[bezalmighty]

When calculating energy requirements on a flat plane of travel, the weight of the load is only relevant* when accelerating. On a flat plane, once the load is up to speed, it doesn’t take any significant increase of energy to maintain speed, regardless of load weight. The main reason fuel is required to maintain speed on a flat surface is overcoming air resistance. At the speeds that road traffic travels, only the front and back shapes of the vehicle contributes substantially to air resistance, not “air friction” from an elongated middle body. This means that on a flat plane like a highway, doubling the length of a vehicle to accommodate double the load, does not require anywhere near double the energy to maintain speed. It does however provide much more space to capture solar energy.

By the way, you can take advantage of this yourself on long highways by driving close (but safe) to large trucks, driving in their slip stream. It cuts fuel requirements substantially because the truck is doing some of the work of moving the air for you.

One puzzling thing is why trucks aren’t designed to be more aerodynamic, instead of a giant box shape. Anyone got any thoughts on this?

*aside from small things like added friction on bearings, changed tire geometry etc.

[rcxdude]

>aside from small things like added friction on bearings, changed tire geometry etc

This is rolling resistance, it's generally proportional to weight, and it's usually not negligible for heavy vehicles: it can easily make a large fraction of the energy usage of a truck, though at highway speeds it's rare for it to be the majority of it.

[sottol]

Afaik, it has partially to do with regulations. In Europe the max length of a truck includes the cab, thus it's as short and upright as possible to leave max room for cargo (cab over engine), whereas US trucks have the long "hood". Old trucks were quite angular but the new ones are a lot more aerodynamic.

Second, those "tapered" add-ons in the back can help a lot to approximate a teardrop shape better given that there are a lot more constraints in general in the front.

[reaperman]

> The main reason fuel is required to maintain speed on a flat surface is overcoming air resistance.

Yeah, 200 horsepower's worth.

Wind resistance at semi-truck highway speeds is very significant. Multiply the numbers here[0] by 9.5x to convert from BMW to Semi-Truck (accounts for increase in Cd and frontal area). So semi trucks might take on the order of 180hp at the wheel just to maintain speed. With drivetrain losses you'd need to be generating 200hp.

0: https://www.e31.net/resistance.html