Since Tredecillion was pondering using feedwater flow and enthalpies directly in calculations of turbine balance, I thought I’d help “lower the bar” by spewing some pent-up biking geekspeak. Really, I just need to get this out of my system. I’ll cook something later this week. Promise.
One of the things I disliked about the original configuration of my Bike Friday was the shifting. Switching among front chainrings was clunky, sometimes evoking profanity if the chain slurped down past the small ring and wedged itself between the hinge and the bottom bracket. The sloppiness is a result of some compromises made related to the small wheels and the limited range of gears I can push without knee pain.
It’s often convenient to refer to quantify the drivetrain in “gear inches,” the distance moved for each full rotation of the pedals. It’s defined by the simple formula:
# teeth in front chainring circumference distance traveled
X of = per revolution # teeth on rear freewheel the wheel of the pedals
A low number means pedaling is easy, but I don’t go as far. A high number means I go farther, but it takes more effort. It’s just like the low gear on your car’s transmission. If the number is too large, I’ll need ice and ibuprofin for the rest of the week.
In the original setup, I had front chainrings of 39-48-60 and rear freewheel of 11-34. Sparing my gentle readers the tricky math, this is a gear-inch range of 22.9″ to 109.1″, or 6.1 to 29.2 miles per hour. It’s a comfortable range.
Unfortunately, there are three engineering problems with this setup. The first one you can sort of see in the picture. The arc of the front derailleur is tighter than the arc of the biggest chainring. The front derailleur was designed for a 52-tooth ring. To make it work with a larger, 60-tooth ring, the derailleur has to be positioned much further than it should normally be, creating extra slop in the way it kicks over.
Second, there’s some manic handwaving necessary because (as my mechanic explained, I don’t quite see it) the Vuelta chainrings don’t have the little grooves on every fourth tooth that kick the chain up to the larger ring easier. Finally, a lot of extra chain needed to handle the shifting possibilities — 21 teeth up front (60 – 39) and 23 (34 – 11) in the back.
An extra bonus feature, only available on the overpriced DVD version, and which you can easily see in the photo, is the rear derailleur hangs very low to the ground. This sucks up leaves and other organic debris. In the morning, the noise freaks me out.
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Ideally, what I need to do is reduce the rear freewheel’s size, which would also let me reduce the front chainrings’ size. It just so happens that Shimano makes, but doesn’t advertise well, a component group for small-wheeled bikes under the “Capreo” label. The upshot is there’s a 9-26 freewheel available. Plugging the numbers back into the formula above, thereby sparing you all mind-numbing boredom, this means I can use a standard Ultegra triple: 30-42-52. Shifting problem solved!
Sort of… it turns out my original bike was made with an Ultegra double crank with spacers to add a third ring. I managed to barter with someone for a triple crank (and the rings). The next road block was having to get a wheel built because the Capreo freewheel only works with the Capreo hub. It has something to do with crossing the streams – cats and dogs, living together.
The final stumbling block came when Tom (of Issaquah Ski and Cycle), my mechanic, discovered the braze-on (the little bolt hole that I use to secure my fender and pump) was right where the front derailleur ideally should hang. This, it turns out, was simple to resolve thanks to some photos from the folks at Bike Friday: grind off a small, braze-on-sized semi-circle from the front derailleur clamp.
The results are amazing. The bike shifts well, and I haven’t picked up any large pieces of decaying, organic matter. The new setup has a gearing range of 23.1 – 115.6″, or 6.2 – 30.9 miles per hour.
With that settled, the next thing I’m fiddling with is tires. I originally had Schwalbe Stelvios. These were great, up until they started wearing out. Any aerodynamic/friction advantage they had was furiously negated by the time I spent changing flats. It… wasn’t… fun.
I’ve had a pair of Primo Comet Kevlars on since just before STP last year. They’ve fared better, but the rear tire is worn out. Schwalbe’s Marathon Slick is supposed to be “Road Touring,” which is mostly what I do. The tire has a slick tread, reinforced with a kevlar layer. It feels somewhere between the thin Stelvios and the heavy Comets. I plan to put a few hundred miles on it before passing judgement and adding another on the front. I also picked up a Continental Top Touring tire, which is legendary for longevity. If the Marathons work out, I should be due for a replacement just before Ride Around Washington in August. If it doesn’t work out, I’ll try the Continental.
August 2005 update: I’ve had a lot of problems with rim wear and think this is a symptom of overheating as I have a gnarly, serpentine downhill on the way to work. I put my old wheel with the 11-34 cassette back on. Meanwhile, I’ve been shopping for replacements.
None of the local bike shops — Il Vecchio, Sammamish Valley Cycle, Issaquah Ski and Cycle, Gregg’s — can build 406 wheels. Three mail order places I contacted who can make 406 wheels do not have access to Capreo parts or even the proprietary tool to extract the cassette. Bike Friday is the only option, and while their prices are reasonable, they’re quoting me 3 weeks for new wheels.
After some more thinking on this, I’m going back to a non-proprietary hub and cassette. I’m having a set of wheels built at Hostel Shoppe with the SRAM 11-34 cassette.