Wednesday, May 20, 2009

14 Ventilation Design In Velomobiles

I've not ridden a velomobile myself as yet. But if my observation about their design is not too ignorant, I don't see how a fully enclosed recumbent will be comfortable riding in hotter temperatures. For one thing, the recumbent now acts like a greenhouse, trapping the sun's heat in, while the other source of heat is the rider himself. Since the rider is actually doing work to pedal, heat is given out. How much heat? Its depends. To put things into perspective, even while at rest, an adult radiates heat at the rate of 80-90 watts (BMR = 1.2 watts/kg for a 70kg individual) as a result of basal metabolism. But now because of the recumbent being a greenhouse, the ambient temperature inside the vehicle rises. At higher temperatures, the body cannot radiate heat effectively. But the human body is a remarkable temperature regulation machine in that we have temperature sensors in our brain that activate the evaporative cooling mechanism when the ambient temperature is in excess of 30 deg F. But the catch here is that in order to get blood to the skin to aid in this process, there is some amount of cardiovascular strain on the rider's body. Irony also has it that as you sweat and lose more salts from the body, you face additional strain on your cardio system.

Bottomline : someone sitting in a velomobile to travel extensive distances must have a cooling system already built into the vehicle, or he or she must be able to tolerate such temperatures and cardio strain within an enclosed space for long periods of time. I mean, what are the odds that you won't die of a heat stroke riding something like this? Let's not forget that one of the ultimate joys of riding a conventional bicycle is the soothing feeling of a cool breeze skimming your body.

Are velomobiles designed to have vents in them as an escape mechanism for heat? Or must they come with a neat little air conditioning system of their own? I think if designers were to take live heat loads into design consideration (like the design of HVAC in a building), the velomobile will have great potential as a comfortable vehicle for longer distances.

What do you think? Have you ridden a velomobile? Do you find that their ventilation systems are adequate for long travel in hot temperatures? Let's discuss.


Cooling Mechanisms Of The Human Body
The Ventilation Of Streamlined Human Powered Vehicles (Thanks To Alloycowboy)
Speedbike Designers Use Ansys Fluid Simulation To Gain A Competitive Edge (PDF Submitted By Jason)

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  1. First off you can't measure anything in Watt/Hour. Watts is already a measure of energy transferred, if you're dividing by hours you're creating a measure of energy transfer rate rate. I typed rate twice twice.

    Yes, an enclosed bike is freaking hot, astute observation. This is an enormous advantage for people who are trying to ride a bike when it is outrageously cold outside. This is also one of the reasons that no-one drives one around town.

  2. Ron, I thought you might find this article interesting. "The Ventilation of Streamlined Human Powered Vehicles.

  3. Ron, first off I have to say it's a neat and cool little car. I like the idea behind it. However, I have to agree with you. It would not make a good vehicle for long distance drives, especially without some sort of cooling system. But even if it did have a cooling system, I would not want to be in one of those for a very long time because of how cramped you'd be in one.

    Although I didn't understand half of your scientific explanation, I still think I would have to agree with you. Are you going to get one now?

  4. Josh : I see your point. I made a typo. You're right. I think I originally meant to write BTU/hr...which I'm more familiar with. So 90 watts of heat production in a human is equal to 307.09 Btu/hr. I'll correct it.

    Zach : See above for a better perspective. What 307 BTU/hr means that your body can heat up 307 pounds of water by 1 deg F in one hour. This is by definition of BTU. See Wikipedia.

    Alloy : Thanks for sharing.

  5. Anonymous9:15 PM

    Reminds of the movie matrix, where the machines harvested human fetuses for heat energy!

  6. Hi, I made up the stories for Matrix I and II for the Wa something brothers, miserable wretches, and I don't remember harvesting fetuses? III? I did the schooling bots bit for that film and little else.
    I have a Quest (286) velomobile and it is great in the Florida sun, of course I have a motor in mine and don't have to work so hard. The white color, though, is so bright by the sun that shades are required. I also got the flevobike top (slightly modified) to keep the rain and sun off my head.
    There's plenty of ventilation for riding around town and you don't get sunburned.

  7. B. Nicholson : Interesting. I thought the whole script for the Matrix was stolen by the brothers from an African-American woman's SF manuscript back in the 1980's? So what stories did you make up for the movie? If you ever saw the first part, you'll recall the human fetus farm and energy plants. The humans are not born, but grown. The humans are then placed in pods where body heat is derived as a source of energy.

  8. Ron, I love your blog. Thanks for the high quality content.

    Here is some information I have gathered pulled from an email I sent. Sorry about the formatting.

    "Apparently this topic was studied in detail for the Daedalus human powered aircraft. The information below is from the books, "Human-Powered Vehicles" and "Bicycling Science".
    -Acceptable rise in body temperature 2 degrees C.
    -Tolerable limit for pedaling [without cooling] is 12 minutes.

    For the human engine,
    -Each ml /second of oxygen is equivalent to about 4.5 Watts of power to the legs and 18 watts of heat output(Bicycling Science)
    -A liter of oxygen is found in about 5 liters of air.
    -24 liters of air is passed through the lungs for a liter of oxygen to be absorbed.

    -Each liter of oxygen is equivalent to approximately 5 kcal
    -The human engine is about 25% efficient.
    -Air contains 20.9% oxygen, the minimum air required by humans is 17.5%
    -VO2 max levels of 75 to 83 ml/kg/min represent the top of aerobic power for elite endurance athletes.
    -Leg ergometry VO2 (ml/min)=W*6.1*2ml/kg/m+(3.5 ml/kg/min*body weight in kg)

    -A bicyclist producing 450 Watts evloves heat at about 850 W/m^2
    -For a standing human 850 W/m^2 is absorbed by air moving at 3 m/s
    -Air currents at 2m/s are up to 16 times as effective for cooling as air currents at .5m/s. Thus, even relatively small amounts of air flow inside a human-powered vehicle fairing can result in significant cooling."

    From personal experience even with small wheel openings and disk wheels a lot of air is circulated by the wheels (which act as air pumps) But for rider cooling a small duct about 1/2 inch by 2 inches directing air directly over the head provides sufficient cooling at 35 mph and 200 Watts power output. If the vehicle stops moving you overheat rather quickly.

    There is an hour record speed event in July and one of the contending teams has done a lot of modeling for cooling

    This weekend there is going to be an event in Portland (Human Power Challenge) and the West Coast Velomobile gathering is happening concurrently.

  9. Jason : Thank you. I placed a link to the Ansys pdf below the post. Its a good read.

  10. B nicholson,



  11. Conversely, what if it is cold out and the cyclist's body heat fogs it up?

  12. Coluber4210:08 AM

    I have never ridden a velomobile, but I have run across a number of randonneurs using them, and they seemed to have figured out how to deal with cooling even at slower speeds over very long periods of time.
    At this link
    you can search for photos from Paris-Brest-Paris in 2007 by some interesting parameters, one of which is the type of bike. There is a category in the drop-down menu for velomobiles. PBP was cold and rainy that year, not hot, but even still, those riders had to have been prepared for hot weather as well. And they had to have ridden sufficient distances in their velomobiles so as to discover and solve any cooling problems before getting to PBP.

  13. I've test ridden a used early Leitra Sport( velomobile, three years ago in Munich while the last thirds of an summer thunderstorm.

    I was wearing T-shirt, bermuda shorts and sneakers. Despite the fact that is was condensing wet outside ( ~100% humidity, rain on hot asphalt, plus dropping temperatures) I returned completely dry. Neither rain nor sweat wetted any of my clotes. And ventilation inside is IMO close to the theoretical maximum.

    Watch this:

    Bubble texts clockwise, beginning at the top:
    * scratch-resistant windscreen, as close to the eyes as possible (**), manually actuated wiper
    * white colour yields low surface temperature
    * adjustable flaps for body ventilation
    * easy separation of hood from the frame simplifies transport, p. ex. important for defrosting the hood inouse during winter
    * ventilation duct intake for windscreen ventilation
    (** to avoid distortion of sight due to drops on screen, like in a motorbike helmet, which also has no wiper)

    what is not on this image - but equally important - is the "chimney effect": the fin-like little tower on top of the is a) a periscope panoramic rear mirror and b) an exhaust duct


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