DID  YOU  KNOW??

Dr. James Allen Watrous

What are gear-inches? 

Gear-inch is a term that is used by cyclist to describe how easy or hard it is to pedal the bicycle.  When the value of the gear-inch is high, then it is hard to pedal the bicycle.  When the value is low, then it is easy to pedal the bicycle.  This is simple.  But, what is it?  How do you determine it?  What’s it used for?

“Well, I thought you would never ask.”  Actually, the gear-inch involves the relationship between the gears on the back wheel, the chain rings and the diameter of your rear wheel.  The gear-inch is easy to calculate. 

It might be a good idea if we defined a few terms.  The chain ring is the gear attached to the crank on the bicycle.  Most bicycles come with two or three chain rings.  (The gears in front are often referred to as a double or triple.)  For the gears on the back wheel, there are three terms that you are likely to hear.  Two of these terms are known as the free wheel and the cassette.  Both terms refer to the cluster of gears on the rear wheel.  Each of the individual gears is know as a cog with a certain number of teeth.  Rear wheels come with one cog (like a track bike), five cogs, seven cogs, eight cogs and nine cogs.  Some of the latest cassettes on the market are coming with ten and eleven cogs.  One wonders if this is ever going to stop.  At any rate, the three terms are free wheel, cassette and cog.

Now, that we know a few terms, let us list the few steps to determine gear-inch values.  First, count the number of teeth on each chain ring or be clever by looking for this value stamped on the chain ring by the manufacturer.  Next, count the number of teeth on each cog of the rear cassette or free wheel.  By the way, the manufacturer usually stamps the number of teeth on each cog.  However, the cogs are so close together it is often difficult to see these numbers.  So, counting the number of teeth is faster.

Finally, we determine the gear-inch value for each front chain ring and rear cog combination by dividing the number of teeth on the chain ring by the number of teeth on the cog and then multiplying the result by the diameter of your rear wheel.  If you have a mountain bike, then use 26 inches for the diameter.  If you have a road bike, then use 27 inches for the diameter.

Let us look at an example for a road bike.  The front chain ring has 52 teeth and one of the rear cogs has 13 teeth.

                          The gear-inch value  =  108 gear-inches

This is a hard gear to pedal, whereas 30 gear-inches is very, very easy to pedal.

Clearly, it is easy to determine the gear-inch for one or even all the various combinations.  “But, so what”, you ask.  Look at the following table as an example of gear-inch combinations for a road bike with a triple set of chain rings and a nine-speed cassette.

Gear-Inches

  Note: the table cells that are in gray represent gear combinations that should not be used.  These gear combinations create a large amount of stress on the gain and may cause it to break.

This road bike has 27 gears, but 4 gears should not be used.  Thus, there are only 23 gears available.  However, the actual number of useful gears is only 15.  If we rearrange these three columns into the following row arrangement, then you will be able to see the overlaps and redundancies.

Useful Gear-Inches

Obviously, this is a bad design considering the bike has a grand total of 27 gears and there is only 15 useful gears.  In addition, the % difference in gear step change in some of the cases is quite large.  This simply translates into a gear change that is too easy if shifting down (lower gear-inch value) or too hard if shifting up (higher gear-inch value).  When the % difference is less than 3%, the average rider cannot perceive any effect.  On the other hand, a % difference of more than 10% is perceived as a big change.  Ideally, the best range for % difference in gear steps is 5.5 to 7.5 %.  Whenever you have many gear steps that are larger than or smaller than this range, the gear design is not good. 

Notice that some gears require you to use both shifters to get to the next gear.  This translate into complicated shifting patterns.

While riding your own bike, test to see if you have a lot of gear step changes that are too large or too small.  Also, if you find that there are a number of very low gears you are no longer using, then it may be time for you to get a different cluster combination of cogs for your cassette or free wheel.

One can order different sets of chain rings and/or rear cogs to provide more useful gear combinations.

The discussion in this article is not intended for designing gear combinations.  But by understanding what a gear-inch is and determining which gear-inches work well for you, provides you with enough information to successfully communicate your needs to a bicycle technician.  You can create a table of gear-inches for your bike, which you may find useful in deciding what changes you want to make when ordering from the potential gear combinations available at a bicycle shop.  Since manufacturers do not make every gear combination, you have to pick those combinations that come closest to your needs.

The following chart is the gear combination that I have used for many years.

Gear-Inches (using 27 inch wheel instead of a 700 cc wheel)

Note that 700 cc wheels have a smaller diameter than the 27 inch wheel.  Therefore, my gear inches are actually less than shown in this table by 2.86%.  Thus, at the highest gear the 119 gear-inch becomes 116 gear-inches and at the lowest gear the 36 gear-inch becomes 35 gear-inches.

My preference for the rear cassette is a 12 to 20 instead of 12 to 21.  However, Shimano does not manufacture the 12 to 20 cassette for the Ultegra model.  From 1981 to 1998, I used a seven-speed free wheel with cog combination of 13 to 19 or 12 to 18 teeth, which provides 17 different gears when you eliminate overlaps.  From 1998 to present, I use the 12 to 19/21 Ultegra cassette from Shimano, which provides 19 different gears when you eliminate overlaps.

One of the reasons for developing this particular gear design was to create a simple shifting pattern.  Each chain ring represents a range of gears.  You only change chain rings when you want to go change the range of gear selections. 

Copyright, 2003, 1997,1981 to 1995. Watrous' Cycling Enterprises