wrote

[quote myself from another thread]

It's common to assume that recumbents have an aerodynamic
advantage over the traditional diamond frame, but the
advantage is actually limited to fairly extreme recumbents.

[...]

http://www.kreuzotter.de/english/espeed.htm

[...]

mph frontal type
17.1 4.3433 recumbent long wheel base
17.3 4.7889 df hands on tops
18.5 3.3781 recumbent short wheel base
19.4 3.2559 df hands on drops
20.5 2.7111 df triathlon bars
21.2 2.1748 recumbent short wheel base racer
22.3 2.0397 df superman position
23.1 1.5504 recumbent lowracer


Again, the numbers above are apples to oranges, or at least
apples to pears because they use different tire choices.

I quote from my previous response:

[...] the default tire choices on the kreuzotter website for
[non-race configured] recumbents are medium and wide.
The df tire choices are all high pressure, by default.

Here's the list with the high-pressure tire choices for the
LWB and SWB added**. [Also note that the modeled non-racer
SWB and LWB are less aero underseat steering style.]

mph frontal type
17.1 4.3433 recumbent long wheel base (medium slick tires)
17.3 4.7889 df hands on tops
18.0 3.8777 recumbent long wheel base (high-pressure tires)**
18.5 3.3781 recumbent short wheel base (wide slick tires)
19.4 3.2559 df hands on drops
19.5 2.8408 recumbent short wheel base (high pressure tires)**
20.5 2.7111 df triathlon bars
21.2 2.1748 recumbent short wheel base racer
22.3 2.0397 df superman position
23.1 1.5504 recumbent lowracer

I can confirm anecdotally a measurable speed difference in coast
down between a non-extreme SWB "highracer" and a df with
hands on drops.

I agree these numbers do not show the overwhelming aerodynamic
advantage that some may expect. Fast df riders who switch to
recumbents expecting 20% improvements, will be disappointed
almost certainly.

But perhaps a 5% to 8% difference is not unreasonable. And
you don't have to go extreme to gain some advantage and
retain still some comfort.

Jon Meinecke