Gears let you choose your trade: an easy gear turns the wheel a little per pedal but feels light; a hard gear turns it a lot but takes more push.
Most people think a low climbing gear gives you more power or makes the hill less work. In fact it adds no power and the climb costs the same; it just lets you pay in many light pedal strokes instead of a few crushing ones.
What's actually happening
Every gear system on Earth sells the same product: a trade. Your legs produce a certain effort at a certain pedalling rhythm, and the gears convert that into a chosen blend of wheel-turning force versus wheel-turning speed. More of one always costs the other — the chain is a market with no free lunches.
The arithmetic lives in the teeth. Pedal a 50-tooth front chainring connected to a 25-tooth rear cog and every pedal revolution drags 50 links of chain past, spinning the rear cog, and the wheel, exactly twice. You've doubled speed and halved force. Swap to a 34-tooth front and a 34-tooth rear and one pedal turn is one wheel turn: all your force survives, but you crawl. That is the whole theory of the twenty-something gears on a modern bike: a menu of tooth ratios from about 1:1 (steep climbs) to 4:1 (flat-out descents).
Why does "spinning an easy gear" up a hill work? The hill demands a fixed amount of work to climb — weight times height, no negotiating. A low gear doesn't reduce the bill; it lets you pay in many small instalments (light pedal strokes) instead of a few crushing ones. Cars do exactly the same thing: first gear for pulling away with force, sixth for cruising with speed, and a lorry grinding up a motorway hill at 30 mph is just a cyclist in their granny gear, scaled up.
Gears cannot add a watt of power — they only let you choose the trade between force and speed, paying for one with the other.
- 1Flip a bike upside down. Count teeth on the biggest front chainring and the smallest rear cog (just count one and check the stamp on the other — they're usually marked).
- 2Predict the ratio: front teeth ÷ rear teeth = wheel turns per pedal turn.
- 3Turn the pedals exactly once by hand with a tape mark on the rear tyre, and count wheel revolutions. The teeth never lie.
Common questions
The hill demands a fixed amount of work to climb, set by weight and height. A low gear does not reduce that bill; it lets you pay in many small, light pedal strokes instead of a few crushing ones.
A fully loaded lorry drops to a low gear ratio to multiply engine torque at the wheels. It trades away speed it could not have held anyway, just like a cyclist in their lowest gear.
Velodrome riders face no hills and no need to accelerate from rest mid-race, so they fit one enormous ratio and pay its high force cost with heavily trained legs.