Bosch CX Gen 5 / Test: Since the Eurobike this year at the latest, it has been clear: A new Bosch CX motor is in the starting blocks. At the world's largest bicycle trade fair, the new drive was already seen in taped form on many new e-bikes and e-MTBs. Today the time has finally come: Bosch is presenting the successor to what is probably its most successful motor - we have already tested the Bosch CX Gen 5, both on the test bench and on the trail.
It's actually amazing: It's been a full five years since Bosch last introduced a really new Bosch CX motor. In the meantime, the switch to the new Smart System has taken place, but the motor has remained largely unchanged. Despite this age, which is quite considerable for the bicycle industry, the Bosch CX Gen 4 has enjoyed great popularity up until recently and, for us, continues to be one of the best drives on the market.
Bosch CX 2025 (Gen 5): (Un)complicated name
The successor to the Bosch CX Gen 4 was previously mostly referred to as the Bosch CX Gen 5. Officially, however, Bosch wants to say goodbye to this naming and simply calls the motor the Bosch Performance Line CX. The technical name is BDU38 - in contrast to its predecessor BDU37 (= Bosch Drive Unit + serial number). Unfortunately, this will also mean that it will not always be immediately clear in the data sheets or in specialist shops which of the two motors is installed. For the sake of transparency, we would have liked a clear separation in the name here.
Technical data: Surprisingly unspectacular
Given the long "waiting time" for the new motor, expectations were correspondingly high. A look at the technical data of the new motor may therefore be sobering at first. Not much has changed on paper. The drive unit still has a maximum torque of 85 Nm, a maximum output of 600 watts and a maximum support of 340%.
| Bosch CX Gen 4 | Bosch CX 2025 (Gen 5) | |
|---|---|---|
| Weight | 2,9 kg | 2,8 kg |
| Max. Torque | 85 Nm | 85 Nm |
| Maximum power | 600 W | 600 W |
| Max support | 340% | 340% |
Bosch refers to the "triad" of performance and, according to its own statement, deliberately wanted to record these performance data. These have proven themselves and would be more than sufficient for the vast majority of drivers. On the one hand, we can agree with that - a lack of power was never the problem with the Bosch CX - but we would have expected a small step forward here too.
This small step forward is in terms of weight. 100 grams have been saved compared to its predecessor. At 2,8 kg, the new Bosch CX BDU38 (Gen 5) is still not the lightest mid-engine in this power class, but it does reduce the gap to the Shimano EP801, Yamaha PW-X3 and Co. to around 50 g. Speaking of shrinking: A lot has also changed in terms of dimensions. Viewed from the side, the new CX motor is very similar to the Bosch SX light-assist motor introduced last year. This is partly because it is attached using two bolts.
Compact but not small
This immediately raises the question: Are the two motors compatible? Will the new CX fit into a frame for an SX motor and vice versa? Unfortunately, there is no clear answer to this, because since the new CX is significantly thicker than the slim SX, most frames do not offer enough space. As always, exceptions prove the rule, for example the frame platform from M1 Sporttechnik offers space for both motors. However, the vast majority of manufacturers rely on dedicated frames for SX and CX - for us, a small missed opportunity to offer end customers a new level of modularity.
Along with the motor, Bosch also introduced two new batteries – the Powertube 600 and 800 – and a new display – the Purion 400. You can find detailed information about this and the other innovations for the Bosch Smart System 2025 in our dedicated article:
Bosch SmartSystem 2025: New Bosch CX, new batteries and Purion 400
Bosch Smart System 2025 / Product news: For the second time this year, Bosch is expanding its Smart System. With two new batteries with 600 and 800 Wh and the Purion 400 display, the company is further expanding its ecosystem and closing some gaps in its portfolio. The two new energy storage systems in particular are opening up new possibilities. As […]
Bosch CX 2025 (Gen 5) on the test bench: Performance data
We had the opportunity to put the new Bosch CX (Gen 5) through its paces beforehand. For this, we went back to the PTlabs test laboratory to check the performance data communicated by Bosch and also to look at other values such as energy consumption or thermal stability.
A look at the performance values we measured confirms what Bosch had also communicated beforehand: The two CX motors, BDU38 and BDU37, behave almost identically here. In terms of maximum power, they are in the same league as Shimano EP801 and Brose Drive S Mag and can leave the Yamaha PW-X3 and the old EP8 far behind. The Pinion MGU offers a little more power, while the Sachs RS, Panasonic GX Ultimate Pro and TQ HPR120s are clearly ahead. No surprises, then. The same applies to our measurement with "only" 100W input power, which is intended to reflect a more everyday situation. Here, too, the two CX motors are on par. Bosch therefore maintains that the Turbo mode in its standard setting requires significantly more effort from the driver to achieve the maximum power of the motor than is the case with Shimano or Pinion, for example, both of which offer significantly more power here.
This rather sportier basic setting of the Turbo mode is also reflected in our performance curve, where we compare the input power to the output power. The power increases significantly faster with the Shimano EP801 and the motor reaches its maximum power at just over 100W input power, while with the two Bosch CX motors you have to put almost 200W on the pedals. This behavior could probably be changed by making adjustments in the Bosch eBike Flow app. We have planned a corresponding measurement for the future.
For the first time, we have also recorded a cadence curve for the new Bosch CX (Gen 5). Here we compare the rider's cadence to the motor's output power. We carry out these measurements at an average input power of 130W in order to depict an everyday situation. Here, some glaring differences between the individual motors become apparent. It is noticeable that the two Bosch CX drives are again virtually identical and remain incredibly constant from a cadence of around 50. All of the other motors we measured fluctuate significantly more: the Pinion MGU upwards, the Shimano EP801 downwards.
Bosch CX 2025 (Gen 5) put to the test: Derating
Modern e-bike motors are becoming increasingly powerful while at the same time becoming increasingly compact. This is impressive, but it does not only have advantages; no matter how sophisticated the technology inside may be, at some point all manufacturers reach the limits of physics, because high performance always generates a lot of waste heat, which then has to be effectively dissipated in order to be able to maintain this performance over a longer period of time. At a certain point, so-called thermal derating occurs in virtually all motors: the motor throttles its performance.
In order to take this trend towards ever smaller housings into account, we will also always carry out a derating measurement in the future to determine when the respective motors reach their limits. To do this, we call up the maximum power with 250W input power in the highest support level and with a simulated gradient of 10%. We measure the output power over a period of 15 minutes and simultaneously monitor the temperature of the motor housing with a thermal imaging camera.
To ensure better comparability and to keep the "bicycle factor" as small as possible, we removed the engine covers from all test bikes. In addition, a large industrial fan provided very strong, simulated airflow to give the engines the most ideal conditions possible.
This is where a clear difference between the two Bosch CX motors becomes apparent for the first time. While the "old" Bosch CX Gen 4 loses a lot of power after around 8 minutes, the new Bosch CX (Gen 5) lasts much longer. Although derating also occurs after around 12 minutes, this is still a considerable improvement given the smaller housing. Other motors behave in a similar way: The Pinion MGU is a little more stable and can probably benefit from its large housing and the associated additional cooling surface. The Shimano EP801 handles the problem fundamentally differently and throttles suddenly and very strongly, but also brings the power back again. The Brose Drive S Mag was impressively stable in terms of performance, but with a housing temperature of almost 90°C it also got significantly warmer than the competition, which usually settled at around 75°C.
Bosch CX 2025 (Gen 5) put to the test: Energy consumption
We also determined the energy consumption of the individual drives. Here we take two rudimentary measurements at 0% and 10% gradient and measure the energy drawn from the battery. This is only a rough guideline and does not represent a realistic scenario, which is significantly influenced by many factors from tires to driving style, surface, rear triangle and more. Accordingly, the figures determined should be treated with caution, but do provide some information about whether there are major outliers upwards or downwards. That is exactly what we are seeing with the new CX (Gen 5), which in our measurements consumes significantly less power uphill than its predecessor and virtually all other drives from the competition.
Now, such test bench measurements are certainly exciting, but by no means decisive for a good, harmonious driving experience. That's why we also tested the new Bosch CX in practice on different test bikes in a variety of scenarios to get to the bottom of the response, features and noise level.
Bosch CX 2025 (Gen 5): Finally some peace and quiet!
The volume of e-bike motors is a complicated topic, as it is perceived very subjectively and also depends heavily on the riding style. Nevertheless, most people will probably agree that the noise level was probably the biggest drawback of the "old" Bosch CX Gen 4. The penetrating, high-frequency humming uphill was just as unpleasant as the loud clattering on the downhill trail. The Swabians were also aware of this and declared an improvement in the noise level as their number one development goal. The improvements are accordingly large.
E-mountain bikers in particular will be pleased to hear that the rattling motor noise of the predecessor has been completely eliminated. The motor itself did not rattle in any of our test bikes - the fact that none of them was 100% rattling-free is more likely to be attributed to the manufacturers, who now have to do their homework and ensure that battery integration, cable routing, etc. are quiet.
The high-frequency hum has also been significantly improved. Although the new Bosch CX (Gen 5) is not silent, it is significantly quieter and less intrusive than its predecessor. Depending on the situation and surface, its volume is now approaching that of the Brose Drive S Mag, which is still the benchmark in this area of full-power motors. Another noticeable feature: As with the Bosch SX, the new CX also becomes noticeably louder at higher temperatures. At the beginning of the ride, it is almost silent even under high load - after a few minutes of high load, however, the hum becomes significantly louder. Of course, the frame material also plays a major role overall: In a solid aluminum frame, the noise is better dampened than in lightweight carbon constructions, which even serve as an additional resonance body.
Bosch CX 2025 (Gen 5) on the trail: New sensors for more sensitivity
In addition to the volume, Bosch has also worked on the sensors and software for the new motor. This is somewhat surprising, as the predecessor - despite its "age" - is still one of the best drives when it comes to responsiveness and dynamics. Nevertheless, Bosch saw room for improvement here: Among other things, the new CX has been given inertial sensors that measure position and acceleration. Together with torque and speed sensors, the system knows what is happening at all times: Are you driving on a steep forest road, are you smashing down a root trail or are you simply driving at a traffic light? It adapts the support behavior accordingly.
In practice, it doesn't feel that different from the predecessor at first. Only in direct comparison do the considerable improvements become clear: the new Bosch CX (Gen 5) reacts faster and more sensitively. The response when starting off is a big step forward. This is due to the position sensor mentioned above, which makes the performance easier to control, for example when starting off on steep terrain.
The motor also makes use of the new sensor technology when the rear wheel loses grip. If the system registers that the rear tire is spinning, the support level is reduced. This is a noticeable improvement on technical uphills. The same applies to the push assist: since the motor always knows how steep the terrain is, the maximum speed is adjusted and reduced if necessary.
Revolution canceled – is it worth switching?
Overall, the new Bosch CX (Gen 5) continues to drive like a Bosch: Those who got on well with the predecessor will find their way around here straight away. At the same time, a few adjustments have been made to make the already very good handling even better. But anyone who expected groundbreaking changes or fundamental innovations will probably be disappointed.
Bosch has chosen a cautious path of evolution with the new CX (Gen 5): Many of the advantages of the well-known CX motor have been retained, but at the same time improvements have been made where there were clear shortcomings - especially in terms of noise. The new sensors also set the course for the future. This could perhaps be the best argument for the new CX motor compared to its predecessor: even those who would actually be well served with the CX Gen 4 today could be left empty-handed with new updates in the future. So anyone who attaches great importance to future-proofing when buying a new vehicle should definitely think about the new Bosch CX - especially since a lot has also happened with the batteries. On the other hand: The CX Gen 4 is still a very good motor and bargain hunters in particular could find good deals here in the coming months.
