Friday, May 5, 2017

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.

12 comments:

  1. Tires that have very high pressure start to bounce. Bouncing wastes energy and make your wheel(s) with traction (micro)slip.

    Shockabsorbers will eat (bouncing) energy but keep the traction wheel(s) from slipping, but without shock absorbers you will have more (micro)slip between your traction wheel and road surface.

    So losses on variable road surfaces with high pressure tires no matter what. One would expect that the more rigid a vehicle is ( and the more wheels is has ) the more it will have bouncing losses.

    The exception being race tracks with continuous smooth road surface. No bouncing there so no shockabsorbers needed and you will go faster with high pressure tires.

    On uneven surfaces ( real world roads ) broad tires with lower pressure will give lower rolling resistance. In velomobiles you can get away with the higher air resistance of wide tires because they are covered in your streamline for 70 %.

    I do wonder if there is a speed within the general velomobile
    riders perfomance envelope where wide tires actually are worse then small tires due to increasing air resistance. I suspect it will not occur for most riders as not many will exceed 50 km/u or 31 mph, and it is only a very high speed competion racing thing.

    It would be a common misconception to interpret the fastest racing condition tires to make you faster on normal roads.




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  2. I add that most velomobile owners probably can exceed 50 km/h or 31 mph but will generally not spend most or their riding time above that speed, so it will in fact not be a very important factor.

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  3. I performed the 50-60psi tests riding an unfaired Greenspeed trike on a downhill that I did most every ride in the last couple of miles coming home. I deliberately varied the tyre pressure every ride and stopped at a given location. From there I let the trike roll and noted where I rolled to a stop about 1/2 of a mile later. The surface was very broken in places. I was using Tioga Comp Pools - sadly no longer made.
    On *broken* surfaces the suspension effect from the softer tyres was significant with the optimal pressure being around 60psi, with a drop off in distance for both harder and softer pressures.
    On another local section of road with beautiful brand new tarmac I did get the more usual expected 'higher is better' results.
    My conclusion was that it is important to balance pressure for the anticipated road conditions - as mentioned a race track can be considered 'perfect', a mixed surface ride is difficult to optimise for, a known rough ride benefits from the lower pressures.
    I note that this does seem to read across to velomobiles - I have a QV with soft air suspension and an EvoK with an air shock that is set to give a fairly harsh ride. The QV's airshocks appear better tuned to eliminate road shock from high frequency uneven surfaces and give a noticeably smoother and faster ride over broken pavement where as the EvoK which seems more tuned to handle pothole type bumps is generally (as would be expected from a 'sports' machine) a faster machine as long as the road surface agrees with it.
    Unfortunately it is very common here for roads to be top-dressed (shake-and-bake) where the road is painted and small stones thrown at it, some sticking to the surface.
    The softer, higher frequency suspension is happier on this material.

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  4. Thank you for your comments, I tend to agree with Rob that you could see difference in rolling resistance depending on road conditions and tire pressure but it is hard in the abstract to say if it would be better at a given pressure in every case. The impact of a specific velomobile suspension has on rolling resistance is an unknown but it should mitigate the bouncing effect to some extent.

    Roads are very different, on a given ride you can go from a smooth repaved surface to something looking like the course of the Paris-Roubais. Do you set the pressure based on the worst or best conditions you are expecting to encounter?

    Rob made his tests using one particular tire and one would have to check if other tires behave the same before we could come to a conclusion.

    As I said in the post above, there is a lot of research that can be done to demystify the importance of tire pressure on rolling resistance on several road surfaces and condition validated with several different types of velomobile.

    This post is not the definitive thesis on rolling resistance but it provides the basic elements that riders should consider in their quest for the best rolling resistance.

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    1. Exactly so Luc...

      The shake-and-bake here means I am particularly keen on a velo with suspension that handles high-frequency vibration.

      But as the season progresses 'fresh' shake-and-bake goes one of two ways...
      1) At junctions/farm entrances/lorry turnings it gets ripped up and ends up rutted - it deteriorates into field like ruts that needs very soft suspension to cope with, or
      2) The 'shake' stones melt into the surface gradually improving into that race-like surface.

      So even after very careful testing on ONE road the conditions vary over the season and testing is thus a forever ongoing event :(

      Reading the tyre testing of Wim and some of the Dutch riders they are finding variations in behaviour of a tyre between 10C and freezing - with some tyres behaving just the same as normal and others getting very much worse.

      There are many, many variables - no wonder the tyre manufacturers often choose to blind us with tread patterns and fancy names for their 'technologies'...!

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    2. This is a very complex subject that keep tire engineers working. Formula One tire manufacturers have been designing tires for specific tracks under wet and dry conditions taking into consideration the expected temperature. Tires for velomobiles are in different categories and as riders we need to experiment to find the best combination of tire and pressure that will be a compromise for rolling resistance but also for handling, puncture protection and durability. A rider who expects to have a cushy ride and never have a flat tire will have to compromise on rolling resistance but there are still options open to him or her. Hopefully this post and comments will enable that rider to make an informed decision.

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  5. That is another matter : do cycle tires really need tire threads/profile ?

    Below a certain speed and above a certain weight thus wheel pressure and ofcourse how large the tire-road contact surface is it is not necessary to have tire threads/profile to prevent a water film under the tire.

    Downside of tire threads is they give higher rolling resistance. Even a bit more air resistance too. Plus side is they probably prevent some leaks if the sharp object isn't able to punch through far enough.

    It will only really be needed if you cycle through sand, mud or snow. I use summer and winter tires so the summer one can be "flop on/flop off" folding slicks that can be changed without tools in about 3 minutes and are easy to store in the velomobile. In winter i go for anti leak tires that do have tire threads/profile.

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    1. Well I was surprised initially when I saw that the Shredda (with threads) had a lower rolling resistance than say a Kojak or a Durano but when you think about it, the tread is only one aspect of tire construction. The compound, the number of threads, etc. are probably larger contributor to making a fast tire. Do not underestimate the threads when riding on wet or dirty roads. Recently I rode a brevet where the road was covered with sand used for providing winter adherence on snow and ice. I was drifting through the curves and I was spinning on steep hills (10%+). I used to race Karts when I was younger and driving with slicks under wet conditions was like driving on roller bearings so threads under certain conditions can be important. If you ride normally on mostly straight flat roads, it would not be a problem in wet conditions.

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    2. The "rubber" compound is important - for most road riding the tread is just decoration that customers in shops like to feel - this came from the Schwalbe Rep...
      I've found Kojak too 'hard' so gets slippy on wet roads where as Durano and Scorcher are a softer 'rubber' and so grip better in the wet.
      My comparison based on rear wheel 559s on my EvoK - and no doubt Schwalbe change the rubbers every season - I heard the Duranos can now be slippery but they were my grippy tyre of choice....

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    3. If you look closely at the road surface and closely at a tyre tread then unless you've got much better roads than I have then the road lumps are much bigger than any tread pattern I've seen on road/touring tyres...
      And 'gaps' between bits of tread have no contact with the road so provide NO traction... :(
      Oddly I've had success riding soft 'rubber' slicks on my delta trike in the snow.

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  6. I didn't see anywhere where you gave a relative percentage of all the various forces working against a velomobile - relative to speed. That would be hard to generalize - but might give even more in depth discussion.

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  7. Very important informations. Thanks from Brasil....

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