Driving Dynamics - Part 2

David Vespremi, June 12, 2007

In the last installment of the driving dynamics blog, we explored the interplay between static mass distribution and power delivery in the Tesla Roadster. That’s a complicated way of saying that we looked at how the Tesla Roadster drives based on two key parameters: its chassis architecture (mid-“engine,” rear wheel drive) and power delivery characteristics (extremely broad, flat torque curve). By way of a quick refresher on the subject, the Tesla Roadster maintains the bulk of its mass, the battery pack, ahead of the rear axle and delivers its power to the rear wheels in a linear fashion.

Now let's look at some of the further intricacies of weight and driving dynamics.

Without digressing too much, I’d like to tackle a reoccurring question because it dovetails perfectly with a discussion of driving dynamics. “Would four hub-mounted motors have made for a better Tesla Roadster?” In a word: no. Four hub-mounted motors would work great in an electric off road vehicle or rally car – power to each wheel could be controlled for mud, ice, and gravel along with the hill control feature used in off-roading. In a sports car, the added weight and complexity would have compromised the driving enjoyment that makes a sports car a driver’s delight.

Consider the following. When world renowned race car driver and Lotus founder, Colin Chapman, uttered his famous words, "add lightness," the actual quote was, "simplicate, then add lightness." His point was this: Do not add anything to the car that would compromise the purity of the driving experience. In designing the Tesla Roadster, the engineers went to great lengths to ensure that the car is, and would remain, a driver’s car. Why use four motors when one would do? All ingredients were to be kept in perfect balance. As innovative and high-tech as the Tesla Roadster is, at its core there has been a philosophy of KISS (Keep It Simple, Stupid).

Further, and this goes back to weight management theory, while weight compromises every aspect of performance from acceleration to braking and handling, different types of weight are more or less problematic in the design of a car.

Rotating Mass

The weight that engineers most worry about is rotating mass. In other words, anything that goes round and round as the car moves. This includes components like the wheels, tires, brake rotors, and even the lug nuts. Besides the gyroscopic forces that a spinning wheel assembly represents as a car turns, this is mass that needs to be spun up to speed for acceleration and slowed back down again for braking. More mass here means relatively slower acceleration and braking.

In designing the Tesla Roadster, the engineers incorporated forged aluminum wheels to help reduce rotating mass while at the same time providing a wheel that would better withstand bending than its cast counterpart. By forging the aluminum under enormous pressure, the molecules are more closely packed and thus it takes less aluminum to make a significantly stronger (and lighter) wheel. In addition, the wheels and tires were sized to provide optimal lateral grip, without any excess beyond what the chassis needs to achieve optimal cornering. A 175 series front tire mounted on a relatively small sixteen inch rim may seem very narrow for a performance car, but this careful scrutiny of tuning the chassis for a specific wheel and tire configuration allowed engineers at Tesla Motors to extract maximum tire grip for spectacular performance without adding flab back into the car. Sure, the engineers could have specified steamroller-sized wheels and tires to put even more rubber on the road and inflate the lateral grip numbers for the magazine statistics. That would have compromised the Tesla Roadster’s tactile response, and created a car that tramlines over grooves in the pavement and thuds over bumps and potholes, but posts amazing dry skid pads numbers. At the end of the day, shouldn’t a car be fun to drive and not simply a way of generating numbers that look good in a magazine, especially if you can hit the high numbers without resorting to caveman tech to do it?

Unsprung Mass

The next concern moving up the weight ladder is unsprung mass. This represents any mass that goes up and down as the suspension compresses and decompresses, for example tires. This includes elements of the rotating mass described above along with the hub assemblies, brake calipers, and suspension control arms. The more weight carried here, the longer it takes the car to regain its composure as it soaks up bumps and variations in the road surface. A car with less unsprung weight will feel lighter on its feet, while an increase in unsprung weight will make the handling feel lethargic and uncommunicative. By the time the driver feels what the car is doing, his or her reaction time will have greatly diminished and the important tactile feedback will have been lost. Here too, the engineers would have been compromising the handling of the car by mounting heavy hub-mounted motors (drastically increasing the unsprung mass) at each of the four corners.

Sprung Mass

In relative terms, the least significant weight is sprung mass, and this includes everything on the car’s body and chassis that is held aloft by the suspension assembly. Even here there are distinctions to be made. Generally speaking, mass above the car’s belt line (the imaginary halfway point that runs horizontally along the perimeter of the car) is more critical than mass below as this affects the center of gravity (how top heavy the car is). The lower the center of gravity, the better from a handling standpoint. This is why engineers go to great lengths in race cars to mount the engines as low as possible in the chassis and using lighter weight alternatives to traditional glass in windows to move the center of mass downwards. In the Tesla Roadster, examples of reduced mass include the optional carbon fiber hardtop (as opposed to heavier alternatives) and the extensive use of carbon fiber body panels on nearly all exterior surfaces (minus the bumpers).

Likewise, weight that is at the polar ends of the car (ahead of the front axle and behind the rear axle) is generally less desirable from a handling standpoint than weight that sits between the axles. Here, too, the Tesla Roadster has short overhangs and carefully managed mass at its extreme ends to help reduce even this form of static mass. The ultimate curb weight of the Tesla Roadster will be among the lightest on the road. In final trim it is expected to weigh on the order of 500 to 1000 pounds less than some of the most esteemed exotics on the market, making the Tesla Roadster a true driver’s delight entirely in line with Colin Chapman’s philosophy. The fact that engineers were able to accomplish an estimated 2,690 curb weight with a 950+ pound battery on board is truly a remarkable feat.

Fluid Mass That Moves

There is also an ingredient not yet addressed in the mass discussion, and that is mass that moves around and changes as the car is driven. Consider the following: In a typical car, a gas tank can hold twenty or more gallons of gasoline. At around 6 pounds a gallon, this represents 120 pounds or more of weight somewhere in the car when full and absent from the car when empty. On a handful of cars, like my MR2, the gas tank is in the middle, so as the gas level drops, the front/rear weight distribution doesn’t change. However, for many cars, the tank is located in the rear and moves the weight distribution forward as the gas is depleted.

Further, the oil – 5-8 quarts on a wet sump system – sloshes in the pan under cornering. At 2 pounds a quart, this adds roughly 10 to 16 pounds of weight that sloshes around. For a dry sump system like a Porsche 911, which holds 24 quarts, this is 48 pounds in oil alone (although in that car, it is spread evenly along the perimeter of the chassis). Other than the obvious point that the Tesla Roadster doesn’t have either a wet or dry sump oil system, since it doesn’t use engine oil, another point worth mentioning here is that the Tesla Roadster will not scavenge or pull air when the pick-up runs dry for either oil or fuel under extreme cornering using sticky tires.

As a side note: in a combustion engine car, the result of pulling air under cornering as the fluid moves around can be a disruptive hiccup under power delivery, all the way to full-fledged engine failure. To date, we have not logged any electron sloshing issues.

The Future of Mass Management

In a car like WhiteStar, Tesla’s forthcoming sports sedan, there is no rule that says that the battery pack needs to be situated in a centralized location. One could imagine a scenario where the bulk of the mass sits lower in the chassis than even the most aggressively prepped race cars with combustion engines. The future looks bright for Tesla’s handling dynamics.