Drivetrain
Gear ratios, chainline, crank length — the mechanical interface between rider power and forward motion
What is Drivetrain Design?
The drivetrain converts the rider's pedalling motion into rear-wheel rotation. Every component — chainring, cassette, derailleur, cranks, chain — affects efficiency, ground clearance, and suspension behaviour.
Drivetrain design is often treated as a simple matter of which groupset the bike came with. But the details matter enormously: crank length affects knee angles and ground clearance, chainring tooth count affects Anti-Squat and pedal kickback, and chainline alignment affects shifting performance and drivetrain longevity.
In the context of BikeSPEZL's kinematic and ride dynamics analysis, the drivetrain is not just a power delivery system — it is an integral part of the suspension kinematics. The chain line directly determines Anti-Squat, and chain growth from suspension travel directly causes pedal kickback.
Gear Ratios & Range
The gear ratio determines how many rear wheel revolutions you get per crank revolution. The range covers everything from steep climbs to flat sprints.
The gear ratio is simply the number of teeth on the chainring divided by the number of teeth on the selected cassette cog. A ratio of 2.0 means the rear wheel turns twice for every crank revolution. Lower ratios (smaller chainring, larger cog) are easier to pedal — good for climbing. Higher ratios (larger chainring, smaller cog) are harder to pedal — good for speed.
Gear ratio = T_chainring / T_cog
Development (m) = (T_chainring / T_cog) × π × D_wheel
Speed (km/h) = Cadence (rpm) × 60 × Development (m) / 1000T_chainring = chainring teeth · T_cog = cassette cog teeth · D_wheel = wheel diameter
- 11x drivetrains (single chainring): Simpler, lighter, no front derailleur. Range is limited by the cassette. Typical: 30–34T chainring, 10–52T cassette.
- 2Gear range: The ratio between the easiest and hardest gear. A 10–52T cassette with a 32T chainring gives a range of ~5.2×.
- 3Chainring size: Affects Anti-Squat (changes chain line angle), ground clearance, and gear ratios. Smaller chainring = lower gears and more ground clearance.
- 4Cassette cog size: The largest cog determines climbing ability. The smallest cog determines top speed. The jump between adjacent cogs affects shifting smoothness.
Chainline & Alignment
Chainline is the lateral alignment of the chain between the chainring and cassette. Misalignment causes friction, wear, and poor shifting.
In an ideal world, the chain would always run perfectly straight from the chainring to the cassette cog. In reality, the chain sits at an angle whenever you are not in the middle of the cassette. The chainline is defined as the distance from the bike's centreline to the chainring (front chainline) and to the midpoint of the cassette (rear chainline).
- 1Ideal chainline: Front and rear chainline match, so the chain runs straight in the middle cassette cog.
- 2Boost spacing (148mm rear hub): Widens the rear chainline. Requires specific chainring offset to match.
- 3Chainline offset: Chainrings come in different offsets (e.g. 3mm, 6mm) to match the bike's chainline standard.
- 4Excessive chain angle: When in the extreme gears (smallest or largest cog), the chain angles laterally. This increases friction, accelerates wear, and can cause poor shifting.
Crank Length & Biomechanics
Crank length affects knee angle, ground clearance, pedalling leverage, and even suspension behaviour through chain growth.
Crank length is traditionally 170–175mm for mountain bikes, but the trend toward shorter cranks (160–165mm) is driven by several factors: better ground clearance (critical for enduro and downhill), reduced knee flexion at the top of the pedal stroke (better for tall riders and riders with knee issues), and minimal loss of power output.
- 1Longer cranks (175mm): More leverage, more torque per pedal stroke. But less ground clearance and more knee flexion.
- 2Shorter cranks (160–165mm): Less leverage but more ground clearance. Opens hip/knee angles. Studies show minimal power loss for most riders.
- 3Ground clearance: A 10mm shorter crank gains ~10mm of pedal clearance — significant on technical terrain.
- 4Rider height: Taller riders can tolerate longer cranks; shorter riders benefit from shorter cranks to maintain proper hip-knee-ankle geometry.
Drivetrain Efficiency
Every watt of power the rider produces passes through the drivetrain. Friction losses — though small — add up over a long ride.
A clean, well-lubricated 1x drivetrain is typically 95–98% efficient, meaning 2–5% of the rider's power is lost to friction. The biggest factors are:
- 1Chain angle: The more the chain deviates from straight, the more friction. Extreme gears are less efficient than middle-cog gears.
- 2Chain tension: Higher tension (harder gears) is slightly more efficient than lower tension (easier gears).
- 3Lubrication: A dry or dirty chain can waste 5–10W. Regular cleaning and lubrication is the single biggest efficiency gain available.
- 4Derailleur pulleys: Oversized pulley wheels reduce chain articulation friction, but the gains are small (1–2W) for the cost.
- 5Chainring tooth profile: Narrow-wide chainrings (for 1x) hold the chain more securely but add slightly more friction than standard tooth profiles.
Drivetrain in BikeSPEZL
The drivetrain configuration is a key input to kinematic analysis — it directly affects Anti-Squat and pedal kickback calculations.
In BikeSPEZL's Kinematic Editor and Ride Dynamics simulation, the drivetrain settings (chainring teeth, cassette cog, crank length) are used to calculate the chain line for Anti-Squat analysis and the chain growth for pedal kickback analysis. Changing gears in the simulation updates both curves in real time, showing how your gear selection affects suspension behaviour.