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DIY Speedo calibration apparatus.


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Hello All,

 

First a warning:

If you try this, and completely ruin your speedo, please don’t blame me. Anything you do is entirely at your own risk.

 

Last year I made a “Quick and dirty” rig out of bits of junk I had lying around (You can probably tell that by looking at the picture.)

Just a variable speed drill (Set to “Unscrew”), a cheap digital rev counter, a short bit of old speedo cable and some bits of wood and metal.

You can alter the speed of the drill with a hose clip and you attach the digital revcounter to the base with a bit of double sided tape.

 

I won’t go into details of how a speedo works, because there are plenty of sites on the Internet that can explain it better than I can. All you really need to know is that the speedo cable causes a bar magnet to spin around inside an aluminium cup. The cup then starts to turn due to the magnetism, but is restrained by a hairspring that tries to hold it back. The speedo needle is attached to the aluminium cup.

The stronger the magnet, the more the needle moves for a certain speed of the speedo cable.

To calibrate the speedo you need to alter the strength of the magnet. Professional calibration machines do this in various ways by having an electromagnet placed around the speedo mechanism. My machine does it by wafting a magnetic “Magic wand” in the direction of the speedo bar magnet. (Just a strong magnet attached to a wooden stick)

To be totally accurate the strength of the hairspring has to be altered, but I don’t think that this is done by the professionals, as it is a bit of a faf to change it. (I may be wrong though.)

As the speedo does not have to be 100% accurate by law, this method will work OK.

Any inaccuracies that you find will probably only show up in a non-linier range of readings. I.e. if you set it right for 60 mph it may well be slightly wrong above and below that reading.

I found that by wafting the magnet (magic wand style, along with the words “Izzy Wizzy let’s get busy”) close to the rotating magnet I could get the reading on the speedo to vary considerably. It was just a matter of stopping at the right point when the speedo read the correct MPH. (Just trial and error really. Sometimes I turned the “Magic wand” magnet around, so that North and South were reversed, but I had no real calculated plan of action.)

Eventually a road test of the speedo comparing it to a satnav proved OK.

Will the altered magnetism last? I don’t know, but I don’t see any reason why it would fade away quickly.

 

Note: This alters the needle reading only. It does not do anything to the odometer accuracy. To do that you need to alter the gearing in the odometer. When you do that you may well drop some or all of the small pieces on the floor, never to be seen again. Far easier to work out the true mileage using a pencil and paper.

--------------------

Here are some calculations I used to work out what the speedo should read for various drill RPM

 

Back axle  4.1:1.  (4.1 turns of propshaft = 1 turn of back wheels)

Gearbox 2.5:1   (2.5 turn of propshaft = 1 turn of speedo drive cable into of gearbox.)

Rolling circumference of a 165/80 tyre 79.72ins (12.69 radius)

 

1 mile = 63360 ins

 

Therefore it takes 63360/79.72 =794.78 turns of the rear wheel to go one mile.

Therefore it takes 794.78*4.1 = 3258.6 turns of the propshaft to go one mile

Therefore it takes 3258.6/2.5 = 1303.44 turns of the speedo cable to go one mile. (Also referred to and printed on the speedo face as TPM)

 

At a speed of 1 mph the speedo cable will turn 1303.44 turns in an hour.

Therefore at 1 mph the speedo cable will turn 1303.44/60= 21.72 turns in a minute

 

Therefore with a drill speed of 1000RPM you should see a reading of 1000/21.72 = 46.04 MPH on the speedo.

 

(I did all these calculations last year. Please feel free to tell me if they are wrong.&

 

And yes, you can get it done professionally for about £70.

But where is the fun in that?

 

Charlie

 

 

SpeedoRig.jpg

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Charlie.

I did something similar, but use my lathe as the driving force. 1st I measured the chuck speeds in all the possible belt positions. I did this using the odometer of my speedo, knowing the turns per mile, by zeroing the trip, then running the lathe for a set period of time, then calculating how many turns it had done from the mileage accrued. So turns divided by time - RPM.

Moving the end stop of the hair spring does not alter the calibration, it just moves the rest position of the disk. As you say altering the strength of the spring (very difficult) would alter the calibration. Like you I have altered the strength of the magnet, but by stroking it with a stronger one to increase (make speedo read faster), or, to reduce the magnetism I use a de-gausing coil which I bought years ago for de-gausing tape recorder heads. Takes some trial & error to get it right.

Here is a chart I made up for speedos with various different turns per mile

Speedo checking with lathe.pdf

Bob.

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47 minutes ago, Lebro said:

As you say altering the strength of the spring (very difficult) would alter the calibration.

Bob,

I do remember reading somewhere (possibly on this forum) about a bloke who altered the strength of a hairspring by leaving it in a glass of acid for a certain amount of time. I do believe that he came from a family of clock makers, so I guess he had experience of such things!

I once found a catalogue of hairsprings (I think it was by the people who now make Smiths branded gauges.) There were hundreds of different strengths etc.

 

As you have done the mag/demag thing I guess you were happy with the results just doing it that way.

Here is a link to the sort of RPM meter I used.

https://www.ebay.co.uk/itm/Digital-Motor-Speed-Meter-Tachometer-DT-2234C-/274412498836

(Claims to come from Hayes, not China.)

 

Charlie.

Edit: Just found the spring catalogue. Which one takes your fancy?

http://www.britishprecisionsprings.co.uk/html/stock_spring_list.html

 

 

Edited by Charlie D
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Accuracy: ±(0.05%+1 digit)

The percentage figure typically refers to the full scale deflection (FSD) or equivalent in digital, not of the reading displayed. So if a meter has a FSD of 10,000, 0.05% is 5 so a reading of 800 on that scale could in reality be 794 to 806, a potential error of about 0.75%. 20 to 25 times greater than an initial read of the specification.  I do realise that in this application, tyre wear will create a greater source of error.

Regards

Bill 

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Charlie--

Nice setup.

There is typically a number on the face of the speedometer that tells how many revolutions of the input cable will result in one mile on the odometer.  A little thought will reveal that the number will also indicate what RPM of the input cable should result in a 60 MPH indication on the speedo.  On my TR6 speedo, that number is 1120.

My calibration setup was essentially the same as yours.  Varying the strength of the magnet changes the slope of the graph that relates input RPM to indicated MPH.

Ed

SDC12244a.JPG

 

Graph3.jpg

Edited by ed_h
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You know

the technical ability of you guys never ceases to amaze me. 
 

engineering, electronics, bodywork, paint, race set up etc etc

brilliant

I’ll get back to trying to work out what’s a bolt or a screw or a nut on my mecano set. 
 

keep up the good work. 

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Am i right to assume that the same technique would work for re-calibrating a tachometer? I had my speedo professionally calibrated by Speedograph Richield at least 30 years ago and it is still absolutely spot on at all legal speeds according to the wife's satnav. However, I know that my tacho is over-eading since at a steady indicated 30mph in top with the standard 3.7:1 axle it reads 1,650rpm as against a theoretical 1,500.

Tim

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45 minutes ago, tim hunt said:

Am i right to assume that the same technique would work for re-calibrating a tachometer?

Tim

Yes, the mechanisms for the tach and speedometer are essentially identical.

Ed

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Hello Ed,

Yours looks a bit more professional than mine.

I guess you have a better class of “Junk” in your workshop spares box.

 

Bob,

I bet that one of the first things you will do with your new electronic tacho when it arrives is to compare it with your TR tacho.

 

It's like when I got an infra red thermometer.

"New toy syndrome". I went around checking the temperature of everything from the hot water pipes to the cat's ears.

Charlie.

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On 4/5/2021 at 4:01 PM, Lebro said:

Useful looking tacho, have just orderd one !

Bob.

Yes I have one of those digital things and it confers with my Smiths unit.  Use both on my Boxford lathe headstock spindle to check speeds after renewing belts and motor.

 

 

DE083728-BEBE-43BF-B8E6-3E4FD231C4D6.jpeg

Edited by BlueTR3A-5EKT
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