Art of Driving: Rear vs mid engine sports carsContent
With some focus on Porsche products over the coming weeks, we thought you may like to find out some more about the difference between the rear-engined and mid-engined Stuttgart offerings available through Garage Sportique. Our resident performance driver coach, Neil Furber, will be explaining the physics behind marvellous mid-ship motors and the iconic traction-focused thoroughbred born from the beetle lineage; the Porsche 911.
The 911’s handling has a long-established reputation. During many a discussion about the classic rear-engined setup it doesn’t take long for somebody to say the word pendulum, or sometimes even, widowmaker. As for the mid-engined layout seen in the Boxster/Cayman/718, there is generally nothing but praise from all but the most die-hard 911 fans. If you’re after a ‘proper Porsche’ and the more modern kit excites you, you may well be stuck in the quagmire that is the ‘mid vs rear’ dilemma. Perhaps you had a 911 on the wall as a child, or maybe the past twenty-five years of motoring press praise for the Boxster/Cayman pairing has you leaning that way. In this article we’ll explore what differentiates these two layouts. Whilst there aren’t many options for cars with the engine hanging out over the rear axle, you’re almost spoilt for choice when it comes to modern mid-engined motors. From the lightweight Lotus Elise to Honda’s superb NSX, most modern Italian exotica, and the new Alpine A110, sports cars with an engine just behind the driver come in many flavours. But what makes these cars so nimble and what is it about Porsche’s near-patented rear-engined recipe that keeps them knocking out 911s?
Skateboards and mass distribution
An engine position behind the rear axle and the skinnier tyres of earlier 911s make for a near-unique spirited driving experience. Although still possible to get in a pickle with the latest high-powered cars, it tends to be more through higher speeds or dodgy driving than through chassis layout. Of course, the most modern kit has a suite of electronics to try and help protect the driver from himself. Or herself (!) Even then, these systems are not infallible and, contrary to popular belief, good driving technique is more effective than electronics.
Over the years, the 911 has seen huge amounts of development, with the latest levels of refinement and handling, frankly, superb. Regarding its ability to lose the tail during everyday driving, it’s certainly less of a concern than for early cars. This is thanks, in part, to significant development in tyre compound technology and increased size, but there has also been continual tweaking of architecture and tuning. Perhaps unexpectedly, during poor weather conditions, I’d prefer to drive a rear-wheel drive 911 over a Cayman any day of the week. The rear-biased weight distribution champions rear grip and, when we talk inertia, the 911 is initially more forgiving if you steer gently.
You may have heard the term ‘moment of inertia’, or rotational inertia. It’s a measure of resistance to an increase (or decrease) in rotation. This is the main property behind why rear-engined and mid-engined cars behave differently when turning-in to a corner. To help explain what it means, let’s consider a skateboard as a simplified chassis. If we pop a strimmer engine on the rear end, behind the wheels, a tin of paint in the middle to represent the driver and a large pack of batteries over the front wheels to simulate the fuel tank, we have a very crude representation of the 911. If we move the engine forward of the rear wheels so it touches the paint tin, we now have an equally crude Boxster. In this case, the total mass of each model is the same, but the individual masses that make up the vehicle are distributed differently along its length. The more concentrated the individual masses, the less rotational inertia and easier the vehicle will rotate when turning-in to the corner.
A mid-engined car has its major masses nearer the centre and can rotate very quickly when steering. Rear-engined cars have these masses distributed over a greater length and react more slowly when steering. It’s just like if you spin on an office chair. Tucking your legs in will make you rotate faster; if you kick them out you’ll slow down.
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Many of us have heard stories about early 911s. You’d be forgiven for thinking that the rear-engined layout is much easier to spin and that this happens very easily. The truth is a little more complex. In fact, the mid-engined setup will steer, pivot or spin much more easily due to the reduced rotational inertia. Yet, if (excess) oversteer is detected and corrected quickly it is also much easier to bring back under control. As for the 911’s pendulum analogy, I prefer to think more in terms of a sledgehammer. To swing that over your shoulder will take quite some effort, but once it’s going you’ll struggle to stop it mid swing. Fortunately, the rear tyres do just that. If you stay within their limits!
Consider your front tyres as destabilising and your rear tyres as stabilising. When you turn the steering wheel a fixed amount for a corner, the front tyres steer, deform and generate lateral force, the bodyshell rotates (yaws) a little and, therefore, the rear tyres receive an induced angle, generating their own lateral forces. The front tyres started a spin about the centre of rotation, the rear tyres provide the counter-balancing. They support. It’s a sort ofdynamic see-saw.
For equivalent vehicle weight, the rear-engined format will generate greater rotational momentum during cornering, but it’s less keen to change direction than mid-engined equivalents. That’s one of the reasons the Boxster feels more agile than the bigger brother. Thankfully, when a 911 is driven in a stable fashion, wide rear tyres and all that extra weight at the back provide the grip to help resist a spin. Progressive acceleration upon corner exit will continue to load the rear to really glue it to the road. This is one of the 911’s truestrengths. And all is fine and dandy unless you push it too far...
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Life changes at the limit
Despite a common opinion that the 911 engine is in the wrong place, in terms of performance, it’s actually rather good. During high-speed cornering at the limit, a front-engined car will tend to understeer (run straighter than desired), whereas a rear-engined car will tend to oversteer (spin). Although this oversteer is the very behaviour that may worry many road drivers, it is generally preferred by the racing driver. And let’s not forget the
massive gains in traction to help get the power down. Mid-engined machines are fairly neutral, but are easy enough to spin.
Once we start flirting with the limit during circuit driving, we are looking to get similar behaviour from both mid and rear-engined layouts. If no longer worried about oversteer and spinning, but instead looking to promote rotation when required, things become more interesting. The mid-engined Lotus layout makes for nimble cornering with quick changes in direction and easy pivoting at the expense of rear traction for acceleration. The rear-engined layout makes turn-in a slower process, but can more than make up for it at the exit of slow corners.
If you’re not familiar with trail braking, don’t worry, we’ll cover that in a future article, but mid-engined cars don’t support as much as rear-engined alternatives. Even less weight on the rear means the tail can break away that much more easily. For quick lap times in a 911, trail braking is essential but challenging. The potential to build rotational momentum is much greater, so when you do reach the ultimate rear tyre limits, that sledgehammer follows through and you’ll need some very fancy wheel-work if you’re to gather things up.
Of course, things aren’t quite as simple as mid vs rear. Overall vehicle weight, wheelbase (distance between front and rear wheel centres) and the actual position and weight of engine, gearbox, fuel tank etc. all play a part in the overall agility-stability mix. A lightweight Lotus is more nimble than a fuller-fat Ferrari, for example, and handling characteristics are tuned to suit customers differently across brands. Back-to-back sampling of Porsche products give you the closest comparison should you wish to feel for yourself.