Efficiency
Velomobile efficiency
is one of the best reasons to own a velomobile.
The subject is fairly complex and I will try to cover the subject in a
few posts here. Today I will an overview
of what characteristics of makes a velomobile efficient and address
aerodynamics.
Out of the box,
velomobiles are fairly efficient when you compare to an unfaired bike but there
are still many things that will give you an even better performance including
several options available from the manufacturers.
How efficient are production
velomobiles? Again this depends on the
velomobile you compare with but if you have a Milan SL or a DF, you have one of
the most efficient velomobiles out of the box.
As a rule of thumb I say that since 80% of the effort on a road bicycle
is pushing the air, a velomobile should improve this by halving this
effort. This gives a 40% advantage (50%
X 80%) to the velomobile. As a result to
maintain a certain speed a bike requires 300W while the velomobile requires
240W X50% +60W = 180W. I tried to see if
my rule of thumb was close so I looked at segments that I rode and compared
them to the next rider assuming he/she was riding a road bike. In one example a rider is pushing 255W for
12.5km on a somewhat flat segment, no wind and a speed of 39.5km. For the same segment, I rode the same segment
with a 10km/h headwind. I needed 155W to
travel at a speed of 47.5km/h. Leaving
the speed difference and wind, my rule of thumb would give me an effort of 204W
X 50% + 51W = 153W. While it appears to
somewhat confirms the rule of thumb, the difference in speed and the extra work
due to the wind has not been accounted for so the efficiency appears to be even
greater on flat ground. Of course the difference
on a hillier course would be somewhat different, as the power to climb the
hills needs to be much greater for a velomobile.
I get it, many people
are happy with the weather protection and carrying capacity of
velomobiles. Others do not see a need
for improvements or are unable to fully extract the performance of a stock
velomobile or they opted for e-assist that gives them the boost they need. Still there is a large segment of riders who
want to get as much speed as possible for the power they can generate. Getting
the most performance out of your velomobile requires paying attention to the
details.
There are four areas
where improvements can normally be made: aerodynamics; rolling resistance;
weight and mechanical efficiency.
Aerodynamics
There are many ways to
improve the aerodynamic efficiency of a production velomobile but it depends on
the velomobile you ride. It may be the
low hanging fruit for improving velomobile efficiency. Every thing that sticks out like mirrors or
holes in the shell like the hatch for the cockpit creates drag. I will provide some ideas on how drag can be
reduced but these recommendations have to take into account your own velomobile
and circumstances. Beyond these tips
there could be many more improvements that may be possible.
I was reminded of this
recently as I started my season riding without any significant modifications on
my DF, it was essentially a stock DF.
The only small improvement was a mini visor. I rode a few weeks until I decided to put on
the hood. That single improvement
provided me a 3 to 5km/h average speed improvement on the same courses; this is
very significant in the order of 10 to 12% for the same power output. Going down hills where I had a maximum speed
of 62 to 65km/h, I was now maxing out at 72 to 74km/h, a 10 to 14% increase.
Open hatch velomobile
and the rider’s head/torso have a major impact on aerodynamic efficiency. Even a rider’s helmet increases the
drag. First let’s look at the
obvious. Many open hatch velomobile
usually have a neoprene skirt that covers the hatch leaving only room for the
head to stick out. This provides a
significant improvement but it can be uncomfortable to ride with the skirt
regularly. Another small improvement is
the mini visor that is attached to the front of the hatch with a Velcro. The visor deflects some of the air around the
rider. There is a small aerodynamic
impact. A hood that covers the open
hatch of the velomobile is probably the best way to minimize the drag of the
velomobile. Some velomobiles like the
Quest have several different hood designs and manufacturers to choose
from. Each has its own advantage and
disadvantage and have differing level of efficiency while others have a more
restricted choice. The hood has the
added advantage of weather protection in cold and wet conditions. On the other hand, visibility can be somewhat
more restricted, it may be difficult if not impossible to wear a helmet, the
cockpit can become too hot for some or it can fog-up. For racers, commercial hoods have also been
modified to make them even more efficient for example by reducing the frontal
area. While hoods are not created equal,
as I mentioned above the gain from a hood can be significant, it is probably
the single biggest aerodynamic improvement that can be made.
Open wheel velomobiles
like the Strada, Mango, DF, Evo-K suffer to different degree from the
turbulence created by the front wheels and wheel wells. There are several ways this can be addressed
with different levels of performance improvements. First wheel cover help for
open wheel velomobiles by covering the turbulence from the spokes. Some wheel covers are made of Lycra while
others are made of fiberglass or carbon fiber discs glued to the rim.
The wheel wells are a
source of air turbulence. Some
velomobiles have tighter space between the wheel well and the tire to reduce
this turbulence. Small plastic
deflectors that essentially cover the gap in the wheel well between the shell
and the wheel leaving just enough room for the tire to pass when turning can be
added to minimize turbulence. Deflectors are normally taped to the shell just
around the wheel wells. This provides a small but noticeable improvement in
efficiency.
If you would like to
further, increase efficiency wheel pants are the solution. Wheel pants essentially cover the whole wheel
well making the airflow past the wheel well following the shape of the velomobile. This minimizes the turbulence
significantly. Only the bottom of the
wheel is visible. Unfortunately, there
is a small price to pay because the wheel pants are restricting tire size and
increasing turning radius. Some racing
wheel pants or wheel pant extensions can be installed on racing velomobiles to
further reduce drag as the wheel pant/extension are made to cover the whole
wheel leaving less than a centimeter of air space between the ground. Even velomobiles where the wheels are covered
like the Quest could see performance improvement with extensions of the wheel
covers effectively hiding the bottom of the wheel and closing the bottom of the
wheel well.
Velomobile with foot
holes can see improvements by closing the foot holes with special covers that
have bumps under the shell to give room for the feet to move freely on the
pedals. While I don’t have a number to
give, the improvement can be significant.
The perfect tail for a
velomobile is shaped like a wedge.
Unfortunately in order to provide a surface for rear lights and
reflectors to increase visibility, most velomobile tails are somewhat
truncated. Many riders have found a
solution and added a tail extension transforming the blunt tail into a wedge
using transparent plastics. The reflectors
and lights are still visible but the airflow is better reducing turbulence. The
improvement is small but noticeable for racers.
The nose is something
that has attracted attention lately. The
DF, for example is a velomobile that has an air intake at the front. While it is designed to minimize the aerodynamic
drag but it still has an impact. Closing
the hole will result in a warmer cockpit but that may not be an issue for a
race or when the temperature is cold but may lead to an increase in fogging up
inside the hood. To get the
improvements, the cover has to follow the shape of the current nose and using
clear plastics would not impede the headlights hidden inside. While this small modification provides
improvements, you can go further.
Recently I was made
aware of one rider who modified the nose of his Milan SL by extending it making
it pointier and saw speed increase. This
modification is not without drawbacks.
The modification makes the velomobile more susceptible to side winds and
this could make the velomobile difficult to control at high speed under windy
conditions.
Beyond these
modifications, those racing may also try to use tape to close seams and other
holes and cracks in the shell. Anything
protruding from the shell like lights, cameras and mirrors especially large
ones are also creating drag. While I
would not recommend removing any items used for safety, riders may choose to
reduce their impact or removing them for special events like races.
IntercityBike
Daniel Fenn is hard at
work on the DF-4, the 4-wheel DF prototype.
He recently went on a 170km ride with the prototype. Daniel even took his dog in the velomobile on
this journey, the dog can fit in the luggage compartment just behind the seat
is much larger than the DF. ICB has
posted several pictures and video on their blog. The prototype will see more refinement before
a decision is made to go to production.
There are a number of
interesting innovations in the design.
The pedals will drive the left wheel of the velomobile while an optional
motor providing assistance will drive the right wheel. The DF-4 prototype has a mid-drive
Rolhoff and a 10 speed cassette at the
back. The wheel wells are larger than
the DF/DF-XL and would enable the use of popular larger tires like the Shredda
and the F-Lite.
Busy as usual, Daniel
is also in the process of producing a new racing hood for the DF with no side
windows. This is the type of hood that
Milan riders have used in record attempts.
The hood is apparently 33% lighter and hopefully more efficient than the
original DF hood.
