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Comment by jack_riminton | Hacker News Reader

I've often wondered why European trucks don't have some sort of mechanism to make them more aerodynamic when they're on the highways i.e. an inflatable nose.

The boxy design must cost a fortune in fuel

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In terms of aerodynamics, a blunt nose isn't actually that bad - the real drag is caused by flow separation at the rear of the vehicle. It's far more important to have a gently tapered tail, to avoid a stagnant low pressure zone forming.

Think a typical "teardrop" airfoil shape, they are blunt at the front and gently taper off at the rear.

In fact most cars are more aerodynamic in reverse than they are going forwards for exactly this reason - on most cars the front end comes in more gradually (the hood portion comes in first, then enlarges to the passenger section) whereas the rear end is usually much blunter (to maximize storage space).

Ok wow, that last point really fascinated me:

> most cars are more aerodynamic in reverse than they are going forwards

I tried to find more about it, but I could only find two sources. The first was a Mythbusters episode about a particular Porsche, and the second was a Top Gear episode about a different car. Can you point me to an article that substantiates this?

https://en.wikipedia.org/wiki/Kammback

https://en.wikipedia.org/wiki/Drag_coefficient

He's BSing or just ill informed.

If you taper properly before the flat cutoff a squared off back (Honda insight is the quintessential example) is fine because the real turbulence happens a few feet behind the vehicle. At speed a blunt (even if it's rounded) nose just isn't great no matter how you cut it but with careful attention to corners, shape transitions and whatnot you can mitigate that quite a lot (modern crossovers with their bulbous front bodywork make heavy use of this).

This is a subject with a million gotcha's that add up to a lot. Be weary of anyone peddling rule of thumb solutions.

Okay, perhaps I over-generalized somewhat. If one was to exclude the little details and edges, intended to optimize the airflow in the forwards direction, I contend that most cars would be more aerodynamic in reverse due to their general shape.

Here's one data point, merely removing the rear spoiler an SUV has the same drag in reverse as it does going forwards: https://airshaper.com/blog/mercedes-eqc-drag-coefficient

If the same car had the fine details massaged for the reverse direction in a similar way to how they had already been done for going forwards (i.e. instead of the rear spoiler, putting a similar spoiler facing the opposite direction at the front of the car), the Cd would be significantly less in reverse.

I drove an early Honda Insight with and without the rear wheel baffles; it was really interesting how you could feel the difference in the drag. Little car had hands on its hips holding it back, without the skirts.

I'd love to get it running again but cant find decent instructions for swapping the battery system out for a normal alternator and battery.

I've heard multiple times that people will put themselves behind trucks while driving on the highway and consume only half of the gas they would have used if they weren't behind them. It's probably related to that.

I remember seeing either Mythbusters or TopGear episode where they tested this.

It was a Mythbusters episode, and they concluded that you needed to be following the truck extremely closely (I believe within a couple of feet) to see a gain in fuel efficiency. The driver was unable to keep that small distance consistent for a continuous period of time causing more speed variations, resulting in a loss of fuel efficiency.

It could also be because trucks drive efficiently with little speed variations, at speeds of high fuel efficiency, 80-90 km/h. Drag is actually much more complicated than that, the air being is turbulent.

May be a little, but classic hypermiling technique is to do that anyway. So the slipstream gains are real since absent the truck a hypermiler is going to do the same thing anyway. I never hit the heady heights of good hypermilers, but I had a car I could reliably do 23 km/l, 55 mpg in.

Funny, my current car does 24 mpg on the highway at the speeds I like. Half the efficiency and probably mostly due to me.

Semi under 12 tons can go up to 110km/h in France, and I think most of them drive around that speed.

It's called slipstream I believe.

Hypermilers do this - pretty fun / a bit dangerous.

Bikers do this too BTW if you follow the sport, makes for interesting team type tactics.

There is some thought that automated road convoys of trucks following closely might generate fuel savings this way as well.

Cyclists ("bikers" is an ambiguous term) enjoy a 30% reduction in required power output to move at typical race speeds when within a reasonable size peloton compared to riding out on their own.

That's mostly untrue

Here's a study which pretty much confirms my idea of slightly rounding off the nose would be the best thing https://www.transportenvironment.org/wp-content/uploads/2021...

Image here: https://ibb.co/FgZvz6p

From your article: "fuel savings of 3.2 to 5.3%" from rounding of the front

The article itself states there are bigger benefits to be had modifying the trailer:

"Further aerodynamic improvements are possible, notably on the trailer. For example aerodynamic add-ons, such as tails and side wings, could help deliver even higher significant fuel savings. Their potential is explored in another T&E briefing. [23]"

The referenced article: https://www.transportenvironment.org/wp-content/uploads/2021...

That article shows a variety of tail modifications which have higher effectiveness than rounding the front.

From my understanding, the drag caused by the flat back of the trailer causing a vacuum is much more impactful than the front shape. I believe a "pillow" of air kind of builds up in the front.

It also looks like truck in EU are limited to 55mph. When I tow our trailer (just with a normal vehicle), there's minimal difference between fuel economy between 45 and 60. Significant drops come into play at higher speeds.

Drag is proportional to the square of speed

Ah. Yes! I forget about this!

In vehicles, there's a lot of additional, interesting factors at play - like engine efficiency.

Yes there will be many nonlinear factors which combine to a maximum efficiency speed, which can, to an extent, be chosen by design.

From my understanding, the drag caused by the flat back of the trailer causing a vacuum is much more impactful than the front shape. I believe a "pillow" of air kind of builds up in the front.

A lot of long-distance American tractor trailers deploy fold-out cones on their back doors to improve their aerodynamics while driving. They fold up for low speed driving and loading.

Also, fold-out flaps under the trailer for the same reason.

I've always wondered the reasons behind the regional difference in popularity of these. Trucks in the middle of the country seem to have them, trucks on the coasts seem to not. And you'd think longer haul trucks would be the most likely to have them, and so you'd end up seeing some on the coasts regardless.

Is it something with state by state safety regulations, like prohibitions on protruding structures that aren't part of the main body? Or differently-defined overall length restrictions? Does a given long haul truck have them deployed in the middle of the country, but then they're removed before getting to the coastal states?

> I've always wondered the reasons behind the regional difference in popularity of these.

There's an interesting mis-alignment of incentives. It comes down to who owns the truck vs the trailer.

* Truck owner pays for fuel.

* Trailer owner pays for the flaps.

So, unless you own both the truck and trailer, there's little incentive for trailer owners to install them.

That by itself isn't enough to explain the stark discrepancy though. Surely in the regions where tail flaps are extremely rare, there are still some operators that own both trucks and trailers.

You have often wondered why trucks do not come with inflatable noses?

yes

A blunt front isn't that bad for aerodynamics as long as it's rounded enough for the air stream not to separate. A lot more can be gained at the back of a truck. Like this: http://learntoflyblog.com/wp-content/uploads/2016/04/4-5.png

Actually having a smooth undercarriage and a smooth transition from truck to trailer and some wind breaking fins reduce drag a lot.

https://en.vda.de/dam/vda/publications/FAT-SCHRIFTENREIHE%20...

And the front aerodynamics dépend more on the details than it's length. Some aerodynamic designs have shockingly steep fronts.

https://en.m.wikipedia.org/wiki/Schl%C3%B6rwagen

It does, but the key heare is the length limitation. Spending 1m on a more aerodynamic shape means 1m less cargo space. So the fuel efficiency per cargo volume actually goes down.

Trucks are allowed to be a bit longer now (0.5m iirc), but only for aerodynamic measures. The first few trucks taking advantage of the new rules are on the road now, but it'll take a while before fleets have been replaced.

The most effective aerodynamic changes are on the back though.

European trucks seem to generally have better fuel economy, but this might simply be because fuel economy isn't a focus of US manufacturers.

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