Efficiency (3)

Efficiency (3)

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

If you did not have a chance to attend SPEZI this year, there are several who reported on the annual show including The Laiback Report with an excellent Spezi video tour and interviews.  . Wim Schermer reports on SPEZI on his blog  and Ligfiets reports on several items of interest