Derailleur Gearing Explained

Derailleur Gearing Explained

Q: “I bought a new bike that came equipped with two big sprockets in the front (chainrings), and nine gears (cogs or sprockets) in the back. Okay, so what does that mean to me?”

“How to identify and shift derailleur gears” is a common and important question for new cyclists, so I decided to write a post about it.

Let’s start with the basics:
The Number Of Gears On Your Bike
Since you have 2 chainrings and nine cogs (sprockets), there are a total of 18 selectable gears on your bike. If you had 3 chainrings, you would have a total of 27 gears!

For each chainring, every time you shift to a different cog in the rear, you select a different gear.

As you go from a smaller cog to a larger cog, you are selecting a lower gear.

The lower the gear, the easier it is to pedal, and the easier it is to go uphill; that said, the lower the gear, the faster you have to pedal to maintain the same speed.

Lowest Gear
The bike’s lowest gear is engaged when the innermost (smallest), chainring is selected in conjunction with the innermost (largest) cog in the rear.

Inner chainring selectedLargest rear cog, or sprocket

Middle Gear
To find out which gear is your middle gear, you need to do some calculation.This  isn’t usually necessary, unless you absolutely want to know.  Otherwise, good approximations are made:

  • If you are on your largest chainring, pick a rear cog that is 3 or 4 smaller (down) than your largest cog.
  • If you are on your smallest (smaller in this case since there are only 2) chainring, pick a rear cog that is 1 or 2 larger (up) from the smallest.

I f you really want to know, you need to count the number of teeth on each sprocket, front and rear. Then calculate your lowest gear ratio by dividing the number of teeth on the inner chainring by the number of teeth on the largest rear cog. For example, if your inner chainring has 40 teeth, and your largest rear cog has 21 teeth, then your gear would be
40 teeth/21 teeth = 1.90.

Next calculate your highest gear, which is the combination of your outer (largest) chainring and your outermost (smallest) rear cog. If your outer chainring has 52 teeth, ant your outermost cog has 11 teeth, your highest gear is 52 teeth/11 teeth = 4.73.

Your middle gear would then be the difference of those two numbers, or 3.35.  If you calculate all of your gear ratios, you can see which gear comes closest to the 3.35 number.

Shifting Gears
But as I said, most people don’t want to get that detailed. So how do you shift gears, anyway?
There are different style shifters, so it would be good to have someone explain to you what style you have. Regardless of the style, the shifter on the left side of your bike is for the front derailleur, which shifts between chainrings. And conversely, the shifter on the right side of your bike is for the rear derailleur, which shifts the chain from one rear cog to another.

To shift, you need to be pedaling!  The chain needs to be moving to actually shift from one cog to another, as it is being physically pushed to the side. If it isn’t moving, it can’t ride up onto a larger cog, or down onto a smaller one.

Also, plan ahead when you are going to need to shift. It’s easier to shift when the road is relatively flat; if you are going up a steep hill and have to shift, it’s harder for the chain to change cogs because of the high tension on the chain.

Shifting just takes a little practice. Modern derailleur shifters are indexed, meaning they click when you shift gears. Older shifters, like on my first “ten speed”, relied on friction to maintain their position, and there was no clicking into gear; consequently, shifting form one gear to the next took more learning.

Happy shifting!