That assumes the instrument can be set to such a small diameter, but if you know the cycle wheel diameter it is intended for you could design gearing from axle to speedometer. Since the power transmitted is very low, a friction-drive using a hard rubber roller might give the initial coupling to the railway-wheel tyre.

However there is another way, used by a member of my own club on the driving-trolley he built: a type based on satellite systems.

Since these devices are mea nt for using over miles of raod I don't know how accurate they would be in the relatively confined area of most miniature-railways, but they obviate worrying about wheel diameters so can be moved from one driving-trolley to another, or say, to a traction-engine.

It might be worth investigating. Do you know a cyclist using one who'd be happy to lend it you to try? You could calibrate it against the length of track if that is known.

I bought a cheap unit on the well known auction site recently gambling that it would do exactly what you say; it did. My wheels are 4" and it just asks you to put in the circumference in mm.

If it is not too ‘disruptive’ for the driver if the wheel size is too small to be accommodated, a speed reading of x10 could be divided by ten for the true speed?

I'm probably wrong, but I thought that GPS speedos worked by measuring the time between map location points, which are, of course, dependant on the map resolution. I understand that non military GPS resolution is 25m, so you will get false speeds if you don't travel in straight lines between resolution points.

Gary makes a very good point …

The basic issues are nicely summarised here: **LINK**

https://www.gps.gov/systems/gps/performance/accuracy/

The timekeeping of the GPS system is amazing … but its location resolution [with a single receiver] is clearly inadequate for ‘speedometer’ use over small distances.

GPS Surveying Instruments do things differently.

MichaelG.

Not quite the same market sector as using an Android App though, is it, John ?

That’s an honest question … I would be delighted to be proved wrong.

MichaelG.

Yes it can indeed be done.

Although I never submitted it, (couldn't see any reason for it as someone else had done more or less the same thing), I found that a cheap cycle speedo could indeed be set such that it assumed a wheel diameter of about 4 inches, and as such it showed the lathe speed more or less correctly. There were a few idiosynchronacies (have a spelt that correctly?) in that the least significant digit appeared as, I think, a small character (usually a decimal speed at normal cycle speeds) and at high speeds, it seemed to go a bit awry. But it worked. For calibration checking, I used the (free running) lathe which has nine fixed speeds. From that, I was then able to get some indication of the speed of my milling machine which has an electronic controller but no speed display.

Also, someone did recommend another cycle speedo which actually had an rpm range built in. I have the device, but never used it, indeed, I think it is still in its original packaging.

Cheers,

Peter G. Shaw

I have very little knowledge of, and no particular interest in, miniature railways [so please forgive me if that is the wrong terminology], but I am interested in the general problem of measuring relatively low speeds reasonably accurately.

So … Would I be right in assuming that the speed-range for such miniature railways would be in the region of zero to 15mph [which equates to 22 feet per second], and that the display would preferably update sufficiently rapidly for the driver to perceive it as being ‘real-time’ ?

Just trying to scope the problem.

MichaelG.

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Posted before I saw John’s latest … but hopefully still a relevant question

Edited By Michael Gilligan on 03/11/2021 11:59:35

I use a GPS speedo reading to 1dp bought off eBay and it works quite well on our track which has an overall speed limit of 5 mph and an ungated crossing over a car park where the limit is 2 mph.

Updating can be a bit slow, but you know whether you are heading for 5 and it has arrows to show whether the speed is dropping or increasing which is useful as well.

It is surprisingly difficult to keep down to the slower speed from memory/experience alone so the device is a very useful assistance.

The cycle speedometers with which I have been associated require the wheel diameter to be keyed in, but being for cycles, the range may well be limited, possibly only covering the normal sizes of cycle wheel, so 4" may be an impossibility.

The particular instrument that you have in mind needs to be investigate, to check that it can accommodate 100 mm wheels.

Howard

Most bicycle electronic speedos I have used allow either the exact wheel diameter or circumference to be entered in order to generate an accurate speed reading. As it is a simple mathematical function there should be no problem if the value entered is significantly smaller than a bicycle wheel.