Wednesday, March 14, 2012

If I need two wheels in the back...

After two test rides, I have found myself correcting some details on the handcycle.  The steering stops constrict the turning to too large a radius.  I have reworked the stops to allow a smaller turning radius.  The carbon fiber has required a few days of curing.

Additionally, one of the inline-skate wheels, when under a lot of loading, was scraping against the carbon fiber.  When I designed it, I probably should have given couple of millimeters of clearance.  I cut it pretty close.  No matter,  I think the problem is solved now.  The rubbing really slowed me so it was difficult to approach a speed in which I could balance the bike.

With these two problems solved (hopefully), I will give the handcycle another spin.  If the bike is too difficult to control at low speeds, then I have a backup plan.  I have designed a parallelogram tilting rear wheel assembly that will allow the rear wheels to tilt into turns.  The benefits of the tilting rear wheels are quite numerous.

I am a novice at the physics of the problem, but here is an explanation that makes sense to me. When the center of gravity (CoG) moves outside the triangle formed by contact points of the three wheels, the handcycle rolls over. When cornering, the front wheel turns and that triangle is now shifted to the side of the turn since the contact point of the front wheel has shifted.  The CoG then may fall outside that triangle -- to the opposite side of the turn.  That is when I roll.  I attempt to lean to the inside of the turn thus shifting my CoG along with the triangle.  In doing so, I am normally limited because my head hits the rear inside wheel.  If I can keep the CoG within the triangle, then I would spin out instead of rolling (if I am going too fast).  I have yet to spin out -- but I have rolled far too many times.

That triangle does have a height formed by a single point from the three corners. Think of a pyramid with a  three-cornered base.  By keeping the CoG very low (I have 1.5" clearance), then the potential of rolling is lessened.  But as the CoG moves higher (as it moves up the pyramid), then the triangle cross section becomes smaller -- thus I have a greater chance of rolling since the CoG may fall outside the pyramid.

So why a parallelogram tilting rear wheel assembly with two wheels?

(1) Rear wheels can be much closer together -- even within the widths of the shoulders.  If the rear wheels are within the width of the shoulders, then the rear wheels are no longer considered "front wheels" (unprotected).  The difference between a protected and unprotected wheel (two rear wheels in this case), can add up to many watts of power depending upon the type of wheel. In my case, I would think that I would be saving 35-50 watts or so with two protected rear wheels in comparison to two unprotected ones.  That is not a lot to a legged cyclist, but for a handcyclist, it surely is. At race pace, I am probably generating about 235 watts of power.  Fifty extra watts of power would be the world.

(2)  Higher speed in turns.  My last race had 60 turns over 42K.  I rolled the bike once when I did not slow down below 17 mph on that turn.  That cost me probably 30 to 45 seconds.  I probably had to slow down for another 40 turns.  One can only imagine if I never had to slow, what type of speed advantage I would gain.  Additionally, the process of sprinting out of a turn takes a lot out of me. Maintaining a single speed is far easier.

So I am attempting to put together a list of materials required for the above design. It will be constructed mainly from carbon fiber (I still cannot weld).

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