OT: Digital Speedometer

[Harry Bloomfield, Esq.]

It happens that JohnP formulated :

..and what happens if a speed limit sign is hidden by vegetation?

[NY]

Or if the sign has faded to a blank disc, as is te case with several signs near me. All of them are national speed limit signs - in other words, where the speed limit *increases*, where it is not dangerous if the sign gets ignored by a human driver or a camera-reading cruise control. Reduced-speed limit signs are maintained a lot better ;-)

[Clive Arthur]

Mine has a built-in hedge trimmer.

[Harry Bloomfield, Esq.]

Fred laid this down on his screen :

It does seem counter intuitive, but it makes surprisingly little difference. The diameter of the wheel doesn't change much, because the entire diameter of the tread still has to make full contact with the road, for the entire revolution - unless the tyre comes free of the rim.

[Harry Bloomfield, Esq.]

NY was thinking very hard :

Sorry, yes!

Yep!

It is a left over rule from when all speedos were mechanical devices, quite unnecessary now.

My conventional looking speedo uses a stepper motor to move the needle, no springs involved.

[Harry Bloomfield, Esq.]

Dave Liquorice laid this down on his screen :

Which confirms there is some sort of digital processing going on, between wheel and display. Mine is entirely electronic, but over-read by a percentage throughout the range, rather than a fixed subtraction from the speed. I heard that someone hacked into the software, to correct the deliberate error in indicated speed.

[Harry Bloomfield, Esq.]

Fred was thinking very hard :

They can book you for 71 in a 70, if they wish.

[JohnP]

Harry Bloomfield, Esq. [e-mail]> wrote in news:rt2i93$lns$ [e-mail]:

Having one foot "free" eanables you to brace yourself better during braking.

[Dave Plowman (News]

Yes - it was perfectly possible to make a speedo many many years ago that was far more accurate than the tolerance allowed.

At one time, decent road tests (like say Autocar) also tested the speedo. If it were just manufacturing variations, you'd expect some from the same car maker to fall anywhere between the limits. But with many makers, they were always at the upper limit. Which would appear to make the car go faster than it actually did and if the odometer has the same error, apparently better MPG too - and servicing needed more often. A win win win

- for the maker.

[Roger Mills]

Don't forget that a rolling wheel isn't a circle - there is a flat bit in contact with the road. I've heard it claimed - I think quite reasonably - that the distance covered by one revolution is equal to the length of the radial belt, and doesn't vary with tread wear.

[Roger Mills]

I suspect that mine does, too. It, and the digital speed display, both read high *by design".

[Clive Arthur]

Yes, the entire diameter of the tyre has to make contact with the road and the tyre doesn't slip on the rim, but nonetheless a flat tyre will move the wheel along less per rotation than an inflated tyre...

Consider a normally inflated tyre. For every 1 degree of wheel rotation the wheel will move horizontally in proportion to the distance between the ground and the axle. For every complete rotation it will move 360 times that amount.

Now consider a flat tyre where the wheel rim is almost touching the ground. For every 1 degree of wheel rotation the wheel will also move in proportion to the distance between the ground and the axle which is now considerably less, as of course is the distance covered by one complete rotation.

But the tyre perimeter length can't change very much, and the tyre doesn't slip on the rim.

But the tyre does slip and deform on the road. (Tyres must be able to slip or you couldn't turn corners.) Which is why flat tyres wear more quickly than properly inflated tyres and don't give as much grip. The tyre pressure is less so the contact area is bigger, so any particular piece of rubber on the ground has less downward force on it.

[Dave Plowman (News]

If you had things thrashing about in your IC vehicle, perhaps you should have considered maintaining it?

But you may well prefer the whine of an electric motor. Like London black cabs. Not a loud sound, but not a pleasant one either.

[Dave Plowman (News]

If your tyres are so flat they have a noticeable effect on the speedo reading, best not to go fast enough to need a speedo.

[Dave Plowman (News]

Of course that needle is still on a taper like all such things. So does need to be positioned correctly on its shaft - unlike a digital display.

[Dave Plowman (News]

They could try. A court and a half decent lawyer would throw it out.

[Dave Plowman (News]

Does that mean an F1 driver only uses the one foot? ;-)

[Michael Chare]

I check my speedos with a GPS (phone these days) and they have always over read. For the motorway 50mph speed cameras I set my cruise control to 52mph. Very few vehicles want to pass me. My current car always shows the analogue display and the central panel can be set to display the speed digitally in large figures.

[Fred]

Thanks for that, I have been considering a Honda for the next car and am getting more reluctant about them.

The cruise control working even in a traffic jam with the car stopped is interesting but I don?t get in that situation often enough to matter.

Yeah, cant see it myself.

That?s why I prefer the system to use a speed limit database and only override that with the sign when the car can see a speed limit sign or speed limit marked on the road surface itself.

Agreed.

[Fred]

Yep, all modern well designed ones do. You can set the distance to the car in front and it measures that with radar. So you can sit behind a car in front until you decide to overtake and it will let you overtake and then run at the set cruise speed again.