Simanaitis Says
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STILL OFF-SCALE AFTER ALL THESE YEARS
AUTOMOTIVE ENTHUSIASTS of a certain age (and retentive memory) still celebrate the Tapley Meter. This British piece of instrumentation thrilled us no end when it measured that a car’s performance was “off-scale.”
Illustration by Leo Bestgen, R&T, April 1988. See http://wp.me/p2ETap-rO for other Bestgen whimsy.
Truth is, when introduced to R&T Tapley data back in the 1950s, I never understood what off-scale performance implied. But it sounded impressive, and I suspected that no car I drove would qualify.
Since then, I’ve learned that a Tapley Meter measures the change in speed. It’s an accelerometer/decelerometer (i.e., measuring positive or negative change, respectively). What’s more, it measures these dynamic phenomena in the most basic of ways: with a pendulum.
An original Tapley Meter. Similar equipment is still available from Tapley Instrumentation, a division of Bowmonk Limited, www.tapley.org.uk.
Within a Tapley Meter is a finely balanced pendulum that rotates its readout scale through a quadrant gear train. To damp out vibration, the gizmo is filled with a temperature-stable liquid.
Accelerate and the pendulum swings aft; brake and it swings forward.
Don’t try this at home, kids. We’re professionals.
Imagine a loosely held wrench swinging longitudinally as a car speeds up or slows down. (Be careful it doesn’t bean you in the beak.)
The classic Tapley Meter has dual calibrations, lb/ton and percent grade. The first gives an indication of the car’s ability to lug itself around; the second, its ability to climb hills.
R&T chose to report its Tapley data in lb/ton, which explains the logic of Leo Bestgen’s whimsical Jaguar being pulled by a butcher scale. Suppose, for instance, the Tapley Meter read 240 lb/ton when the 3000-lb Jag accelerated full-bore in top gear. Exercise: Justify that the butcher scale would have read 360 lb.
Newton’s Second Law of Motion, F = MA, states that force equals mass times acceleration. Thus, with a little algebraic hand-waving, F/M = A, and hence 240/2000 would equal an acceleration of 0.12g.
So what about “off-scale”? The Tapley Meter’s highest calibration is 600 lb/ton, equivalently 0.3 g or, if you prefer, 0.3 x 32.2 ft/sec/sec = 9.66 ft/sec2. Any acceleration greater than this would be in the Tapley’s off-scale region.
The mathematics of acceleration, velocity and distance.
Back in 1988, I invoked a little calculus to analyze how quick this is. A constant acceleration of 0.3g (9.66 ft/sec2, 6.59 mph/sec) results in a 0-60-mph time of 9.1 seconds and, if continued long enough, quarter-mile postings of 16.5 seconds at 108.7 mph.
Cars of the era are put in perspective by my observing back then, “Not bad performance.” Today, I note that a Toyota Prius does 0 to 60 in the mid-9s and the quarter mile in around 17 sec.
Returning again to April 1988, I observe there was one test vehicle decidedly off-scale and another at the other extreme: See http://wp.me/p2ETap-13X for details of the BAC Concorde Supersonic Transport and Cunard Line Queen Elizabeth 2.
The “Sainted R&T Tapley Meter” was an important part of each evaluation.
As stated in April 1988 Technical Tidbits (and cribbed here), things are “still off-scale after all these years.” ds
© Dennis Simanaitis, SimanaitisSays.com, 2014
Simanaitis Says 
Still have mine! Makes a great automotive conversation piece.
i have 1 0f these for sale , how much is it worth , i have the leather case as well, thanks.