How to tell how fast a car is going?

This is done routinely with vehicle-mounted police RADAR systems (obviously using RADAR and not ultrasonics).

I believe you can get cheap RADAR modules that will do something similar

- they're commonly used in factories for measuring conveyor belt speed etc.

This is supposed to be for low speeds ...say 25-65

GPS?

Rgds, Chris

What moves at a rate proportional to speed? Wheels, and thence the transmission system, up to the clutch (or automatic transmission slush box). So you can sense the speed anywhere in that path. KISS - keep it simple, stupid.

"Rodo" schreef in bericht news:JagJj.8133$zb3.1776@trnddc01...

thing

You could use an accelerometer (ADXL202 for example).

Accuracy would be terrible, would be useful only for first boost of acceleration. Then you would have to stop it once you stopped or reached a constant velocity. The drift is to big.

Perhaps you could modify an optical mouse to focus on the road...

.......or just mount it off the hub and read tyre surface speed.........

Ammiravo la mia cresta allo specchio quando "Robbo" ha detto :

that's like every tacho worked 'till early 90's. The problem is that precision changes while tyres get consumed.

No, no speedometer on any production car works that way. Also this method is insensitive to tire size (due to wear or otherwise) as the surface of the tire will always move at the same speed as the road as long there's no skidding. A big problem with this method is that the 'mouse' will get dirty real fast and the dirt will interfere with the optical measurement.

Every car I know of measures speed (and distance) by measuring the speed of the output shaft of the gearbox. This method is sensitive to tire size, but wear will only increase the reading by a few percent.

Ammiravo la mia cresta allo specchio quando Stef ha detto :

sorry, I don't know why but I read "surface" and thought "diameter", that's how old tachos work. Dyslexia? :D

-- Nuno on zx-6r '04 & CR 250 '98 working in progress... E abbiam montato l'autovelox e fatto multe senza pietà A chi passava sopra i 50 fossero pure i 50 di età

My GPS works pretty well for for me.

w..

Monitor the differential gear noise.

bullshit. they used a signal off the distributor. Do you even know what a tachometer is?

They use a pickup off of the transmission.

Maybe this is a language problem. "Tachometer" in English is a device which measures the output speed of the _engine_ in rpm. "Speedometer" in English is a device which measures the road speed of the _vehicle_. All modern cars use the output rpm's of the transmission (or a divider/ multiple thereof) along with a guess at the tire diameter to drive the speedometer.

Ammiravo la mia cresta allo specchio quando AZ Nomad ha detto :

take it easy.

-- Nuno on zx-6r '04 & CR 250 '98 working in progress... E abbiam montato l'autovelox e fatto multe senza pietà A chi passava sopra i 50 fossero pure i 50 di età

Was this an interview question?

How about installing an accelerometer and integrating to find velocity?

Or GPS? I like the Garmin Nuvi. :)

Derek

The simplest solution is to mark the ground position of the driver's side front wheel each morning before leaving for work/school/etc. The average daily speed would then be the distance between consecutive marks divided by 24.

exceed the speed limit. Somebody will stop you and tell you how fast you were going. Or maybe you can just monitor the police radio and see if they say how fast when they call in your license number.

Then they'll call for backup, and spike strips.

-- John