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#1 Big Jim

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Posted 31 January 2019 - 16:34

I just finished watching a video of Brian Redman driving the Gurney Eagle at Goodwood. It reminded me of something that has bugged me for many years. Why do the tachs of that age in sports car racing jerk instead of smoothly sweep?
Back in the day here in the USA, I know that the tachs that Sun made swept, so, it's not like it was unheard of. Those jerky tachs would have driven me nuts! As a matter of fact...they still do!



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#2 Garsted

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Posted 31 January 2019 - 17:02

Those rev counters are purely mechanical in operation and are know as chronometric tachometers.  An on line dictionary gives the following definition "A tachometer which repeatedly counts the revolutions during a fixed interval of time and presents the average speed during the last timed interval."

They are jerky in operation when the engine of a stationary car is being blipped, but when you glance at it during actual driving the needle is not moving around all over the place, as the engine speed is not changing so rapidly.

I think they were favoured even into the electronic era because, on a car with a rigidly mounted engine, the vibrations would send the needle of an electronic tach into wild and meaningless oscillations.

A lot of impressionable youths (including me) thought they were rather cool back in the day, and period ones sell for quite a lot of money.  Modern electronic ones are much smoother, but I believe you can even get an electronic one that simulates the movement of a chronometric !

Steve


Edited by Garsted, 31 January 2019 - 17:13.


#3 Allan Lupton

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Posted 31 January 2019 - 18:54

Not only tachometers but speedometers of the chronometric system were used. A little-known disadvantage was discovered when a 1930s Triumph crashed into some press tools when being driven rather fast along the peri track of our aerodrome. The speedo was still indicating the speed at impact when the wreckage was being sorted out on the following day!



#4 fyrth

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Posted 31 January 2019 - 19:59

I have a period 'chronometric' on both my 70's F3 and 60s FJ, both with tell tales. Jerky, yes, but if you enjoy a period racer it's one of their charms. They tend to read slightly slow although I'm looking at the road most of the time once I know my gear change points. Electronic are more efficient, like most new things, but don't make you any quicker.  



#5 plannerpower

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Posted 31 January 2019 - 20:09

An explanation may be found here;

 

http://hells-confett...c mechanism.pdf



#6 ed holly

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Posted 31 January 2019 - 21:12

John Frankenheimer, the great film director who gave us the movie Grand Prix - featured a chrono tacho in the opening of this wonderful movie. So not only does the tacho have its place in every 50's and 60's racing car - but it was seen to have artistic merit as well !  I go to a lot of trouble to install chronos in my 60's cars whether road or race. Just love that motion they have. Mind you in the Brabham MRD notes to new owners it says that gear changes in the lower gears should be made earlier than revs indicated due to an inherant lag in fast acceleration, but at steadier revs they are almost 100% accurate when adjusted properly. They use a clock mechanism to count revs over time and display that, hence the jerking effect when not at steady revs, they are also much more robust than electric or eddy current type and as such better able to cope in a vibration prone environment.



#7 Andrew Fellowes

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Posted 31 January 2019 - 22:09

A good electronic tacho can be programmed to replicate the chronometric movement and perhaps without as much delay as an original.



#8 Lee Nicolle

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Posted 31 January 2019 - 23:16

Cable drive tachos are a blast from the past and are  far less accurate as well.

Many passenger vehicles as well as trucks had them. We had one on my fathers 1962 International truck and it was jerky and broke the cable every year or two as well. All that  astranomic 3000 rpm from the 354 Perkins!!


Edited by Lee Nicolle, 02 February 2019 - 01:55.


#9 David Birchall

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Posted 01 February 2019 - 02:18

I love 'em!  However as we seem to be learning TNF does not approve of clockwork mechanisms  (watches etc...)



#10 E1pix

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Posted 01 February 2019 - 08:18

I fail to recall a recent example of a lovelier set of posts than those above.

Nothing to add, thanks everyone.

#11 uffen

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Posted 01 February 2019 - 23:55

A good electronic tacho can be programmed to replicate the chronometric movement and perhaps without as much delay as an original.

"without as much delay"... well, then they're not replicating the chronometric movement!



#12 Lee Nicolle

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Posted 02 February 2019 - 02:00

"without as much delay"... well, then they're not replicating the chronometric movement!

So a jerky inaccurate tach is better? All historic racecars use better components, pistons, bearings, rings valves, valve springs generally as that is all that is available. And are cheaper as well. Plus consumables such as fuel lubricants, tyres, brake material, brake fluid etc as well. 



#13 Big Jim

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Posted 02 February 2019 - 13:29

Thanks for the " chronometric" lesson. And, yes, that "Grand Prix" artistic representation was probably where I first saw the "Herky-Jerky Spin Machine".



#14 uffen

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Posted 02 February 2019 - 15:11

So a jerky inaccurate tach is better? All historic racecars use better components, pistons, bearings, rings valves, valve springs generally as that is all that is available. And are cheaper as well. Plus consumables such as fuel lubricants, tyres, brake material, brake fluid etc as well. 

It was meant as a joke! A nit pick.



#15 Peter Morley

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Posted 04 February 2019 - 16:31

So a jerky inaccurate tach is better? All historic racecars use better components, pistons, bearings, rings valves, valve springs generally as that is all that is available. And are cheaper as well. Plus consumables such as fuel lubricants, tyres, brake material, brake fluid etc as well. 

 

For a long period most UK cars used Smiths chronometrics, so whichever figure the instrument read just before the engine blew up is what mattered, the chances are it wasn't totally accurate but everyone would have used the same figure.

 

All the components you list aren't visible (while the engine is still working!) so don't change what the spectators/enthusiasts see, modern instruments might be better but don't look right in the same way as carbon seats & steering wheels, air bags, low profile tyres etc. which should stay in the modern cars they were designed for.



#16 Emery0323

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Posted 05 February 2019 - 00:25

John Frankenheimer, the great film director who gave us the movie Grand Prix - featured a chrono tacho in the opening of this wonderful movie. So not only does the tacho have its place in every 50's and 60's racing car - but it was seen to have artistic merit as well !

 

I seem to recall that they are also visible in the in-cockpit shots from Steve McQueen's LeMans as well.  Back when that movie was in first release (1971), I remember reading a letter to the editor of Competition Press and Autoweek  asking why the in-car tachometers in that movie had a "stopwatch-like" motion.   The editors of Autoweek did not  seem to know the answer!  When I viewed the movie again, years later, I saw what he was referring to.  I didn't know the reason either, but now I do!



#17 Lee Nicolle

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Posted 05 February 2019 - 01:15

For a long period most UK cars used Smiths chronometrics, so whichever figure the instrument read just before the engine blew up is what mattered, the chances are it wasn't totally accurate but everyone would have used the same figure.

 

All the components you list aren't visible (while the engine is still working!) so don't change what the spectators/enthusiasts see, modern instruments might be better but don't look right in the same way as carbon seats & steering wheels, air bags, low profile tyres etc. which should stay in the modern cars they were designed for.

Yeah take off the rollbars and seat belts and wear a pudding basin lid.

I like historic cars as well as most but  commonsense and liability insurance decrees that all the modern stuff is to be used. I have no idea how they get away with it in England.

And a round tach that works will save the rare and expensive engines,, if the driver looks. 

No monster tachs and shift lights.Or digital tachs. You can do without those but a proper tach and oil and temp guages are an essential.

And for a driver a decent seat, rollover protection and harness



#18 Andrew Fellowes

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Posted 05 February 2019 - 07:41

Yeah take off the rollbars and seat belts and wear a pudding basin lid.

I like historic cars as well as most but  commonsense and liability insurance decrees that all the modern stuff is to be used. I have no idea how they get away with it in England.

And a round tach that works will save the rare and expensive engines,, if the driver looks. 

No monster tachs and shift lights.Or digital tachs. You can do without those but a proper tach and oil and temp guages are an essential.

And for a driver a decent seat, rollover protection and harness

 

No, no, no! If you feel unsafe then don't drive it! It's not a question of 'getting away with it in England' perhaps you should look at CAMS requirements for pre war and early post war cars.



#19 Peter Morley

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Posted 05 February 2019 - 15:37

No, no, no! If you feel unsafe then don't drive it! It's not a question of 'getting away with it in England' perhaps you should look at CAMS requirements for pre war and early post war cars.

 

Exactly, if you want to race or just drive old cars then realise they have always been dangerous, accept the possible consequences and treat them appropriately.

If you want to race something that is safe then get a modern car, which is also more compatible with modern circuit safety features.

Don't try to change an old car into a modern one, it really defeats the object of the exercise (which isn't winning meaningless trophies).

 

The people who fly Spitfires don't fit ejector seats etc.!



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#20 Peter Morley

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Posted 05 February 2019 - 15:46

Yeah take off the rollbars and seat belts and wear a pudding basin lid.

I like historic cars as well as most but  commonsense and liability insurance decrees that all the modern stuff is to be used. I have no idea how they get away with it in England.

And a round tach that works will save the rare and expensive engines,, if the driver looks. 

No monster tachs and shift lights.Or digital tachs. You can do without those but a proper tach and oil and temp guages are an essential.

And for a driver a decent seat, rollover protection and harness

 

As you say the most important issue is does the driver look at the gauges and respond appropriately, I've seen several examples of cars in the last race of the season smoking like hell and the driver continuing because it is going to have a winter engine rebuild anyway - the words more money than sense come to mind.

 

Rebuilt original gauges are accurate enough, or there are modern copies, so there is no need for modern gauges and add-ons.

There is an argument about the benefits of using data logging (most of which can be hidden away) to get a better understanding of what has happened to the engine and enables preventative maintenance but such systems can also be used to improve the car or driver's performance which is inappropriate.

 

Back to the original issue about wobbly tachos, most over-revving is caused by gearchange mistakes rather than accelerating beyond the rev-limit - no rev-limiter or tacho can prevent that.



#21 GMACKIE

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Posted 05 February 2019 - 20:09

Spot on! If you pick up first instead of third, and let the clutch out - goodnight Rose...



#22 Andrew Fellowes

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Posted 06 February 2019 - 06:07

A Long Time Ago in a Galaxy Far, Far away............

 

A6-GCS-at-Snetterton.jpg

and totally off topic for this thread!



#23 ed holly

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Posted 06 February 2019 - 22:03

Might be off-topic Andrew, but it is drop dead gorgeous ... see where the open face helmet and goggles comes from now.



#24 Peter Morley

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Posted 07 February 2019 - 09:20

Might be off-topic Andrew, but it is drop dead gorgeous ... see where the open face helmet and goggles comes from now.

 

How irresponsible to drive something that lovely without a roll over cage, seatbelts, hans device and so on!

Can't believe the pair sat on the pitwall aren't gawping at it!



#25 hatrat

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Posted 08 February 2019 - 01:03

I have the original 9000rpm Smiths Chronometric tachometer in my 1960 FJ. Last month I buzzed the engine when it jumped out of gear and the needle and tell tail stuck at 9300rpm (engine is fine). Due to the delay in getting the Chronometric repaired I had to purchase a Smiths electronic "look-a-like" tachometer which I used for the first time last weekend. I found it much harder to read than the Chronometric as the needle was moving at high rpm where the Chronometric would have had a steady needle. Additionally, it was initially inaccurately reading due to a faulty earth connection. Went and saw the instrument repairer yesterday and begged him to repair the Chronometric in time for racing next weekend - I want my Chronometric back!  



#26 Bikr7549

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Posted 08 February 2019 - 02:10

Years ago I taught my sister to ride my Honda motorcycle, that had only a kick starter. So we went thru the start cycle a few times and I started it too so she could see the process. She then hopped on, and rather than sitting and pushing the lever she stood up with foot on the lever and stomped it down. Well it started all right but in moving her whole self down with the starter lever she also rotated her hand turning the twist grip so it was essentially floored when it started! I saw the tach (mechanical) needle zoom all the way up at supersonic speed and bounce off the peg at the end of the red zone and I hit the kill switch as fast as I could. After regaining my nerve I restarted it and it ran just fine, and did so for the few more years that I had it. Never seen anything like that since, hope never again. She did ride it ok, but wasn't all that interested.