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Cycle speedometer

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Speedy Builder502/11/2021 21:54:31
2594 forum posts
208 photos

Does anyone know if a cycle speedo would indicate correct speed if set to a wheel diameter of 4". I am thinking of using one to indicate locomotive speed for a driving trolley.

If it couldn't be set for such a small wheel, I suppose I could set a false size and multiply the reading by 10 or some such factor.


Nigel Graham 202/11/2021 22:10:59
2053 forum posts
28 photos

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.

John Haine02/11/2021 22:21:07
4639 forum posts
273 photos

There are GPS cyclometers which clip on the handlebar and will work fine down to walking speed and below.

Jon Lawes02/11/2021 23:03:23
895 forum posts

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.

Paul Lousick03/11/2021 04:43:49
2018 forum posts
712 photos

To check the accuracy of a cycle speedo, download an app to your mobile phone or use a car GPS to compare the speeds.

not done it yet03/11/2021 07:33:38
6744 forum posts
20 photos

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?

John Haine03/11/2021 07:57:49
4639 forum posts
273 photos

Look up GPS Speedometer on the android play store. Free app that shows speed. Lots of surplus android phones about.

Gary Wooding03/11/2021 07:58:22
969 forum posts
254 photos

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.

Speedy Builder503/11/2021 08:04:47
2594 forum posts
208 photos

Thank you, it seems that John L has answered the question in that a very small diameter / circumference can be entered on some Cyclometers.


Michael Gilligan03/11/2021 08:26:35
20108 forum posts
1043 photos

Gary makes a very good point …

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

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.


John Haine03/11/2021 08:31:29
4639 forum posts
273 photos

GPS speed measurement combines position differentiation with doppler measurement. The receiver has to measure doppler anyway to track the signal.


Edited By John Haine on 03/11/2021 08:31:57

Michael Gilligan03/11/2021 08:49:06
20108 forum posts
1043 photos

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.


Bizibilder03/11/2021 09:48:18
120 forum posts
7 photos

I seem to recall that Keith Wilson wrote up using a cycle speedometer for a loco in ME some years ago (Maybe sometime in the 80's or 90's??). He gave all the details and how to calibrate the thing.

Edited By Bizibilder on 03/11/2021 09:48:42

Peter G. Shaw03/11/2021 10:45:10
1413 forum posts
44 photos

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.


Peter G. Shaw

John Haine03/11/2021 11:27:36
4639 forum posts
273 photos

Well I thought I'd do a quick check on GPS on my phone, which is a Pixel 4a. I installed "GPS Speedmometer and Odometer" from COOL NIKS on the Play store and went for a little walk. It gave a correct speed of 0 when standing still and was very quick to respond when I started walking, showing my speed within a few steps. One snag is there's only one significant figure at low speeds. I checked its reading against another app, GPS Status, which agreed as it should but gives another decimal place. My conclusion is that the speedo app works well even at low speeds, though it may depend on the GPS implementation in the phone.

My CatEye GPS cyclometer also behaves well at low speeds. These devices will use standalone GPS chips which often work better than phones.

Michael Gilligan03/11/2021 11:57:13
20108 forum posts
1043 photos

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.



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

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

John Haine03/11/2021 12:06:45
4639 forum posts
273 photos

Assuming that's the case (seems to me to be reasonable) then it would be a similar range as leisure cycling though this would go up to maybe 25.

Speedy Builder503/11/2021 13:51:18
2594 forum posts
208 photos

By my calculation, 15 mph would be about 1.5 revs / second of a 4" wheel. and 15 is quite fast, some tracks are limited to 5mph. I would have thought that that a cyclometer would cope with that (Unless it was relay driven ?).

(1760 x 36) / (4 x 3.142) = revs per hour. Divide that by 360 for revs per second - unless my schoolboy maths have let me down.


Michael Gilligan03/11/2021 15:44:43
20108 forum posts
1043 photos
Posted by Speedy Builder5 on 03/11/2021 13:51:18:


… 15 is quite fast, some tracks are limited to 5mph.


That’s great, Bob … Thanks yes


Nick Clarke 303/11/2021 16:04:25
1397 forum posts
61 photos

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.

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