top of page

Performance Metrics: Rationality vs. Vanity

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 rare needed, hard to master and use, thus dangerous for the ordinary drivers, and above all harm environment. However, this feature sells, and marketers push it especially because it is just a byproduct of engineering of another requirement for driving on one charge 500km. This is not reasonable for the urbanites whose daily mileage rarely exceeds 50km. When an ICE car with fuel for 500km heavier than it is for 50km by two percent, a battery, that is up to 40% of EV's gross weight for 500km, is 500% heavier than it is for 50km. That's why it's called "range anxiety" and this emotion is not conducive to reason. Well, they say, can't change the market, just follow it. This marketers' attitude became unaffordable in the era of the Climate Crisis. Here, you can learn what reasonable speed and agility are when it comes to an urban vehicle.

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's the computer that suggests numerical requirements rather than hot adjectives of buzzers from media or marketing. No engineering, let alone design can save a project when requirements are wrong. Even worse, when wrong requirements or values get physicalized in the popular product that 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. The speed profile I've used is similar to the FTP75 schedule for energy or fuel efficiency tests of a new car.

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 "closed" or analytical solution easy to interpret even before 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. However, whenever the quantification of our experience is practical it's a must to substantiate requirements of critical implications for personal and national economics: requirements' redundancy is unaffordable in Climate Crisis and overpopulation era.

Here are computed results:

If, in heavy traffic, we stops on every intersection 200 m apart for 10 sec, to “cut the chase” by 20%, you have to 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 limits of about 60km/h. So, what top speed 250km/h of Tesla S100 is for? The 5x speed demands 125x more power to sustain. By the way, modeling confirmed again what all drivers know: it’s much better to drive with fewer stops than faster. With speed rise, travel time is getting less defined by the speed and more by the traffic.

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

Buses and heavy trucks technically are capable of a tenth of free fall i.e. 1m/s^2. ICE cars are normally 2-4x agile - the 3.4m/s^2 ("controllable braking") is used by road engineers for stopping sight distance. EV’s “instant torque” enables 10m/s^2, i.e. the tires' ultimate shear stress. If technology easily can, doesn't mean we lightheadedly should because 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”.

In highways, agility higher than normal for merging doesn't matter at all as you can see on plot below - green curve.

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

Designed top 70km/h, 5m/s^2 for acceleration, of minimal size and weight, all-terrain, 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 glad to learn your objections in box below.

20 views0 comments

Recent Posts

See All
bottom of page