Sunday, June 16, 2013

Gearing Choices - How do I know what to choose?

When custom spec'ing a bike for a customer build, one of the things I consider is the gearing that will go on the bike, namely the front chainrings and rear cassette (cogs).  Factors that play a role in this decision include riding terrain, cycling ability, wheel diameter, the rider's normal cadence range, application (TT vs. road), and also the possibility of rider improvement.

Generally speaking, there are about 4 choices for front chainrings (from biggest to smallest):
Standard (130 BCD):
1. 54/55/56 big with 42/44/46 small
2.  53 big with 39 small

Compact (110 BCD):
3. 52/53 big with 36/38 small
4.  50 big with 34 small

In regards to cassette choice, there are several choices, but in practical terms more like three:
1.  11-23 (or 11-25 if it's an 11-speed cassette)
2.  11-25 or 11-26
3.  11-28

(Note: There is really no advantage in going with a 12 - XX cassette, as the 11-tooth, and hence faster downhill speeds, is lost, and very little gained in exchange.  The only circumstance where a 12-XX, such as a 12-21, 12-23, or 12-25 might be considered is actually on flat terrain where speed variability is minimal and you will be using your middle cogs (14, 15, 16, 17, 19 tooth) more of the time.)

How do I know what to choose?  The bigger the 'big' ring, the faster you can go on the downhills and on sections with strong tailwinds.  The smaller the 'small' ring, the easier it is to climb steep uphills or moderate uphills into strong headwinds.

Example 1: A roadie with average cycling ability (avg. of 14 - 16 mph on most training rides with ~800 - 1,000 ft. climbing per hour of riding, akin to St. George area terrain)
-- enjoys doing century events 
-- lives in the St. George area that has a lot of hills, with a number of steep climbing sections, 
-- likes to pedal at a high cadence range of 90 - 100+
-- fairly stable projected cycling fitness based on limited training time and age
-- 700c wheels

I'd likely spec either 52/36 front rings paired with 11-28 cassette, or 50/34 rings paired with 11-26 cassette for this rider.

Why?  the most important thing to consider is making certain this rider has the SMALLEST gear they will ever NEED while enjoying the longer century events containing substantial climbs.
Thus, a 36x28 combo or a 34x26 combo should give them an ample "granny" gear for the steepest of climbs, and also allow them to "spin" at a relatively high cadence range which they are accustomed to.

Example 2: A Triathlete with above-average cycling ability (avg. of 19 - 20 mph on most training rides with ~1,000 - 1,200 ft. climbing per hour of riding, akin to St. George area terrain)
-- enjoys Sprint and Olympic Triathlons
-- lives in the St. George area that has a lot of hills, with a number of steep climbing sections, 
-- likes to pedal at a lower cadence range of 75 - 85 most of the time
-- cycling fitness likely to improve 
-- 700c wheels

I'd likely spec either 55/42 front rings paired with 11-28 cassette, or 54/39 rings paired with 11-23/25 cassette for this rider.

Why? As a fairly strong triathlete focusing on shorter race distances, the speeds obtained by this athlete will be pretty darn fast.  A bigger 'big' gear will be used fairly frequently on big downhills as well as slight downhill sections with tailwinds.  In addition, the athlete uses a somewhat lower cadence range which means he tends to push bigger gears.  
Lastly, the smallest 'small' gear he will have will be adequate with either a 42x28, 39x25, or perhaps even a 39x23 combo.

Some of you reading might be asking, why even bother with an 11-23, or 11-25 if you can get both the big '11' and the very small '28'??  It boils down to gear SPACING.

Gear SPACING is obviously large on an 11-28 cassette (esp 9 or 10-speed cassettes).  As a result, as you shift through your gears, you may not find the optimal 'feel-good' cadence for a particular gearing need.  However, with an 11-23, where the gears are more closely spaced, you will have better luck finding your 'sweet spot' for cadence.  In particular, the 11-23 10speed cassette (11-23 and 11-25 in 11speed cassettes) contains the oh-so-sweet 16-tooth cog.  If your bike uses 700c wheels, and you are riding in the 53/54/55 -tooth big ring paired with the 16-tooth cog, you are moving at around 22 - 25mph, depending on cadence.  Thus, this is a 'sweet-spot' for riding on flat terrain for an above average triathlete riding in aero position.  When shifting through your gears, from 19 to 17 to 16, with 16 as your final destination, having the 16-tooth cog means you get a nice jump down from the 17-tooth.  However, with an 11-25/26/27/28 10speed cassette (or 11-26/27/28 11speed cassette), you LOSE the 16-tooth cog and make the 'massive' jump from 15 to 17 or vice versa.  Sometimes that 16-tooth is just the "sweet spot" of what you want for your cadence range.  


This ends a short, but hopefully informative discussion on gearing choices.

As always, let us know if you have any questions!

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