A new speedometer and/or rev counter?

[Luke]

In my post on a new coil tester @jab35 enquired about an electronic speedometer and/or rev counter for the model T.

I was thinking about this overnight and have decided this would be a fairly simple thing to do, but I'm not sure if it's anything of significant interest to others or not?

FWIW, as I like things to be 'period', I'm not sure how keen I would be on a digital readout, although that would probably be the easiest. However some time ago I developed some instruments for a Flight Training Device (a.k.a. flight simulator) that were a reasonable facsimile of the actual instruments we use in a 'plane. I attach an image of some of these instruments here:

It occurred to me that it shouldn't be too hard to do something similar that looked like a 'period' instrument for a T. One could then write some code for an Arduino or similar that would sense/calculate road or engine speed and drive a suitable pointer in the instrument (via a stepper motor or servo).

What do you think?

[TRDxB2]

I had been looking at a product that uses a GPS signal to regulate motor driven cable to a mechanical speedometer. https://www.classicautoinnovations.com/classic Now there are some similar products emerging or at least GPS sending units to drive electronic speedometers example of one https://www.summitracing.com/parts/sum- ... gLKdfD_BwE
Could just need to build a "black box" to drive a speedometer cable, adjustable output, using the GPS signal as input. There is one such device available but its output is not adjustable. https://www.speedhut.com/ecommerce/prod ... -Included).

[Luke]

Interesting, I hadn't thought about producing a drive to power a mechanical instrument - thanks for pointing this out!

Thinking about it I feel there's no reason why one couldn't do this with an inexpensive stepper motor for example. These are easily obtainable from old printers etc. I'll have to consider how best to go about that.

In terms of getting a signal to determine road speed and drive the instrument I agree that a GPS is very accurate, but it's also fairly trivial to use something like a hall-effect sensor and a magnet on a spoke, or even against the gear points of an old speedo drive wheel.

Either way that part's easy and reasonably inexpensive IMV. For me I guess it's more about how to display the speed in a form suitable to the vehicle that I was wondering about.

If you've already got a suitable instrument but just need to drive it, then you're most of the way there, however down here there's not too many of those around (that I'm aware of), hence contemplating how one might go about making something new, but period in look...

[George House]

Oh Nooooo.... that’ll drive Russ out of bizness

[Mark Nunn]

There are tachometers for outdoor power equipment that have a digital readout. I bought one about 25 years ago and used it on my mower and 9N tractor. They have a pickup wire that wraps around a spark plug wire to get a signal to determine rpm. They are all over Amazon and many are less than $30.

[TRDxB2]

Luke, I'll have an extra working speedometer I can send so you can design a solution. We can discuss details via email. Frank

[ewdysar]

There are tachometers for outdoor power equipment that have a digital readout. I bought one about 25 years ago and used it on my mower and 9N tractor. They have a pickup wire that wraps around a spark plug wire to get a signal to determine rpm. They are all over Amazon and many are less than $30.

Unfortunately, those “add-on” tachometers count the sparks coming down the wire with the sensor, which for most 4 stroke engines is 1 spark per 2 crank revolutions. And if your car has been converted to a distributor (modern or period), this will probably work fine. But for most of us, with our buzz coils, how many sparks are generated each time the cylinder fires? Does the number of sparks per revolution change with engine speed? The timer spends less time on each contact at higher rpm.

Given the electrical variability of our ignition systems while running, I think that an electronic tach reading the spark plug wires would have to be purpose built for our engines and pretty difficult to make accurate. The timer wire is only connected once per cycle (minus contact bounce), but is low voltage which again will require a custom sensor and electronics to make it work.

That leaves us with mechanical tachs without an obvious place to connect to the engine in stock configuration. I think that the most “common” tach connection is off of the back of the camshaft, but that takes additional modifications before you start adding the tach and drive cable.

This is why you don’t see modern aftermarket tach kits in any of our normal supplier’s catalogs.

Keep crankin’
Eric

[Steve Jelf]

...something like a hall-effect sensor and a magnet on a spoke...

This works great, until it doesn't. I've been using a bike speedometer with the sensor mounted on a bracket by by the left front wheel and a magnet on the wheel. I've found that sooner or later road hazards damage or destroy the sensor. In the last failure I found that the sensor was completely gone. So recently I installed a sensor inside the rear brake drum with the magnet held by a hub bolt. We'll see how well that setup survives.

[Luke]

Quick reply as I've a bit to get through today:

• George - oops, I wasn't aware anyone was doing this already! It's not my intent to compete with anyone, and as with the coil tester I'd see this as a 'free' community project for those that felt like having a go.

• Mark, a digital readout is simple, but seems a bit crass in something like a Model T? Also not sure how well it'd work, as ewdysar as commented.

• Frank, thank you, I've got your email and will respond.

• ewdysar, although I've not yet constructed such a thing I don't see there should be any insurmountable issue with this. With regard to a tach no sensor would be required, and it should be quite accurate. Mind you, perhaps I should knock one up first before shooting my mouth off too loudly!

• Steve Jeff, that sounds a good idea. I had envisaged utilising existing spoke bolts, but what you're doing gives some protection. That said I've had another idea around this that might make it easier/better still, just need to think it through a bit more

[TRDxB2]

As far as counting revolutions - why not tap from one of the wires, #1, from the commutator to coil box. Functionally its just an on/off switch.

[BE_ZERO_BE]

RPMs could be derived directly from Magneto frequency with a frequency to voltage converter.
No sensors required.

[Steve Jelf]

Here's the speedometer setup on my touring. I believe the only reason it's still there is that I haven't driven this car on any trips like the runabout. This sensor will be going inside a brake drum too.

[ewdysar]

RPMs could be derived directly from Magneto frequency with a frequency to voltage converter.
No sensors required.

Bob, you’re right. One can accurately read RPM from the magneto output, as shown by the ECCT in magneto test mode. All that is left is how to translate that data stream to drive a physical tachometer needle with reasonable accuracy and precision.

With the replies in this thread explaining how simple a Model T tachometer would be, I’m surprised that none are available from our vendors.

Eric

[Luke]

Sneaking in a comment at lunchtime ...

For a tach any of the outputs from the timing commutator, or the magneto output would give the requisite RPM data. I'm less inclined to solely use the magneto 'cos I understand some cars don't have one (!), or they don't sometimes work well, but you do need spark. Mind yo as it should be fairly trivial to provide an input for either I'll probably do that.

That said the mag may require a prescaler, dependant upon frequency - does anyone know if they were same throughout the model years? When I say the same, I mean did the mag always have the same number of poles, or did they change at any point? I've only got a sample of one to trial so need some input on this!

Thanks.