Gearing & Gear Inches
A version
of this article was originally
published in Triathlete Magazine
After being a long time supporter of Ironman Canada, I have decided to attempt the distance. This is actually my first attempt at any distance triathlon. I have been riding the typical cycling drivetrain set-up (53/39 with a 11-23 rear cog) and am now considering switching to a compact. I hope to go sub 12 hours with a 5:45-6:00hr bike leg. What are your thoughts on compact cranks for Ironman distances and specifically Ironman Canada? Thanks for the advice!
Chris
Kelowna, B.C., Canada
Chris,
It sounds like you have your work cut out for you… Regardless of gearing, consider doing a few shorter distance races before doing an Ironman. From nutrition to transitions, the experience you gain in shorter events can be invaluable to helping make your Ironman go as smoothly as possible and enhancing your overall experience and results.
The gearing you use is important and specific to the course you are riding, as well as your individual riding style, power and pedaling technique. The ideal gearing for a flatter course can be quite different from a hillier course and a rider’s cadence and power can also play a large role in what is best. The goal is to know your body and the course and to select a gearing range that covers the range you will need, without compromising the commonly used gears in the middle that you want.
One of the best places to start exploring gearing options is with your current set-up. While doing your long training rides on similar terrain to what you are going to ride in Ironman Canada, ask yourself whether you are searching for a higher (top speed) or lower (easier to spin) gear than your current 53/39 and 11-23? A 23 tooth rear cog is higher than many athletes need to encourage spinning up the hills and to provide a “bail out” gear just in case things do not go according to plan. If this is true for you, there are two main options to get lower gearing: 1) A wider range cassette. 2) A compact crank.
Cassette Changes – Changes in rear cassette teeth provide a more substantial gearing change than changes in front chainring size. While it is even more pronounced in higher gear combinations, in lower gearing combinations, a one tooth change in rear cog is equal to a 1.5-1.7 tooth reduction in chainring size. This means that you may not have to change to a compact crank to get the gearing you want as just changing your 11-23 to a cassette with a bigger rear cog (11-25, 12-27…) will provide distinctly lower and more climbing friendly gearing than what you have now.
Again, your initial goal is to find gearing that covers the full range of options you could need for a course. Think about how much lower you need and, if you switch to a cassette that eliminates your 11 tooth small cog, whether you are willing to give up your top speed gear in exchange for a lower climbing gear. If you make this change and still want lower gearing options, the compact crank would be the next step.
Chainring/Compact Crank Changes – A more complete discussion of compact cranks can be found in the December 2005 issue of Triathlete’s “Tech Support”. Compact cranks (commonly geared 50/34 from the factory) do not have the extra weight, wider stance or mechanical complexity of a triple, but are geared almost as low. However, remember that when you switch to compact, you need to not just look at the high and low gears, but also what is in-between. Unlike a triple, compact will notably change middle gearing combinations as well as the extremes. The riders that do not like compact gearing usually find that it is because the middle gear combinations they frequently rode in when they had a 39 tooth chainring are no longer the same.
How can you know what gears you will gain and lose when switching to compact or changing cassettes? Gear inches let you compare gearing options before you make the change and thus can be very valuable.
Comparing Gearing and Gear Inches – The origin of gear inches can be traced back to the front wheel diameter sizing used on high-wheeled/Penny-farthing bikes in the late 1800’s. Penny-farthers are driven off the front wheel directly (no gears) by the crank, so the distance traveled per crank revolution is the same as the wheel’s circumference. Bikes with bigger diameter wheels travel further per crank revolution than ones with smaller wheels and thus the front wheel diameter was used to standardize sizing between bikes.
With the advent of the rear chain drive “safety bike”, things became more complicated – one crank revolution no longer added up one wheel revolution. However, people wanted to still compare chain drive gearing to their Penny-farther and thus the “gear inch formula” was born:
Gear Inches = (Front Chainring Teeth x Wheel Diameter) / Rear Cassette Teeth
Do we still care about how our bike compares to a Penny-farther? Of course not, but these same “gear inches” still provide an accurate method of comparing gearing options across wheel sizes and tooth combinations and are still used today. Gear inches are even practical numbers as you can find out how far your bike is going to travel in a given gear, by multiplying them by Pi (3.14). If you want to skip the math, and get an even more accurate result, use an on-line gear calculator (that also takes crank length and tire size into account), like the one found at www.sheldonbrown.com/gears. If you are really into data, http://www.machinehead-software.co.uk/bike/gears/gear_calculator.html shows you how your gear inches relates to your cadence and speed/distance traveled as well.
With gear inches, you can compare the gearing range you need and which gears you most commonly use to different cassette and crank options to make sure you get the gearing you want and the range you need for your riding and racing. This allows you to eliminate the guesswork with gearing and make an educated decision as to what combination will work best for you as an individual.
Enjoy the ride and train hard and smart!
Ian
originally published December 2007/Copyright © 2007
