What is correct, Speedo or sat nav?

epninety

I haven't checked C&U, but the IVA Testers handbook (Revision 1, May 2009) says :-

"Note 1: The speedometer must be of a type suitable for M1 vehicle use. The types manufactured for bicycles, racing only, those that rely on GPS or those that require switching as a separate function to that of operating the vehicle or those that have a separate power source from the vehicle or where they do not operate as an automatic function when the vehicle is driven are not permitted. Where the device is in question the presenter may provide evidence as to the suitability of the device for road use from the manufacturer of the device."

This is for M1 class vehicles - "passenger vehicles with four or more wheels and not more than eight seats in addition to the driver’s seat."

Strider590

The only way to accurately measure speed inside a vehicle is to use optical sensors to pick up set distance markers on the road surface.... However these markers don't exist in the real world!

You need a fixed point outside the vehicle to measure speed accurately....

Measuring rotations of the drive shaft is inherently inaccurate because it's dependant on too many variable, wheel/tyre diameter (and therefore tyre wear is a factor), level of grip, weight of the car (affects actual wheel/tyre diameter).

Sat-navs use some 24 GPS satellites which are in orbit around the planet, these satellites are the closest thing you can get to a fixed point.
They move around the earth in formation and at a known speed of 3.87km/s (2.405 miles per second OR 8657mph), they'd fall from the sky if they didn't!
So using this fixed speed as a constant and knowing the distance between satellites you can calculate exact speed as though the satellites were a stationary object.

Science done, back to work

jase1

Strider590 wrote: »

(they'd fall from the sky if they didn't!).

Or rise/fall into a different orbit, or leave orbit and disappear off into deep space, depending on the speed in question.

adouglasmhor

Cheers eppinety that clears it up a bit for me. So as long as it comes on when you start up, do not have to switch it on and it does not rely solely on GPS it is ok for cars. I can't see it being much different for motorcycles. So a gyroscopic + GPS one is still legal and my radar one was legal* nothing about being "linked" whatever that was supposed to mean, unless it was that it turns on with the ignition.

*It scanned a point on my front brake disc.

birkee

cyclonebri1 wrote: »

The point I was making is that a satnav is an acceptable method of speed indication so a working speedo then wouldn't be an issue, they don't even check them at the annual MOT even though they use the mileage figure derived from it. (seems very strange that, obviously not seen as a safety issue as is everything else checked during the test).

Also I did have a one of car pass it's SVA, single vehicle approval as it was a few year ago, guess what it had a satnav speedo


The point I felt you'd missed is that speeding is speeding, end of story, the feds/plod/old bill or whatever you want to call them don't give a monkeys how you measure speed, as long as you/we do.

The point you missed, was that I KNOW speeding is speeding.
You also missed the point that a speed indicator must meet certain criteria as required by law. Whether they are checked or not, doesn't negate that law. They clearly should be tested, to pass the MOT. Also, it must be a speed INDICATOR, not an overall speed calculator as in a satnav.

Lum

epninety wrote: »

I haven't checked C&U, but the IVA Testers handbook (Revision 1, May 2009) says :-

"Note 1: The speedometer must be of a type suitable for M1 vehicle use. The types manufactured for bicycles, racing only, those that rely on GPS or those that require switching as a separate function to that of operating the vehicle or those that have a separate power source from the vehicle or where they do not operate as an automatic function when the vehicle is driven are not permitted. Where the device is in question the presenter may provide evidence as to the suitability of the device for road use from the manufacturer of the device."

This is for M1 class vehicles - "passenger vehicles with four or more wheels and not more than eight seats in addition to the driver’s seat."

Thank you. That was the exact regulation I was on about when deciding how to approach the problem of getting an MPH display on my car which only had KMH on it's speedo.

As it happened, it was over 10 years old so didn't need to go through IVA, and thanks to a !!!! up by the MOT tester (reported the mileage as 28,000 miles not 28,000 km) the DVLA never asked for proof of conversion and I was able to do it after registration using this thing which is a legal way of displaying your speed.

It would also seem from that, that different regulations apply to bikes.

Strider590

jase1 wrote: »

Or rise/fall into a different orbit, or leave orbit and disappear off into deep space, depending on the speed in question.

Well the faster they travel compared to earth, the lower the orbit, but it's only our perception of speed relative to us. A satellite in higher orbit doesn't over take a satellite in low orbit, it's position in the sky is the same but it simply travels faster to keep up (relative to earth).

Either way, factoring it's speed into a long an complex calculation it can be effectively used as a stationary reference point.

Like me driving a long at an exact 60mph (as measure by an external device like a speed gun) and you driving along (with no speedometer) behind me, measuring the distance between vehicles versus time. If im doing 60 and your falling behind at a rate of 2.23 meters per second, then you can calculate your speed as being 55mph

The same calculation for a network of satellites travelling at 8657mph is not far different....

adouglasmhor

From the Legislation.

19 Speed-ometers 1. The vehicle shall be fitted with a speedometer capable of indicating speed in mph at uniform intervals not exceeding 20 mph at all speeds up to the maximum speed of the vehicle and capable of being read by the driver at all times of the day or night.
2. For all true speeds up to the design speed of the vehicle, the true speed shall not exceed the indicated speed.
3. For all true speeds of between 25 mph and 70 mph (or the maximum speed if lower), the difference between the indicated speed and the true speed shall not exceed—
V/10 + 6.25 mph
where V = the true speed of the vehicle in mph. In this item “maximum speed” has the meaning given in regulation 3(2) of the Construction and Use Regulations. This item shall not apply to vehicles having a maximum speed of less than 25 mph.
http://www.legislation.gov.uk/uksi/2001/25/schedule/3/made

Lum

That excludes GPS simply by the fact that when you are accelerating, the lag before the speed updates means there are times when your true speed is higher than the indicated speed. Also the requirement for being capable of being read at all times of day and night excludes GPS as it cannot be read while getting an initial fix or in a tunnel

birkee

adouglasmhor wrote: »

Can you please point out where in the regs it says that?
Also how do they define linked, my radar one was attached to the bike and powered by it but like many modern speedometers was not mechanically driven by it. Some read a pulse form a cable others read when the magnet passes the cup sensor on the cable.

Linked to the vehicle means just that. Whatever the link method. Mechanical or electronic detector. The vehicle speed is what determines what the speed readout is on the indicator. Whether it is mechanical or digital.

Turn off all the satellites, and see what speed readings you get.
Satellites are detached from the vehicle, therefore they cannot indicate your speed, particularly (as pointed out earlier) in tunnels and other situations.
Or perhaps, you think you can drive through the Mersey Tunnel at 100mph, because your sat nav isn't reading your speed?