Monday, September 1, 2008
long distance bent blog
D: I dislike fairing design as it exists.
Keep in mind that a fairing is a design compromise.
It is not only to minimize drag.
It is also to keep the rain off sometimes.
This means it often has a larger front cross-section than is needed.
I imagine production methods (the blown bubble) has something to do with that.
D: there are 2 concepts to be understood.
1) laminar flow (or else turbulence) and
2) air pressure
D: the first step and what 'bents do is to minimize front cross-sectional area.
For the standard bike in the upright position, we have this:
At 250 Watts, the upright bike goes 29 Km/Hr or 8.0556 m/s. To go 40 Km/Hr or 11.111 m/s, the upright bike needs 622 Watts.
For the recumbent bike with the full foam fairing, we have this:
At 250 Watts, the bike goes 51 Km/Hr or 14.167 m/s. To go 40 Km/Hr or 11.111 m/s, the bike needs 135 Watts.
For the recumbent bike with the full hard shell fairing, we have this:
At 250 Watts, the bike goes 69 Km/Hr or 19.167 m/s. To go 40 Km/Hr or 11.111 m/s, the bike needs 75 Watts.
D: practical, my ass. Recumbents get enough cross-wind, are top-heavy and are plain heavy enough already.
The typical 'bent partial fairing is a nose cone and tail box.
Sadly, one is better off with
1) maintain laminar flow over >1/2 the length
2) suddenly truncate to induce turbulence once body taper has reduced the rear to < 1/2 the surface area.
To avoid pressure spikes on the front and delaminated flow (turbulence) on the rear half, the surface angles should all be gradual. The surface texture needs to be fairly smooth.
Road vibrations will also disrupt it.
1) Pantour hub elastomer on a stiff frame
2) ? special padded fairing mounts or
3) full suspension
After all, a poorly designed fairing is just so much dead-weight.
I prefer a total-body-length cover for the TOP and bottom.
With open sides.
Ergo, no crosswind.
There is risk of being top-heavy, though.
Aggressive design to place all mass as low as possible is wise.
The chopper handle hands-out is the lowest drag.
The Bachetta bikes are a good example.
Under-seat-steering increases front cross-section.
Of course, a fairing that is one blown bubble will occupy far more front area than is needed.
For this reason, some fairly complex shapes are justified.
The additional perk of over-the-top fairings (and window) is sun/rain use.
A simple non-moving version would be the transparent middle fairing on the mid-down pic.
Then chop out the sides on a diagonal, narrow at the bottom.
A body sock lends itself to use with a top/bottom fairing. It has plenty of anchor points.