This will be the last of my blog posts, for a while anyways, describing modifications I’ve made to my bike to better accommodate my riding preferences.

Below is a description of bicycle gear ratios. Ignore if you don’t care…

Bicycle Gear Ratios

Bicycle gear ratios are calculated using the number of teeth on the front chainring divided by the number of teeth on the engaged sprocket on the rear cassette. For example, if you have a front chainring with 36 teeth and a rear sprocket with 36 teeth, the ratio is 1:1, meaning that for each revolution of the crank, the rear wheel will also have one revolution. Likewise 36 teeth on the front chainring and 18 teeth on the rear sprocket yields a ratio of 2:1, meaning that for each full revolution of the crank there are 2 revolutions of the rear wheel – which means that it’s harder to pedal, but you go farther with each pedal stroke.

Multi-speed bikes have one or more chainrings on the front crank and a cassette of sprockets on the rear wheel. Modern bikes have cassettes with 11 or 12 sprockets of varying sizes, providing a range of gear ratios to choose from depending on the terrain being ridden.

My bike has two front chainrings with 36 and 50 teeth and originally had a rear cassette with sprockets ranging from 11 to 36 teeth. A simple calculation shows that my lowest possible gear ratio was 36:36 or 1:1.

I wanted a ratio lower than that, and thus begins my tale.

Adding the Granny Gear

As I suspect every rider does, from time to time and always when grinding up some onerous hill I found myself wanting just one set of gears with a lower ratio than what is on my bike. Last summer I decided I would modify my bike to fulfill my wish for a “Granny Gear”, (no offense to grannies out there) that is a gear ratio where each revolution of the crank yielded a fractional revolution of the rear wheel. This means I would go less distance with each pedal stroke, but pedaling up hills would be easier.

Where I live, in the southeastern United States, the last part of Summer becomes quite hot and humid and not as conducive to riding as other times of the year. That is a good time to do some maintenance or modifications to the bicycle. I decided that would be a good time to change out my rear cassette to provide the desired hill climbing “granny” gear.

After some research, I found this Shimano Xt Cs-M8000 11-40 mountain bike cassette.

The smallest sprocket matched the size of my existing smallest sprocket. The largest sprocket, however, had 40 teeth which provided a 36:40 gear ratio, or 0.9 revolutions of the rear wheel for each full pedal stroke. That might not seem like much of an improvement, but in practice it has made all the difference!

I removed my existing cassette and installed the new cassette, only to find that my derailleur didn’t have the vertical travel required to properly engage the new, largest sprocket on the cassette. I attempted to make the existing derailleur work by installing a RoadLink extender from WolfTooth Components, but this attempt yielded less than optimal shifting – likely due to my meager bike mechanic skills.

Thus I needed a new rear derailleur; one that was compatible with the the new mountain bike cassette. After some additional research I found this Shimano Deore XT M8000 11Spd Shadow+ Rear Derailleur

I mounted the derailleur on the bike and then discovered that mountain bike derailleurs don’t do indexed shifting like road bikes do. Specifically, the index adjustment on mountain bikes is on the handlebar shifters while the index adjustment on road bikes is on the derailleur. So, this derailleur was incompatible with my Shimano 105 indexed shifters.

Yikes!

I turned to my local bike shop, Moab Bikes, for help – you should always maintain a good relationship with your local bike shop! – where Jeremy, who originally helped me when I was buying the bike, knew a solution!

Jeremy recommended installing a Tanpan from our old friends at Wolf Tooth Components. The Tanpan is specifically engineered to allow Shimano 10 and 11 speed shifters to work with mountain bike rear derailleurs.

Here is what my derailleur looks like with the Tanpan installed.

The result of all of this was that my bike now had the desired hill-climbing gear. Here is what the whole assembly looks like.

The first big test of this was to come later in the fall at the Gravel Revival event. The full account of that event will be in a later blog post, but I will note than my granny gear performed excellently, allowing me to ride up hills that found younger riders walking their bikes.