BMW on Motorway??

[FMurtz]

two feet twenty seven inches ???

[Tim+]

Why do you believe that your friend?s speedo was accurate?

A wrong cable can?t change the the accuracy but it?s not unknown for the wrong speedo drive gears to be fitted to the box resulting in under-reading. Happened with my wife?s old Citroen ZX and it had to be recalled.

Tim

[Harry Bloomfield]

NY laid this down on his screen :

I'm not familiar with current practice, but I doubt they use still use magnet/spring speedos, or GPS.

I expect the standard traffic car is equipped with just the same system has our cars have - pulse taken from the ABS of one wheel, then electronics feeding a stepper motor, except their systems will be set to display the true speed to +/- 1 mph.

For calibration check, they used to use a process of checking time with a stopwatch, over a fixed distance at a fixed speed. The tyre wear makes surprisingly little difference to the calibration. They used to do that daily.

I used to have a mate who worked on traffic, he then progressed to the helicopter. So I able to keep in touch with their methods and etc..

[tabbypurr]

It seems unlikely that they'd use a system that depends on tyre inflation, correct tyre size & tread wear for legal cases. Doppler can be far more accurate.

I expect now it's done automatically electronically many times a day.

NT

[The Natural Philosopher]

Yes. MOST cars today seem to use that system,

IIRC my jaguar speedos were spot on the money +- tyre wear.

In fact it went beyond that as well - the temeperature gauge was always once warmed up EXACTLY in the dial ceter even when the warning lights for overheating gearox came on!

I'd question ABS from ONE wheel. More likely a pair to allow for cornering etc.

[Robin]

shows Harry is right. That is, the police use routinely - and courts convict on the basis of evidence from - speedometers.

-- Robin reply-to address is (intended to be) valid

[NY]

I've always thought that timing over a fixed distance is subject to lots of random errors such as the reaction time of the observer to press the button, and his judgment as to the exact instant when he has passed the fixed features such as the white squares painted on the road.

Yes, but are the speedometers measuring wheel rotations as for a conventional speedo (but more accurate), or do they use GPS, or do they use doppler measured off the road surface (the way that a mouse detects movement over a mouse mat or desk)?

I suppose a speedo, re-calibrated fairly frequently to adjust for tyre wear, will be the simplest and most accurate - as long as the spring tension in the analogue meter is constant throughout the range: I'd have thought that was the thing that is least easy to control in the manufacture of any analogue gauge. If the display is digital then as long as the timing crystal doesn't drift over time, you've eliminated that variable.

[Dave Plowman (News)]

Which shows it is possible. Even with ancient technology. Meaning modern car makers design in the permitted tolerance. Giving, apparently, a faster car with does more MPG - and also needs servicing earlier. A win win - for them.

[Dave Plowman (News)]

Bosch engine management does this on lots of makes. The gauge isn't showing true coolant temperature at all. Stays exactly on 'the mark' over a wide range of normal coolant temps. Ideal for those who can't understand the true coolant temperature can vary due to driving conditions. A BMW I had allowed you to read true engine temp via the OBC display - if you knew how to do it.

However, that same BMW had an air to fluid transmission cooler. So no reason the coolant temperature gauge should show the transmission overheating.

[NY]

That's interesting. I have always been surprised at how consistent the indicated temperature of my car is (once it's got up to temperature): I've never ever seen the needle go above the vertical half-way position, even when crawling along in a queue of traffic after previously driving at motorway speed, when previous cars have shown the temperature go up a bit at first because there's no longer the car's forward-motion draught over the radiator to supplement the action of the fan in all dumping the heat from the hard-working engine. Nor does the temperature rise when the engine has to work hard climbing a long hill.

I'd put it down to a very responsive thermostat and radiator cooling that was more than enough to keep the coolant down to temperature even in extreme circumstances.

Maybe the needle is telling porkies... Hopefully it *would* rise into the danger zone if the coolant really *did* get hot - eg if a hose burst - otherwise it's not a lot of use in indicating a fault. And I know very well what happens if a car is run for a long time with no coolant: my sister trashed the engine of my mum's car when there was a coolant leak. In that case, she didn't notice that the temperature had gone into the red zone because the gauge and its warning light were (for some bizarre reason) down by the gear lever rather than being on the dashboard with the other gauges and with the warning lights. That was a Renault 14, in case you are wondering which car had its temperature gauge in such a stupid place.

[Harry Bloomfield]

on 15/02/2019, NY supposed :

Back then, what better method could they use? A fraction of a second late or early pressing the button, would not make that much difference to the check over a 1/4 or 1/2 mile, but they used to allow a bit of leeway anyway - making sure you were well over the limit.

[Harry Bloomfield]

It happens that The Natural Philosopher formulated :

Again, the error is small enough to be disregarded, were it not, your ABS would be constantly triggering.

[Harry Bloomfield]

on 15/02/2019, Dave Plowman (News) supposed :

Mine does that, it is absolutely useless.. It shoots to dead centre as it warms up and just stays there giving no clues until it has over heated - entirely pointless.

Mine can be accesses via OBD, or via dash diagnostics - a sequence of button presses on the odometer reset button.

[Harry Bloomfield]

No doubt, but it wouldn't give much warning, just a sudden movement up to max after the engine has seized. I doubt it would be much help even if the loss was slow.

That is why they did a little gadget which plugs into the OBD, indicates true temperature and sounds an alarm over a preset temperature.

[Andy Burns]

Mine has reduced the coolant temperature from a gauge on the prior car (no doubt controlled by the computer rather than a direct read of a thermocouple) to 8 LEDs, which very quickly reaches the 90°C mark.

Hidden on an 'extra' page of the trip computer is an oil temperature gauge which takes much longer to reach 90°C, I thought oil would have a lower specific heat capacity than water, so heat up quicker? Or is the circulation of oil within the engine that much lower?

[DJC]

I have a speedo display driven from the OBD2

It has an ajustment for accuracy, by default that is +8%, and very much the same as the car's speedo. Setting it to be ±0 gives a speed reading that's consistent with both a handheld GPS and my phone GPS.

[NY]

I don't know. The car was a Y-suffix so 1983/4. Were OBD ports (and after-market devices to decode information on the OBD) available in those days? Goodness knows what Renault were smoking the day they put the temperature gauge down there. All other cars that I've ever seen, older or newer, have had the temperature gauge (or at least an over-heat light) on the dashboard near the speedo, fuel gauge, oil pressure and dynamo/alternator lights. Even other Renaults had the gauge/light in a visible location.

I see that the Wikipedia article

mentions it: "The placement of the temperature gauge on the transmission tunnel behind the gear-lever, rather than on the instrument panel where it was directly in the driver's field of view, led to incidents of engine damage if the engine overheated and the driver failed to notice."

[2987fr]

yep, and no thermostat either.

[tabbypurr]

No

snipped suitably. Some makes I just wouldn't buy.

NT

[Dave Plowman (News)]

They tend to rise very quickly once the normal range is exceeded. The aux air con fan on mine failed, and it went from the normal reading to near maximum very quickly when idling in traffic.