Mercedes Aero Trailer
 

I found this article on Autoblog. Mercedes is claiming an 18% reduction in wind resistance. claiming over 500 gallons of fuel saved if truck drives an average 92,000 miles. At least someone is coming up with an idea. Hopefully it will find its way to the US.

Mercedes creates "aero trailer" concept to improve big rig fuel efficiency

Cross posting:
New trends in European truck efficiency, post #16

The wheel skirts look great, but the Kamm back looks way too small and the angles look a bit too steep. The whole top (and sides) should taper to help the air close the pocket more smoothly.

I have been noticing this too, particularly on the Honda Fit where there is an "Aero Kicker" type of taper at the end. I wonder if it isn't a feature that does what everyone Thinks air tabs and vortex generators are supposed to be doing, which is to fill the wake area more quickly.

It seems that the extra drag created by going "Sub Template" is made up for by decreasing the size and turbulence in the wake. I have noticed this same feature on a modern Sprinter based RV as well.

http://i39.tinypic.com/2ljq4hx.jpg


http://i43.tinypic.com/vdpp2c.jpg

Below template? Barely, just at the back.

http://i41.tinypic.com/9lh0ec.jpg

Still the M-B seems to be a bit steep. Who knows. If they wind tunnel tested it I don't think they'd do anything stupid. Would they?

It would be great to have an extendable kammback. I've been working on this a little and it seems like a workable solution- like a retractable hardtop but not so heavy.

Sven, I know this is nit pickish, but yeah, it's below template and I think rather significant.

http://i41.tinypic.com/2pza740.jpg

When you overlaid your template, it was too low and a bit too far forward. According to Phil, we want to put the top of the template at the highest point on the roof, this is to prevent the arbitrary placement of the template. I have the underside of the my line right at this point, and it is above the rear door. Your line is into the body work at this point.

I checked the wheels for level, and they are (this can throw you off). I'm looking at where the bottom of my line is to judge the deviation, it does not do any good to cover up what you want to analyze in my mind. Also, I can assure you, that my template is dead nuts on to Phils illustration of the template from the peak back. It does not match the front precisely but I think we can agree that A) It isn’t nearly as important as the back, and B) It isn't really in question.

I have gotten very good at applying this template since I have done it many many times over the past year. (I see I made a mistake on this again in that I didn't hit the bottom of the wheels with it, but that, were it corrected, would make the gap a wee bigger on the back)

Below is a link to a high resolution image of the low res one here so you can more clearly see what you and I have done.

Svens Fit with Charlies Template Detail

I feel the 2½" to 3" deviation at the back of the roof line is significant. And when a Fit is seen in the wild, and you know what you're looking for, it is pronounced. My thought is still that this is an intentionally engineered feature that somehow works to help reduce the Cd, otherwise, I see no reason for Honda to have done it, they could have followed the template and would probably have been cheaper and easier to do.

If I remember correctly, and I can't recall or find the proper thread, Phil has said that the template was a safe conservative way to lay out the taper. He never spoke to the amount of deviation that would produce significantly degraded drag. It is pretty clear to me in looking at many modern cars that even the dedicated manufacturers are using slightly steeper rears than the template predicts. Examples are the Insights(both), the Prius, the Fit and any number of others.

I think that what may be happening is that manufacturers have wind tunnels, and we don't. They are testing their shapes in detail and we can't. From an earlier tuft test on my Insight 1 it is clear that there is some slightly turbulent flow across the rear hatch, but the flow is still attached. Honda obviously made a compromise that they felt was not overly costly, and probably produced the lowest drag vs. dozens of other factors such as total length, interior room, weight, visibility, aesthetics, styling, etc. - all factors that a "real" world car manufacturer must consider. Who can argue with their results? They produced a street car that had the lowest Cd available for many years.

I suppose the real question, the one that interests me, is not where perfection lies, but rather, what is the aerodynamic cost of deviationg from the ideal, and how quickly does that cost mount?

They did that because it is a Euro vehicle.

Here in the U.S., we are allowed 60" length for a boat-tail per the FMCSA.

However, most designs you see here use 48" long side panels because the doors are 48" wide and they stow the boat-tail behind the doors when loading and un-loading.