Why These Four
These four drive units are not competitors in the usual sense. They sit at four different answers to the same question: how much of the work should the bike do?
The Bosch Performance Line CX Gen 5 is the reference full-power unit, fitted to more trail eMTBs than any other motor and backed by the deepest service network in the category. The Bosch Performance Line SX is the same company's answer to the lightweight segment, sharing the ecosystem and much of the software but trading torque for a two-kilogram package and a narrower stance. The TQ HPR60 is the purist light-assist option, built around the premise that the rider should still be the engine. The maxon BIKEDRIVE AIR S is the newest argument: full-power output figures in a package that weighs and integrates like a light-assist system.
Torque is what the spec sheet argues about. Response, ramp, and overrun are what you feel in the first ten minutes.
All four systems have received significant firmware updates within the last twelve months, and two have altered their headline torque figures multiple times. Verify current specifications before making any build or purchase decision.
System Snapshot
Power & Output
| | Bosch CX Gen 5 | Bosch SX | TQ HPR60 | maxon AIR S | |---|---|---|---|---| | Class | Full power | Light assist | Light assist | Full power output, light assist packaging | | Peak torque | Up to 120 Nm | Up to 60 Nm | 60 Nm | 90 Nm | | Factory default torque | 85 Nm | 55 Nm | 60 Nm | 90 Nm | | Peak power | 750 W | 600 W | 350 W | 620 W | | Max support ratio | Up to 600% | Up to 400% | 200% | 400% |
Weight & Drivetrain
| | Bosch CX Gen 5 | Bosch SX | TQ HPR60 | maxon AIR S | |---|---|---|---|---| | Motor weight | Approx. 2.8 kg | Approx. 2.0 kg | 1,924 g | Approx. 2.0 kg | | Q-factor | Not published | 160 mm | 135 mm | 155 mm | | Transmission | Multi-stage geared | Simplified two-stage geared | Harmonic Pin-Ring, single reduction | Planetary gearhead, inline layout |
Battery & Electronics
| | Bosch CX Gen 5 | Bosch SX | TQ HPR60 | maxon AIR S | |---|---|---|---|---| | Batteries | PowerTube 400-800 Wh | CompactTube 400 Wh | 290 / 360 / 580 Wh | 400 Wh, 600 Wh | | Range extender | PowerMore 250 Wh | PowerMore 250 Wh | 160 Wh | 250 Wh | | Claimed efficiency | Not published | Not published | Not published | 85% | | Overrun | Extended Boost, adjustable 0-10 | Extended Boost, adjustable 0-10 | None to speak of | Short, fixed | | Rider app | Bosch Flow | Bosch Flow | TQ app | BIKEDRIVE Connect |
Key notes: The CX Gen 5 launched at 85 Nm and 600 W. A mid-2025 OTA update raised it to 100 Nm and 750 W. The May 2026 Performance Upgrade 2.0 further raised torque to 120 Nm and support ratio to 600%. The SX ships at 55 Nm and can be upgraded to 60 Nm and 400% support through the app. The 120 Nm ceiling on the CX is a drivetrain limit, not a motor limit.
Power Delivery
Reaction Speed and Engagement
Bosch CX Gen 5. Bosch attacked this at the sensor level. The Gen 5 samples pedal input at up to 1,000 times per second and adds a six-axis IMU reading incline, tilt, and vibration. The most recent firmware adds Drivetrain Tensioner, which while coasting rotates the chainring slightly to close the freewheel and eliminate drivetrain slack before the next pedal stroke. Limited to Gen 5 CX and CX-R units, rolled out mid-2026.
Bosch SX. The most cadence-dependent unit here. Full power arrives only above roughly 100 rpm, so the wrong gear on a slow technical section will feel thin. Sprint mode detects high cadence combined with high rider power and brings support in more quickly. Bosch also halved pedaling resistance compared to its other systems, which matters above the cutoff and for riders who want the bike to feel analog when the motor is off.
TQ HPR60. Responds quickly with forgiving start-up behavior, but the 200% support ceiling means quick response and strong response are not the same thing. The motor is prompt about joining in. It is not going to carry you.
maxon BIKEDRIVE AIR S. Engages smoothly and predictably with no surge. Support comes on gently and stays proportional, translating directly into rear wheel traction on loose climbs. The important qualifier, as with the SX, is cadence. At low cadence you get noticeably less assistance than a conventional full-power motor, and the full-power sensation arrives when you are spinning and putting in real effort.
Overrun
Overrun is how long the motor keeps driving the chainring after you stop pedaling. Regulation caps this at two meters. It is the single most divisive characteristic in this comparison.
| | Bosch CX Gen 5 | Bosch SX | TQ HPR60 | maxon AIR S | |---|---|---|---|---| | Behavior | Extended Boost, scaled to input length | Extended Boost, scaled to input length | Essentially none | Short, noticeable, fixed | | Rider control | Slider 0-10 in Flow | Slider 0-10 in Flow | Not applicable | None | | Best case | Ratcheting cranks to clear a ledge | Same with less torque | Predictability, no unwanted drive | Small obstacle clearance | | Worst case | Unwanted drive in flat, fast corners | Same, less forceful | Hard work on tech climbs | Not adjustable either way |
The May 2026 update moved Extended Boost control into rider hands as a 0-10 slider per mode in the Flow app, and extended the same control to the SX and fourth-generation CX units. For real-world satisfaction, that single change matters more than the torque increase it shipped alongside.
TQ has no meaningful overrun and treats that as a feature. The maxon sits between the two: short but perceptible, audible as a soft freewheel click, not adjustable.
Feel Under Sustained Load
The CX builds support gradually and evenly. Dynamic Control in eMTB+ functions as effective traction control. At the cutoff, support fades smoothly rather than dropping off a cliff.
The SX rewards sustained high-cadence effort and punishes the opposite. Spun properly, it feels like considerably more motor than 55 Nm suggests, which is the whole design thesis.
The TQ amplifies rather than replaces. Its Harmonic Pin-Ring transmission is described by TQ as near loss-free and free of backlash, which is the mechanical basis for the directness riders report.
The maxon delivers linearly and modulates finely through pedal input. Very precise control, but fitness is a prerequisite for accessing the top of its range.
Efficiency and Thermal Character
Only maxon publishes an efficiency figure, at 85%. The TQ Harmonic Pin-Ring transmission is the most mechanically distinctive design, eliminating conventional reduction gearing in favor of a double tooth ring where many teeth engage simultaneously. The practical effect is very low unpowered drag, which matters disproportionately on a light bike ridden above the cutoff.
The maxon's inline layout, with part of the unit extending into the down tube, is what allows a narrow Q-factor alongside full-power output figures. Bosch's SX drops one transmission stage relative to the CX, making it roughly 20% more compact and 800 grams lighter.
Battery strategy is the real variable. Battery selection determines range far more than motor efficiency does.
- Bosch CX offers 400-800 Wh plus a 250 Wh extender. The only system here where a customer can meaningfully choose their own range and weight trade-off at purchase.
- Bosch SX is built around the CompactTube 400 Wh. Battery compatibility across SX and CX Gen 5 is a genuine shared-platform advantage.
- TQ offers 290, 360, and 580 Wh with a 160 Wh extender. The 580 Wh option paired with a 200% support ratio is the strongest range combination of any system in this paper.
- maxon offers 400 Wh and 600 Wh with a 250 Wh extender (850 Wh combined). Drawing 620 W from a 400 Wh battery is a short conversation, so battery selection on an AIR S build matters more than on most.
Pedal Feel
Q-Factor
Stance width is the most consequential fit variable that eMTB motor selection imposes on a rider, and the one least discussed at point of sale.
| System | Q-factor | Note | |---|---|---| | TQ HPR60 | 135 mm | Comparable to most analog mountain bikes | | maxon AIR S | 155 mm | Narrow for the output; inline layout keeps width down | | Bosch SX | 160 mm | Reduced from 178 mm for a more natural pedaling feel | | Bosch CX Gen 5 | Not published | Gen 4 was 175 mm; Gen 5 housing described as narrower |
Stance width mismatch is a real driver of lateral knee and hip symptoms. The direction of the error matters less than the size of the change from what the rider is already adapted to.
A customer moving from an analog bike to a TQ-equipped build faces essentially no change in stance width. The same customer moving to a CX faces a step change of roughly 40 mm from a typical analog Q-factor, large enough to be symptomatic on its own. Cleat lateral position and pedal spindle length provide roughly 10-15 mm of adjustment range in each direction.
Drag, Noise and Mechanical Character
The TQ is the quietest of the four by a clear margin, following directly from the Harmonic Pin-Ring design and its low rotational speeds. The maxon is close behind, particularly at partial load. The SX is quieter than the CX and was designed with lower noise as an explicit goal. The CX Gen 5 significantly improved over the Gen 4 by eliminating the freewheel clatter on descents that was its most common complaint.
What the App Actually Lets You Change
| Capability | Bosch CX Gen 5 | Bosch SX | TQ app | BIKEDRIVE Connect | |---|---|---|---|---| | Adjustable modes | Custom riding modes | Custom + Sprint | Eco, Mid, High individually | Cruise, Sport, Blast individually | | Peak torque | Yes, up to 120 Nm per mode | Yes, up to 60 Nm | Not exposed | Not exposed | | Overrun | Extended Boost 0-10 | Extended Boost 0-10 | Not applicable | No | | Navigation | Yes, offline | Yes, offline | No | No | | Third-party head units | Garmin via Live Data Interface | Garmin via Live Data Interface | Not published | ANT+ and Bluetooth to Garmin, Sigma, o-synce | | Battery care modes | Not published | Not published | Yes, gentle charge and storage | Not published | | Paid tier | Flow+ | Flow+ | None | None |
The Bosch Flow ecosystem covers both CX and SX, which is a genuine practical advantage for a shop or rider running both. The TQ app stands out for battery transparency: charge cycle count, self-discharge rate, and time spent at temperature extremes are all visible, which is unusually honest and useful when evaluating a used bike.
Where Each One Fits
Bosch Performance Line CX Gen 5. The right answer for the widest range of riders. Intuitive in technical terrain, excellent traction through Dynamic Control, mature software, the broadest battery selection, and a service network nothing else here approaches. A rider can run it as an 85 Nm bike with overrun disabled or a 120 Nm bike with maximum Extended Boost, from the same hardware. Trade-offs: 2.8 kg, conventional eMTB silhouette, mid-pack noise, and the widest stance of the four.
Bosch Performance Line SX. The bridge product. Two kilograms, 600 W peak, 160 mm Q-factor, half the pedaling resistance of other Bosch systems, and full access to the same customization including adjustable Extended Boost. Battery compatibility with the CX Gen 5 is a real ownership advantage. Wrong motor for a rider who grinds. Peak power above 100 rpm is a genuine constraint, not marketing language.
TQ HPR60. For the rider who wants to still be the engine. The most natural ride feel, the quietest operation, the narrowest Q-factor at 135 mm, the lightest unit at 1,924 g, and the strongest range combination through the 580 Wh battery. Wrong motor for anyone wanting shuttle-like assistance. Fitness is not optional.
maxon BIKEDRIVE AIR S. The most interesting product here. Approximately 2.0 kg with 90 Nm and 620 W, 85% claimed efficiency, near-silent operation, invisible integration, and a 155 mm Q-factor narrower than the Bosch SX despite full-power output. Enables bikes that look and handle like analog trail bikes while climbing at full-power pace, with builds under 17 kg achievable on either battery. Caveats: cadence dependence and battery capacity. The ecosystem is also younger than Bosch's, with a smaller service footprint.
Performance specifications are manufacturer ratings published by Bosch, TQ, and maxon, together with TGI Cycling's assessment of how those systems behave in use. Not based on TGI bench testing. Firmware in this category changes frequently. Verify against current manufacturer documentation before any purchase or build decision.
Beyond the headline torque and power figures, the thermal management strategies of these systems create distinct performance ceilings. The TQ HPR60 incorporates cooling fins directly into its underside to maintain efficiency during prolonged efforts at the cost of a minor weight penalty. Conversely, the maxon BIKEDRIVE AIR S relies on its integration into the frame to manage heat without active cooling, making its performance envelope more susceptible to ambient conditions. Meanwhile, the Bosch CX Gen 5 utilizes its larger physical mass as a heat sink, allowing it to derate far more gradually than its lighter counterparts during sustained climbs in warm weather. Understanding these thermal characteristics is essential for riders who prioritize consistency over absolute peak output during long, high-exertion backcountry rides.