Sunday, September 7, 2014

Handcycle -- New Base with Improved Aerodynamic Design

Small changes in shape can have a big impact on aerodynamic drag.  Surprisingly so.

Cd of 0.18
The 3D design:



I am testing a new base to my handcycle for which the design has been put through hundreds of iterations of Computation Fluid Dynamic (CFD). That is, I take the 3D design, put it into a virtual "software" wind tunnel, see the results, make changes to the 3D design, and do it again -- hundreds of times. As small changes improve the aerodynamics, I maintain those changes in the 3D design.  My challenge is to reduce the aerodynamic drag on the handcycle.

The base is connected to my existing front fork (the front fork takes much longer to build).



Some of the rules of thumbs of design that I thought would hold true, did not.  For instance, a "longer tail" did not give the best results.  A "narrower wheelbase" did not give the best results.  "Closer to road" did not give the best results.  That is not to say that the rules of thumbs are wrong -- but only that the best overall design results may be achieved with otherwise.

That being said, the best measure of aerodynamic design is of course, "How fast is the handcycle on the road?"  This handcycle is easily the fastest I have ridden.  

PR's:

In the first three days of taking the new bike out on the various rides (that I have done many, many times) I broke three of my course records easily.  The first ride was a 25 mile "time-trial" with something like 14 stop signs and a 450 foot elevation gain. I averaged 18.4 mph.  The next ride was a 42 miler for which there is about 500 foot gain and too many stop signs to count.  I averaged 17.1 mph,  The third ride was a 72 miler with a 800 foot ascent with 30+ stops including numerous stoplights.  I averaged 17.4 mph. None of the rides were "all out" -- but they were definitely above a leisurely pace (for which I know little about).

Of course I do not stop completely at all stops -- but I have to slow to a a max of 5 mph since the corn, soybeans and grass is high -- and I am low to the ground.  Most stop-sign corners are not mowed -- so I have to slow considerably.  And I do literally stop at stop lights. Of course then I have to speed back up -- the hell of handcycling.  On the flats, without wind, I am guessing that I average in the high 19's -- maybe even into the 20;s.  But there is nothing really flat -- even 20 or 30 feet ascent over a mile makes quite a difference.

Why faster:

So why is the bike faster?  Through the many iterations of CFD on the 3D design, I found that the small changes added up quickly to a better overall drag coefficient.  Most importantly, I was able to: 
  • Reduce the trailing drag by reducing the compression of air under the handcycle;
  • Replace the rear axle with a "wing" shape;
  • Contour the headrest/hydration bag area (many! iterations);
  • Keep rear wheels vertical.  A 5-10% slant of the rear wheels increased drag.
  • Reduce frontal area under the legs.  The base comes to nearly a point at its front edge.
  • Much lighter.  The base lost 15 lbs!

Flex:

The above times were completed with a fair amount of flex in the base.  I added four more layers of  carbon fiber (two on the top, two on the bottom).  This helped -- but the bike still had too much flex when  I was climbing hills.  So I added a 2-part urethane foam to the interior of the base as a structural element. The urethane can withstand 20 lbs./sq. in.  At all out, I might be producing 200 ft-lbs/sec. Given the huge surface area of the base, the urethane foam appeared to be more than adequate. The urethane (2 lbs. per cu.ft.)  greatly reduced the flex -- and was a simple, simple solution to the problem,  The addition of the urethane only added a quarter pound to the base and required about an half-hour of work -- mix the 2-part liquids, pour it into the base, and in 5 minutes, it expands about 20-25 times the liquids volume. In fours hours, the urethane is hard.

Added features:

2 rear disc brakes!!!  Man, this bike can slow or stop much quicker.  Thus I can come into a turn (or stop sign) at higher speed and reduce the speed quickly.






Large hydration bag.  I now have a 6 liter hydration bag instead of two smaller ones. 

Positionable hydration line holder. I can place the holder out of the way for when I enter or exit the bike.Plus the water line is always available by moving my head forward on inch.


Regular wheels (thus cheaper!!!). These wheels cost a total of $290 with the Shimano hubs.  I used a 1/2" x 20 tpi hex-head SS bolt going into an aircraft-aluminum axle holder wrapped in about 30 layers of carbon fiber fitted into the "wing" axle.


I have hit some big-ass holes, ridden over miles and miles of newly "tar and chipped" roads (for which my teeth nearly vibrated out of my jaw).  The axles appear to be quite strong.  My axle holders are but 1/2"x 20 tpi automobile aluminum wheel hub nuts -- 2" long, costing about $4 apiece.

This may be my last base I build since it might be very difficult to improve upon it...

USB Rear Light:

The base holds a large lithium-ion polymer battery that can hold a week or two of charge for the 15-LED rear light.  There is a USB line hanging out of the rear (see pictures above) to allow for EASY charging!

3 comments:

  1. Looks good and inetersting results on whats faster and whats not. Wheres the 2 wheeler now?? robbo100bike@hotmail. co .uk

    ReplyDelete
  2. Bruce I need your help with some handcycle aerodynamics questions. Could you email me I'd like to take our discussion offline. Thanks, Chris

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