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Performance Metrics: Rationality vs. Vanity

Updated: Feb 3

Often, especially in ads, you learn that a new car, lately electric, makes 0-60 mi (or 0-100km/h) in three seconds. Those accelerations are rarely needed, hard to master, and dangerous for ordinary drivers of conventional cars, and, above all, harm the environment.

However, this feature appeals to "macho-image," and marketers push it, especially because it is just a byproduct of engineering to meet another vanity requirement for driving an EV 500km on one charge in daily commutes. This is unreasonable for urbanites whose daily mileage rarely exceeds 50km. When an ICE car (GC) with fuel amount for 500km range is heavier than it is for 50km by two percent, a battery for 500km range adds up to 40% of EV's curb weight. Accounting for its bearing structure, such a battery is 600% heavier than it would be for 50 km. That's why so-called "range anxiety" is unreasonable.

Well, when it fits a business, they say, one can't change the market. Follow it. When ripe enough technology can profitably surprise the market, they say the opposite: people will love it when they see it. This marketers' attitude has become unaffordable in the era of the Climate Crisis.

Here, you can learn an urban vehicle's reasonable speed and agility.

Computer modeling will help me avoid the "dush-bag" or "an idiot" title from Tesla worshipers. It's not like I value their opinions; it is the computer that suggests calculable specs from reasonable requirements and technological readiness rather than hot adjectives and absurd metaphors of buzzers from media or marketing.

No engineering, let alone design, can save a project when requirements are wrong. Even worse, when a popular product developed from wrong requirements or values gets wildly adopted, it degrades human or natural resources, e.g., drugs, booze, etc.

We don't use "meta-" or machine learning algorithm that would protect us from the "garbage in, garbage out" because our case is just the simplest yet adequate approximation of city intermittent traffic of average non-stop runs s[m], idling time on each stop t[sec], speed limit V[m/s] and applied acceleration or stopping a[m/s/s] rate, i.e., agility. And adequacy of the model is proved by matching its output (travel time) with real-world stats given input data like city block size (200m), and distance (95% trips are within 20km) in some metropolitan areas. I've used the speed profile similar to the FTP75 schedule for a new car's energy or fuel efficiency tests.

The efficiency or energy costs of an urban vehicle's dynamics is a subject for another post where reasonable agility obtained here will be used.

More accurate than linear speed profile shows no difference in resulting trip times. However, the simplicity of our model gives a "closed" or analytical solution that is easy to interpret even before computer-aided numeric experimenting.

You don't have to be a seasoned driver to know that the more often you stop, the longer it takes to your destination. Still, whenever the quantification of our experience is practically feasible and physically adequate, it's a must to substantiate requirements of critical implications for personal purse and national economics because requirements' redundancy is unaffordable in the Climate Crisis and overpopulation era.

Here are the calculations:

If, in heavy traffic, we stop at every intersection 200 m apart for 10 sec, to “cut the chase” by 20%, you must cruise 50% faster. Speeding 3x along empty streets (during pandemics) yields almost the same-fold cut in trip time. Still, you can't exceed the speed limit of about 60km/h. So, what top speed of 250km/h of the Tesla S is for? The 5x speed demands 125x more power to sustain. Computer modeling has confirmed what all drivers know: driving with fewer stops is much better than faster. With speed rises, travel time is less defined by speed and more dependent on traffic.

Indeed, we do need power reserves for agility and driving on rough or hilly terrain. In this post, however, let's examine only the role of agility on flat streets, and it's less intuitive and more surprising.

Buses and heavy trucks are technically capable of a tenth of free-fall acceleration, i.e., 1m/s^2. ICE cars are normally 2-4x agile, so road engineers use 3.4m/s^2 ("controllable braking") in their designs for "stopping sight distance".

The EV’s “instant torque” enables 10m/s^2, i.e., the tires' ultimate shear stress of "exciting smoking start" or stupid drift drills. If technology easily allows, it doesn't mean we lightheadedly should: Tesla's “ludicrous” agility of 5x already palpable 2m/s^2 would reduce your normal travel time only by one-fifth if you get away with being “ludicrous” without consequences.

Higher than normal agility for highway merging doesn't matter, as shown in the plot below - green curve.

And again, practically, if your vehicle enables routes with fewer stops, you would be better off.

Designed to top 70km/h at 5m/s^2 acceleration, of minimal size and weight, all-terrain, and regulated as a bike, FELA makes you the swiftest in a city due to its specific permeability by using routes restricted for cars.

I would be glad to learn your objections in the box below.

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