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Old 11-15-10, 11:50 AM   #1
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Join Date: May 2007
Location: Vancouver, Canada
Bikes: Brodie Force w/ Xtracycle, Dahon Helios, Merida Folding, Pacific Carryme, Softride Classic
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Dahon: Using an MTB derailleur on a narrow spaced road cassette\cogs

Summary: The SRAM 3.0 derailleur (MTB long cage) will work with a cassette with narrow road spacing. Little to no adjustment is needed.

Long writeup below.

I have a Dahon Helios P8 (circa 2006). This is a Shimano-hub based rear freewheel with 8 speeds. The original equipment on the bike has the gearing spaced as follows:

Gear Teeth Gear Inches
1 32 31
2 28 35
3 24 41
4 21 47
5 18 55
6 15 66
7 13 76
8 11 90

This yields a gear range between 31" and 90" gear inches. I found myself seldom using gears 1, 2, 3 and 8. In fact the difference between 31" and 35" is so close at the low end of the range that it is redundant for my type of riding. Gear 8 was only used once a day coming down the Cambie Bridge into downtown Vancouver. In addition, a comfortable cruising speed for me was about 20 mph (32 km/h) using gear 6 at 66 GI. If I tried to go any faster, the sudden increase of 10 gear inches in Gear 7 to 76 GI felt too hard on the legs. In a headwind or if the gradient steepened slightly however, I would downshift to gear 5 but the 20% decrease in GI was too much leading to too light a pedal effort and a subsequent loss of speed. The effect was jarring. What was the solution?

I knew that I could climb the largest hill on my commute with gear 4 (47 GI). So I searched for a Shimano-based cassette that would give me tight narrow spacing between gears so I could keep my cadence in a narrow range for optimal efficiency. This turned out to be an Shimano HG50 12T-21T cog set as follows:

Gear Teeth Gear Inches
1 21 47
2 19 52
3 17 58
4 16 62
5 15 66
6 14 71
7 13 76
8 12 83

My lowest gear has been reduced to 47 GI while I have lost the high gear of 90 GI replaced with 83 GI and much tighter spacing in between. This is NOT cassette for touring but daily commuting on paved roads with hills and bridges. With narrow spacing between gears, it is possible to "trim" the gear selection for varying conditions such as head winds, inclines, rough surfaces all the while keeping the legs moving within a small cadence window. I also find it less frustrating as I seldom feel I am "between" gears anymore.

When I first approached this option, one bike shop and some websites said that a MTB derailleur's parallelogram would not work with a narrow spaced cassette. Well, it works fine. I did not even need to adjust the High and Low setting on the derailleur. What I needed to adjust was the gear cable tension using the barrel adjuster by the shifter to make the shifting smooth throughout the new gear range.

The other choice would have been a 12T-23T. This would have yielded one lower gear at 43 GI to aid in climbing hills but would have resulted in a drop from 66 GI to 58 GI. I really felt the 66 to 62 to 58 option found in the 12-21T was a more useful range.

So instead of an original cassette from 11-32T where I only used 4 gears, I now have an 8-speed cassette where I am more likely to use the full range of gears within the normal parameters of my riding. I ordered the 12-21T from my LBS and in my neck of the woods, it cost CAD $38. With all the appropriate tools, it took about 30 minutes to work the change (taking my time). Hardest part was shortening the chain since the largest rear cog went from 32T to 21T.

Last edited by puppypilgrim; 11-15-10 at 11:55 AM.
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