Aerodynamics

Aerodynamics

At normal bikes the lion´s share of the total resistance on the flat is air resistance. To build a faster bike you´d have to try to reduce this air resistance first of all. This can take place to a small extent as e.g. the use of a streamlined helmet or triathlon handlebars. But you can reach the optimum only by fairing of the whole bike including the driver´s head and the wheels.

Magic Scooter 2

Introduction to the aerodynamics of fully faired recumbent bikes

The shape of Magic Scooter 2 is based authoritative on its predecessor. That´s why I`ll start from the beginning and describe first of all the

development of the shape for Magic Scooter 1 .

As already mentioned under Concept the first model "Magic Scooter 1" was entirely successful, but not yet competitive with the international top teams. The main reason for that was that the air resistance was still too big, because the performance of a fully faired recumbent drops or stands with its aerodynamic qualities. In addition MS1 was difficult to control in gusty crosswind conditions.
So I quickly had the wish for a new, aerodynamically opimated following model.
Of course I had also cast my eyes over the competitors. Fully faired ( singletrack ) recumbent bikes were on the whole split in two types :

r those with a big cockpit and a relative simple basic form and

those with a head lid on the top of a basic form.

But in the rules of the international association IHPVA it was already established at that time, that a video system is permitted to control the bike. This means no big cockpit or lid with visor is necessary any more. You can design the recumbent position to be even more extreme and you can place your head completely behind your body. For the view outside there´s a camera installed. The driver has a small monitor in front and he can follow then his ride on it like a computer game. By that naturally reveal extreme possibilities to improve in aerodynamic view once again, and that´s why there are also bikes of this kind in the meantime, here the model of the American Matt Weaver "Virtual Edge" :

But the drawback is the extremely limited area to use, i.e. record attempts and some time trials. The thought of such a bike whizzed around in my head a long time, but then it got condemned again. In my opinion a bike, whose development and construction takes several years, should be able to take part in many different competitions. And you can´t imagine what happens if the monitor fails ...

As in MS1 also for the follow-up a big glass cockpit was not acceptable. Instead a small head lid should be used again.

The following changes have been made based on Magic Scooter 1:

20 cm lower:
I took a compromise in this item. On the one hand the frontal area should get reduced and the wheels should get integrated in the main stream body. In addition the sensitivity to crosswind will be reduced with a lower lateral area because of the smaller lever and the crosswind being less dangerous at the bottom by the boundary layer.
On the other hand I didn´t want to lower the center of gravity so near to the bottom that the straight running suffers too much by nervous steering behaviour. Besides, the landing gear should have room, and for that a certain height was necessary.

shape of the front :
To keep the hole for the swinging front wheel as small as possible I pulled the fairing from the front as far to the bottom as convenient, so at the front wheel there´s less than 5 cm to the bottom. For this reason I naturally did without a suspension system at MS2.
I tried to form the front in the bottom area as sharp as possible, because, by the proximity to the bottom, the stream at the bottom of the fairing is accelerated specially quickly. By that not so much air comes under the fairing and instead past the flanks, although this could have negative effects on the crosswind sensitivity.

shape and size of the head lid :
The lid decreased moderately in width, because the lid in MS1 was wider than really necessary. In addition I raised the lid a bit in the tail region and formed it more "smartly".

shape of the tail :
I lengthened the tail in such a way that a normal 28inch rear wheel has room again which otherwise wouldn´t have been possible by the 20cm lowering.

breathing air supply :
At the ventilation of the interior I attached great importance to an optimal breathing air supply this time. So I placed an admission in front of the head lid and led the air through the handlebar directly to the mouth. To use this air also for cooling the interior is tied to the back / the bottom. So the air has perforcely to flow past the body and the legs to the front wheel.

wheel fairing :
To avoid eddies inside the fairing the wheels are also faired in MS2. Because of the expected high speeds I also made sure the hollows were tight as well as possible. By that I wanted to avoid the air in the wheel fairings pressing radially to the outside as in a centrifuge and a steady air flow arising then.

Comparison of the aerodynamics of different bikes with zero angle of attack :

	cd*A	cd values	frontal area A
upright race bike, dep. on driver and equipment	0,23 - 0,40	0,5 - 0,75	0,4 - 0,55
unfaired recumbent, low	0,24	0,74	0,32
low recumbent with tail fairing	0,187	0,54	0,35
M5-Lowracer, fully faired	0,047	0,16	0,29
Cutting Edge	0,032	0,125	0,26
Pinguin ( Frank Lienhard )	0,0245	0,098	0,25
Varna Diablo 2002	0,0185	0,097	0,191
Magic Scooter 2	0,0225	0,067	0,335

The values have different sources, so you can compare them just partially and you should use them just as a clue.

Comparison of resistances

The following values have standardly been used for roll resistance :
crr= 0,004
weight bike+driver = 85 kg

You have to add mechanical losses as well as resistances by gradient and acceleration to the power in the upper diagram. Moreover ideal conditions have been presupposed, i.e. even further resistances can be added by wind, bike pendular movements, increased pressure in turns, etc.