UNDERSTANDING GEAR INCHES
by Coach Troy Jacobson
The bicycle is a wonderful human powered machine. It enables us to propel our bodies forward and travel with a relatively low amount effort and force on the flats and up and down the hills. This is a result of bicycle gearing and something known as a 'gear inch'.
The bigger the gear inch, the harder the bike is to pedal and the further the distance it moves with each pedal revolution. For example, try to put your chain on your 'big ring' on the front and the '12 tooth cog' on the back and then attempt to ride your bike from a complete stop. It's not an easy task!
On the other hand, the opposite holds true if you use a 'small gear inch' such as the small front chain ring on the front and the large cog on the back. Now try that same drill and you'll see the bike rolls very easily while not traveling very far per crank arm revolution.
The front of the drive train on most road bikes contains a big ring and one or two smaller rings. The manufacturer typically stamps a number to the inside of each ring indicating the number of 'teeth' the ring has. Most stock bikes nowadays have a 53/39 (or a 52 big front ring and a 39 tooth small front ring).
The 'rear sprocket cluster' or cassette typically has 8 -- 10 'cogs'. Each cog also has a certain number of teeth and again, the number is stamped on the inside of each cog by the manufacturer. Most modern road bikes come with 10 cogs starting with a 12 tooth small cog and working up to a 25 tooth large cog in various jumps such as 12, 13, 14, 15, 17, 18, 19, 21, 23, 25. Again, this can vary from bike to bike and component manufacturer to component manufacturer.
To determine gear inches when riding, you need to perform a simple mathematical equation.
Gear Inch Value = Number of Teeth on the Front Ring / Number of Teeth on the Rear Cog x Diameter of the Wheels.
Most road bikes have a 27" diameter wheel while most Mountain bikes (and some smaller road bikes) have 26" diameter wheels.
Let's use our formula to calculate the gear inch of a few common gear ratios on a standard 27" wheel road bike.
BIG RING EXAMPLES
53 (front ring) divided by 15 (rear cog) x 27 = 95 (Gear Inches)
53 (front ring) divided by 12 (rear cog) x 27 = 119 (Gear Inches)
53 (front ring) divided by 21 (rear cog) x 27 = 68 (Gear Inches)
SMALL RING EXAMPLES
39 (front ring) divided by 15 (rear cog) x 27 = 70 (Gear Inches)
39 (front ring) divided by 21 (rear cog) x 27 = 50 (Gear Inches)
As you can see, the smaller the gear inch, the easier the bike will be to pedal. This is why you climb a hill in the small ring and push the flats in the big ring... in most instances.
It has been noted that in our Spinervals DVDs, we sometimes 'cross over' and use the small ring as an 'easy spin' gear when sometimes the large ring and a larger cog in the back accomplishes a similar if not even smaller gear inch. As such, pedaling in the 39x15 (70 inches) is actually more difficult than pedaling in the 53x21 (68 inches) Why do we do this? The main reason is to keep it simple and not confuse the process of gear selection on the trainer by shifting all of the time from the large to the small ring and vice versa (not to mention the increased chance of dropping your chain). In addition, many cyclists are not aware of the characteristics of gear inch selection. The way we determine gearing in a Spinervals workout 'makes sense' and is more intuitive, (i.e. small ring equals soft-pedaling or high cadence work whereas big ring training equals higher power output levels). The key of course is to choose the gearing when doing the workout DVDs that best suit YOUR personal needs but stay within the general framework of the workout structure. In other words, a 53x15 for one person may be entirely different than a 53x15 for someone else due to trainer resistance, tire pressure and other factors.
I hope this overview is helpful and clarifies a few things regarding the magic of the bicycle and gear selection. Good luck and train smart!
