This blog explains the basics of all gearing systems then specifically the ins and outs of bike gearing. It will help you decide if your current bike’s gearing is suitable for your intended climbing and riding adventures or maybe it's time for that new bike! The focus is on road bikes versus gravel, mountain, time trial, or e-bikes.
Gearing Basics
Invented in 3 BC by the Greeks (Archimedes), gears have wide use in machines including cars, wind turbines, and variable speed gadgets in the home. The most common uses of gears are controlling speed (eg a wind turbine) and to reduce the force required to move an object (eg your bike). Some tasks would be near impossible without gears, for example climbing a 25% grade on your bike or jacking up your car. Gears are almost always two different diameter gear wheels. In the case of bikes, when the larger chainring drives a smaller cassette sprocket, you’ll go further per revolution but will require higher effort. When the smaller chainring drives a larger cassette sprocket, you won’t go as far per revolution but will be lower effort, So gears essentially trade distance traveled for effort required.
Using bike gears as the example:
- A 50 tooth (largest) chainring and 12 tooth (smallest) cassette sprocket is a gear ratio of 50/12 or about 4:1, meaning that 1 revolution of the cranks produces 4 revolutions of your wheel which is about 30 ft of travel. The distance traveled is large but the effort to turn the cranks on a steep climb would blow your knees apart
- A 34 tooth (smallest) chainring and 34 tooth (largest) cassette sprocket is a gear ratio of 34/34 or 1:1, meaning that 1 revolution of the cranks produces 1 revolution of your wheel which is only about 7 ft of travel. The distance is smaller but the effort to turn the cranks is much less
Understanding and Using your Bikes Gears
- A typical road bike has 2 chainrings and 12 sprockets on the cassette. Referred to as 12 speed, it gives a choice of 24 gear ratios. 12 when on the inner chainring plus 12 when on the outer
- Cassettes are designated by their minimum and maximum tooth count, ex 11-30 or 11-36. Double chainrings are simply designated by the number of teeth on each chainring, eg 36/52. 3 chainrings (triples) have thankfully all but disappeared due to cassettes having grown from 5 to now 12 or 13 sprockets
- For climbing, the ratio that's most often quoted is for your 'easiest' gear combination ie the number of teeth on the smallest chainring divided by the number of teeth on the largest cassette sprocket. The lowest gear ratio for a 34/50 tooth chainring with a 11-34 tooth cassette sprocket is 34/34, ie a 1:1 gear ratio (wheel rotates 1 time for each crank revolution). This will be be slightly easier than a larger 36 tooth chainring with a 34 tooth maximum cassette sprocket ie 34/36 or a 1.3:1 gear ratio (wheel rotates 1.3 times for each crank revolution)
For hills, we change gears to trade distance traveled for reduced effort. Less distance traveled per crank revolution for less effort to turn the cranks. The lower the gear ratio, the lower is the effort you feel with each pedal stroke.
A simple climbing guideline is to shift up or down as the gradient varies to maintain a cadence between 60 and 80. Anticipate the gradient of each hill. A few bike lengths before the hill starts, shift into the gear that will give you a cadence of between 60 and 80 when you hit the climb.
Advances in derailleur design and cassettes have enabled bikes to span a much broader range of conditions from fast downhill to steep climbing. In days gone by, it would require changing the cassette and/or chainrings to suit the conditions. Electronic shifting, cassette and chainring tooth profiles and ramps have resulted in faster and more reliable shifting, especially when climbing under load (although, anticipating an easier gear before the climb is always the better choice). When derailleurs went electronic, they became smarter. They have options to prevent you riding cross chain (large chainring/largest cassette sprocket and vice versa have the highest rate of chain and tooth wear). Electronic derailleurs can also automatically change the chainring (synchro shifting) as you shift through the cassette. This minimizes large changes in gear ratio thus avoiding disruptions to your cadence. Lastly they take care of preventing chain rub with micro movements of the front derailleur (feathering) as you shift on the rear cassette.
Choosing Optimal Bike Gearing
Whether buying a new bike, upgrading an existing bike, or trying to decide if your current bike can handle steep hills, the choice of gearing is important. Your leg strength, cardio fitness and how competitively you expect to ride also factor into your gearing choice.
Most new bikes are standardized with 12 speed cassettes and usually don’t give options for chainring and cassette sizes. Higher end bikes are coming with 36/52 (“mid-compact”) chainrings and a 12 speed 11-30 or 11-34 cassette. Lower end road bikes also use 36/52 chainrings and a cassette with a larger 36 tooth sprocket to make steeper climbs easier.
Bottom line is new road bikes configured with 36/52 chainrings and an 11-34 cassette will get you up all the hills in Wisconsin, including for events like BCETS, Horribly Hilly Hundreds and Dairyland Dare. 34/50 chainrings will make them even easier as will an 11-36 cassette. Stronger, fitter and more competitive riders can consider more aggressive 39/54 chainrings and an 11-30 cassette. It will be up to the bike shop for the cost of chainring and cassette swaps.
Upgrading an Existing Bike's Gearing
This can get involved and expensive depending whether you can upgrade with a direct swap of chainrings and cassette to make climbing easier.
- Chainrings. Most of the time these can be swapped for smaller/less teeth versions but may require a different chain length and/or a longer rear derailleur cage
- Cassette. If you want to change the number of sprockets, eg a 10 to 12 speed, the derailleur and its shifter have to be replaced (expensive). You can't for example use a 10 speed shifter with its 10 speed derailleur with a 12 speed cassette. Chains are generally narrower for 12 speed cassettes, check with the cassette manufacturer to determine if a narrower chain is required
An easier and cheaper option is to swap the cassette with one having the same number of sprockets but a larger maximum sprocket size, eg from 28 to 34 or 36 teeth. Derailleur gear capacity must be checked first:
- Derailleur
- Smaller chainrings and a larger maximum cassette sprocket size may require a derailleur change or changing the jockey wheel cage from a small to a medium or medium to large cage. This is determined by the rear derailleur’s ‘capacity’ (Difference in the number of teeth of the two chainrings + difference in number of teeth between the smallest and the largest cassette cogs). For example:
- (50-34) + (36-11) = 41 gear capacity. In this case, both Shimano and SRAM specify a long cage derailleur. Derailleur manufacturers’ websites show the capacity ranges for Short, Medium and Long cages
- Chain. A longer chain may be needed to accommodate a larger chainring and/or a larger maximum cassette sprocket
Extreme Gearing Examples
Below are two examples that emphasize how gearing is used for record-setting endurance speed on the track versus extreme climbs.
- A single speed track bike focused on the 1 hour distance record uses a very high gear ratio. It requires a much higher force (ie massive quads!) to rotate the cranks but will produce a much larger distance per crank revolution. For example, a 60 tooth chainring and 12 tooth single fixed gear sprocket, has a gear ratio of 5:1 (5 rotations of the rear wheel for every revolution of the crank). At an average cadence in the mid 80’s, the distance traveled with each crank revolution is a whopping 35 ft with a resultant speed of about 40 mph.
Source: Cycling Weekly, Campenaert’s Hour Record bike
- Conversely, a bike focused on extreme climbing will have a front chainring that is smaller (28 teeth) than the largest cassette sprocket (40 teeth). This is a gear ratio of 0.7:1 (0.7 rotations of the rear wheel for every revolution of the crank). This trades a much reduced effort at the pedals for a distance per crank revolution of only 7 ft at a speed of about 5 mph
Some Fun Videos Illustrating Gear Ratio Extremes
Words and photographs: Graham Rae