An internal gear hub (IGH) failure under high torque happens when the rotational forces from a mid-drive motor exceed the mechanical limits of the hub’s internal planetary gear sets, clutch plates, or drive pawls. Unlike standard external derailleurs that flex under load, an enclosed hub transfers all motor and pedaling force directly through miniature sun and planet gears. When overloaded, gear teeth shear, pawls round off, and shifting mechanisms bind, resulting in loud popping sounds, lost drive traction, or a locked rear wheel.
Fast Fix
Stop pedaling immediately to prevent sheared metal fragments from destroying the internal gear cluster. Shift into the hub’s direct-drive ratio (such as gear 5 on an 8-speed hub or gear 11 on a 14-speed hub), inspect the external shift cassette joint for aligned indicator marks, and gently apply pedal pressure without motor assist to check if the drive engages.
Quick Status & Riding Safety
- System Severity: High (mechanical drivetrain failure).
- Safe to Ride? No. If gear teeth or drive pawls are fractured, riding under power will circulate hardened steel fragments throughout the gear assembly, cracking the outer hub shell or jamming the planetary carrier.
- Immediate Concern: A hub that slips into a false neutral under hard acceleration can cause you to lose balance, while an internally jammed hub can lock the rear wheel mid-ride.
How High Motor Torque Breaks an Internal Gear Hub
A mid-drive motor drives the bicycle’s front chainring directly. When you select a low mechanical gear to climb a steep hill, the mechanical advantage multiplies the motor’s torque output at the rear cog.
[Mid-Drive Motor] ──► Front Chainring ──► Chain ──► [Rear IGH Sprocket]
(60-95+ Nm) │
Torque Multiplied by Cog Ratio
│
▼
[Planetary Gear Carrier]
• Sun gears take shear load
• Stepped planet pinions bind
• Pawls round off against ratchet ring
Standard city-bike internal gear hubs (like the Shimano Nexus 7 or 8) are designed for human pedaling outputs of roughly 250 to 300 watts with peak torques around 40 to 60 Nm. When coupled to a mid-drive motor delivering 65 to 100 Nm, combined peak torque spikes can exceed 150 Nm at the crank axle. This excessive load causes three distinct mechanical failures:
- Pawl Stripping During Loaded Shifts: Shifting gears while the motor is applying full power forces the shifting clutch to engage the next drive dog before the previous gear has released. The driving edge of the pawl catches the spinning ratchet ring at an angle, chipping the hardened steel lip.
- Planet Gear Tooth Shearing: In the lowest planetary reduction gears, torque is focused across small gear teeth. Sustained steep climbing under full electric assist strips teeth clean off the planetary carrier.
- Roller Clutch Slipping: Hubs that use roller clutches instead of ratchet pawls to deliver quiet operation rely on wedge friction. High torque forces the rollers to ride up out of their ramps, mushrooming the internal clutch race and causing permanent slip.
For troubleshooting motor-side drive issues that can stress the rear hub, read Mid-Drive E-Bike Motor Problems: Common Internal Issues.
Diagnostic Path: Isolating Hub Failure Modes
Work through these steps to determine whether your problem is an external adjustment error or irreversible internal damage:
[Drivetrain Slips / Skips Under Load]
│
Check Shift Cable Tension & Alignment Marks
│
┌────────────────────┴────────────────────┐
MARKS MISALIGNED MARKS ALIGNED
│ │
Realign barrel adjuster. Does the hub slip in ALL gears,
Test ride under light pedal. or only specific ratios?
│ │
┌─────────┴─────────┐ ┌─────────┴─────────┐
Fixed? Slipping? ALL GEARS SPECIFIC GEARS
│ │ │ │
Cable slack Internal wear. Drive pawl / Stripped planet
was the cause. Check gears. clutch ring gear stage on
failure. that carrier.
1. Check Cable Indexing Marks
Before declaring an IGH dead, check its external shifting alignment. An out-of-adjustment shift cable holds the internal gear selector halfway between two ratios:
- Shift the bike into the designated index gear (e.g., Gear 4 on Shimano Alfine 8 / Nexus 8; Gear 6 on Shimano Alfine 11).
- Inspect the yellow or red alignment lines on the hub’s cassette joint bracket.
- If the lines do not form a straight, continuous marker, turn the handlebar shifter barrel adjuster until the two colored marks align perfectly.
2. The Direct-Drive Gear Test
Internal gear hubs have one “direct-drive” ratio where the planetary gears do not multiply or reduce rotational speed; the rear sprocket is locked 1:1 directly to the hub shell.
- Shimano 7-Speed: Gear 4
- Shimano 8-Speed: Gear 5
- Rohloff Speedhub 14: Gear 11
- Enviolo / NuVinci: Mid-range flat ratio
Shift into this direct-drive gear and pedal with the motor turned off. If the hub drives the wheel cleanly without slipping in this gear, but slips, grinds, or free-spins in the lower climbing gears, the internal planetary reduction gear teeth are stripped. If the hub slips even in the direct-drive gear, the main drive pawls, freewheel body, or roller clutches have failed completely.
Preventing Torque-Induced IGH Failures
If you ride a high-torque mid-drive bike equipped with an internal gear hub, follow these mechanical guidelines to prevent repeated failures:
- Ease Off Pedaling While Shifting: Treat an IGH like a manual car transmission. Stop pedaling for a split second when shifting gears. This cuts motor torque, allowing the internal gear selector to seat fully into the next ratio before load is reapplied. Many modern systems use a gear sensor that temporarily pauses motor power during shifts.
- Respect Minimum Sprocket Ratios: Hub manufacturers establish strict minimum primary drive ratios (front chainring teeth divided by rear cog teeth). For example, running a 38T front chainring with a 22T rear cog yields a safe primary ratio of 1.73:1. Installing an oversized rear cog (such as a 26T) to get lower climbing gears multiplies input torque beyond the hub’s design limit.
- Choose E-Bike-Rated Hubs: For mid-drive systems exceeding 65 Nm of torque, use components engineered specifically for e-bike loads:
- Shimano Nexus 5E (C7000 series): Designed specifically for e-bikes, with wider gear teeth and heavy-duty pawl carriers rated for up to 60–85 Nm.
- Rohloff Speedhub 500/14: Rated for high input torque (up to 100–130 Nm depending on primary drive ratio) with an oil-bath planetary gear set.
- Enviolo Heavy Duty / Cargo: Uses rotating steel traction balls in an elastohydrodynamic fluid rather than toothed gears, eliminating gear-tooth shearing entirely.
For ongoing preventative maintenance, follow Internal Gear Hub (IGH) Maintenance Guide for E-Bike Users to clean and relubricate the internal assembly.
Repair vs. Replacement Context
Servicing an internal gear hub that has suffered structural torque damage depends on the brand:
- Shimano Nexus / Alfine: Shimano does not sell individual planetary pinions or gear teeth. If a gear stage is stripped, you must replace the complete internal carrier assembly (internal gear cluster). You do not need to rebuild the wheel spokes; unthread the right-hand drive side cone, pull the old gear core out of the wheel, and drop a new factory carrier into the existing hub shell.
- Enviolo: The internal core is a factory-sealed cartridge. If it slips or loses drive internally, the core cannot be rebuilt and must be replaced as a sealed unit.
- Rohloff: Designed to be disassembled, inspected, and rebuilt with individual replacement components, typically serviced by authorized rebuild centers.