How to Choose Cycling Gear Ratios for Smart Trainers
1. What is a Road Bike Gear Ratio?
Gear Ratio = Number of Chainring Teeth ÷ Number of Cassette Cog Teeth
This ratio represents the number of times the cassette (and consequently the rear wheel/tire) rotates for every single full revolution of the chainring (pedal stroke).
- Cassette (Rear): Composed of multiple stacked cogs, sized by the number of teeth.
- Chainring (Front): Sized by tooth count; road bikes typically feature single, double, or triple chainrings.
Road Bike Speeds = Number of Cassette Cogs × Number of Chainrings
While riding, cyclists adjust their gear ratios based on the terrain to maintain a steady cadence:
- On flat roads: A higher gear ratio is generally used. Pedaling requires more effort, but the bike travels faster.
- When climbing: A lower gear ratio is preferred. Pedaling is easier, but the bike travels slower.
2. Understanding Road Bike Drivetrain Power Transmission
The gear system is responsible for transmitting rotational power. Without increasing the total energy, it allows for the exchange between rotational speed and torque. The chainring (driving wheel), cassette (driven wheel), and chain (gear connector) make up the road bike's drivetrain, transmitting the rotational torque generated by pedaling to the rear wheel to drive the bike forward.
While riding (ignoring friction), output power can be calculated using the following formula:
Power = Torque × Rotational Speed
- Power: Energy transmitted per unit of time.
- Torque: The rotational force causing an object to spin.
- Rotational Speed: How fast the object spins.
When power remains constant:
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Choosing a low gear ratio (e.g., small chainring, large cassette cog): The small chainring drives the large cassette cog. The large cog spins slower, which means the torque it endures increases.
Example: On a road bike with a 50/34T chainring and an 11-34T cassette, using the small chainring and large cog gives a gear ratio of 34/34 = 1. One revolution of the chainring equals one revolution of the cassette. The rotational speed is slow. -
Choosing a high gear ratio (e.g., large chainring, small cassette cog): The large chainring drives the small cassette cog. The small cog spins faster, which means the torque it endures decreases.
Example: Using the large chainring and small cog gives a gear ratio of 50/11 ≈ 4.545. One chainring revolution equals 4.545 cassette revolutions. The rotational speed is fast.
3. How to Choose Gear Ratios on a Direct-Drive Smart Trainer
When using a direct-drive smart trainer, the trainer replaces the road bike's rear wheel, and the torque from the bike's drivetrain is transmitted directly to the trainer's motor.
While there are no strict gear ratio limits when training on a direct-drive trainer, the motor has an optimal rotational speed range. Depending on your training scenario, it is highly recommended to ride using a large chainring and a middle cassette cog. This makes it easier for the motor to stay within its optimal working speed, maximizing motor performance and reducing component wear.
4. Avoid Low Gear Ratios in ERG Mode
When using ERG mode, the smart trainer automatically adjusts resistance based on your cadence to maintain a constant target power output.
If you ride in a low gear ratio (small chainring and large cog) in ERG mode, your target power remains constant, but the cassette's rotational speed slows down. Consequently, the torque endured by the gear system is amplified, and the trainer's motor speed also slows down.
When your cadence drops, the trainer automatically increases resistance to maintain that constant power. This resistance is further amplified into massive torque by the small-chainring/large-cog setup, making it very easy to exceed the maximum torque limit of the components.
At this point, because the torque is exponentially amplified beyond what the components can handle, you may experience a "slip" or a sudden loss of resistance sensation.
* Recommendation: During ERG mode training, always use a large chainring and a middle cassette cog.
5. Gear Selection for Virtual Shifting
Virtual shifting allows the smart trainer to adjust motor resistance by receiving signals from a virtual shifting controller, eliminating the need to physically shift your gears.
When using virtual shifting, you should keep your bike in the large chainring and middle cassette cog for the following reasons:
- Reduced Torque Load: Constantly using the large chainring and middle cog reduces the physical torque endured by the road bike's drivetrain, minimizing the chances of slipping or chain skipping.
- Optimal Motor Performance: Using this gear combination at a standard cadence of 80-90 RPM easily places the trainer's motor in its optimal rotational speed range, allowing it to perform at its best.
- Less Wear and Tear: The large chainring and middle cog combination provides a straighter chainline with a smaller deflection angle, significantly reducing chain wear.