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Look at the design page for info on the plywood velomobile construction.

Saturday, 28 January 2012

On mass and aerodynamics

Is there a trade off between mass and aerodynamics? A streamlined velomobile may reach a higher speed at the same power input because of the decreased aerodynamic drag but the fairing adds extra mass to the vehicle. Is there a speed penalty in adding mass? The rolling resistance of the wheels will increases but also the time and energy needed to accelerate the vehicle. While setting a new hour record only the first minutes are used for increasing the speed. But in normal every day traffic we may have to stop every km...
I calculated the total trip time for a 5 km trip and varied the number of starts. In the next diagram you find the total trip time and the maximum speed for different vehicles. The coefficient of rolling resistance was held constant at 0.005. The QuestSL is a Quest with the mass of an MTB. For the calculation of the plywood velomobile I estimated the effective area to be equal to that of the Versatile. The mass of the Plywood Velomobile was very optimistic chosen to be 18 kg (The current proto is 23 kg).
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I assumed a cyclist that delivers 75 W power. At that level the MTB would reach ~20 km/h which is more that the most of us do... We can conclude that at 7 starts in 5 km the Quest and the Plywood Velomobile perform equal. The mass reduction compensates for the compromised aerodynamic quality. At an increased cyclist power the break even point moves to the right. 7 starts in a 5 km trip is quite high but not unrealistic for an urban trip. 
All in all we can conclude the most apparent difference exists between the MTB and the other vehicles. The differences between the velomobiles are small. It would be interesting to see what happens when a small slope is taken into account. Another interesting thing is recuperative breaking and start assist.

On second thougth: After a 5 km trip in a Quest with one start only the total work done is 48.2 kJ. The Quest reaches a maximal speed of ~42 km/h after 3.5 minutes. The kinetic energy of the vehicle and rider at 42 km/h is ~7 kJ. Now it becomes very clear what happens when we have to stop and accelerate again: we loose the kinetic energy of 7 kJ which is 7/48=15 % of the energy needed with one start only. Would we have to stop 7 times our energy usage more than doubles ! The MTB uses 104 kJ for the 5 km trip with one start only.  His kinetic energy reaches 2 kJ only (2/104= ~ 2%). An extra stop doesn't bother him to much... 
Depending on the number of starts in our trip, recuperative breaking and start assist would give a significant increase in performance of the Quest...

Remark: decreasing the mass of the velomobile is not helping much as it is dominated by de rider.

Posted by Titus van den Brink at 18:30

17 comments:

  1. Unknown28 January 2012 at 19:16

    That is an interesting point of view. I hadn't found any information that shows how much of a difference there is between velos. I guess that would mean that the overall shape of the velomobile is not as important as it is getting people off their mountain bikes. And I have noticed, many commuters using bicycles seem to prefer mountain bikes.

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  2. demetrius11 February 2012 at 04:38

    Velomobile only makes sense when it is electrified. Especially if it is to work in a city. With electric -I really becomes the perfect vehicle.

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    1. Titus van den Brink19 February 2012 at 11:31

      Please explain why you think that a velomobile without assist doesn't make sense.

      Delete
    2. demetrius21 February 2012 at 03:22

      A Velomobile has two outstanding features - aerodynamics and weather protection. When you have to stop and go in a city commute, you have just too many stops to actually get any advantage from a velomobile (as your graph above shows). On my city commute I have 10 stops in 5 km. Also, a bicycle is much more manoverable in traffic that a velomobile.

      Electric assist addresses that stop and go issue and compensates for the extra mass when going up hills.

      Ofcourse I still want to have one - but inevitably, you have to spend 7,000 dollars to get one. Which is the last reason to not use one in a city - how could you leave a 7,000 dollar velo locked up in a city?

      Delete
    3. Titus van den Brink14 March 2012 at 21:22

      I agree totally with you but why is it that we leave our 70000 euro car at the street without worrying and have to park our 7000 euro velomobile in the hall...

      Delete
    4. demetrius20 March 2012 at 07:58

      A velomobile is an elegant vehicle and it would make so much sense but it is not as manoeuvrable in the city. I have a recumbent bike that I enjoy but it also is harder to ride in the city although I still do. I recently put a bionx system on it and now it is incredible because I don't have to go up and down the gears as much and I can also store regen electricity as I ride. This system would work very well on a velomobile and in fact most velomobiles can be set up with a bionx.

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    5. Maciej Dlugosz11 July 2012 at 12:53

      Well, You could almost make it with without electric assist. But then take care about:
      - weight ---> 23 kg is not bad
      - rolling resistance ---> 28" wheels have 30% less rolling resistance than 20" wheels (velomobiles ususally loose here a lot)
      - chainline --> in velomobiles chainline is usually not good. In velomobiles there are usually chaintensioners on the upper side of the chainline. That is bad. You loose some energy. When you pedal the upper part of the chainline stretches. Straight line is efficient for transfering stretching. Stretching and flexing upper chainline causes loses. Chaintensioners should be on the down side of the chainline only.

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    6. BertQuest21 August 2012 at 14:54

      Chaintensioners are placed in the return line only. Putting one in the pulling side makes no sence at all.

      Delete
  3. Ries Bouwman17 February 2012 at 12:10

    Hi, my experience from the automotive industry was that aerodynamics plays a major role above 120km/h. Interesting to see that with these vehicals, the effect is visible at much lower velocities!

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    1. Titus van den Brink19 February 2012 at 11:43

      This comment has been removed by the author.

      Delete
    2. Manouchk22 July 2012 at 03:11

      Yesterday, I remembered that aerodynamics is very important. Using an upright position with my common commuter bike I can ride easily 25 km/h. If I do get a more aerodynamical position, I can easily reach 30 km/h with about the same power. It's just a bit less comfortable! Probably with any velomobile, I could ride at 40 km/h with a similar effort!

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  4. plywood29 February 2012 at 10:13

    Thx a lot of we are Sesa Plywood

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  5. Manouchk11 March 2012 at 15:38

    What an interesting article! You just get it right. You looked at the speed and then analyzed the energy! I love that. I always that I could propose exercice to my student that uses bicycles.
    Do you know any succesfull recuperative breaking that uses a flywheel in a velomobile or a bicycle. I did some research few months ago and did not found any thing very convincing. I started this research because here, in Brazil I use a pedal assist bicycle. I did observe that what I do need more is assistance for each acceleration after each starts. A flywheel that would store kinetic energy may be could help even if it increases the weight a little bit, especially if there no way up on the commute. Could the flywheel be in around the rim and between the spoke? I really don't know what would be the transmission control between the wheel and the inside flywheel...

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    1. demetrius20 March 2012 at 07:48

      Another option is an electric assist with regen feature. I own the bionx system. It allows me to regenerate electricity whenever I am braking but also I can do a light regen when I am on a slight incline or even on the flats in anticipation of an upcoming hill. It is interesting but using this system I can actually travel without loosing any electricity from my battery.

      A flywheel would be very interesting but you could only store a few seconds of energy and if it weighs more than 10 kilos, you might as well get electric assist.

      Delete
  6. Titus van den Brink14 March 2012 at 18:10

    For more info on flywheels take a look at this page: http://www.ccm.nl/en/component/content/article/22-energy-storage-system

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    1. Manouchk22 July 2012 at 03:13

      Thank you! Infact it is driven by electromagnetic transmissiom/motor. I didn have any notion of that. It is really a way to store energy with a great power and without battery!! Quite interesting! Don't know if the electronics is complicated, I'm just a physics teachers and my electronics knowlegde deals more with quantum mechanics than real electronics!!

      Delete
  7. OSr Group16 April 2014 at 08:48

    AMAZING SIMPLY AMAZING!!
    Plywood Company in India, Plywood Manufacturer in India


    ReplyDelete

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