Category Archives: Bikes

Single Speed Gearing (part 2)

My previous post on single speed gearing covered all of the convoluted ground of figuring out the “gain ratio” to compare gearing between different types of bikes.  But it can be simplified to just the “tooth ratio”, as long as you’re comparing bikes with reasonably similar wheel/tire sizes.

I mean, maybe you care more about the details if you’re racing the thing, but I’m talking about riding around town, right?

So the tooth ratio I’ve stuck with on my daily rider is 2.67.  Some may call it laziness, but I prefer to think of it as, uh, Salsa’s excellent build design- I’ve stuck with the 48/18 gearing that came standard on my Cassaroll.

As it turns out, this ratio is quite similar to the popular single speed setup of a 42 tooth chainring and 16 tooth freewheel cog, which clocks in a tooth ratio of 2.63.  In fact, that’s the setup on my rain bike, but because that beast (a Marin 29’er) has bigger wheels the gain ratio is very, very close to that of my Salsa.

However, there’s no need to stick with what comes standard – a new freewheel cog costs about $10-$15 and can be replaced in minutes.

Here’s a handy chart for comparing tooth ratio across different single speed gearing setups.  I’ve highlighted the most common configurations:

single speed gearing chart

Single Speed Gearing

It took me months from the time I first started thinking about going single speed to when I finally plunked down the cash for my Salsa Cassaroll.  What finally put me over the top was:

  1. Realizing I was 42, and that it wouldn’t be too long before ascending Capitol Hill on a single speed would be a physical impossibility; and
  2. Finally getting a grip on gearing, “gain ratios” and what it would mean to do the ascent in a relatively high gear.

Trying to figure out gearing was HARD.  I really didn’t want to walk my bike up the hill.  I finally stumbled on Sheldon Brown’s site, which contains a highly-illuminating formula for figuring out the gain ratio of gearing (thus allowing me to compare the gearing on my 9-speed urban destroyer commute bike with the single speed of my dreams). Here’s how the formula works.  It looks more complicated than it really is:

  1. Determine the wheel radius.  Note that wheel radius is influenced by tire depth; a 700 x 28 road tire has a radius of 336 mm, vs a 44 mm tire on the same wheel which has a radius of 354 mm.  Refer to Sheldon’s handy-dandy chart.
  2. Divide the wheel radius by crank length (use consistent measurements for each – millimeters or inches) to get the “radius ratio.”  For the old urban destroyer, this yielded a radius ratio of 1.906 (324 mm wheel radius/170 mm crank length).
  3. Divide the number of teeth on the chainring by the number of teeth on the rear sprocket (let’s call this the “tooth ratio”).  For third gear on the destroyer – the gear I typically used to climb the hill – this was 38/18, or 2.11.
  4. Multiply the radius ratio by the tooth ratio to get the gain ratio – which can then be compared, apples-to-apples, to the gain ratio on any other bike.  Third gear on the destroyer? 1.906 x 2.11 = 4.024.

Now, most of the single speed road bikes I looked at had radius ratios around 2.00.  That meant that getting gearing equivalent to what I was currently climbing the hill with would require a tooth ratio of around 2/1.  While that would make it easy to climb the hill, it would be impossible to ride at any speed with such gearing.

The trick was to find a tooth ratio setup high enough to allow some speed on the flats but not so high I’d be unable to climb.  In talking to people riding single and fixed gear bikes up the hill, this seemed to indicate a ratio of a little less than 3.00 (one popular single speed configuration is 42/16).

I saw one guy riding a track bike up Pine with a setup of something like 52/12, but he looked like he weighed in at about what I did at age 9, so I ruled that configuration out.

Here’s where it got daunting:  a 42/16 configuration has a tooth ratio of 2.63.  Multiplying that by a radius ratio of 2.00 equaled 5.26 – which is about what 7th gear was like on the urban destroyer.  Gulp.  I tried climbing the hill in that gear a couple of times, but always had to gear down after a block or two.

Ultimately, I just took a leap of faith and went for it.  More on that later.

Tired of my Rain Bike

The summer solstice is almost upon us, yet I’ve had to break my rain bike out every day this week – and today is shaping up no differently.  Despite its cushy ride and cargo-carrying capacity, I’d rather be on my more sprightly Cassaroll at this time in June.