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  1. #1
    a fat old bus driver bikerjohn's Avatar
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    Specialized '05"Globe"; Royce Union "Express"; Dahon "Cadenza"; Bianchi "Castro Valley commuter"; Cruzbike "Sofrider" FWD recumbent;
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    Converting a single chainring to a multi chainring crankset

    I have a 9speed single chainring bike (2005 Bianchi castro valley)

    I am looking for more downhill speed. I have a 48t crank with 170 mm crank arms and 11t on the smallest cassette ring right now. I can cruise for short distances at 20 -25 mph as the bike is currently set up, peak downhill speeds around 33mph. I think a 52t or 53t chainring would be better for top speed but I need easier gearing for going up the "hills" too. So.. What are the mechanical considerations to make a conversion to a triple crank? Specifically, what difficulties can I encounter setting up the front derailleur? Are "bolt on brackets" available to attach/position the derailer cable to the frame? Any suggestions?

    I also have toe clip interference with my front wheel. This situation seems correctable with shorter crankarms. Would shorter crankarms solve the speed problem by reducing the leverage associated with longer crank arms? Would less of a pedaling circumference mean I could pedal at a faster rate with the same effort? Has anyone out there ever tried using 160mm or shorter crank arms? How will shorter crank arms effect pedal leverage/ pedaling effort? Seems like I could gain "leg speed" a faster spin with shorter crankarms.

  2. #2
    sch
    sch is offline
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    Unless you can generate power outputs in the 350-400 watt range I don't think you are going to get much more speed out of larger chainwheels. The power required to go faster than 33mph is up in the professional cyclist range. You could peak output for a short time and perhaps exceed 33mph, but in my experience very few cyclists do much pedaling when they can coast above 30mph. Your present top gear is about 117, at 90rpm this is 31.8 mph, at 100 rpm 35mph. To think you will be able to exceed these speeds with a 52x11 or 53x11 suggests Lance type metabolic output ie power outputs in the 450watt range. This is Merton territory. Most road bikes have the potential for toe interference with the front wheel at some point in the range of motion of the cranks and the front wheel. This is almost never a problem in actual riding. ALL three of my road bikes will touch toe of shoe to the wheel but it has never happened in the ~32,000 miles I have ridden them while riding. Be not concerned.
    Finally, shorter cranks do allow a higher cadence and for some, improve climbing thereby. There have been lots of threads on this on other forums. Recumbent riders particularly are interested in shorter cranks as the heavier bents are (for 99% of bent riders anyway) hill challenged. BMX dealers are sources for shorter cranks that are fairly cheap. As I implied above higher cadence in higher gears runs into motor problems so it depends on the motor.

    There are bolt on bits to act as cable stops both on the downtube and at the bottom bracket. I gather the Castro Valley is standard steel frame tube diameter. These bolt on bits are less common now than they were. You would need a L brifter, triple crank and CW set, front derailleur and triple length BB axle fo make the shift.
    www.loosescrews.com is one site to look for bolt on cable stops.
    Steve

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