First published December 4, 2020
It’s the weekend and you’re out on the club ride when one of the old boys asks something like, “When I was your age, I’d climb that in a 91 inch gear; what you pushing?” Or, maybe, it’s the mid-week time-trial and one of the skinsuit clad wannabes boasts, “Yeh, anything less than 130 inches and I spin out!”. Do you nod and grin with inane agreement. Worry no more, you’re about to discover the mysterious world of gear inches.
Back in the days of the penny farthing, if you wanted to know how hard or easy the gearing of your companion’s bike was, it was simple: you just looked at their front wheel. If their wheel was smaller than yours, they were a wimp pushing an easy gear, and if their wheel was larger, you knew they were insane or had legs of steel. Alas, once we entered the modern age and chainrings and sprockets arrived, it all became a lot more confusing. Who’s pushing the bigger gear now: Mr Compact on the little ring with a 16 on the back or Mr Big Chainring who’s riding at the extreme end of the block? Luckily, our desire to hold on to the past allows us to make this comparison. Gear inches allow us to take any modern gearing combination and express it in terms of the diameter of a penny farthing wheel—useful, isn’t it? So, how do you calculate gear inches - here’s the formula:
Gear Inches = Wheel Diameter x (Front Chainring Teeth / Rear Sprocket Teeth)
The wheel diameter is the diameter of the wheel including tyre and is expressed in inches. A simple way to work this out is to take the rim size (622mm for a road bike or 559mm for a mountain bike) and add double the tyre size (printed on the side wall of the tyre). So, for example, a typical road bike has 700 x 23 tyres, which gives us:
622mm (rim size) + (2 x 23mm (tyre size)) = 668mm (26.3 inches)
If that seems too much hassle, don’t worry. To make things easier, people tend to not worry too much about rim and tyre sizes; they simply use 27 inches. This is a more recent historical reference but, this time, to pre-metric road tyres, which measured 27 x 1 ¼. This means that the calculation for Mr Compact’s gear is:
27 inches x (34 Teeth / 16 Teeth) = 57.4 inches
And Mr Big Chainring? Simple, it’s:
27 inches x (53 Teeth / 25 Teeth) = 57.2 inches
So there you have it, Mr Compact was actually pushing the harder gear by the tiniest of margins.
Before I go, two things for you to note before you go wild with your new powers:
- Gear inches do not relate to the distance the bike has travelled. They simply represent a bike’s gearing in terms of the size of a penny farthing wheel. Of course, we can calculate the distance travelled, but that’s for us to discover some other rainy day.
- Remember the words of Stan Lee: “With great power, comes great responsibility.” You have a powerful tool at your disposal, but please don’t abuse it. You’ll be surprised to learn that other riders might not share your passion for gear inches, and you’ll soon become Boring Bob, the guy everyone avoids!
Happy Calculating
Photo credit: Greg Boll