Speedo gear size

[Dave HalfordParticipant @davehalford22513]

Posted by Speedy Builder5 on 29/09/2023 06:48:18:

Thanks for all the comments and links. From Michael's link, I think I know what the engine is and correct output ratio of the gear box. Need to see if the TPM is on the speedo next.

Pete did you mean 15 / 16 tooth gear ?

And just for clarity, we are in France with a LHD French marketed mini and KPH. As an aside, during its life, the bolts are now a horrible mix of UNF/UNC and metric combinations – spanner night mare !

Bob

I guess you have seen this already Mini Speedo Calculator, how accurate is your speedometer? – Tom's Mini Site (tomsminisite.co.uk)

[martin haysomParticipant @martinhaysom48469]

don't know if this helps, i had the same problem with a ford fix was simple. just changed the gear on the cable. found a calculator on line which told me which gear to use .

[Speedy Builder5Participant @speedybuilder5]

Thanks Dave, problem is knowing exactly what I have got. It's 44 years since the car left the factory and who knows what has been done to it. Certainly the suspension has been tweaked – its hard rally style suspension, front hubs replaced for disc brakes, gearbox is suspiciously clean etc etc. Still its fun finding out.

[Howard LewisParticipant @howardlewis46836]

As an aside, it was standard practice for vehicles fitted with Eaton 2 speed axles to have a small two ratio gearbox inserted into the speedo drive, at the gearbox end, so that when the axle ratio was changed by the driver, a solenoid changed the ratio in the small gearbox.

One ratio provided direct drive to the speedo, whilst the other compensated for the change in diff ratio.

Getting the wiring for shift motor and speedo gear box out of phase produced some interesting speedo readings!

Howard

[Clive SteerParticipant @clivesteer55943]

Hi Speedy Builder5

One thing you might like to consider is that the speedo itself may have been changed. The speedo is often swapped out if it starts to give unsteady reading for one that looks similar but not designed for that car.

I believe the Mini were fitted with a Smiths Nemag speedo. On the speedo dial, either at the bottom or above the Odometer digits, there are the type number and calibration value. The type number will be something like SN618/90 and a calibration number like 1376.

The calibration number represents the input shaft RPM needed for the speedo to indicate 60 mph or the equivalent KPH. You can probably ask a Mini forum which type should be fitted.

A Nemag speedo has two separate mechanisms that need to be correct. The pointer and the Odometer. The Odometer is corrected by changing a worm gear as was shown in the link. However the pointer requires a hairspring change which is slightly more complicated.

For the most knowledgeable advice I would recommend contacting Mike Flannery of Flannery Speedometer Repairs.

CS

[Dave HalfordParticipant @davehalford22513]

Posted by Speedy Builder5 on 29/09/2023 21:17:07:

Thanks Dave, problem is knowing exactly what I have got. It's 44 years since the car left the factory and who knows what has been done to it. Certainly the suspension has been tweaked – its hard rally style suspension, front hubs replaced for disc brakes, gearbox is suspiciously clean etc etc. Still its fun finding out.

Look at the site I linked to

If you input your current wheel size.

Then do as I suggested to get your final drive ratio and input that.

Then try each gear set in turn on the drop down and see if you get a speedo match in the results.

If you are still miles off, try a different wheel size etc etc

Hopefully your speedo will be a match to the rsult

If nothing matches you have something really odd

[Speedy Builder5Participant @speedybuilder5]

Thanks Clive and Dave.

The speedo does have a serial number and is marked 800 and as its in KPH would be 800 Turns / Km.

The wheels are off the car waiting for brake cylinders, so I will have to wait before counting wheel revs over 100 meters against speedo turns will be one measure and then it will be radiator out, speedo gear out and counting the number of teeth on the speedo drive.

Bob

[John ATTLEEParticipant @johnattlee20632]

Bob,

Clive has reminded me of what I have forgotten from my days at Smiths Industries many moons ago. He is absolutely right on what he says about Nemags etc and the RPM with one exception. It may be that different hairsprings were fitted. However, my recollection is that calibration was achieved by altering the magnetism of the mechanism so that there was more or less drag pulling the needle around.

I think that you can put a pointer on the speedo cable and count the number of rotations of the road wheel to get one of the pointer (after taking into account any effect of the diff).

I will note Mike Flannery for my own purposes.

John

[Robert Atkinson 2Participant @robertatkinson2]

I agree with John. Normally with eddy current indicators the magnetic field strength is adjusted to change the calibration. Changing a return spring (normally a hairspring) will give coarse range change. The field can be adjusted by movable poles or how far the poles cover the disc or cup but often the actual magnet strength is changed.
On some aircraft instruments I've calibrated this often meant taking the magnet out, putting it in a magetiser / demagnetiser taking an educated guess at the energy setting required, zapping it, putting it back and re-checking the calibration. It you were doing a series of similar instruments you would get quite good at setting th level. If it was a one-off you might have many iterations…. If i awas a electric remote tachometer you had to do it twice. Once to set the output voltage from the generator and then the eddy current to calibrate the indicator.

You could always fit one of the modern electronic programmable speedos:
https://www.competitionsupplies.com/smiths-programmable-speedometers

Not cheap though…

Robert.

[Dave HalfordParticipant @davehalford22513]

Posted by Speedy Builder5 on 30/09/2023 19:17:13:

Thanks Clive and Dave.

The speedo does have a serial number and is marked 800 and as its in KPH would be 800 Turns / Km.

The wheels are off the car waiting for brake cylinders, so I will have to wait before counting wheel revs over 100 meters against speedo turns will be one measure and then it will be radiator out, speedo gear out and counting the number of teeth on the speedo drive.

Bob

lifted from the Mini forum

Well, 1 mile = 1.609344 km, so 800TPK would mean 800 x 1.609344 = 1287 TPM

Which has no direct swap though the 1280 is close

[Clive SteerParticipant @clivesteer55943]

On the Nemags the rotor is magnetised to give a reading of between 10-15% of required reading and then incrementally demagnetised to fine tune the calibration. However the range of adjustment is relatively small if the speedo is to remain accurate over its lifetime. There are quite a few hairsprings to cover the range of turns per unit distance from 560 to 1850 that Smiths speedos cover. Similarly there are quite a few worm and ratchet wheel tooth counts for Odometer to cover the same range.

A common fault with old speedometers is that the pointer arbor shaft wears where in runs in the drive spindle bearing making the drag disc run closer to the magnet causing the speedo to over read. If the wear is excessive the disc can catch on the magnet causing the pointer to "flicker".

In most cases owners take care to ensure their vehicle is appropriately lubricated but this doesn't seem to extend to the speedometer and like other types of clocks not thought about until they stop working.

CS

[Clive SteerParticipant @clivesteer55943]

The speedo dial calibration number is turns per unit indicated. So 800 turns will make the Odometer increment by 1 unit and if the input spindle is turning at 800 rpm the Odometer will increment 1 unit in 1 minute. Also at 800 rpm the pointer will indicate 60 whether it is kilometres or miles per hour.

To match such a speedo the vehicle gearbox/wheel configuration must make the speedo input spindle rotate 800 times for the appropriate travelled distance.

Vehicles have a vast number or wheel/tyre size, gearbox and differential ratios so the range can be quite large. The vehicle designer may also wish the flex drive to the speedo to run at a specific speed for a number of reasons I'm not aware. I would imagine that a long flex drive with a number of curves would need to run slowly to prevent wear or achieve a smooth drive to thee speedo.

You don't have to physically move the vehicle to check the calibration just be able to rotate one wheel or hub assembly. You need to determine the wheel circumference to work out the number of wheel rotations that represent the unit distance the speedo indicates. Don't forget that with one of the drive wheels stationary the differential will multiply the spindle turns by 2. This gets you into the ballpark but a tyre compresses when on the road and loaded.

I believe there are similar instructions offered on various vehicle owners websites.

CS

[Speedy Builder5Participant @speedybuilder5]

Clive, just to make sure, with one wheel off of the ground and a transmission with a differential does the count still read true ?

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Clive Steer on 01/10/2023 11:19:32:

.

[…] This gets you into the ballpark but a tyre compresses when on the road and loaded.

.

Ahh … I remember a fairly convoluted discussion about that in the RSSOC *

Final consensus was that the circumference doesn’t change.

MichaelG.

.

*

Reliant Sabre and Scimitar Owners Club

[Clive SteerParticipant @clivesteer55943]

I believe with one wheel off the ground the count will be twice that for normal two wheels moving. You only need to work out the gear ration between the wheel and the speedo takeoff which may not be an integer. I am not a Mini expert but the ratio is likely to be fixed whether a metric or Imperial speedo is fitted. In which case the metric speedo would be an 800 and the Imperial one a 1275.

To check thee calibration of the Speedo you will need a tachometer but bear in mind that if you use a car tachometer it is usually driven from the camshaft so the input is rotating at half indicated RPM.

CS

[Keith LongParticipant @keithlong89920]

Speedy – if you've got a local friendly tyre fitter/garage ask if you can have a look at the "Technical Data Handbook" for the make of tyre that you've got fitted. The 1981 version of the Michelin Tech Data book that I have has a table listed by tyre size (eg 145-10 ) and against that it lists what rim sections it can be fitted to, tread pattern/s, speed category, inflated section, overall diameter, static laden radius (not just half of the overall diameter ) and revs per mile. That lot SHOULD help you with deciding what speedo gear ratio you need.

Keith

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