How to shift a bicycle

I recently heard that less than 10% of new cars are sold with manual transmissions. Many drivers can't drive a car with a stick shift these days. This reliance on automatic transmissions may be why I see cyclists standing on their pedals with bikes stuck in high gear, trying to get up little hills: they don't get how multi-speed bicycles work. It wouldn't be polite to slap them upside the head out on the trail to show them how to shift a bike, so I guess I'll try it here.

A bicycle drivetrain

No matter whether you ride a $125 department-store "mountain" bike or a $9,000 carbon-framed road racer, the principles are the same. What makes bikes move is a bunch of levers and pulleys. Your legs push a pair of levers, which make a big, toothy disk turn. That disk is your chainrings. A chain connects the chainrings - two or three bolted together - to similar, smaller disks on the back wheel, like a pulley system. The smaller disks are cogs, and cogs come in "clusters" of five to ten.

Chainrings

Cogs

The number of "speeds" a bicycle has is the number of cogs times the number of chainrings. You used to be able to buy ten-speed bikes (five rear, two front); now you can get up to 30 speeds (10 rear, three front). The bike pictured here is an old fourteen-speed road bike, with a double chainring and seven-speed cluster.

Shifting systems

To change speeds (shift gears) you move the chain between chainrings or from one cog to another. Spring-loaded devices called derailleurs push the chain in the right direction. Shifters mounted on the handlebars allow riders to shift on the fly.

On all bikes, the shifter controlling the front derailleur is mounted on the left handlebar and the shifter for the rear is on the right. Mountain bikes and hybrid bikes use a shifter design with two thumb levers mounted parallel to the ground behind and below a flat handlebar. The larger lever pushes against the spring, moving the chain from a small chainring or cog to a larger one. The smaller lever triggers a spring that allows the chain to move from larger to smaller.  The arrangement is the same on both shifters.  Most modern shifters are indexed so that they automatically move one gear per click, so you don't have to feel your way as you shift. The system needs to be kept properly adjusted for this to work smoothly

Modern road bikes have shifters integrated into the brake levers. The brake lever pivots from side to side and pushes  the chain from a small chainring or cog to a larger one. A second, smaller lever mounted behind the brake lever triggers the spring-loaded derailleur to move it from larger chainring or cog to a smaller one. This arrangement is also the same on both shifters.

Shifter designs

Shifting physics

No, don't worry! This pretty simple stuff!

The teeth on your chainrings and cogs are all the same size and the length of the chain can never change. That means that the chain will move one tooth on the cog for every tooth it moves on the chainring. The chainrings are different sizes, so to keep the teeth the same size a big chainring has more teeth than a smaller one. Aha! the cogs are different sizes, too, so they must also have different numbers of teeth. Now you're catching on!

Since the chain has to move one tooth on the cog for every tooth it moves on the chainring, this gives you different ratios between front to back. The smallest chainring on a mountain bike often has about the same number of teeth as the largest cog, a 1:1 ratio. At the other extreme, some road bikes have a ratio of 5:1 between the largest chainring and the smallest cog - or five times as many teeth on the chainring as on the cog.

What makes bike gears work for you is this tradeoff:

• the lower the ratio of chainring teeth to cog  teeth, the easier it is to pedal, but the slower you move and the less distance you cover for each pedal stroke.
• the higher that ratio, the farther the  bike moves with each pedal stroke and the faster you go. but the harder it becomes to push the pedal.

A ratio of 5:1 means that for every revolution of the pedals, the rear wheel turns five times.

How to shift

Shifters aren't intuitive at first, but once you've ridden for a while your hands will remember how to shift up and down without asking your brain how. The key is that pushing against the derailleur's spring always shifts the chain to a larger cog or chainring, while triggering the spring always moves the chain to the next smaller cog  or chainring. Once you've figured this out, you're on automatic pilot.

To start, climb hills, or ride into a headwind you sacrifice speed for power, and the physics (that word again) of gears says you need a low ratio of front to back. Shift the chain to a smaller chainring by triggering the front derailleur spring with your left shifter; and to a larger cog by pushing the rear derailleur against its spring with your right shifter. As you pick up speed, shift the chain to smaller cogs (push against the rear derailleur). At some point, you'll want to shift the chain to a larger chainring (push the front derailleur with your left shifter); which usually means downshifting the rear derailleur (pushing it to a larger cog with your right shifter) a step or two to keep from overworking your legs.

When you come to a hill, slow for a turn, or face a headwind you may need to downshift. It makes it easier to pedal, though you'll go slower. Trigger the front derailleur to shift to a smaller wheel (your left shifter) and you'll be rewarded with more leg power. You may need to shift to a smaller rear cog to keep from downshifting too far and losing momentum. To climb, keep downshifting (front to smaller chainring, rear to larger cog) as necessary to keep your pedals spinning at the same speed.

Wasn't that easy? Now, go out and practice!

Rear cogs

A few closing thoughts

1. You should always ride in a gear that is comfortable for you.
2. Always "let up" a little on the pedals when shifting gears, instead of mashing them like a maniac. The chain and gears will last longer.
3. Don't ride with the chain on the largest chainring and largest cog or smallest cog and smallest chainring. This puts added stress on the chain and can wear it out before its time.

See you on the trail! (after I clean that poor bike...)