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Speedo
designer - 1/10/08 at 08:15 AM

I know that the speedo is checked in the SVA, but are there any rules on how information for the reading is gathered?

This:

http://www.instructables.com/id/NonTraditional-Speedometer-for-Used--Homebuilt-Ca/

could be an option. But it would be impossible to check on SVA.

Mr Whippy - 1/10/08 at 08:30 AM

soooo you like saying fit a radar gun to your car and use it as a speedo? but that will pick up any oncoming traffic and you'll then you'll be doing 140mph!

if you point it at the ground what’s the beam reflecting off? cats eye's?!

for about £12 you can buy a push bike digital speedo that stores total miles and uses a magnet and reed switch on the front wheel as I have on my push bike


[Edited on 1/10/08 by Mr Whippy]

ravingfool - 1/10/08 at 10:34 AM

quote:
Originally posted by Mr Whippy
soooo you like saying fit a radar gun to your car and use it as a speedo? but that will pick up any oncoming traffic and you'll then you'll be doing 140mph!


No you point the beam down so there would be no interference from traffic etc.

quote:
Originally posted by Mr Whippy
if you point it at the ground what’s the beam reflecting off? cats eye's?!


I thought you were a physics bod? no it would just be reflecting off of the ground, you know, in the same way light does so that we can see the ground!


quote:
Originally posted by Mr Whippy
for about £12 you can buy a push bike digital speedo that stores total miles and uses a magnet and reed switch on the front wheel as I have on my push bike


To be honest, even a bike speedo has got to be preferable to a radar based custom speedo. These aren't small units, but you'd want to mount it completely under the car which would be difficult, and realistically you're either going to mount it somewhere it's going to get hit occasionally and eventually damaged, or it'll get killed by moisture.

It sounds like a very expensive and short lasting toy. Don't know about the legality for SVA though.

pewe - 1/10/08 at 10:34 AM

^^^ What Mr. Whippy said about a bike speedo as you can calibrate it pretty accurately - must be no more than 10% over/under the correct speed.
Also I think it needs to be visible at night - probably there's a bike speedo with illumination as night-time off-roading is becoming popular amongst mountain-bikers (but only those with a death-wish!) or I'm sure you could rig up an LED or similar without too much trouble.
Cheers, Pewe.

Rosco - 1/10/08 at 11:00 AM

I guess the other consideration is the Mr SVA needs to be able to test it against a calibrated standard, which I presume they do on a rolling road, so you're limited to measuring wheel speed rather than road speed.

02GF74 - 1/10/08 at 12:06 PM

quote:
Originally posted by ravingfool
quote:
Originally posted by Mr Whippy
soooo you like saying fit a radar gun to your car and use it as a speedo? but that will pick up any oncoming traffic and you'll then you'll be doing 140mph!


No you point the beam down so there would be no interference from traffic etc.

quote:
Originally posted by Mr Whippy
if you point it at the ground what’s the beam reflecting off? cats eye's?!


I thought you were a physics bod? no it would just be reflecting off of the ground, you know, in the same way light does so that we can see the ground!



You are gonna have to explain how this works.

Radar sends a beam of something and measures time for reflection to come back. Doing this repeatedly on a moving object will result in the time for reflection changing. The difference in time can then be used to determine the relative speed.

So unless the car is changing the distance between itself and the ground, like taking off in an air-plane-like fashion , I fail to see how it is gonna work realiably, if at all.

The radar needs to point at some stationary or very slow moving object in the distance horizontally e.g. tree, large house, stationary traffic, giant tortoise, pedestrian, guide dog etc.

Mr Whippy - 1/10/08 at 12:12 PM

really using a radar gun as a speedo is quite a crazy idea and would never pass an SVA test, full points though to the sales guy for even comming up with such a sales pitch

[Edited on 1/10/08 by Mr Whippy]

ravingfool - 1/10/08 at 01:52 PM

quote:
Originally posted by 02GF74
You are gonna have to explain how this works.

Radar sends a beam of something and measures time for reflection to come back. Doing this repeatedly on a moving object will result in the time for reflection changing. The difference in time can then be used to determine the relative speed.

So unless the car is changing the distance between itself and the ground, like taking off in an air-plane-like fashion , I fail to see how it is gonna work realiably, if at all.

The radar needs to point at some stationary or very slow moving object in the distance horizontally e.g. tree, large house, stationary traffic, giant tortoise, pedestrian, guide dog etc.



OK, what you say about repeated measurements to establish a speed for a moving object is correct.

However you can also measure the speed of an object by firing just one pulse at it, or from it and measuring the returned signal, because of the doppler effect, which is what causes the 'red shift' experienced looking at distant galaxies. With the distant galaxies because they are moving away from us you can imagine that the light waves get stretched, which equates to a lowering in their frequency and a shift towards the red end of the spectrum.

The same effect can be used in any other kind of wave to measure the speed of objects. If the radar is set up to look slightly ahead of it's position the ground will in effect be moving towards the radar gun. The reflected waves will effectively be squashed and there will be a measurable increase in frequency. The difference in emitted and received frequencies will allow calculation of speed.

Whether it is the radar gun or the object being targeted that moves is irrelevant as the same effect is caused.

I hope that helps, also, consider cars driving towards and then away from you. As they rush towards you the sound increases in pitch because the frequency increases as the waves are being compressed. When a car passes you and then drives away the pitch of the sound decreases as the frequency of the waves decreases with the waves being more spread out as the distance between you and car increases. (although it is not the distance that is important, but the fact that the distance is CHANGING)

[Edited on 1/10/08 by ravingfool]

jeffw - 1/10/08 at 02:00 PM

Just to clarify ..this will be a Doppler Shift radar rather than a conventional radar. It doesn't take repeated readings to measure speed.

I don't have time to explain so you'll need to look it up in Wikipedia or similar to see what I mean.

[Edited on 1/10/08 by jeffw]

RK - 1/10/08 at 02:57 PM

I don't know, the bike speedo thing sounds a lot easier and probably a lot cheaper in the long run. Small too, and easy to set up.

I think I should go that route myself, even though I have bought a fancy set of gauges with my own custom faceplates etc. I have no idea where the thing plugs into the car! A few things to sort before I get down to it in earnest.