You are here

Cycle Gearing

Gearing is a personal choice. Your best gearing is not necessarily my best gearing. And what Shimano offers is likely not best for either of us. As we evolve as cyclists we can better identify what suits us. Usually these needs can be met with off-the-shelf components. But not always. Here is a brief view into the longish process that helped me define what suited my style best and how it was accomplished.

The role of the road bicycle in question is a cross between Winter Bike and Randonneuring Bike. A Shimano eight speed drivetrain was chosen for reliability and relative durability. A double crankset was chosen for simplicity. Within this sixteen ratio constraint the gearing needed to have a low enough low gear and a decent selection of cogs while cycling fast-ish. A ten speed drivetrain coupled to a triple crankset would have easily covered all bases but cutting the number or ratios nearly in half to sixteen meant that a little more thought was necessary.

The first couple years saw the use of an XT 12-32 cog cassette. The cogs and percentage differences (AKA range) between cogs are shown below:

12 tooth
16.7 %
14 tooth
14.3 %
16 tooth
12.5 %
18 tooth
16.7 %
21 tooth
14.3 %
24 tooth
16.7 %
28 tooth
14.3 %
32 tooth

The largest cog of 32 teeth gave a nearly one to one ratio when necessary. This allowed the bike to be ridden up nearly any hill. The smallest cog of 12 teeth allowed a top speed of over 30 mph. If the only requirements were a wide total ratio range than this cassette might still be installed. But they weren't. So it isn't.

While riding at my moderate pace on flattish ground I found myself on the 18 tooth cog quite often. When I was fresh or motivated or had a slight tailwind I bounced back and forth between the 18 and 16. This 12.5% change was small enough that I never wanted for a 17 tooth cog. But late in a ride the chain was more likely to be on the 18 or 21. And too often the 18 felt too tall and the 21 too short. The percentage change of 16.7% between these two turned out to be one of the largest changes in ratio (and cadence) for this cassette while the 12.5% change between the 16 and 18 was one of the smallest. Spending half of my flat road pedaling wishing for a 19.5 tooth cog didn't make sense. So it was time for a change.

Although my home region of the Puget Sound is not considered flat there is enough flat ground that the majority of the pedaling is in the middle of the cogset. All that was needed was a cogset with adequate range and reasonably close ratios. Of course those two requirements are in conflict. But a possible solution is for closer ratios in the middle where the chain spends the most time and wider ratios at each end which are used much less of the time.

Shopping for such a cassette was largely a waste of time. It appears that Shimano tends to choose cogs to keep the ratios as constant as possible across the cassette. After using some online tools to calculate percentages I decided that a 13-30 cog would fit my needs best. This theoretical cassette looses some range compared to the 12-32 XT cassette. It's cogs and percentage differences look like this:

13 tooth
15.4 %
15 tooth
13.3 %
17 tooth
11.8 %
19 tooth
10.5 %
21 tooth
9.5 %
23 tooth
13.0 %
26 tooth
15.4 %
30 tooth

The middle percentages are now delightfully close. The biggest percentages are at the extreme ends. This seemed light a good idea. But, because this cassette didn't exist as a stock item it would need to be custom. In this case "Custom" translated to "dissassemble two used cassettes and assemble one used cassette".

Both donor cassettes were older Shimano Hyperglide cassettes. One was an eight speed 13-26 and the other was a seven speed 13-30. Shimano Hyperglide cogs are designed to have indentations to help the chain climb up from the next smaller cog. The number of indentations is dependent on what the number of tooth change is from that smaller cog. Those indentations dictated which cog was used when there were duplicates.

My first impressions after riding the new cassette were centered around the range. The difference between 32 and 30 was barely large enough to be noticed. The difference between 12 and 13 cogs _could_ be felt. But as a non-racer I was content to coast when the speeds exceeded 30 m.p.h. After a few longer rides I became more enamored with the ratios. Steep hills were slightly tougher. But I never missed the 12 tooth. And having the tight ratios in the middle allowed my cadence to stay in its sweet spot.

Another option to create a similarly spaced cassette would be to subtract two teeth from every cog and utilize a similarly spaced 11-28 tooth cassette. (A similarly spaced 12-29 is possible but 20 tooth cogs are not common.)

The concept for this cassette (tight ratios in the middle) is not likely to go very far. Few riders choose to limit themselves to just 16 ratios. But, after spending a fair amount of time thinking about what works for my style, I thought it might help others evaluate their needs and hopefully find their best fit.