Efficiency (2)

Efficiency (2)

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.   In my last post I reviewed how aerodynamics played a role in making velomobiles efficient. 

I received many comments and some pointed to items I did not mention.  There are radical ways that could potentially further improving aerodynamics but the potential gain would come at a cost like difficulty to manufacture, complexity, weight, etc.  What I tried to identify were practical and safer ways for improvements. 

Something I forgot to mention however is the potentially positive impact of velomobile shells while riding in the wind.  Not only does the shell reduce the effort when riding up wind but when the wind comes from the side or the back, the shell can act as a sail further improving performance.  On a regular bicycle, the effect is only felt when riding with the wind is coming more or less directly from behind.

In this post I will discuss rolling resistance; again, this is a high-level view at the issue, not a comprehensive thesis.  If readers want to explore this subject, there are many sources that can be accessed on-line but there is still much to be discovered.  Riders may want to perform rolling resistance tests to contribute to the discussion.

Following aerodynamics, rolling resistance has probably the second largest impact on performance.  In my opinion, paying attention to rolling resistance may be even more beneficial for a velomobile compared to a bike.  There are several things that affect rolling resistance that velomobile owners need to take into consideration.

Since the tires are effectively the points of contact with the road, therefore they are the point we measure for rolling resistance.  Several factors affect rolling resistance including weather, temperature, rolling surface but also tire construction, tire size, weight, inflation and type of inner tube/tubeless.  I will try to address most of these factors, more specifically their influence on velomobile performance.

Compared to a bike, a velomobile has three or four wheels increasing the points of contact with the road by 50 to 100%.  Furthermore velomobile wheels have a significantly smaller diameter that make the same tire brand/model potentially less efficient compared to a road bicycle with 700 wheels (size 622).  Most velomobile use significantly smaller wheels such as size 406 (20in) all around or a combination of 406 (20in) tires at the front and 559 (26in) at the rear.  More recently some velomobiles that can accept wide 406 tires have been able to replace those with narrow tires size 451 to provide more tire choices.  Australian Pedal Prix velomobiles use 349 (16in) all around or 349 (16in) at the front and 406 (20in) at the rear. There are exceptions for example the LeMans but these configurations cover most velomobiles and some riders have experimented with other sizes like 650B (size 584) and 24in (size 540).

The heavier weight of the velomobile may also increase the impact of an increase in rolling resistance.  While the weight is distributed over 3 or 4 wheels vs two, think of a heavy cart that you have to push empty vs fully loaded.

One has to recognize that most; maybe all tires are not developed for velomobiles, since the market is too small.  Tires are developed mostly for BMX, MTB, e-bikes and to a lesser extent trikes.  As a result, it is difficult to find the perfect tires for velomobiles.  Tires are adapted to velomobiles, to find the best tires for velomobiles involves research and experimentation.

A smaller diameter tire will be more affected by road surface than a large one.  To illustrate this, imagine you are driving a truck with large tires where you can drive over a pothole while the driver of a Smart car can fit the entire wheel in the same pothole.  The energy of the rolling wheel is almost completely lost when the small wheel gets in the hole while the larger wheel skims over with only part of the tire entering the pothole resulting in a minimal energy loss.

To a smaller extent, tires encounter bumps, holes and cracks even on regular road surface.  While it can be hardly noticeable, it will affect how much power is required to move the velomobile.  One advantage of the velomobile is that the wheels and tires are mostly hidden behind the shell while unfaired bikes wheel and tires are subject to aerodynamic loss.  That loss prompts unfaired bike riders to use narrower tires to reduce the aerodynamic effect while velomobile riders can use wider tires with negligible aerodynamic penalty.  Using slightly wider tires will help minimize the impact of road surface and compensate for the use of small tires.   Unfortunately some velomobiles may not be fit on all velomobiles that have narrower wheel wells.  A wider tire gives more contact surface and provides better weight distribution putting less pressure on the road.

As I mentioned above, all tires are not created equal.  Tire construction incorporates several components that will affect how the tire performs under different conditions.  Tire construction is a set of compromise to meet a perceived market demand.  In order to explain, I will re-purpose some information related to tires I previously posted on BROL.

Tire/tube/sealant weight also has an impact on rolling resistance; as a rule of thumb, if you compare tires of similar construction, the lighter the faster.  Changing the inner tube to a lighter and subtler tube will lower rolling resistance in the same tire and the improvement can be significant.

Tire characteristics
Engineers determine the physical characteristics that will be used in the construction of a tire, this will in turn determination the tire’s different qualities. It is important to note that two tires by the same manufacturer using the same name may have a different construction depending on size or manufacturing plant.  You have to examine tire specs carefully to spot this.

Some of the physical characteristics including:
• number of threads per inch (TPI) - the higher number of threads normally the better;
• foldable bead - faster tires normally fold;
• subtleness of the tire is a factor in rolling resistance;
• Protection rolling band to make the tire more durable and puncture resistant;
• Compound is the rubber mixture used to make the tire;
• Thread is the pattern of groves in the tire - a tire with more groves will evacuate water better but may also increase rolling resistance;
• Weight of the tire will impact the performance a heavier tire will take more power to move it;
• Most tires require tubes but some of the better tires are tubeless.

Some of the tire characteristics to consider include durability, puncture protection, handling, rolling resistance, noise. You also have to take into consideration that these characteristics may be significantly different under hot vs cold and wet vs dry or even on snowy and icy conditions. Unfortunately when it comes time for shopping you may fall victim to marketing hype. Many website will provide an assessment of the characteristics but for the most part the information is biased.

The tube inside the tire will affect the characteristics of the tire.  There are two main type of tubes used in bicycle tires Latex and Butyl What are the advantages/disadvantages of the latex tubes vs butyl.

Butyl Tubes

• Butyl are inexpensive
• Available everywhere
• Available in all sizes
• Available with Schreader and Presta valves
• Have a slightly higher rolling resistance compared to latex tubes
• Pressure remains for longer periods

Latex tubes

• Much more expensive
• Not available everywhere
• Not available readily in velomobile wheel size
• Available mainly with Presta valves
• Have lower rolling resistance compared to butyl tubes
• Requires frequent pumping to maintain desired pressure

Weather conditions and temperature also have an impact on rolling resistance.  As a rule of thumb, rolling resistance increase as the temperature gets.  Wet conditions also increase rolling resistance.  Some tires have provide better rolling resistance in low temperature or in wet conditions so it is important that you consider this when choosing a tire for your velomobile.

Tire pressure is critical to rolling resistance, the higher the pressure the lower the rolling resistance.  For best rolling resistance tires should be inflated to the maximum pressure for the type of tire used.  Some riders in order to further reduce rolling resistance, exceed the maximum recommended pressure but one has to be careful as blowout could occur.  While appears to be true in tests, tests performed by the BHPC found from roll-down tests that a wider tire at lower pressure e.g.: 50-60 psi actually gave lower rolling resistance on a broken road surface than higher pressures, presumably this is due to the suspension effect.  It is not clear if this is also true for fully suspended velomobiles.

Rolling resistance tests:

Several people and organizations perform rolling resistance tests of velomobile and bike tires.  Here are a few places where you will find this information.  There are generally 2 main methods where the rolling resistance of tires is tested.  The most common is to measure the rolling resistance of tire by spinning the tire against a friction wheel/roller.  Rolling resistance is calculated in Watt.  The second is to use a pendulum on a rig where two tires are mounted with the arm of the pendulum at the top.  The arm of the pendulum is pushed to the maximum and released.  The time that the pendulum takes to stop provides a relative measure of the rolling resistance.  The longer the pendulum swings the lower the resistance of that tire.

Some of the tests have compared rolling resistance at different temperature even in snow/ice conditions but I don’t think that the same tests were performed on different rolling surface.

For more information on measuring rolling resistance of tires, you can check Carboro (www.velomobil.ch/ch/en/rollenpruefstand) – The site in German and provides very good information on measurements and tire rolling resistance in general as well as some measurements.
 
Wim Schermer  (wimschermer.blogspot.ca/search/label/bandentest) – Wim Schermer has a blog (in Dutch) where he regularly tests only velomobile appropriate tires size 406 and 559 uses a pendulum to test relative tire resistance.

 
The Bicycle Rolling Resistance website  www.bicyclerollingresistance.com - the site has few tires test for tires in the sizes used by velomobiles. If the same brand/model is tested, check with manufacturer’s site to see if they the velomobile size has the same construction/compound. Use a friction wheel.

On the German velomobile forum, there is a 20in (406) tire testing thread (www.velomobilforum.de/forum/i...t.41020/) where Daniel Fenn and other velonauts discuss tires and post results. Daniel uses a friction wheel.

There is a thread on BentRiderOnline where velomobile tires are also discussed but there are no objective tire tests performed. (www.bentrideronline.com/messageboard/showthread.php?t=133279)

For the more technically astute, it is also possible to measure a specific velomobile overall rolling resistance through roll down tests or with velomobile equipped with a power meter.  These tests help you calculate not only the rolling resistance (Crr) but also the aerodynamic resistance (CdA).  Robert Chung developed a methodology that can be used to perform these calculations.

One important aspect of rolling resistance not mentioned above is to ensure that your velomobile alignment is correct.  When the alignment is not correct the rolling resistance will increase.  Remember that for the lowest rolling resistance, all (3/4) wheels should be properly aligned not only the front wheels.  After your alignment you can test your alignment using a roll down test.  To perform this test you need a road with small hill followed by a long flat section.  On a day when the wind is very calm, take your velomobile to the top of the hill, identify a spot where you can get the velomobile to coast down when you release the brakes and mark that spot.  Perform your first coast down and mark the spot where your velomobile came to a stop.  Climb back to the top of the hill and adjust the alignment of both front wheels very slightly about ½ turn.  Perform the same coast down if you go further, adjust another ½ turn and try again if you go further continue the same if you don’t go back to previous setting.  This should be the optimal setting for the lowest rolling resistance.

Velomobile tire and tube price/availability

On a related note, I searched for tires and latex tubes recently and was very surprised that latex tubes in sizes other than 700 (622) and 26 (559) were basically not available in North America and velomobile tires were significantly more expensive in North America.  Several tire models are only available several months after they are available in Europe and often not even offered by North American distributors. It certainly is unfortunate that manufacturers and distributors policies put North American velonauts at a significant price/availability disadvantage.

IntercityBike

The 4-wheel DF prototype DF-4 made its first public appearance at SPEZI alongside the DF and DF-XL.  The DF-4 had a wheel pants a tail extension.

Sitko-velo

The sitko-velo was another new velomobile making its first public appearance at SPEZI 2017.  The new velomobile made of plywood comes with electric assist.  In my opinion the Sitko-velo will compete with similar velomobiles like the new Cab-Bike and the Orca.