Wil reviews the Bosch SX motor

After much anticipation, the Bosch SX motor has arrived as the German brand’s first system designed specifically for lightweight electric mountain bikes. As exciting as it is, the SX motor does arrive somewhat late to the party with the likes of Specialized, TQ and Fazua having dominated the lightweight e-MTB category over the past four years.

Of course Bosch isn’t one to move too quickly, preferring to exercise patience over panic. After all, the current Performance Line CX Gen 4 motor was introduced back in 2019 and five years on it’s still regarded as one of the best and most reliable full-powered motors on the market.

Has that same patience paid off with its entry into the lightweight category? We’ve been testing the new Bosch SX motor across several e-MTBs over the past few months, and here we’ll be covering off what we like about it, what we don’t like, and how it compares to the competition.

Watch our Bosch SX video review here:

An overview of the Bosch SX motor

The Bosch Performance Line SX motor has been purpose-built for the lightweight e-MTB category. Whereas the CX motor is designed to be paired with larger batteries for maximum power and range, the new SX version prioritises weight and efficiency.

Featuring a magnesium casing, the drive unit itself is smaller and lighter. Bosch claims the SX motor weighs just 2kg, which is close to the TQ HPR50 (1.85kg), Specialized SL 1.2 (1.95kg) and Fazua Ride 60 (1.96kg), though it’s considerably lighter than the Giant SyncDrive Pro 2 (2.7kg) and Bosch CX (2.8kg).

It’s also much narrower than the full-powered CX motor. Bosch has managed to shrink the Q-factor down from 178mm to 160mm, which produces a more natural pedalling stance that’s closer to a regular mountain bike.

In terms of output the SX motor produces less peak torque (55Nm vs 85Nm), but it’s claimed to deliver the same 600W of peak power as the CX motor. You’ll need to be pedalling with meaning as it requires a cadence of over 100RPM before you’ll hit maximum power.

Still, that 600W peak power output is vastly greater than the TQ HPR50 (300W), Specialized SL 1.2 (320Wh) and Fazua Ride 60 (450W) motors. As we’ll discuss shortly, that difference is very noticeable on the trail.

CompactTube 400Wh Battery

To go along with the SX motor, Bosch developed the CompactTube 400Wh battery. Weighing a claimed 2kg, it incorporates newer cells that feature a higher energy density compared to the bigger PowerTube batteries. It’s also narrower in profile, which allows frame manufacturers to engineer a slimmer and lighter downtube.

It’s worth noting that the SX motor can be paired with the larger 500/625/750Wh PowerTube batteries. However, we’ve mostly seen it combined with the 400Wh battery, which results in a claimed system weight of just 4kg. To put that number into perspective, it’s actually lighter than a PowerTube 750Wh battery on its own.

In addition to the internal 400Wh battery, Bosch offers an external range extender called the PowerMore 250. This increases total battery capacity to 650Wh, which is similar to a lot of full-powered e-MTBs on the market. More on that in a bit.

Simple & sleek user interface

While Bosch offers a range of different controllers and display options for its e-Bike systems, our SX-powered test bikes have elected for a clean and simple user interface that sees the System Controller integrated directly into the frame’s top tube. This incorporates five LEDs to display the battery status in 10% increments, with each LED turning white first before disappearing altogether.

You’ll find the wireless Mini Remote for changing the assist modes. You don’t necessarily need the remote however, as you can use the mode button on the System Controller instead. On the last few Bosch test bikes we’ve removed the remote to tidy up the cockpit and provide more freedom for setting up the brake lever and dropper remote in the optimal position.

As with the bigger CX motor, the Bosch SX system offers the same Eco, Tour, eMTB and Turbo modes out of the box. Using the Flow app you can customise the four available modes and remove any that you’re not using. For example, with our test bikes we removed the Tour mode and replaced it with the adaptive Tour+ mode.

You can also use the app to tune the SX motor’s power output. We have a lot of information about this in our Bosch Tuning Tips article if you’re keen to know more.

What e-MTBs does the Bosch SX motor come on?

There is currently a small but growing list of lightweight e-MTBs that have adopted the Bosch SX motor system. We’ve tested three so far including the Norco Fluid VLT, the Canyon Neuron:ONfly (review incoming) and a prototype that we can’t talk about just yet.

All of these bikes feature the same Bosch SX motor, 400Wh CompactTube battery, System Controller and wireless Mini Remote. They also make use of the wireless speed sensor that’s built into the motor casing, which is tripped by a magnet attached to the valve on the rear wheel. The result is a pleasingly clean setup that’s free of any external electrical wires.

In regards to weight, all of these test bikes have come under the 20kg barrier. The lightest option would be the Norco Fluid VLT C1 130, which has a claimed weight of just 18.1kg. This is a key point of difference to the bigger CX motor, which is typically found on burlier e-MTBs in the 22-27kg weight range. As we’ve discussed in many reviews of lightweight e-MTBs, this difference in mass has a big impact on handling and overall performance.

Outside of the bikes we’ve tested most Flow readers will be familiar with the Mondraker Dune, the Whyte E-Lyte and Cube AMS Hybrid ONE44. There’s a number of other Euro brands that are employing the Bosch SX motor including Centurion, Bulls and Flyer.

Given what the new motor offers, we expect there’ll be a load more bikes being announced in the near future.

On the trail with the Bosch SX motor

Across the three test bikes I’ve ridden, the Bosch SX motor has impressed with its powerful and responsive performance that sets a new bar in the lightweight e-MTB category.

It packs a serious punch, offering significantly more power and faster pickup at the pedals compared to the TQ HPR50, Specialized SL 1.2 and Fazua Ride 60 motors. Indeed in the Turbo mode it actually feels very close to the bigger CX motor. The assistance comes on fast and strong, providing fantastic acceleration. That sensation is accentuated on a lightweight e-MTB, which typically comes with quicker-rolling tyres.

I’ve mostly been using the adaptive Tour+ and eMTB modes, which scale the motor’s assistance based on how hard you’re pushing the pedals. This provides more natural and progressive power delivery that sees the motor better matching what your legs are doing.

Compared to the full-powered CX motor, it’s worth noting that the SX drive unit is more sensitive to cadence as well as the input torque. If you’re just ticking the pedals over the power output is quite mellow, which helps to conserve the battery. Pedal a bit faster though and you’ll feel and hear the SX motor take off.

As mentioned earlier, you’ll need to pedal over 100RPM before you reach the full 600W of peak power. That sounds fast, and indeed not many riders are going to sustain that kind of cadence up a long fireroad ascent. But on a technical singletrack climb, there are plenty of instances where you’ll be spinning well over 100RPM. The increase in assistance is particularly welcome when approaching steeper sections and awkward features, and so too is the Extended Boost function. This sees the motor continuing to supply power to the rear wheel for an additional metre after you’ve stopped pedalling, which can often mean the difference between making it over a rocky ledge or stalling out.

Controlling that power

The way the Bosch SX motor ramps up its power is really impressive, but its responsiveness does take a few rides to get used to. Over time you’ll find yourself more regularly downshifting to spin at a higher cadence, which is really what the motor wants.

I think this is a great attribute for newer riders, as it encourages you to make use of the available gears. Speaking to a variety of bike shops, it’s fairly common to see a beginner wear out a single cog on the cassette of their e-MTB. This is because most full-powered motors will provide plenty of pull even in the highest gears. While the Bosch SX motor does offer loads of power, it really benefits from you spinning the pedals faster in order to unlock its full potential. Not only is that better for the motor, it’ll also improve the lifespan of your drivetrain too.

Once you get a feel for how it behaves, the power is easy to modulate on loose and technical climbs. I’ve found the SX motor to be smoother and more natural compared to the bigger CX motor, and it’s more controllable too. Setting off from a standstill partway up a climb, there is minimal lag to the assistance kicking in but it never feels overwhelming like the bike is going to loop out from underneath you. Bosch has clearly done a lot of work on the firmware for this drive unit, and the result is an intuitive connection between your legs and the output to the rear tyre.

Downsides

While the Bosch SX motor is smaller than its full-powered sibling, its profile is still on the larger side compared to other lightweight drive units. That’s especially the case when comparing it to the tiny TQ HPR50 motor, which can be more discreetly integrated into the frame design. Aesthetics are of course subjective, but there’s something to be said for a lightweight e-MTB that doesn’t look like an e-MTB.

In terms of performance, for me the biggest downside of the Bosch SX motor is the clacking noise on the descents. This is due to the internal freewheel that disconnects the motor from the drivetrain when it isn’t supplying power. This helps to reduce drag, which Bosch says is 50% less than the CX motor. However, it also results in an annoying clacking noise when you’re coasting across rough terrain.

The CX motor seems to have become quieter in this regard over the last couple of years, and that made the issue all the more noticeable with the SX motor on the test bikes we’ve been riding. While you do mentally tune it out over time, we’d much rather it wasn’t there to begin with.

Otherwise the Bosch SX motor is pretty decent when it comes to general noise output on the climbs. It certainly isn’t as whisper-quiet as the TQ HPR50, but it’s comparable to the Fazua Ride 60 and it’s for sure less audible than the Specialized SL 1.2 motor.

It’s no free ride

Although the Bosch SX motor delivers rapid and powerful assistance, it’s worth acknowledging that it won’t give you a totally free ride like the CX version.

As mentioned before, it requires a decent cadence to continue supplying power as the gradient kicks up. You’ll really benefit from reading the terrain and downshifting in anticipation of steeper pinch climbs. Having the SRAM GX Transmission on our test bikes has been a boon in this regard, as you’re able to shift a lot later and pedal through each gear change to better maintain your momentum.

This is really the biggest point of difference with the full-powered CX motor, which provides a heap of grunt regardless of which cog the chain is engaged with. In comparison, the lower torque of the SX motor means it’s possible to stall out if you’re caught in too high of a gear.

I’d also suggest that riders who are after an e-MTB for self-shuttling purposes will be disappointed by the SX motor. You really need to put in the effort to get the most out of it, which means it’s not as well suited to lazy pedallers.

Then again, those folks will naturally gravitate towards bigger and heavier full-powered e-MTBs in the first place. In contrast, the SX motor is a great match for sporty and active riders who prefer the more natural assistance of a lightweight e-MTB.

How much range can you get with the Bosch SX motor?

As with any e-MTB, range depends on a variety of factors with rider weight and elevation gain being the big ones. A heavier rider taking on steeper climbs is going to drain the battery a lot faster than a lighter rider on flatter trails. Tyre choice, your chosen assistance mode and personal riding style will also have an impact. Just like a car, loads of stop-start accelerations will demand more fuel.

Having predominantly used the eMTB mode, most of my rides with the Bosch SX motor have been around 33-42km in length with 1,000-1,100m of elevation gain. To put it into simpler terms, that’s around two hours of riding before I’ve run the 400Wh battery flat.

On longer rides, you’ll notice the last two LEDs on the System Controller will turn orange once the battery hits 30% to provide you with a visual warning. At 10% remaining the last LED will turn red, and you’ll feel the motor’s torque drop down to help eke out the final portion of the battery. However, unlike some other lightweight e-MTBs that dribble along at this point, the SX motor still provides decent support all the way down to 0% before it switches off altogether.

The standardised range test

To provide some comparative data, I also performed our standardised range test with both the Norco Fluid VLT and Canyon Neuron:ONfly. This involves pedalling up a road climb that features an average gradient of 10%, with the motor set to its most powerful setting. I’ll then take a singletrack descent back down, before repeating the climb to see how many laps I can get before the battery runs flat.

It isn’t a perfect test, but it’s as replicable as I can make it while minimising the key variables. With the same rider going up the same climb, here’s how much elevation gain I’ve been able to achieve on the two Bosch SX bikes alongside all the other lightweight e-MTBs we’ve tested;

• Focus JAM² SL (Fazua Ride 60, 430Wh Battery) – 1,665m climbing
• Scott Lumen (TQ-HPR50, 360Wh Battery) – 1,567m climbing
• Orbea Rise (Shimano EP8-RS, 360Wh Battery) – 1,388m climbing
• Norco Fluid VLT (Bosch SX, 400Wh Battery) – 1,361m climbing
• Trek Fuel EXe (TQ-HPR50, 360Wh Battery) – 1,312m climbing
• Scott Voltage (TQ-HPR50, 360Wh Battery) – 1,311m climbing
• Specialized Levo SL (SL 1.2, 320Wh Battery) – 1,307m climbing
• Canyon Neuron:ONfly (Bosch SX, 400Wh Battery) – 1,292m climbing
• Giant Trance X E+ Elite (SyncDrive Pro 2, 400Wh Battery) – 1,057m climbing
• Specialized Kenevo SL (SL 1.1, 320Wh Battery) – 1,053m climbing

As you can see, both the Norco and Canyon hit around 1,300m of elevation gain before they ran the 400Wh battery flat, which is an impressive amount of climbing.

Neither came close to the Focus JAM² SL, which delivered a thoroughly impressive performance in the standardised range test. However, what the above results don’t show is the average climbing speed. In the case of the JAM² SL with its Fazua Ride 60 motor, I was climbing at around 16km/h. The speed was even slower with the TQ HPR50 motor on the Scott Voltage, which ticked along closer to 14km/h.

In comparison, both the Bosch SX bikes were comfortably climbing at 18km/h. If I lifted my cadence, I was able to push that speed over 20km/h. Yes I was working harder, but doing so means it’s entirely possible to keep up with a full-powered e-MTB on the climbs.

The Bosch range extender battery

For those who are looking to maximise their ride time, the Bosch SX system is compatible with the PowerMore 250 range extender.

The battery sells for $799 AUD and includes the mounting bracket and a separate bottle cage. You’ll also need the necessary cable ($40-60 AUD), which comes in several sizes that are specific to the bike you’re fitting it to.

Confirmed weight for the PowerMore 250 battery is 1,562g including the mounting bracket and cable. It’s simple to fit, though of course you do lose a bottle mount in the process so you’ll need to wear a hydration pack.

Inside are the same 21700 cells that are used in the main 400Wh PowerTube battery. Once plugged in the system reads the two as one big battery and will drain them in tandem. This differs to some other e-MTBs that will drain the range extender first before moving onto the main battery. The advantage of that approach is that if you’re riding a looped trail you can remove the range extender once it’s dead and leave it in the bushes while you complete the rest of the ride.

That isn’t possible with the Bosch system, though for some riders it’ll be made up for by the whopping 650Wh capacity. That is considerably higher than a Specialized Levo SL (480Wh) or a Scott Voltage (520Wh) with their respective range extenders fitted.

Combined with the punchy SX motor, you’ve pretty much got yourself a full-powered e-MTB at that point. This really ups the versatility by giving you scope to take on big backcountry trail rides with very little range anxiety. It also means you can hoon along in Turbo mode when you’re with your pals on full-powered e-MTBs. And if you’re heading out the following day for a solo after-work ride, just switch out the range extender for the bottle cage and you’ve got yourself a sub-20kg bike again.

What about the competition?

Throughout this review I’ve referenced how the Bosch SX motor compares to the TQ HPR50, Specialized SL 1.2 and Fazua Ride 60 systems as well as the bigger CX motor. With e-MTB technology evolving at such a rapid pace however, there’s a growing pool of competitors that Bosch will need to continue fending off.

At Eurobike this year we saw the arrival of two completely new motor brands. DJI arguably made the biggest splash, no doubt due the strong pivot the company has made going from drones and action cameras to an e-MTB system. ZF has also entered the market with its astonishingly small Centrix motor. Both companies are promising the holy grail of low weight with high power, though it’s yet to be seen whether any of the larger bike brands will be partnering with them.

An option that is currently available is the Orbea Rise, which was updated this year with a new frame design and the Shimano EP801-RS motor. Orbea is also hoping to straddle the lightweight and full-powered categories, with the Rise offering up to 85Nm of peak torque combined with a 420Wh or 630Wh battery and a claimed weight as low as 16.2kg. It’s a fascinating option that we’re eager to get our hands on in the near future.

Giant is another brand that already combines a full-powered 85Nm motor with a slim 400Wh battery in its Trance X E+ Elite. It’s a curious combination that will be perfect for certain riders, and compromised for others. Having tested that bike, I enjoyed many aspects about it, but found the SyncDrive Pro 2 motor would chew through the battery if you ran it in the higher assist settings. The power delivery is also quite abrupt, and in certain trail scenarios the motor’s hypersensitivity can be overwhelming. See our Giant Trance X E+ Elite review for the full story.

Those attributes were especially noticeable alongside the Bosch SX motor, which is smoother and more intuitive overall. The adaptive eMTB mode is brilliant, with its progressive power output making it a lot more intuitive to ride compared to the SyncDrive Pro 2 motor. As we’ve mentioned many times before, a motor’s numbers on paper never tell the full story out on the trail.

Reliability & warranty support

Of course any conversation regarding motor comparisons isn’t complete without a discussion regarding backup service and support. e-MTBs aren’t simple machines, with the addition of electronics, hardware and software meaning there’s a greater chance of things to go wrong. It’s when those problems arise that you’ll want things dealt with as quickly and painlessly as possible.

We’ll point out here that while we’ve only had a few months aboard our SX-equipped test bikes, we have years of experience with the CX motor across many different bikes and brands. And aside from some battery rattling issues on the Trek Rail (which is Trek’s fault, not Bosch’s), we haven’t encountered any significant problems with the CX motor or the broader Smart System.

As such, we expect our ongoing experience will be much the same with the SX motor, and there’s every indication so far that its reliability will match its bigger sibling.

If something were to go wrong, it’s worth acknowledging that Bosch has its own service centre in Australia that can provide backup support. The SX motor comes with a two-year warranty, and really any certified Bosch dealer should be able to diagnose most issues and provide necessary software updates whether it’s in or out of that warranty period.

Flow’s Verdict

The Bosch SX motor has been one of the most hotly anticipated arrivals in the lightweight e-MTB category, and we’re happy to report that it more than lives up to the lofty expectations we had for the German electronics brand.

It isn’t the smallest drive unit out there, but the overall reduction in weight and width is still very much welcome. And the fact that it produces up to 600W of peak power gives it a significant advantage over most of the motors you’ll typically find in a lightweight e-MTB. The SX motor is considerably more powerful, and it’s way more responsive too.

You still need to put in a decent amount of effort to unlock its full power output, and the high cadence required means it’s not a motor we’d recommend for lazy pedallers and self-shuttlers. Its dynamic performance really suits a sportier and more active rider, and as such it’s an ideal match for lightweight e-MTBs like the Norco Fluid VLT and Canyon Neuron:ONfly.

Indeed for those chasing maximum power in a lightweight package, the Bosch SX system is as good as it gets. The user interface is simple but effective, and the option of increasing the battery capacity to 650Wh with the range extender provides a load of versatility.

Aside from its noisy clacking on the descents, really the main downside at the moment is the lack of complete bikes that are offered with the SX system. That pool is likely to grow in the near future, and we look forward to seeing how other brands will respond to this genre-blurring motor.

ABOUT THE REVIEWER - Wil Barrett

Hometown

Bendigo, Victoria

Height

175cm

Weight65kg