March 21

March 21, 2005

Greg, what on earth are you doing!!

That's what I hear from most of you after an update like the one on the 17th - I'm not crazy - possibly a bit creatively over inspired at times, but I assure you there is at least a bit of method to my madness.

Including this most recent design:

Wait! Before you start firing off emails to me, let me make a case for the above approach.

Since this new vehicle is for an endurance race this summer - and may have some application for my trans Canada trek next summer, I have determined that these are the most important design considerations, and reasons why the above configuration satisfies most of these restrictions:

1. Good aerodynamics. The fairing has to be VERY efficient and nothing aside from comfort considerations should be compromised in my attempt to achieve the lowest drag possible. A fairing design is in the works, but the general body position, bottom bracket and seat height are 'locked' in.

2. Low rolling resistance. Time and time again, large diameter wheels show lower rolling resistance in Crr tests. Further, high pressure tubular tires also show considerable lower Crr than clinchers. I have used a front 650 wheel and a 700 rear wheel - both with high pressure tubulars. I would use a 700 up front, but there would be no way to turn the front wheel without hitting the chain ring - there is just enough room with a 650.

3. Mechanical efficiency. This is important, but not nearly as important as Crr or drag. I did some gear teeth calculations, and it looks like to get the gearing I need for my planned cruising speed, I will either need a very large front chain ring, or an intermediary drive. Since the large chain ring won't allow me to turn my front wheel, I need to use an intermediary drive. I played with a variety of locations for the intermediate drive - all with the consideration that the chain cannot interfere with the wheel turning either. My options were front wheel drive with a twisting chain, or rear wheel drive with the chain running up and over my left shoulder as shown in the illustration. I can't run the chain below the seat because it will obstruct the turning of the front wheel. Routing it around the front wheel and then under the seat is also a bad idea due to the large number of pulleys required. The over the shoulder approach allows me to run the chain directly from the chain ring to the rear mid drive without any pulleys at all, then from the mid drive directly down to the drive wheel. Further, it has been shown that running the chain around larger diameter gears results in greater mechanical efficiency. To take advantage of this, I plan on using a bottom bracket with two large chain rings for the mid drive. I'll need to add pulleys to the return side of the chain, but they don't interfere with the drive efficiency nearly as much as pulleys on the drive side of the chain.

3. Light weight. The least important consideration.

Here is the donor bike. I bought it at Sport Chek for $99 Canadian. That's $82 US!!

I cut off the rear triangle, and welded it to my prototype frame.

This is going to work pretty slick for the experiment because the derailleur, cables and a brake are already done!


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