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The Porsche 911: the car that shouldn’t work but does
17 June 2011 02:02
Humanity might struggle perpetually with questions like the meaning of life, but it does know one thing for sure: rear engine cars are bad. Like a sling shot, the heavy engine at the back is always trying to overtake the driver in front. Anyone can understand that.Yet the Porsche 911 - one of the fastest cars on the market, and one of the most enduring in history - has its engine at the back. It’s a chewy conundrum; a clash between uncontroversial ‘laws of physics’ and the visible, demonstrable reality of the impossible flourishing in practice.
Are we sure we’ve got this one right?
It turns out we do have a prejudice against rear engine cars, i.e. an opinion not based in fact. The rear engine car was stymied by a couple of unfortunate associations. It’s a political, not an engineering, misfit.This is not the sorry tale of a fatally flawed idea. It’s the unfinished story of promising technology that found itself tragically and possibly terminally tainted after falling in with bad company.
Here’s a 1949 critique of the RR, rear engine rear wheel drive, layout from Bernard W Crandell, writing in the US magazine, Mechanix Illustrated:
‘What are some of the arguments against the rear-engine design? One of the principal ones is weight distribution. With the heavy motor over the rear axle, the center of gravity of the car is shifted to the rear. This means that when not under complete control, the car will have a tendency to turn around and travel backwards, like an arrow shot tail-first. When skidding on ice, for example. Then, too, in case of an accident, the heavy rear moves forward, telescoping the car and its occupants.'
'Even the rear-engine boys recognized this. In almost all models, the front hood has been retained. The driver is minus the protection of the heavy engine and its frame, it is claimed. Steering will not be as positive since not enough weight will bear down on the front wheels. Engine cooling, too, is another problem.What can be said in favor of the rear-engine car? The driver can be moved forward for better visibility. Added weight in back gives better traction. Noise and engine heat are eliminated from the passenger compartment. Fewer engine parts are necessary; one of the first to go would be the long drive shaft and its bulge in the car floor.’
The argument is more or less exactly the same today.
Little Ralph takes a swing at the Corvair
Crandell was talking specifically about General Motors’ Corsair, a concept car. This should not be confused with the 1960 Chevrolet Corvair, the rear engine star of Ralph Nader’s seminal 1965 polemic ‘Unsafe at any Speed’.The flat six powered Corvair was described by Nader as a ‘one car accident’. He alleged the car’s wheel could spontaneously tuck under, causing all kinds of grievous accidents.
You might wonder why a rear engine would cause wheels to tuck under and you’re right, they don’t.That is a trait of the swing axle, the primitive independent suspension system fitted to the Corvair.With the wheel fixed rigidly to the drive shaft, bumps and ruts caused pronounced camber changes, with loss of grip and clear potential for tuck-in. If the bump or rut came in a corner that could mean oversteer, or its malevolent cousin, lift off oversteer.
Yes, rear engine cars like the Porsche 356 and the VW Beetle had swing axle rear suspension, but so did front engine cars like the Mercedes 190SL, the 300SL and the Triumph Herald, and they all suffered from the same faults. Chevrolet brought out an updated Corvair the same year, with fully independent rear suspension, and even sleeker looks, and the problems disappeared overnight.The fame and notoriety generated by the book – and particularly some dodgy spoiler tactics from GM – saw Nader go on to be a hero, a national consumer champion and founder of the US environmental movement. His name is still well known today having contested the last four US Presidential elections.
As April, from hubcaps.com in California says, ‘Though the Chevy Corvairs were popular and maintained sales which exceeded 200,000 for many years, it was decided to discontinue the Corvair in 1969. I’m sure that Ralph Nader’s book ‘Unsafe At Any Speed’ and competition from Ford’s Mustang helped speed the Corvair’s demise.’
The tragedy for The Layout is that the characteristics it was linked with could have been all too plausibly caused by the layout itself. The rear engine car had a habit of making unsavoury bedfellows, bearing in mind it was Hitler’s favourite.
Moment of inertia
This isn’t when you can’t be bothered to get of the sofa to go for a pee, it’s the widely-quoted law of physics that says why rear-engined cars are dangerous. But actually it doesn’t.Armed with a Grade D Physics A level, and a term of Industrial Engineering at Sheffield Polytechnic, I will attempt to explain why.
Not to be confused with polar moment of inertia - that’s to do with twisting in a tube of regular cross section - moment of inertia, or polar moment of inertia of mass, is an object’s resistance to rotation.We could go into this in incredible detail but what you need to know is that inertia increases the further the mass gets from the centre of rotation.
This concept has been popularly illustrated – in many cars magazines I’ve read, anyway – by the Pendulum Effect: a weight on the end of a line swings freely from side to side, alarmingly so in this context.But a Porsche 911 isn’t a pendulum, it’s a sledge hammer. Hold one at the end of the handle – it isn’t so easy to rotate now is it? True enough, if you threw it, the heavy bit would quickly find its way to the front, powered by a deadly momentum.But throwing a sledgehammer is not akin to driving an RR car. The proper analogy would be to throw it from the hammerhead end. Not such a frightening prospect after all.
To get the best handle on how rear, front and mid-engine cars compare in absolute performance terms you don’t need to look any further than Le Mans, surely the ideal, intense environment in which to show up any dynamic shortcomings.
Pound for pound 911s have dominated their categories for years when logic demands that they should be blown off the circuit in the same way four wheel drive cars drove the two wheel drive cars off the rally stages. But it hasn’t ever happened.Of course, purpose-built racing cars are all mid-engined, including Porsche’s own headline Le Mans contender the 911 GT1. This is often held to be the defining moment for the layout – when push comes to shove even Porsche opted for the mid-engine (as it also did with the Boxster and the Cayman). Porsche did cite some weight distribution benefits for the new arrangement but the chief advantage was aerodynamic. Shifting the engine forward allowed a diffuser to be fitted at the rear of the car.
It took a long time for my underlying feeling that 911s are inherently dangerous to go away. I’d probably done 10,000 miles in mine by the time I felt comfortable with lateral grip.My first demo of 911 dynamics was on a test drive. The salesman braked at the last second at a damp roundabout and wrenched the wheel to the right. The car stuck so hard my neck hurt. No weight shift, no stuttering, pattering or edging. Solid.
I confined myself on that occasion to playing with the accelerator - before I knew it we were doing 120 down the bypass and I had to climb on the brakes. My god they were fantastic. Not just because of the consistent meaty response but because, despite full bore braking, the car didn’t stand on its nose.
After 25,000 miles I can honestly say I’ve never been aware of the engine as a malevolent force.Fanatic that I am though, I’m not blind to its faults. Apart from the rock hard ride it’s the front not the back that’s the problem. The great Georg Kacher was absolutely right when he said not long ago in Car magazine that the 911 suffered from poor directional stability at speed. And the front goes scarily light under full bore acceleration. But that just highlights the real mystery of the 911. With no weight on the front, how come turn in is so fantastic?
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