I've wondered why manufacturers still use wheel rotation to calculate speed, You'd have though that by now they would be using some sort of fancy electrickery to calculate a very accurate speed reading.
Like we need more electronics to go wrong and cost a fortune to repair!!!!!!
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Road Angel and other GPS detectors display the "current" vehicle speed, although it is actually a recent average. It rarely agrees with the speedometer. It tends to settle a few mph slower.
However, from time to time it changes satellite, or has trouble getting a good signal, and can give erroneous results. Such as when it said that I was managing 247 mph - in a car park.
Wheel rotation is simple, reliable, and gives an instantaneous figure. It's degree of inaccuracy is trivial; only scamera ppartnerships think there is a real difference between the indicated and actual figures.
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You'd have though that by now they would be using some sort of fancy electrickery to calculate a very accurate speed reading.
I can remember from Mark Evan's "Car is Born" series on Sky he fitted an electronic speedo where the sensor picked up the signal from the propshaft. Would that be more accurate, or does tyre circumferance still come into play?
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Waiting in the wings is a totaly 100% accurate speed measuring device.
Its laser based, and paints a small section of road surface, bounced back up to provide a "picture" of a section of road. The movement of this road surface picture provide perfectly accurate speed. (there is corrections for the varying road / laser distance built in)
The technology comes from your humble PC optical mouse which works in exactly the same way.
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Didnt this get mentioned a while (maybe years!) ago...doesnt it suffer problems due to getting covered in mud/water/salt and getting a reading fromnon-tarmac surfaces eg snow, ice, mud, gravel.
ABS and stability programs all require wheel sensors, so why complicate the speedo sensor?
StarGazer
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Seem to remember a device used on Driven a few years ago that "scanned" the road surface as the car passed over it and gave a more accurate readout than from the speedo.
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andymc
Vroom, vroom - mmm, doughnuts ...
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Wot propshaft? I dont have a propshaft!
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You will have either one propshaft or two drive shafts (ignoring 4x4 complexities) however taking speed reading from either is still subject to tyre variables.
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Exactly I dont have a propshaft.
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The ability to detect vehicle speed absolutely, rather than relying on wheel rotation will help develop better ABS, ASR, etc.
To take an extreme example, consider a 4x4 vehicle trying to set off on ice. With a current system, once all 4 wheels are turning and accelerating at the same rate, the system can do no more. The car itself may not have budged an inch!
The laser based optical system has been in use for many years during vehicle development, they are used to obtain the vehicle's velocity vector during handling tests.
Note, that the laser based systems can give you the velocity vector, which is more useful information than the vehicle's speed, which is more useful information that the speed of a shaft somewhere in the drivetrain.
Number_Cruncher
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There's more;
If you had two such sensors, you could also obtain the vehicle's yaw velocity. Currently, you can only estimate this by integrating the signals from a yaw accelerometer, with all the low frequency drift that the method brings
All of this new info would allow vehicle dynamics to take quite a leap forward.
Number_Cruncher
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Under EU law, speedos can be up to 10% optimistic, but never the over way round. That's why I chuckle when I see folks in front slamming on their brakes to an indicated 35 before a camera in a 40 zone, not realising that they could safely travel through at an indicated 45 and not get caught. Duh!!!
My friend has an Audi and has always gone through 40 zone cameras at an indicated 50, and has never been flashed.
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Under EU law, speedos can be up to 10% optimistic
Note: "up to"
That's why I chuckle when I see folks in front slamming on their brakes to an indicated 35 before a camera in a 40 zone, not realising that they could safely travel through at an indicated 45 and not get caught. Duh!!!
Correction: not "could", more "might be able to". This is the difference between "is 10% optimistic" and "can be up to 10% optimistic". My experience is that the error is iro 3-4%.
My friend has an Audi and has always gone through 40 zone cameras at an indicated 50, and has never been flashed.
Tick, tick, tick ...
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>> My friend has an Audi and has always gone through 40 >> zone cameras at an indicated 50, and has never been flashed.
Tick, tick, tick ...
hmm let me see speed 50mph -10% = 45mph
Camera set @ 40 mph +10% +2mph = 46mph
You had best hope your mates speedo is exactly 10% fast
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There's more; If you had two such sensors, you could also obtain the vehicle's yaw velocity. Currently, you can only estimate this by integrating the signals from a yaw accelerometer, with all the low frequency drift that the method brings
This would have to be inline with a steering position sensor so as to determine whether lateral differences between the two sensors were due to slippage or steering input.
All of this new info would allow vehicle dynamics to take quite a leap forward.
Agreed.
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>> There's more; >> >> If you had two such sensors, you could also obtain the >> vehicle's yaw velocity. Currently, you can only estimate this by >> integrating the signals from a yaw accelerometer, with all the low >> frequency drift that the method brings >> This would have to be inline with a steering position sensor so as to determine whether lateral differences between the two sensors were due to slippage or steering input.
Perhaps a sensor is all that steering wheels of the future will be connected to, maybe with the addition of some kind of force-feedback motor to fool the dummy behind the wheel that he is connected to the car! The pointing of *all* of the car's wheels will be firmly under the control of a vehicle dynamics ECU.
Number_Cruncher
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"Perhaps a sensor is all that steering wheels of the future will be connected to, maybe with the addition of some kind of force-feedback motor to fool the dummy behind the wheel that he is connected to the car! The pointing of *all* of the car's wheels will be firmly under the control of a vehicle dynamics ECU."
Ah that will be the new car built in Tolouse by Airbus Industries then,
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I put that type of electronic speedo in my kit-car, with a couple of magnets on the propshaft triggering a reed switch. It could have gone on a wheel instead, but it was easier to fit it to the propshaft.
You have to program the speedo with the number of pulses per mile, so you need the distance of one wheel revolution (which will be slightly less than the tyre circumference due to the deformation at the bottom) and the diff ratio to work it out.
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