you are travelling on level roads, DG; they under-declare if you are on an incline, up or down.

Ron.
Straight and level. Depending how often your GPS updates it's location it might make a major underestimate of your speed if you are travelling along a winding road.
Straight and level? Like the motorway I did it on then.
Uphill and downhill would make a very small difference, but not enough to make the results void.
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Re: Curiosity question
« Reply #12 on: 11 February 2018, 07:15:51 »

Is that "respectfully" in the "Yes, Minister" sense ot the term, DG?

Ron.
If it helps :P
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Re: Curiosity question
« Reply #13 on: 11 February 2018, 08:40:28 »

You are perfectly correct DG I do not understand the purpose of it[assuming you aimed that bit at me]This then begs the question what exactly is the purpose of it?
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Re: Curiosity question
« Reply #14 on: 11 February 2018, 09:45:29 »

Older cars were 10% tolerance on the speedo, I think newer ones have to be something like 2%, but in all cases, overread.

This is why zero tolerance is possible, and has been around for 15+ years - its nothing new.


As DG says, on most ABS equipped cars, the speedo signal usually comes from the ABS ECU.  Non ABS cars, its often from a sensor on the gearbox.
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Re: Curiosity question
« Reply #15 on: 11 February 2018, 10:57:48 »

As I understand it the cruise control works in conjunction with the speedo.  As the latter is not calibrated the former is also technically inaccurate. As your tyres wear down and your system ages that must have an effect, slight maybe, but still a factor that makes the speedo and cc inaccurate compared say to a police calibrated speed monitoring device.


This, to me, is why there cannot be zero tolerance with speed limits under 10% as of now ;)
Actually the fact that all production speeds are legally obliged to overread, provided your speedo doesn't exceed the posted limit, you can never be caught speeding...

Except by the fluke of physics as displayed in that video posted last week where the chap who never exceeded 72mph yet got pulled for 110 because the police failed to spot the car which the radar had actually picked up.

Thanks DG.  I didn't know that and that is useful information :y :y
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Re: Curiosity question
« Reply #16 on: 11 February 2018, 22:01:58 »

Cruise control is best suited for open motorways... Proper distance driving... ie running up to Scotland late at night, or hacking across the continent ;)

In the UK, people who get the most benefit are lorry its and coach drivers... Whose speed is artificially restricted as it saves holding the accelerator down for four hours.

In a car, it's handy for regulating vehicle speed in restricted roadworks etc. If fitted, I tend to use it alot as it makes the journey smoother. Changes in traffic flow can easily be dealt with by a mere push of a button.  :y
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Re: Curiosity question
« Reply #17 on: 11 February 2018, 22:15:32 »

Since my being "flashed" late one night in a 30 zone and the subsequent speed awareness course, i use it quite a lot in built up areas. ;)
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Re: Curiosity question
« Reply #18 on: 12 February 2018, 09:13:41 »

Older cars were 10% tolerance on the speedo, I think newer ones have to be something like 2%, but in all cases, overread.

According to the IVA (which is, for the most part, the same as type approval) the tolerance is "(true speed/10) + 6.25" along with a table for those incapable of maths.

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Re: Curiosity question
« Reply #19 on: 12 February 2018, 09:28:18 »

I downloaded a GPS speedometer app. At 70mph my car is doing 65-66. I tried it with this supposed 10%+1.5mph =78.5mph. I would need an indicated speed of 84 to be doing that.
So, theoretically, I could sail past all of the cars in lane 2, doing a good 10mph more, and still be ok.
Precisely  :y

GPS speeds are more accurate than 10%, typically 2% due to the signal delay ;)

Its nothing to do with signal delay, its down the accuracy of the position determined on a moving object which will have typically a 1-2m inaccuracy, GPS often reads slightly low  :y
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Re: Curiosity question
« Reply #20 on: 12 February 2018, 09:28:56 »



Ahh, yes. Used to have most of that manual burned into my brain.

The point being the speedo can over-read , but if it under-reads by any amount at all, you fail.

By the way, I assume you've seen this gem?

https://www.gov.uk/government/consultations/road-vehicles-improving-air-quality-and-safety

Time to get the Cobra finished!
« Last Edit: 12 February 2018, 09:30:29 by Kevin Wood »
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Re: Curiosity question
« Reply #21 on: 12 February 2018, 09:33:40 »

As I understand it the cruise control works in conjunction with the speedo.  As the latter is not calibrated the former is also technically inaccurate. As your tyres wear down and your system ages that must have an effect, slight maybe, but still a factor that makes the speedo and cc inaccurate compared say to a police calibrated speed monitoring device.

Its actually even less high tech, the ABS ecu produces a pulse count on the non drive by wire cars and speed value (its not in MPH or KMH) which goes to the cruise controller (non drive by wire) or the ECU (drive by wire) AND the instruments so the two are independent.

What happens is you activate cruise based on what the speedo is indicating and the ECU/cruise controller simply opens or closes the throttle to match the selected speed value (or adds or reduces fuel if it is a diesel)

So in summary, the cruise system is only inaccurate due to the reading of the speedo, add a more accurate speed indication and the cruise will naturally be more accurate.
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Re: Curiosity question
« Reply #22 on: 12 February 2018, 11:41:51 »

By the way, I assume you've seen this gem?

https://www.gov.uk/government/consultations/road-vehicles-improving-air-quality-and-safety

Time to get the Cobra finished!

Yeah, reading up it seems that it's likely to come into force on 01/01/19, so if it hasn't been IVA'd by then I'll be bollarded (well, maybe not, I know someone who regularly gets carb'd FEs through an emissions test with CATs in place)..
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Re: Curiosity question
« Reply #23 on: 12 February 2018, 11:44:08 »

Yes, I'm sure there are ways and means, but it's a ballache that you don't need. >:(
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Re: Curiosity question
« Reply #24 on: 12 February 2018, 11:52:48 »

Indeed! Seems some (or most) of the manufacturers aren't that bothered either with their responses being "fit a modern engine like an LS/ecoBoost/etc*" *delete as applicable depending on if you're talking to a Cobra replica manufacturer or 7-clone manufacturer..
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Re: Curiosity question
« Reply #25 on: 12 February 2018, 12:01:32 »

Speaking of GPS accuracy Mrs Opti bought me one of these to play with a few years ago. Cost around £450 if memory serves.

Extremely accurate. :y 



http://racelogic.co.uk/_downloads/Techical_Specs/PERFORMANCE%20BOX%20-%20Technical%20Specs.pdf
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Re: Curiosity question
« Reply #26 on: 12 February 2018, 13:17:59 »

Which Mrs Opti ?  ??? :P ;D
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Re: Curiosity question
« Reply #27 on: 12 February 2018, 13:18:25 »

That is actually pretty inaccurate from a position perspective, the clever bit is the G-meters  :y

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Re: Curiosity question
« Reply #28 on: 12 February 2018, 13:20:34 »

Indeed! Seems some (or most) of the manufacturers aren't that bothered either with their responses being "fit a modern engine like an LS/ecoBoost/etc*" *delete as applicable depending on if you're talking to a Cobra replica manufacturer or 7-clone manufacturer..

So they've dropped this neatly into the "too difficult to fix" box then  ::)
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Re: Curiosity question
« Reply #29 on: 12 February 2018, 13:26:48 »

Which Mrs Opti ?  ??? :P ;D

Let me think on that. Number 2 , I think. :)
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Re: Curiosity question
« Reply #30 on: 12 February 2018, 14:39:26 »

Indeed! Seems some (or most) of the manufacturers aren't that bothered either with their responses being "fit a modern engine like an LS/ecoBoost/etc*" *delete as applicable depending on if you're talking to a Cobra replica manufacturer or 7-clone manufacturer..

So they've dropped this neatly into the "too difficult to fix" box then  ::)

.. or the "We can sell you a crated LS for just £xxxxx" box. ;D

I think it'll hit some kit car manufacturers hard - especially those making replica kits that need air cooled engines to be anything close to authentic, for example, and those who tend to attract lower budget builders whose engines turn up in a pile of cars at a scrappy not on a pallet from Ford.

Remember also that it effectively ratchets up the emissions standard with each change in MOT emissions - so when that requires a GDI engine and the associated management where will we be then?
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Re: Curiosity question
« Reply #31 on: 12 February 2018, 16:00:48 »

Yup. I suspect Pilgrim will suffer off the back of this, a crated LS is probably more costly than the entirety of my build!!

Somehow I can't imagine a 1970's fiat twincam passing modern emissions, CAT or no CAT!  ::)
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Re: Curiosity question
« Reply #32 on: 12 February 2018, 17:23:17 »

On injection it might, on Webers / Dellortos... not a prayer!  ;D

Injection is probably beyond the means / understanding of a lot of kit car builders, though.
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Re: Curiosity question
« Reply #33 on: 12 February 2018, 17:45:53 »

Yup. I suspect Pilgrim will suffer off the back of this, a crated LS is probably more costly than the entirety of my build!!

Somehow I can't imagine a 1970's fiat twincam passing modern emissions, CAT or no CAT!  ::)


Would it have to? At that age, isn't it MOT exempt?  :y

Ron.
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Re: Curiosity question
« Reply #34 on: 12 February 2018, 19:40:23 »

True, but sadly the kit car it’s in was registered on 1997 :)
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