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copria
Starting Member

USA
21 Posts

Posted - 11/18/2013 :  21:31:37  Show Profile  Reply with Quote
I am member of the Kansas State University Human Powered Vehicle Design Team.

We are investigating a better way to control the overall design of the bicycle. A few sources have considered measuring overall bicycle design by three parameters: rigidity, weight, and aerodynamic drag.

Does anyone know of studies that have looked at how each parameter could be weighed and optimized against each other? (i.e. adding 5kg to the bicycle is equivalent to increasing aerodynamic drag by ___ N)

Any other ideas/sources on controlling overall bicycle design?

W Hilgenberg
recumbent enthusiast

USA
286 Posts

Posted - 11/18/2013 :  22:14:39  Show Profile  Reply with Quote
Hello there,

As the Team Lead for this years Cal Poly HPV team, I was asking the same question earlier this year. The following response is what we have come up with through looking at past design processes of the team and discussing the end results. Below is what we came up with for this year.

First you have to set your requirements for your vehicle. Determine your extreme loading cases, what your lateral loads are, what your vehicle has to do and so on and so forth. This resulting set of requirements will then set your initial design space. From here you can narrow it down a bit by setting limits based upon your manufacturing possibilities.

From here you should give yourself a timeline so you can understand what exactly you are aiming to do and how long you have to do it. This will very much influence your design by showing you how much time you have to develop your vehicle into something competitive.

After all of that is done, THEN you can start to get into the nitty-gritty of what the design tradeoffs are, where the optimal design point is concerning weight and aerodynamics, and other such things. To answer specifically the question of determining the tradeoff between these parameters, you will have to determine the governing equation of your vehicle. Something like;

Power Req = drag f(weight, crr, lateral load, number of wheels) + aero drag f(Cd, A, V, rho) + drivetrain losses f(number of chains, crr, alignment, gearing type)

You can then write up the separate components in different functions (I think of these things in terms of Matlab but you can apply the same logic to excel if you wish) and then call them out with their variables to give you an idea of the various components and how they affect the overall picture. This could be something as simple as an graph with weight and aero drag as the two axis and do a plot in the z-axis of the power so you can get a graphic representation of your design points. Or you could write a code that would optimize the above function using limits on your weight and aerodynamic drag.

There's a lot of ways to skin this cat but I will tell you right now that at the low speeds that you typically will see with these bikes (this is a point of contention with me but I will save that for a later discussion) that aero drag isn't as big of a game-changer as weight is. But you should do the analysis for yourself.

Also, I would look at ways to cut down on manufacturing time so you can spend more time working the kinks out of the bike than trying to fix them at the event.

Hope this helps. And I extend an invitation to e-mail me too if you would like: calpolyhpvpresident@gmail.com

Thanks,
Will Hilgenberg
Cal Poly Team Lead
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copria
Starting Member

USA
21 Posts

Posted - 12/08/2013 :  22:40:54  Show Profile  Reply with Quote
A great resource if anyone is wanting to learn about bicycle performance in terms of rider power:

http://www.recumbents.com/wisil/MartinDocs/Validation%20of%20a%20mathematical%20model%20for%20road%20cycling.pdf

It seems that each decision made on a bicycle could eventually be undestood in terms of a at least one of the measurable parameters of bicycle performance (i.e rolling resistance, aerodynamic drag, frictional loses in the drive train, etc.) described in the paper (see link). For example, understanding rider visibility in terms of aerodynamic drag and rolling resistance; thus equivalently in terms of aerodynamic drag coefficient, air velocity, coefficient of rolling resistance, and component of gravity.

It is very unintuitive. How often would someone on your team consider decreasing the coefficient of rolling resistance to increase rider visibility as a potential design trade off?

I think there is a lot of useful information for overall recumbent design waiting to be discovered using the already clearly defined, measurable parameters of bicycle performance.

Edited by - copria on 12/08/2013 22:41:37
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