I recently had a Mountain bike racer (who's also a motorcycle racer) ask me this question:

Dear Aaron;

If I change from a 11-34 cassette to an 11-32 how much will that effect my effort?  If I'm thinking correctly it will make my slowest speed faster. Right? Thanks for your help.

Sincerely, Thomas M.

Hi Thomas;

That is a great question. You are correct in that if you go from a 34 down to a 32, you will have a higher speed and a slightly higher effort. I can tell you that the bigger the gear, the bigger the ratio change.  i.e. 53x13 versus 53x12, there  is a greater increase in ratio and mph. Granny gears have a smaller ratio change in speed and effort.

As far as the amount of effort, I don't have an exact answer so I'll do what I can to help. You want to choose your gearing based upon your ability to provide power to the pedals and stay within your "zones" (out of the red zone).  And that is dictated by conditions, internally and externally. So internally, if you feel good that leaves external: elevation, temp, wind.

Here's some info on gear ratios.
I'm assuming one of your chain rings is a 36 so we'll use that as baseline for explanation.

Here's the math:  Chainring x Wheel diameter divided by rear cog = your gear inches
i.e. 36 x 27 / 32 = 30.375 inches traveled per stroke of the pedal

This means you travel 30.375 inches per stroke of the pedal

So to compare a 32 and 34 tooth cog
The 34 cog comes out to 28.58 traveled inches per stroke of the pedal

The difference seems small but here's the true difference.
At 100RPM in 36x34 you will go 8.5mph
At 100RPM in 36x32 you will go 9.03mph

The difference between the two gears is only  0.53mph which overall is small but as we both know that could be a big change in racing conditions. Whether you are climbing up a 18% grade or time trialing into a headwind,  0.5mph could effect your effort level.

So I guess what I'm saying is that if you are climbing and not using your 34 then you could and should step it down so you could have another intermediate gear to use in your "usable power" range.

Old school method is to take all your gearing with you, pre-ride the course and then adjust/change your gearing based upon your observations.

Like I was saying earlier about bigger gears having a bigger change in ratio.
The difference between a 53x13 versus 53x12 at 100rpm is 32.75mph versus 35.48mph. That' a big effort change!

I hope this answers your question.

Aaron

As I learn how to navigate the SpeedTV site, I'll begin posting and blogging on a regular basis. You can keep up with my training, how it fits into motorcycle racing and what's going on with some of the athletes (racers) that I am training and coaching. I'm looking forward to making lots of friends and hearing back from you.