Velomobile efficiency
is one of the best reasons to own a velomobile.
This is the third post, the first was on aerodynamics, the second on
rolling resistance and today I will cover weight and mechanical
efficiency. Each velomobile is different
so the discussion has to be general in nature.
I received many good comments
on my previous post regarding rolling resistance and some pointed to items I
did not mention. First, it is clear that
a tire/tube combination at a given pressure may work best in specific
conditions but may not be optimal in all conditions. There needs to be experimentation to find
what is best in your riding environment.
All the measurements will provide some avenues to explore or give you a
quick gauge for comparing but they do not provide a definitive comparison for
your conditions.
Weight
Weight is an important
characteristic of velomobile performance.
Weight has an impact on acceleration and ability to climb hills. There are significant differences in the weight
of commercial velomobiles, are in the 22-23kg range while others are in the 40
to 60kg range. This is something to
consider when choosing a velomobile because the rider has to move that
weight. Currently a racing velomobile
can be in the 12 to 15kg range and we could see a sub 20kg production
velomobile in the not too distant future.
Weight in cycling is
very important. Several years ago the
UCI has mandated that road bikes should not be lighter than 6.8kg, this is to
ensure that riders do not have an undue advantage when climbing or accelerating
and it was also to ensure that manufacturers are not cutting corners affecting
rider safety in the process of making a bike lighter. Nowadays, with better techniques and
materials bike manufacturers can easily make safe bicycles that are less than
the 6.8kg limit and the UCI could easily revisit that rule to help increase the
performance of cyclists.
As you can see the
weight of a velomobile is significantly more than the weight of a good road
bike. Even the lightest racing velomobiles mentioned above are nearly twice the
weight of a good road bike.
When climbing up a
hill, a production velomobile is at a significant disadvantage compared to a
road bike. A rider in the best
production velomobile has to push the equivalent weight of more than three road
bikes up a hill and it could be almost 8 road bikes for the not so light
velomobiles. Since there is no aerodynamic
advantage on steep hills and the weight and rolling resistance of the 3 smaller
wheels makes the effort more difficult for the velomobile. As a result, the rider has to expend more
energy to climb steep hills and the velomobile will not be able to match the
speed of a road bike.
Fortunately, the added
weight will be an advantage on descents and increase momentum and that is
desirable when the road has a number of short rolling hills. The energy stored will enable the experienced
rider carry the momentum of the descent into the next climb almost effortlessly
and at great speed while a road bike rider would need to climb each one.
Of course if one could
reduce the weight of the velomobile, there could be important performance gains
to be made. Top velomobile designers
will use their skills to choose the best materials to reduce weight and improve
performance. In choosing a velomobile
many people will place the looks over other important characteristics such as
the weight. There are a few things the
average rider can do to reduce the weight of a velomobile including some
choices can be made at purchase time like the type of drivetrain. But unless you are racing, some things like
lighter wheels and tires for the velomobile may not provide a significant
improvement since the resulting savings would only reduce the weight by a few
percent of the overall weight of the velomobile, the same change would be three
or more times as important for a road bike.
A velomobile used on public roads may encounter several hazards and
velonauts should be careful when considering lighter components since they can
potentially trade-off weight for sturdiness, it depends on the material used
and the type of construction. A light
wheel for example could fold more easily in a pothole.
However rider who
would like to improve performance can decide if they should be installing
optional equipment like a sound system. An
even easier way to reduce weight is looking at what they carry in the
velomobile. I’m probably guilty of
carrying too much as I like to be prepared for most eventualities. Leaving at home the kitchen sink, personal
anvil or other stuff seldom used could probably help you get better
performance. It is very likely that you
carry a kg of stuff that is not really required.
Mechanical Efficiency
Mechanical is the last
element of efficiency; power needs to be transferred from the pedals to the
wheel with the smallest loss possible.
Most of this is determined by the design of the velomobile and components
but for the most determined rider, it is possible to make some
improvements. I make my comments based on velomobiles with rear-wheel
powered tadpole trike configuration because they are by far the most popular, however
some but not all of these comments will apply to other models (delta or
4-wheel).
Getting power to the
wheel efficiently requires stiffness.
The crank, boom and swingarm are all elements that contribute
significantly to stiffness but the monocoque shell or the frame, as the
backbone, is the glue that holds everything together. When pressure is applied on the pedals, any
torsion or unwanted movement of any components will result in a loss of power
transfer. The unwanted movement can be a
very small lateral movement of the boom or the swingarm but the loss will be
noticeable. The movement can be so small
that you would not be able to see it while riding. Well-designed components made of materials
like carbon fiber instead of aluminum normally provide more stiffness.
People realising the
issue have modified their velomobiles to increase stiffness. Being the most popular velomobile, many
changes were reported for the Quest in particular in order to address this
issue. Some of the changes include the
replacement of the aluminum coat hanger used to provide stiffness in the
turtledeck area and the point of attachment for the rear shock. The replacement made of carbon fiber material
creates an attachment point at the top of the wheel well. A second change is the replacement of the
aluminum swingarm with the newer carbon fiber version. A third change is the installation of a
carbon fiber pillar to give rigidity to the aluminum boom. A number of riders also have added carbon
fiber ribs in several areas of the shell.
Many of these improvements could also be made on different models of
velomobile with similar results. These
changes also have the potential of adding extra weight with minimal stiffness
improvements so one has to be careful in making this type of modifications by
carefully researching, planning and implementing them. In the case of plastics like the Rotovelo,
while I cannot confirm this, I expect that the plastic material would be much
less stiff and may contribute to a loss of performance that may be significant,
especially in the case of the Rotovelo where there is virtually no additional
metal frame.
In addition to stiffness,
the drivetrain can be a source of power loss.
Chain line rubbing, unnecessary chain tube, damaged or missing idlers,
bad alignment of chain line components are possible sources of power loss.
Suspension can also rob
significant power; every push on the pedal can contract the rear suspension to
some degree. This is particularly
noticeable when climbing a steep hill at low cadence. There could be a significant bobbing effect. Instead of using the power on the pedal to
turn the wheel, part of the energy is used to compress the rear shock. The problem can be larger when the rear shock
is soft. Adding more air in an air/oil
shock or installing stiffer springs or polymer in a mechanical shock could
improve efficiency.
Bearings play an
important role in the velomobile efficiency.
Since all bearings are not created equal, for the same size bearing,
there are several categories, some have looser tolerances and there is a bit
more movement inside so they do not roll as well as others, I would call them
cheap but there are others that are of tighter tolerance and are more efficient. In the United States there are 5 classes of
bearings (ABEC1, 3, 5, 7, 9 with 9 being the best) but since bearings are made
in many countries, they may use another standard such as the ISO and DIN that
have similar levels, see table below. Bearing classes explained
|
ANSI Standard 20
|
ISO 492
|
DIN 620
|
|
ABEC 1
|
Class Normal
|
P0
|
|
ABEC 3
|
Class 6
|
P6
|
|
ABEC 5
|
Class 5
|
P5
|
|
ABEC 7
|
Class 4
|
P4
|
|
ABEC 9
|
Class 2
|
P2
|
The different classes
take into consideration several factors that will affect performance internal clearance, surface finish, ball accuracy, torque,
noise, cage type, and lubrication Some racers use the lowest
friction bearings to lower resistance and improve performance but the gain
could be small relative to the increased cost for those bearings. A 406 wheel equipped with 28mm tires will
turn almost 10 times a second at 50km/h and small movement could make you lose significant
power but the loss is even greater if those same bearings are worn or damaged.
It may surprise some
people but bearings in the front wheels can be subjected to intense heat
generated by hub brakes. Excessive heat
can also contribute to bearing failure.
Repeated long descents under load may cause bearings to fail.
There are bearings in
wheels, pedals, bottom bracket, cassette and some can be changed some cannot
because they are sealed inside the unit and you may have to change the whole
unit (cassette, pedal, bottom bracket) if they are worn or damaged. Idlers could have bearings or bushings and
the bearings could be sealed too. A
damaged bearing or bushing will typically have one of many of the following
unwanted movement (lateral or perpendicular), difficulty turning, noise
(grinding, rubbing) or other damage like missing or damaged seal.
A key aspect of power
transfer is the drivetrain. There are many different types and all are not
created equal. Velomobile drivetrains borrow
elements of several bicycle systems but they are different in several ways from
traditional bicycles. First difference
is the chain length; velomobiles have the equivalent of 3 standard lengths of
chain. To function properly the
drivetrain also needs idlers, tensioners, chain tubes to transfer the power
from the pedals to the wheels, something only recumbent bikes use. Second, because velomobiles have higher top
speed than unfaired bikes, velomobiles need higher gearing but due to their
higher weight, they also need lower gearing than most road bikes to climb steep
hills. As a result, the required gearing
range is much greater than the gearing used by other bikes. This means that components are often near or
at the maximum capacity. To meet the
requirements, velomobile designers will mix and match different components made
for different types of bikes (grupo) for example road and mountain bikes and
even urban bikes. They have larger chain
rings some pushing 75 teeth, wider range cassette e.g.: 11-36 or more.
Most often, high
capacity long cage rear derailleurs will be chosen to handle the range. I expect that designers will soon take
advantage of the 11 and 12 speed rear derailleur developed for mountain biking
with wide range gearing and install them in velomobiles soon. It may not be a slam-dunk for velomobile
applications there are several potential issues. First, since the internal space required for
the extra long cage derailleurs and the smaller chain may create some
issues. It is also not clear if the rear
derailleurs designed to be used for single ring applications could handle the
extra chain required for a 2 X11 or 2X12 configuration especially if the big
and small ring has a big difference in the size of the rings e.g.: 62-34.
When operating at or
near the limit of components that were not designed together, one has to be
careful that efficiency is not affected significantly. Some of the new wide range cassettes may not
fit standard hubs but if they did they may create issues. There are cassettes with 9 and 10 teeth that
are used to provide the speed for mountain bikes racing down hills or for
trikes with smaller wheels. In their
applications they could be fine because these gears are only used seldom going
down a hill for example but they are highly inefficient. In velomobiles, we will tend to use the
higher gearing even on flat ground and for much longer periods of time. The angle of the tooth spacing increases and
the chain does not sit in the grove very well; to illustrate this at the
extreme, imagine a 4-tooth cog, the teeth would be spaced 90 deg. The chain would get stuck or skip when you
would try to turn the crank. I have a
bike with a Capreo hub with a cassette that has a 10-tooth cog and I find it awkward
to pedal and I can feel the inefficiency.
Some road racers don’t even use 11-tooth cog on their cassette because
they feel they are inefficient but I think they are OK in velomobiles. As a side note, it is not certain that
cassettes with 9 and 10-tooth cogs could be fitted on to velomobile axles as
they are typically require installation onto smaller diameter hubs.
Velomobile with
smaller rear wheels (20in/406 or 16in/349) at the rear will require higher
gearing, to compensate because the smaller wheel means that it has to turn
significantly more than a 26in wheel to achieve the same speed. To address this issue, manufacturers will
often install a mid drive; a secondary gearing that can be as simple as a 3-speed
cassette and derailleur to Internal Geared Hubs (IGH) like a Rohloff. Mid-drives add some amount of drag but can provide
a much larger gear range.
As mentioned above, it
is possible to use other gearing system like the IGH. There are different make
and model of IGH with their pros and cons, there are also a few pedal-based
internal gearbox and even hybrid systems that have an IGH coupled with a
cassette and there is also crank based systems like the Schlumpf that multiply
your gearing. They each have significant
advantages including providing a wide range of gears, some have the ability to
change gears when at a standstill and they have low maintenance. Unfortunately, some of those are noisy and
can be heavy and may have torque limitations that can be exceeded by
velomobiles under certain conditions and can’t be shifted under load. As far as efficiency, some reports indicate
that there is a 6% penalty for IGH compared to derailleur systems. This can be significant for a performance-oriented
rider and something to think about when making your choice for gearing.
There are other
factors that could create power loss. In my previous post on rolling
resistance, I mentioned ensuring proper alignment contributes to mechanical
efficiency but so is truing of the wheels.
A wobbly wheel will sap your power because you will need to push harder
to maintain your speed. While more rare, brake adjustment could be
over-tightened and touch ever so slightly when the brake handle is released and
this is another thing that could sap your power. Brake adjustment should be part of proper
regular maintenance. In short, if it rubs, squeaks or grinds it probably robs
your power and needs to be fixed.
Lastly for a good
velomobile, you also need efficient brakes to keep velomobile rider safe. Most velomobile use drum brakes on the front
wheels. There are 70mm and 90mm drums
and both will stop a velomobile efficiently on flat ground but when the road
gets hilly, 70mm brakes will probably be unable to meet the challenge, the
brakes will quickly overheat. The larger
90mm version will fare better but they will eventually overheat and lose their
braking efficiency. There are 90mm drum
brakes with added fins to cool brakes faster and some riders have designed
simple water injection system to further cool brakes. Riders in mountainous areas should consider
these enhancements. There are some
velomobiles equipped with disk brakes that may offer better braking in some
conditions but they require more maintenance and many have switched to drum
brakes.
Through this three
part series on velomobile efficiency I think that I have covered most elements
affecting efficiency and performance. While I’ve touched on many things, the
information is fairly superficial and there is much more to investigate to
deepen your knowledge; an ocean wide but a foot deep. There is still a lot to be researched and I
would hope those who can add apiece to the puzzle would publish their findings
through public forums so everyone can learn.
I hope that these post provided you a glimpse into what makes a
velomobile efficient.
Effigear
A new pedal-based gearbox
designed for mountain biking is now available. EFFIGEAR's has a gear ratio of 444% with 9-speed providing a similar
range as derailleur-based, double chainring 24/36t with an 11/34t cassette. This system provides the ability to shift
under load contrary to other IGH and gearbox systems. The gearbox system weighs 2.8kg including
cranks and shifters. The gearbox can
also be used with a rear derailleur to further increase the range. Unfortunately to install in a velomobile
would require the design of a new crank mount.
This system is similar to the Pinion gearbox that offers several systems
from 9 gear 568% ratio, 12 gear 600% ratio to an18 gears and 636% ratio.
www.effigear.com
SPEZI
