Smith's crono tac

[joe7]

Before I break something, I thought it best to ask. I have a Smith's chronometric tach I'm going to do a drill test using a cable to see if it works. Looking at the tach from the rear, does the cable/drill turn clockwise or counter clockwise? OK, I'll go out front, I think its clockwise! Thanks Joe

[Andy8421]

Joe,

I had a look at these things a few years back. They are miracles of clockwork inside and I would imagine won't respond well to being run backward.

I had understood the cable direction followed the direction of the needle - hence the rather strange 'anti clockwise' displays on some chronometric tachs when the cable was driven anti-clock by the engine. I have however discovered that some chrono tachs had reversing gearboxes, so that an anti-clock cable could drive a clockwise display.

A long answer to say I am afraid I don't know. I would look to see if there is a serial number you could lookup on the web to find a direction - or perhaps find its original application and figure it out from there.

These a remarkable pieces of engineering, and worth looking after.

[nmauduit]

does the needle move when turning by hand ? I'm planning to install one of these when I eventually source the suitable one with tell tale...

[rgh0]

The cable is turned by the jackshaft in a twin cam. The jack shaft turns clockwise when looking at it from the front of the engine. The means the cable turns anticlockwise when looking at the back of the tach.... I think

cheers
Rohan

[Mike Ostrov]

My Climax Elites use a Smiths mechanical tachometer, cable driven from the back of the Lucas C39 dynamo. The direction of rotation is counter clockwise, as viewed from the rear of the tachometer. I use a variable speed drill with the squared end and the nylon stop from a junk cable (2 inches long) to test the tachometers and speedometers, which also rotate counter clockwise. Same for my Smiths Lotus Cortina speedometers. You might post a picture of your tachometer. A small slotted screwdriver should easily turn the internal square shaft counter clockwise by hand and you will see a minor movement of the needle off the peg. If your Smiths mechanical tachometer drive ratio is 2:1 for example, a max. 2500 rpm variable drill should register 5000 tachometer rpm. The tachometer needle should move smoothly, both up and down the rpm range. Hope this is helpful. Cheers. Mike.

[joe7]

Thanks all for your replies. Here's what I found. The cable turns in the direction of the pointer. So with the pointer turning clockwise viewing from the front, the cable turns anti clockwise viewing from the rear. BUT, as noted above, some Smith's tachs have a reversing gear although those are very uncommon. I followed this advice and I didn't break it!

[Andy8421]

Mike,

The chronometric speedo / tacho work differently to the 'eddy current' tach you are describing. Chronometric displays 'jump'. See the video below.

https://www.youtube.com/watch?v=pWw426X1VXE

[Andy8421]

Thanks all for your replies. Here's what I found. The cable turns in the direction of the pointer. So with the pointer turning clockwise viewing from the front, the cable turns anti clockwise viewing from the rear. BUT, as noted above, some Smith's tachs have a reversing gear although those are very uncommon. I followed this advice and I didn't break it!

Joe,

Glad you got it working. Just out of interest, what are planning on using it in?

[Mike Ostrov]

Hi, Joe. Yes, had a few chronos in my various race cars. Lovely to watch the needle. Have found the Smiths mechanical tachs easier to read for me are accurate to 50 rpm at 6500 on my Climax Elites and TCs. Fun video. Glad you are moving in the "proper" direction. Cheers. Mike.

[joe7]

Its a 12k which I picked up in a "package deal" but not using it for anything. I needed an 8K but couldn't find one. The seller had a 9K and after hunting around for a very long time I gave up and bought it. But I had to buy it along with the 12K Smith's and a 9K Raceparts. It was either all of them or no Smith's chrono for me.

[Craven]

Old joke, a chrono tach hits the red line just after the engine has blown.

[Andy8421]

Old joke, a chrono tach hits the red line just after the engine has blown.

Its a good point. It was supposedly one of the good points of the tach - the mechanism that moves the needle uses the rotation of the input shaft as the driver. If the engine blows up and the input shaft stops, the needle won't reset to zero but instead indicates the rpm of the engine at the time it blew up. I am sure many an embarrassed driver was caught out by this.

Modern stepper motor / microcontroller tachs can be programmed to mimic the display characteristics of the chronometric tachs if you really like the look of them.

[Craven]

No Andy, the display is a retrospective calculation, small delay but is there. Don’t forget you need time from somewhere, internal clock.