Gearing Part II: In praise of low gears

I may have mentioned in previous posts that there is some topography around here.  In town, 6% slopes are routine, 10% climbs are common, and there are even some block-long hills around 20%.  Rural roads are usually a little gentler, since lower road density allows routes that avoid the steepest hills, but there is little flat road and still plenty of 6-10% and steeper climbs. 

I am not as young (or as fit) as I used to be.   I don’t alway ride with the express purpose of causing myself cardio-vascular distress in an effort to recapture my lost youth.  I am formerly (I hope) handicapped by a bad heart valve, and occasionally get injured (the knee is healing up nicely, thanks for asking).

I like long rides.  Jamming up hills out of the saddle works for short trips around town or even relatively short rides in the country, but I don’t have the fitness (or probably the potential, much less the motivation, to develop enough fitness) to use this tactic for rides longer than 25 miles or so.

I like to be comfortable and reasonably safe on my rides, so my bikes will never be as light as a stripped-down racing machine. And on tour, I carry appropriate loads, including camping gear. I also like unpaved roads, though there is a lot more rolling resistance on them than on smooth pavement.

I am a spinner.  I tend to stay around 90 rpm on the flats (such as they are) and I feel best when I also maintain this cadence on climbs.  If I go much under 70 rpm it  feels like a grind, beats up my knees, and tires me quickly.

All of this has led me to install very low gears on my bikes and use them frequently.

 I consider myself a 100 watt cyclist (see this previous post) though I probably average closer to 125 watts these days.  Of course I can push it up higher for short periods or for training rides.  But I might also slow down and enjoy the scenery.  Pacing is important on day-long rides, and on a tour, you have to leave enough energy to set up camp, cook dinner, and do it all again tomorrow, and  35 watts gets you down the road.

So how low is low enough?  This table gives speed and gear as a function of slope and rider power output for a 175 lb. rider on a 25 lb. bike.

  100 Watts 200 Watts
Slope (%) Speed (MPH) Gear at 90 RPM (inches) Speed (MPH) Gear at 90 RPM (inches)
3 7.3 27 12.6 47
4 5.9 22 10.7 40
5 4.9 18 9.2 34
6 4.1 15 7.9 30
7 3.6   7.0 26
8 3.2   6.2 23
9 2.8   5.6 21
10 2.6   5.1 19

The lowest commonly-available low gear is 19 inches (24 tooth front, 34 tooth rear, 27 inch [nominal] wheel).  A fellow 100 watt cyclist runs out of gear (or has to reduce cadence) on a paltry 5% slope even with this gearing that is extremely low by current convention.    Even the 200-watt sportster who wants to maintain a good cadence on a long hill might find the usual 39×27 (39 gear inches)  or “compact” 34×27 (34 gear inches) too high for a mountain ride.   And this chart does not take into account  camping loads or rough surfaces. 

 The lack of low gears in hilly terrain can turn a pleasant rural ramble into a gruelling test of strength and endurance.  While I enjoy a little gruel now and then, it is nice to have options.  It would be difficult to have gears that were too low.

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  1. #1 by martinsj2 on December 29, 2010 - 12:06 pm

    Very interesting, and to a Flatlander such as myself a little foreboding. I have a standard compact gear and am looking forward to tackling some Appalachian centuries this summer/fall. This post just confirms for me the need to get on some serious hills as soon as possible and see how bad this is going to be!

    Steve

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