Wednesday, June 02, 2004

All about gearing

Over the weekend I installed a new SRAM cassette (the gears on the back :). I picked up a 12-26 model, meaning the gears range from 12 teeth to 26 teeth. On the front end I have 39 and 52 tooth chainrings. Using a handy gear table and a little math, you can find out that with one revolution of the crank I can travel anywhere from 8.61 to 20.42 feet. That is a pretty reasonable range. Those gear ratios will get you up just about anything around here.

Now let's convert those figures to actual speed. Assume I am flying down a hill, 52 tooth chainring in the front, 12 in the back. At a cadence of 110 rpm I will be doing about 38.12 miles per hour.

Cassettes are available in a variety of sizes, some with an 11 tooth cog. Also, most racing setups use a 53 tooth chainring, not a 52. Do a couple teeth one way or another make a difference? Hell yes. If I put a 53 on the front and had an 11 on the back, at 110 rpm I could do 42.57 mph.

You think, big deal, so you can go a bit faster. Well, in my limited experience I've been able to make up good ground on downhills. I'd love having a little extra speed at my disposal. The catch is that I can't find a manufacturer that makes a cassette with an 11 tooth and something as big (or small, depending on your point of view) as a 26 tooth. If you want an 11 tooth cog on your cassette, the biggest you're going to get is a 23, unless you go with a mountain bike cassette which gives you something like 32 (way too much).

I wrote to SRAM about it, but no luck:

"We don't produce an 11-26 model cassette and don't recommend altering the gearing on the cassettes. Cassettes are designed as whole units and exchanging cogs will make shifting inconsistent and can affect wear on the chain and cassette.

Happy Trails."


Let's see, is there another way to approach the problem? If I purchase an 11-23 rear cassette I would have to go to a 34 or 35 tooth front chainring to get a gear ratio similiar to a 39 tooth chainring and 26 tooth rear cog. The catch here is that chainrings smaller than 39 teeth require a new crankset.

Sigh.. I just want an 11 tooth cog. The search continues!

3 Comments:

At 10:48 PM, Blogger Jim Carson said...

A couple o' thoughts on this:

First, Shimano has a 11-21 (Dura Ace, Ultegra), 11-23 (ditto), 11-32 (XT), and 11-34 (XT) See Colorado Cyclist. Sheldon Brown also has a bunch of data on gearing (http://sheldonbrown.com/k7.html)

If you wanted to totally go fringe, there's an obscure Shimano hub/freewheel combination under the Capreo label that will do a 9-26.

Second thought is the basic physics of speed. What's your typical course look like? How much uphill vs level vs down? The reason I ask is in the randonneuring community, there's a lot of emphasis on uphill speed because it affects your overall time more than downhill speed.

Consider the simple problem of a mile ascent and a mile descent. If I go up the hill at 15mph, it takes me 4 minutes. Assume I go down the hill at 30mph, the same hill takes me 2 minutes. Total elapsed time to cover the two miles is 6 minutes, averaging 20mph.

Suppose I wanted to average 22.5 mph, meaning I do the hill in 5:20. There are two extreme combinations:
1) increase my uphill speed to 18mph (uphill time is now 3:20) Or, I can increase my downhill speed to 45mph.

My point in this obviously contrived example is depending on your course, downhill speed may not be as important as that on uphill and flats.


- Jim
http://www.jimcarson.com

 
At 8:41 AM, Blogger Doug W said...

I totally agree with you. Uphill speed is very important and that is really where you want to put your emphasis. That said, I've noticed that people tend to take breaks on the downhills so it is an opportunity to attack (or catch up, in my case).

I'm hoping to pick up a set of Power Cranks over the winter to work on power development. They aren't cheap, but when I look at places where I can waste my money it seems like they will have a high roi. They have some very positive reviews.

 
At 6:06 AM, Anonymous Don said...

I don't know the date of this, but Nashbar.com has a few selections of cassette shimano 11x21, 11x23, SRAM 11x21 9spd, 11x23.

I ride on a fairly flat course, and am cranking out around 90-100 rpm consistently but can't seem to get past 26mph! I'm also thinking of a more aggressive high gear. Would it be a crime to put an ATB cassette on a road bike?
-Don

 

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