I use a half-step gearing system because it provides a wide range of gears, with fairly uniform steps between gears, and no duplicated gears.  The basic principle of half-step gearing is to use a geometric sequence for the cogs, with chainrings such that the step between the outer and inner chainrings is one-half of the ratio between successive cogs. My current setup (development given in meters) is:

In addition to the advantages mentioned above, there are no "bad" combinations; even when using the inner chainring/outer cog combination, the chain doesn't rub against the inside of the outer chainring.  Most shifting is accomplished using the rear derailleur; the chainring shifts are used for fine-tuning the gear ratio.  For a given gear ratio, the stress on the chain is lower using half-step gearing, as the chainring and cogs are larger.

However, there are a few disadvantages to half-step gearing:

1. Because the inner chainring and all but the smallest cog are larger than they would be for crossover gearing, the half-step setup tends to be a bit heavier.  The longer chain which is necessary to accommodate the large cog/outer chainring combination also adds a little to the weight.
2. There may be a tendency for the chain to wedge itself between the chainrings during chainring shifts; this can be prevented by installing spacers near the circumference of the inner chainring to keep it the correct distance from the outer chainring.
3. In the event of a rear derailleur cable break, it may be necessary to ride in a high gear combination unless a spare cable is carried.

Rear derailleur take-up capacity is similar to that required for a crossover system; for example, the same high-end gearing could be used in a crossover system with a 39-tooth inner chainring and a straight 12-19 cogset. Rear derailleur take-up would be similar; 24 for the half-step setup vs. 23 for the crossover system.  Note, however that the lowest gear on the crossover system (39x19) gives a development of 4.3 meters; the half-step system has three lower gear ratios than that. Using a non-straight cogset block with crossover gearing to get a lower gear would result in a larger rear derailleur take-up capacity, and would also entail less uniform gear ratio changes.  On the other hand, the rear derailleur must be able to accommodate the largest cog used in a half-step system, which is typically larger than that used in a crossover system.

Front derailleur capacity is not an issue with a half-step system, as the ratio between chainrings is small.  Adjustment is also simplified for the same reason.

Reading materials:

• Bicycle Gearing: A Practical Guide, Dick Marr, 1989, ISBN 0-89886-184-5 (prepare yourself for a cookie attack if you follow the hyperlink)
  • This book is a bit dated: it only covers 5- and 6-speed freewheels, however the principles are still valid.
• Effective Cycling, John Forester, 1993 (sixth edition), ISBN 0-262-56070-4 (paperback), 0-262-06159-7 (hardcover)
  • Chapter 5 deals with gearing.

Web resources:

• Sheldon Brown's "Gain Ratios"

Back to Cycling