Technical Using a Speedometer as a Tachometer?

https://www.fullthrottlecorvette.co..._lcJHO3W5MR-Ez9S11BB_l7qUwjF54Y-FPCGeCE&gQT=2

 

Yikes!

 

I'm not sure if this is the right thread to ask this, but it seems like as good of place as any?

I recently contacted Classic Instruments about converting my clock into a tach, does anyone have any experiences with having that done?

 

Hey, Kerry;
Thanks for the suggestion. I've seen faces changed/restored using flat or satin paint, & the rub-on letter-sets. If the font & color is close, actually looks good. & the glass, now plastic, keeps it that way. Computer graphics are the way to go, iffen *you* can. Me, maybe not so much... .
I was always making the ass-umption that a new face(for proper usage) would be required in almost every case. It was the driving-speed-change(s) needed that was an issue, as was the point-of-hookup. I was hoping to use 2 speedo's as a matched pair, mostly for looks. & then there's the problem(maybe) of reversing hte sweep-movement(s). I'm guessing that European or Aussie/Kiwi gauges of the same make(ya, good luck finding matching stuff to what I want... ) aren't reversed-sweep, even if they do sit on the opposite side of the car as the US. It was something I never put a great deal of effort into, as I didn't need it "right-now!" . Dawie usually brings up things for me... .
Marcus...

& kelly;
I do believe that this'd be best done w/an adaptation of a different tach, using the clock face, case, glass, & well, at least 1 pointer hand. Just have to clock the tach correctly, & would only go to that rpm level. I do believe that finding a tach w/the same dia(or at least the same spacing 'twixt the # when laid out on a radial-grid) as the clockface would make things easiest. I have also thought about this, but can't use a 12k rpm tach, on a Stude motor... . But, boy howdy, would it match well, & get the wuffos a-goin' . - & that's be worth the effort(& expense) alone. !! .

 

Thanks for posting this Kerry. Absolute dark magic, and an absolutely amazing innovation.

 

I reckon just about everything explored here could cross over to a clock just like that.

 

You could make a clock into a tach if you had the time!

 

The only way to turn a clock into a tach would be the easy way i.e. gutting the clock and putting a tach movement in, because an automotive clock doesn't work anything like an eddy-current speedometer or mechanical tach.

It'd be a pity to lose one of those clockwork clocks which were rewound electrically when you next started the car, because they tended to be premium pieces, unless the one you had was stone dead.

 

sorry I can't help you with the technical side but here is a picture of a mechanical tach Pat Swanson made for me out of a speedometer like the one on the right.
he did his magic with all but the face
that he had someone else silk screen.

 

Hey, Paul;
Thanks, looks good, & matched. !
Any write-up/thread on how Pat did this? More interested in the guts/et-al, than the face. Those SW speedos @ 160 aren't all that common, nor are the 8k tachs; but the 120mph are everywhere, &, iirc, cheap. (read: Stude hawks.), if someone is looking for aftermarket gauges like that - which do look good.
Marcus...

 

sorry, I don't have any specifics on the process, he did explain it but it was years ago and my memory is vague.
he eliminated the odometer and did something with the magnates of course, beyond that we would need to ask him.
I don't think he has been here for a long time but I see him post on **gram as greyghostgmc

 

If he reused the eddy-current mechanism, the way would be to degauss the magnet, and then regauss it to two thirds of the strength, so it'll read 120mph (6000rpm) at 3000rpm and not 2000rpm input. Doing that with a bar magnet is apparently not that hard, but the ring magnet in the VW speedometer seemed a bit too much for me!

 

Hmmm, thanks Ned. More to study n learn. .
Marcus...

 

Related: I tore into an Austin A40 Farina Mk2 cluster I have, to see what kind of magnet its speedometer has. It seems to be a straightforward bar magnet rotating coaxially with the aluminium cup:

I suspect that it's going to take a while for me to steel myself for the degauss/regauss exercise. I also wanted to see how the odometer drive works, as modifying that might make the magnet modifications redundant. But it's a worm driving a wheel driving a crank/arm/ratchet arrangement which apparently pushes the odometer drums on digit-by-digit:

I'm leaving that alone, but for figuring out how to zero the odometer. But for now I'll pop it back together lest I lose most of its bits. I didn't even bother to clean anything.

My purpose with this one is a bit different, i.e. to use it as a speedometer, albeit a 190km/h metric one. It's marked 1376 turns per mile, which I can very easily use as 1376 turns per kilometre, if I can figure out a way to drive it at the right ratio. Only, it'd read up to 95km/h, which is barely 55mph, which is little use where the speed limit is 120km/h. Either I'd have to gear up the odometer and drive the speedometer at 688 turns per kilometre, which is looking impractical, or keep it at 1376 turns per kilometre and regauss the magnet to around half strength. Whichever way, I'd need to print a new speed scale which reads 0-180:

and, of course, change the "M P H" to "k m / h". I can use the fuel and temperature gauges and all the indicator lights just as they are.

 

How can a magnet spinning in an aluminum cup move the needle?

 

Eddy currents i.e. little circular electrical currents in the sides of the cup i.e. not by magnetic attraction.
https://en.wikipedia.org/wiki/Eddy_current

 

Learn something new every day. Thanks.

 

I tested the Austin speedometer using a square bit in an electric drill set to reverse rotation, with a speed stop. I read the drill speed with the optical tach. I took a few readings at different speeds. All seem to indicate an overread of about 8-10%, i.e. well within legal error. That's probably also within factory calibration tolerance.

It was rough and ready but it tells me the magnet is in good shape. The readings weren't high enough to suggest a weak watch spring (which is most certainly the case on my DD.)

 

Yup, lots of speedos read fast. & the change-from-reality is usually greater the faster the speedo reads.
Marcus...

 

I think most jurisdictions that have such a standard have a legal accuracy tolerance for speedometers of +10%, -0%. In practice that means calibrating to 5% fast ±5%. The idea is not to have a speedometer reading the speed limit at anything over the speed limit.

 

That is correct for here in Australia.

 

Yes. I have some cars/trucks that the speedo is correct, at least 'twixt ~ 25mph->80mph, validated by stupid "smartphone app". Depends on tire wear, too. Don't need to test faster than that.
A lot of "jurisdictions" "unofficially" allow(or "allow Officers" that much discretion. Whether they use it or not, well...) ~ 5mph over the limit(since it's proven that speedos can & are wrong, & show on the slow side, from the factory. That'll still buy you a ticket, though - since you're *required* to know & follow the "law"), but when the "officially-sanctioned/licensed-Purveyors-of-Enhanced-Revenue-Creation-Facilitation-&-Procurement" get too zealous in writing at 0->2 mph over, then it gets(or can become) politically driven(read: citizen complaints resulting in possible/probable loss of Office-Holding for incumbent) to lighten-up for abit. Still, I'm not in *that* much of a hurry...
I'd rather know Reality - uncomfortable as that may be... .
Marcus...

 

I don't know if it's something to do with where I am on the globe relative to available synchronous satellites, but every phone speedometer app I've tried has dangerously underread. I mean, around 25%.

There is a whole debate around 85th-percentile speed limits. The old traffic engineering principle is that you take the range of speeds people will drive on an unlimited road, and peg the limit at the 85th percentile of that range. So, if people continue to travel at those speeds, 15% of them will be exceeding the speed limit. This ties the speed limit to the design of the road, which is a sound principle.

The problem comes in when, as is often the case, there are valid reasons for keeping speeds down in a given place, like whenever there are pedestrians around at all. The problem then is that traffic engineers will tend to design for the highest speed for which the standards will physically fit on the site. The idea that travelling slowly on a road designed for far higher speeds is safe might seem intuitive, but it isn't a sound principle, because people are going to drive at the speed which feels right for the road, i.e. that sweet-spot between boring and dangerous. That is always true. You need to get pretty far into a totalitarian dystopia before it ceases to be true.

You want cars travelling at 20mph because you want to encourage pedestrians. The traffic engineers have gone and built a road which is dynamically safe for 150mph, and motorists drive accordingly. So now you limit that down to 20mph by means of people with guns. It makes no sense. There is a growing movement to control speed through road design rather than enforcement i.e. if you want traffic at 20mph, design the road for that speed: few, narrow lanes; parallel parking; kinks and chicanes; frequent stops; etc.

But I digress. Two things cars wouldn't need in a reasonably sane (never mind perfect) world: licence plates and speedometers.

 

I don’t know about the standards way down south but after 68 years of a family owned shop those calibration specs are terrible
Here are my standards. Of course final calibration on vehicle also depends on proper tire size and rear end ratio. I am always there to help my customers get things dialed in once my work is installed.

 

Just a hint and comparison of size on the Lincoln Zephyr speedometer. The 1935 Dodge floating power speedometer is a large dial but nothing compares to the Lincoln.

 

Sorry Ned didn’t mean to hijack your thread
Good luck on your quest

 

Hey, Ned;
No argument. The discussion could very well get into some rather arcane & interesting "rabbit-holes". . Esp concerning Control - by Whom, or What... . & Why??? ;( .
Actually, in the United States of America, you don't need a license - for either you, nor the car/truck/motorcycle/etc.
Unless you are in Commerce. Or you just want to "volunteer". This is getting rather close to OT, although it does pertain to the Past, Current, Future. So's I better quit while I'm merely behind here. The speedo-device, to me, is warranted, simply because I want to know at what velocity I'm Travelling at.

Back to getting the damn things to function the way we want them to, w/some kinda easy(?) workaround. .
Marcus...
I'll drop you a PM.

 

Rusty — I think it's just a case of mine being used-and-abused junkyard/swap meet stuff which hasn't had the benefit of your ministrations!

 

Ned
I didn’t mean that as a criticism or anything personal, I hope you don’t take it that way.
Everyone on this board knows something that other members don’t so I am always trying to learn. I can tell from your threads that you come at things from an engineering background and perspective, which I truly admire and am not really capable of. I hope I didn’t offend and anything I can offer to help you just ask. I know how to fix them but could never dream of coming up with the original idea and inventing one from scratch.

 

Ned here are some of the tools of the trade
I have a large magnito magnet charger and also a smaller charger depending on what I’m working on. As you stated round magnets are a pain in the ass to get poles lined up for overcharging. I have two demagnetizers that are used while the speedometer is spinning on the test bench to dial in speed magnetically.
Two test benches backed by a hand held computer for fine tuning.
Just thought you might like to see some of the tools to achieve charging and discharging the magnets for calibration.