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Pontiac’s Rope Drive: The Flexible Driveshaft That Will Bend Your Mind

I love creative engineering, and I love it all the more when it comes from an unlikely source. That’s what makes the Pontiac rope drive of 1961 such a cool thing: Not only is it bizarre as all get-out, but it came from General Motors at a time when they were the bastion of conservative management.

The 1961 Pontiac Tempest was a font of new ideas. The engine was a 3.2-liter 4-cylinder that looked like what it was: A 389 V8 with the right cylinder bank lopped off. (It was designed to be built on the same assembly line as the 389.) Out back, it had an independent swing-arm suspension surrounding a rear-mounted transaxle. And connecting them was the rope drive of which we speak.

Instead of a traditional drive shaft, the Tempest used a thin steel shaft concealed inside a torque tube — "no thicker than a man’s thumb and curved like a swaybacked horse," explained the September 1960 issue of Popular Science magazine. The Tempest’s engine was tilted upwards, and the driveshaft shaft deflected 3 inches between the center and the ends. This had the happy effect of reducing the size of the transmission tunnel (already decreased by the rear-mounted transmission) and eliminating the need for universal joints.

But these were just added benefits. The real reason for the swaybacked shaft had to do with vibration, a major problem with the big four, which lacked a balance shaft and already had special motor mounts to absorb the vertical shaking. PopSci explains the rope drive’s role: "With a straight shaft, the rotating speed at which its natural frequency might cause vibration would fall within the shaft’s normal operating speed," the mag reported. "Curving it raises the natural vibration frequency above the highest speed at which the shaft must turn." (Ah, physics.)

Another nifty detail about the rope drive was how spindly the driveshaft was — it wasn’t really a rope, but at five-eighths of an inch in diameter (three-quarters of an inch for manual-transmission cars), it looked pretty darn close. Typical prop shafts of the era were on the order of 2.25 inches or more. PopSci explained that traditional drive shafts must carry the heavier torque loads as multiplied by the transmission; the torque load between engine and transmission is significantly lighter, hence the ability to use a thinner, lighter shaft.

One might wonder how a tight-collared company like General Motors would allow such engineering extravagance from one upstart division. The answer: Pontiac’s chief engineer at the time was John Z. DeLorean, a man who was always looking to inject new thinking into the corporate hive-mind that was GM. (I highly recommend "On A Clear Day You Can See General Motors," DeLorean’s authorized-then-unauthorized biography, which goes into entertaining detail about how the sharply dressed John Z continually confounded the suits at GM.) John Z was responsible for a long list of engineering advances, including the "wide-track" ’59 Pontiacs, the division’s overhead-cam straight six, concealed wipers, plastic "Endura" bumpers and antennas embedded in windshield glass. He developed the GTO and the "personal luxury" Grand Prix, and he was a driving force behind the Chevrolet Vega. And, of course, we all know the gull-wing-door sports car that bears his name. (Shame about that whole cocaine thing.)

The Tempest won accolades from the press: Motor Trend named it their Car of the Year for 1961, and Road & Track was ebullient in their praise. It was one of the most innovative American cars of the era, and its split powertrain and independent rear end gave it outstanding balance. But the car had its problems: Tricky at-the-limit handling (including a snap-oversteer problem) and harshness from the big 4-cylinder made it less than endearing to the general public.

Such radical engineering at GM was, as the British would say, "Not the done thing, petal," and in 1964 the Tempest was enlarged to share its platform with other GM intermediates. Of course, by then, DeLorean had moved on to other things: He was figuring out how to bend GM’s rules and get a 389 (6.4-liter) V8 into the new Tempest, resulting in the legendary Pontiac GTO.

And what of the rope drive? So far as I can tell, it hasn’t been used in another GM product since the Tempest (or any mass-produced car sold in the U.S.; please tell me in the comments if I’m wrong). But the split-powertrain idea would be revived for the 1997 C5 Corvette, which has retained the rear-mounted transaxle ever since. Find a Pontiac for sale

Photo: Lars-Goran Lindgren

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