Wednesday, May 26, 2010

Final Gear Inches?

My fitness level can be improved over the next year in preparation for the Aids Life Cycle, but so can my trike. Looking at gear inches was an objective way to analyze the capabilities and limits of the Tornado.

Without a place to attach a derailleur, my choices for modifying the gears is limited to the Schlumpf drives. Needing lower gears, I'm chasing down the Mountain Drive crankset.

After researching the web and discussing experiences and opinions of members of the forum, I've come up with this gear inch chart.

The 40T chain ring in the middle column is my current GI's. It'll be eliminated in the final design, but I've posted it here for convenient comparison. It will be replaced by the 21T and 52T chainrings.

The range will be 12.4-94.5GI. 12.4 will by my granny gear. The intention is for the 17.5GI combination to be used for big hills. The 12.4 will only be engaged for super steep bite my lip and cry hills. It'll be reserved as my emergency backup gear.

Changing the gears in the rear cog is enough to think about during my ride. In order to minimize the number of times I have to change from one chain ring to another, I am attempting to reserve the 27t ring for hills and 52t ring for regular riding. For this to work, my first gear under the 52t ring must be easy enough for me to start from a standstill. While riding, I've noticed that I'm most comfortable starting from stop in either 3rd or 4th gear, 38.1-44.4GI's.

This means my first gear on the 52t must be within that range. After calculating gear inches for 54t chain rings and 56t chain rings, I found the 52t chain ring fits the best. Since the 34t cog in the back is so far off from the 24t one, I'm designing the 24t cog to be my first gear. It therefore needs to fall within the 38.1-44.4GI range. Only the 52t chain ring makes this work.

The Mountain Drive has internal gears and creates a virtual chain ring that's 40% of the physical chainring. Doing the calcs, I find that my virtual chainring will be a 21t.


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