Driving Dynamics

David Vespremi, April 6, 2007

It doesn’t matter where we were going, on which freeway on-ramp we were merging, or even how fast the car was actually traveling. All that matters is that I was driving, I had a journalist riding shotgun, and I was just passing the imaginary apex of a decreasing radius turn with speed to spare. A lot of speed to spare. Oh, and there was a very solid looking concrete barrier occupying pretty much all of the real estate just ahead of the Tesla Roadster’s short red snout.

And then it happened…

As my journalist companion began to squirm and brace his legs for impending impact, I tapped the throttle and gently turned the steering wheel and, without effort, the Engineering Prototype (EP) began to turn in more sharply, adjusting its trajectory away from the barrier. The journalist – an otherwise very composed gentleman from a Bavarian newspaper – was giggling like a school girl as the car rocketed towards a sea of anonymous freeway commuters and safely out of harm’s way.

I may feign serenity while scaring the most jaded journalists in an electric super car, but this really is more fun than anyone deserves to have at work. Merely a month into my role as the Tesla Roadster Director of Public Relations it truly began to sink in – this is a dream job. I have here the unique opportunity to give a bit of insight into what it is like to actually drive the Tesla Roadster, as well as the engineering and design choices that went in to making it such a thrill ride. I’ll concede that my insider’s view cannot take the place of a hard nosed journalist giving his or her assessment of the car, and can assure you that we are well underway to arranging for all of the top magazines to have seat time to make that a reality in the very near future. Until then, I’ll do my best to share a bit of my perspective as one of the newest voices on the Tesla Motors team.

The touch points on the car are exactly as one would expect. A whiff of leather from the seats, primary and secondary controls falling easily to hand – the result of solid ergonomic planning – and a very traditional analog style gauge cluster that should be familiar to anyone who has driven a sports car in the last 50 or so years. The only clue that anything is slightly out of the ordinary is the oddly high redline on the tachometer. What is this? A motorcycle? For anyone expecting an interior like Kitt of Knight Rider fame with flashing multi-colored buttons and a digital voice that makes witty comments, the Tesla Roadster fails to deliver – a mid-1980s Mitsubishi Colt seems more “car of the future” than this. In a recent team meeting, I half-jokingly suggested a user selectable soundtrack of different combustion engine noises ranging from a lumpy American V8 to the whir of a German flat six, but none of that exists in the Tesla Roadster. Instead, there is a green light letting you know that the car is on and ready to go and a whole lot of silence when you turn the key.

Once underway, the hum of the turbine-like AC motor begins to make its presence known, creeping into the cabin from behind and gaining volume as the car picks up speed. Immediately, visions of Blade Runner (or insert your favorite Hollywood SciFi flick) should come to mind. At first incongruous with sitting in what otherwise feels like a traditional sports car, after just a few minutes behind the wheel the sound begins to “fit” the driving experience and become every bit as much of the tactile character of the car as the mechanical buzz of a combustion engine. Each time I am out in the Tesla Roadster, I find myself pushing harder just to hear the turbine whir up the octave scale to an increasingly higher and more frenzied pitch... fweeeeeeeeeeeee! Those operating under the misguided assumption that the Tesla Roadster lacks automotive “soul” are in for a pleasant surprise.

The suspension, brakes, lights, and horn all work like any other car, and, as best I can tell, the car does not levitate. There is a touch screen video display system (VDS) by your left knee that seems to know how often you have opened the trunk, how much air is in each of your tires, how many g-forces you were able to pull on your favorite corner, your average energy consumption, and presumably your daily horoscope, but I find I rarely look at it once underway. That’s all well and good, but it is when you get beneath the skin of the Tesla Roadster that the real engineering vision shines through. Drive the car with aggression, and you’ll know straight-away that despite its relatively quiet and unassuming demeanor this is not a car to be trifled with. Here’s a glimpse into the inner workings:

The Figure Skater Has Her Arms In

For those that have had the benefit of taking a high performance driving course, feel free to skip ahead or stay along for a quick refresher.

A car’s handling is essentially determined by how weight is managed over the four points of contact with the road (the tires). The static mass of the car over the tires can be configured one of four ways.

  1. Front engine, front wheel drive (FF): This puts the heaviest part of the car – the engine – over the drive wheels. Think of cars like the Honda Civic or the Volkswagen Golf. As the car brakes, the weight shifts forward to the wheels that steer. As the car accelerates, it shifts backwards away from the wheels that steer – the same wheels that serve the dual purpose of determining the car’s trajectory via input from the steering wheel and also delivering power. The gyroscopic effects of the front wheels attempting to simultaneously steer and turn, combined with uneven power delivery to the front wheels, can cause a condition called “torque steer” which results in the steering wheel jerking and squirming under acceleration. When going around a corner at speed, the mass of the car pushes forward, making the car take a wider arch around the bend – something called understeer.
  2. Front engine, rear wheel drive (FR): Here, the bulk of the weight is again over the front wheels (the wheels that steer) but the power is being delivered to the rear wheels. Think of cars like the BMW 3 Series or the Ford Mustang. When the car accelerates and the weight shifts back, FR cars are more apt to gain cornering traction under acceleration, and more apt to lose traction under braking as the weight shifts forward. While a number of these cars split their weight 50/50 front and rear, this is often accomplished by carrying the heaviest mass at the two ends of the car with relatively little mass in the middle. Even though a 3 Series can boast of 50/50 weight distribution, that weight is distributed in a barbell fashion, with the heaviest parts, the engine and differential, at either end of the car.
  3. Rear engine, rear wheel drive (RR): Now we are talking Porsche 911 or VW Beetle (the old one). Here, the bulk of mass sits behind the rear axle, and it is essentially a car that does the opposite of the FF car above. The rear weight bias gains traction over the drive wheels on acceleration as the car sits down on its haunches and springs forward, but it loses rear traction on deceleration and tends to change steering trajectory as the mass in the back shifts laterally in corners.
  4. None of these is the Tesla Roadster.

  5. Like the Ferraris, Mclaren F1, Pagani Zonda, and other high-dollar exotics of the world, the Tesla Roadster has the bulk of its mass squarely in the middle. Unlike those cars, this mass is technically not the motor (which weighs a mere 100 pounds or so) but the Energy Storage System (a giant battery to most folks), which rests 900 pounds of the approximately 2,500 pound car’s weight directly in front of the rear axle (as opposed to behind the axle, as on an RR). So, the mid-motor (mid-battery) rear wheel drive Tesla Roadster has neither a front nor rear pendulum effect (unlike the Civic and 911 above), nor is it a “bar bell” like the 3 Series BMW.

From a driving dynamic standpoint, this means that the moment of inertia is quite small. In other words, it takes very little steering or throttle input to make the car change direction. Expressed in terms of physics, one might say:

Or, one could say that the car is nimble and be done with it.

In between a physics formula and a one word descriptor is a visual analogy that helps tell the story. Picture a figure skater spinning in a circle with her arms extended. This is a traditional front engine, rear wheel drive car with the mass on either end. She will spin (change trajectory) but will do so slowly. As she pulls her arms in closer to her chest, she spins more quickly (the mass is now more centralized) and as she extends them back out again, she slows down.

As a footnote, for those that wonder why I haven’t included separate descriptive categories for front engine all wheel drive, like the very capable Evos and WRXs of the world, nor mid/rear engine all wheel drive, like 911 Turbos or Lamborghini Gallardos, it is essentially because the chassis dynamic analysis above is pretty much the same as it relates to mass. The big difference with all wheel drive is the presence of a mechanical or electro-mechanical coupling device sending power to the wheels at the opposite end of the weight transfer.

Back to that Concrete Wall

Speaking in terms of broad generalities, had I continued to accelerate with the journalist in an FF car (1), the mass of the car would have caused me to turn wide and plow nose first straight into the wall. In car (2), the FR car, I may have made the turn, but only if I reacted quickly enough. I would have had to resist the urge to brake or lift off the throttle, and would have instead needed to gently dab the throttle as I let the steering wheel unwind, just past the clipping point of the turn. Done quickly enough, I would have made it. Too slowly, I would have broadsided the wall. In car (3), the RR car, it would have taken…ahem… nerves of steel to apply throttle when already going too fast in order to put more steering angle into the car. Lifting off the throttle would have induced the rear end’s pendulum effect and put me into the retaining wall going backwards. By contrast, in the Tesla Roadster, the car reacts almost telepathically and turns with your line of sight. I looked through the turn, held my line, and with a touch of the throttle, accelerated right on through to the other side.

So the car turns like any other mid-engine exotic, is that all?

At this point, one might assume that entering that turn in a Ferrari F430 or Tesla Roadster would feel about the same. Here is the next part of the equation – power delivery.

To change the cornering attitude with weight transfer, a car needs to be able to both slow down quickly and accelerate quickly. In other words, the mass must transition fore and aft as quickly and efficiently as possible.

With the Ferrari, the power delivery becomes more pronounced as the revs climb. Catch the car with the revs down, and the engine needs a moment to gather itself up and deliver the most potent part of its punch. With the Tesla Roadster, maximum torque is available from 0 rpm on and there is no guesswork in getting maximum power. A mere downward adjustment of the driver’s right ankle, and the car responds instantaneously. For those accustomed to reading dyno charts, picture the offspring of a diesel truck and a super bike and you are not too far from reality.

Living in that Area Under the Curve

Reading a dyno chart is a bench racer’s dream. Without ever having slipped behind the wheel of a car, it enables one to argue about whose daddy would beat up whom in a good old fashioned power throw-down. The problem is that the vast majority of bench racers never get past rattling off a list of peak horsepower numbers – 100, 200, 500 or 1000 horsepower – the bigger the number, the bigger the win. The more sophisticated bench racers will throw in weight to come up with a power to weight ratio so that at least the amount of mass that the power needs to move is factored into the hypothetical race. This is a nice concession to reality, but fails to tell the whole story.Those “in the know” will tell you that far more important than the peak number is the area under the curve – in other words, where is the “meat” of the car’s power delivery? One clue is to look at where the horsepower and torque lines intersect and to find out how high those numbers are relative to engine speed. So, in other words, if I have a small displacement four cylinder turbo charged tuner car that makes 600 horsepower, is that really faster than a car that makes one or two hundred fewer horsepower, but does so over a significantly larger spread? The big turbo begins to come to life at 6,000 rpm but until that point, without the turbo’s assistance, the car has only 150 horsepower. Since the rev limit is 8,000 rpm, it then stands to reason that driving this car would entail optimizing the amount of time spent in the 2000 rpm worth of useable power that makes up the meat of its power band. In other words, in day-to-day driving I am limited to enjoying a 150 horsepower car, but when I really wind it out, I get a brief taste of what 400, 500, and 600 horsepower feels like… and then it’s gone as the revs drop and I grab the next gear up to find the “sweet spot” again. For the muscle car guys who argue that there is no replacement for displacement, the opposite frustration applies. Here, we are rewarded with smoky burnouts and lots of power down low, but the party comes to screeching halt early on as the jumbo sized engine peters out and begins to run out of breath at wheezy 4,000 or 5,000 rpm.

By contrast, the Tesla Roadster makes useable, healthy power from 0 rpm – more quickly than even a muscle car – to 13,500 rpm – far above the point at which a Ferrari or Acura NSX (two high revving masters) would suffer catastrophic engine failure.

Combining the “area under the curve” power with a mid-engine platform nets a car with few, if any, peers. Of course, a car with lots of power on demand and razor sharp handling is not for novice drivers. I secretly (well, not so secretly anymore) wonder how many Signature One Hundred Roadsters will surprise their proud owners, who misjudge the capabilities of their “cute” little sports car. The Tesla Roadster may not look or sound like a Dodge Viper or Ferrari Enzo, but it needs to be driven with the same level of respect. As with any other car, and peculiar to the Tesla Roadster, powering into the apex of a corner too early – because the power came on more quickly than expected – and then lifting off the throttle, or worse still, braking at the apex, will upset the weight balance of the car and cause it to break traction at the rear. For a driver with cat-like reflexes, this can be minimized with a glorious tail-out power slide by counter-steering and re-applying throttle. For those that panic at this stage, the car will spin until something solid (like that concrete wall) intervenes to stop the spin.

I continue to believe that attending a weekend autocross or driving clinic should be a must for the new owner of any super car, including the Tesla Roadster. Not only will this minimize the risk of costly carbon fiber repair, but it will also enable new owners to realize the truly spectacular level of performance that the Tesla Roadster has to offer.

Back to My Journalist Friend

Having passed the barrier of doom without a scratch or care in the world, I’m confident that my driving companion that afternoon went back to Bavaria with some stories to tell. No doubt, when we finally invite automotive journalists to drive the cars (as opposed to riding shotgun) they too will be able to heap on the superlatives describing what a delight the Tesla Roadster is to push to its limits. In the meantime, I’ll cut this already long blog short so that I can get started populating the Tesla Motors website with new images and video of the Tesla Roadster in the real world and share some of my enthusiasm for what is now very close to the finished vehicle.

In a future posting: For those interested in a discussion of the static vs. dynamic mass, and in particular, the distinction between sprung, unsprung, and rotating mass as it applies to the design and driving experience of the Tesla Roadster, please stay tuned for the second half of this blog to be published at a later date.