After much speculation and many a spy shot, SRAM has unveiled the Powertrain, powered by Brose. To be clear, this is not just another eBike Motor. Designed to work optimally with the electronic Transmission groups as a complete system, the Powertrain is able to offer heaps more than pedal-assistance. Written into its firmware are some seriously high-tech features; the auto-shift and coast-shift functions are engineered to enrich the ride experience by handling gear changes for you.
Indeed, the Powertrain not only automatically shifts for you while you’re pedalling, it also shifts for you while you’re coasting. And you, the rider, can tailor how it does that.
The SRAM Powertrain will launch on four new e-mountain bikes over the coming months, the first of which is the Nukeproof MegaWatt seen throughout.
Happily, we had the good fortune to test ride this latest eBike tech just last week, at a Bike Connection Agency event in Andalo, Italy. Details on the MegaWatt will remain sparse, for now. Here, we turn our attention to the tech and specs of its engine room, along with our first ride review of its performance up and down the mountain.
SRAM Eagle Powertrain | An Overview
From a hardware point of view, the SRAM Eagle Powertrain is a Brose motor. But, critically, from a software and firmware point of view, it is very much a SRAM creation.
With a maximum torque of 90 Nm, the Eagle Powertrain is one of the more powerful mid-drive motors on the market; that’s 5 Nm more torque than the Shimano EP801, or the Bosch Performance Line CX Systems. Its peak power output is high, too. It comes in at 680 Watts versus the 600 Watts boasted by either of the aforementioned. SRAM refrain from quoting a specific percentage for assistance.
- eBike System: SRAM Eagle Powertrain
- Ride Modes: Range, Rally, and Push
- Extra Features: Auto-Shift and Coast-Shift, for Transmission only
- Maximum Torque: 90 Nm
- Peak Power Output: 680 Watts
- Batteries: 630 Wh, 720 Wh (+ 250 Wh Range Extender)
- Drive Unit Weight: 2.9 kg
- Warranty Period: Two Years
Though the system is at its best when paired with Transmission, the motor is still very much compatible with mechanical groupsets, as well as the other AXS groups. Only with Transmission do you get the benefits of Auto-Shift and Coast-Shift.
Broadly speaking there are three main batteries; a 4.1 kg 720 Wh battery (175.6 Wh/kg), a 3 kg 630 Wh battery (210 Wh/kg), and a 250 Wh Range Extender. The 630 Wh is likely to find its way onto lighter eBikes, offering the more favorable capacity to weight ratio. And, smaller frame sizes of the “full fat” eBikes, of course, where there’s insufficient room for the 720 Wh battery.
Frame manufacturers will have the option to have a main battery that slides in from underneath, pops out via a door on the downtube, or one that is not removable.
On the topic of batteries, the Powertrain system does mean the T-Type derailleur no longer needs one of its own.
The Central Processing Unit
At the heart of the SRAM Eagle Powertrain is the AXS Bridge Display, integrated into the bike’s top tube. It is ever so slightly raised relative to the top tube to ensure the best possible connection between the various components. This is where you turn the system on or off, and where you can see whether you’re in Range or Rally mode, as well as the charge status of the battery (or batteries).
There is no additional display unit on the bar, or wires running into headtube. SRAM packs all the controls into the AXS Bridge Display and two wireless Pod Controllers on the bar, giving an impressively clutter-free cockpit. In my opinion, the SRAM Eagle Powertrain wins hands-down on aesthetics.
Data enthusiasts need not be dismayed; the option remains to pair the system with a Hammerhead, Garmin or other third party device. That way, you’ll still be able to see other stats like speed, cadence, remaining range, rider power and current gear selection.
Being the brain of the system, the AXS Bridge Display collates data from torque and cadence sensors in the motor, and a high-resolution speed-sensor on the rear wheel. As per, that data is used to make decisions on how much torque should be delivered to the rear wheel. And, when paired with Transmission, it is also used to make Auto-Shift and Coast-Shift decisions.
Powertrain Brings SRAM Bang Up To Date
Indeed, Auto-Shift and Coast-Shift aren’t particularly novel. Shimano has implemented similar features on their latest eBike system. I am yet to ride that system myself, so I will refrain from making any ill-informed comparisons between the two – however, should you want more information on how Shimano’s offering performs on the trail, you can read Cory’s in-depth review here.
These are some of the more interesting features of the Eagle Powertrain. SRAM could very well have just launched a Drive Unit alone, implementing high-end features at a later date. But, given the capability of Transmission, and the inter-connectivity of the AXS ecosystem, they’ve gone right ahead with a product set to compete with Shimano’s EP801 XT Di2 combination.
Thirteen years after Shimano first announced the E6000 STePS Unit, SRAM has finally launched its very first eBike motor. Our take? It’s very good, but I’m relieved that some of the bonus features are optional, not obligatory…
First Ride | SRAM Powertrain
Unlike many eBike systems, the SRAM Eagle Powertrain has just two main riding modes, aptly named Range and Rally. As you might expect, Range delivers relatively less assistance in response to the rider’s input, while Rally delivers relatively more. Both modes are tuneable via the AXS App, and there’s rather a large range of adjustment on offer.
Rather than having a number of discrete settings for assistance, SRAM use a continuous slider so the rider can dial in exactly how much assistance they want in Rally and Range. Out of the box, Rally mode is set to 80% assistance, giving 600 Watts at maximum power, but that can be moved right up to 100% for the full unbridled 680 Watts of assist.
I did all of my technical climbing in Rally mode, set to 80% assist. The power was certainly not wildly unmanageable at any point. It seemed nicely measured most of the time and I was able to ride steep, bumpy sections with ease. The exception to that would be the odd occasion where the rear wheel spun out while trying to find grip on wet limestone. That stuff is the epitome of anti-grip, by the way.
For me, an eBike is never going to deliver a “natural” pedaling experience, despite what you may read in marketing material everywhere. That said, the power delivery of the SRAM system never did anything weird or unexpected. It never really felt as though it was running away from me.
There’s a gentle ramp of torque delivered between pedal strokes, so it doesn’t feel like a violent, wild horse. It’s a little smoother than the Shimano EP801 in that regard. Oh, and it’s a fair bit quieter, too. That’s true for going up and down the hill. The motor’s whir is less pronounced, and there’s no rattle while descending. Very pleasing.
When you stop pedaling, the drive unit reacts reasonably quickly to stop delivering torque to the rear wheel. There is a small amount of intentional lag, but very little compared to what you get from the “Extended Boost” feature on the Bosch Performance Line CX Race.
I’ll not pretend i’m any kind of eBike specialist, preferring to do almost 100% of my riding unassisted. But, in the context of what it is? Yeah, OK, I really liked it.
Auto-Shift is a “Nice to Have”
On a first ride of the system, the Auto-Shift function is not quite as automatic as the name might lead you to believe. That’s because there are no fewer than seven cadence modes to choose from.
To name them: -3, -2. -1, MID, +1, +2, and +3. In the -3 mode, the system prioritizes a slow pedalling cadence, and will auto-shift to ensure cadence remains low. At the other end of the spectrum, the +3 mode will prioritize the highest cadence, and will auto-shift accordingly.
Pedaling out of the yard in the -3 setting, I quickly realized the system was forcing a cadence that was significantly below what I like. The setting is easy to change on-the-fly; you simply press and hold the lower button on the right Pod Controller, after which you can use the buttons on that same controller to toggle through the auto-shift settings. Pedaling on a flat road, I toggled to the +2 setting where the system is biased toward a much higher cadence.
Actually, I found the +2 setting to be best for almost all of the afternoon’s test ride. The highest +3 setting was arguably better suited to the steepest climbs, but it would force too high a cadence on flatter sections, or more gentle fire road climbs. I found that if I just wanted to ease along without speeding up, +1 or +2 were best.
I can’t fathom who might settle for one of the slower cadence modes, but SRAM say the modes were decided based upon rider feedback.
Auto-Shift Can’t Predict the Future, But You Can
The auto-shift seemed to perform very well in most scenarios. There is a short lag experienced when the system is required to shift to an easier gear when the gradient tips up. It needs to see that your pedalling cadence has slowed in response to the gradient before it can react. At the risk of stating the obvious, the Eagle Powertrain doesn’t have eyes. It can’t predict what’s coming up, but you can.
Indeed, you can take matters into your own hands at any point in time, over-riding the system by manually initiating a shift in the usual way. Or, you can just wait (no more than) a second, and the system will auto-shift. Shifting is ultra-crisp, either way. Actually, the harder you’re pedalling, the more crisp that shift feels.
It’s quite nice not to have to think about shifting. I was surprised at how quickly my brain adapted to relying on the auto-shift function. Very quickly I forgot I was still the author of my own destiny, forgetting to shift in anticipation of steeper sections. It caught me out a few times, but I soon re-adapted.
Where Powertrain X Transmission Falls Short
For me, the system is let down by its limited shift speed. This is more in relation to Transmission than it is the Eagle Powertrain and its Auto-Shift function, of course. When it shifts, the T-Type derailleur guides the chain onto the next sprocket of the cassette, ensuring it is properly engaged before initiating a second shift.
You can demand multiple shifts of course, and the system will execute on that, but it will still only shift the chain up or down the cassette one sprocket at a time. The benefit of that is a reduced risk of snapping the chain, of course, but it does mean the system is occasionally too slow to react to dramatic changes in gradient.
This resulted in too many forced dismounts for my liking. The natural trails around Andalo are littered with surprise inclines, and I simply wasn’t able to change gear in time.
Coast-Shift is Excellent
I found the Coast-Shift function to be excellent. As you coast into a descent and speed picks up, the system auto-shifts to a harder gear so that the gear ratio better matches your speed. That means when you do finally pedal, you have something to push against. There’s less risk of “pedalling into nothing”, and the inevitable maximum effort push-up that follows.
This is not a trap I ever fall into when riding a regular bike. But, it is a trap I fall into when riding other eBikes. I have a tendency to forget which gear I’m in, likely because gear selection seems less important when the motor is going to help you out regardless. The Coast-Shift function is the solution to that problem.
It works in the other direction, too. As you slowly come to a stop, the system shifts back up the cassette so that you’re in a more suitable gear for pedaling off again.
Coast-Shift works by rotating the chainring independently of the cranks. So, if you look down as you coast along, you’ll see the chainring rotating forwards to pull the chain through for a shift. It’s pretty cool.
There is a noise associated with this. The first few times it happened, I thought I had a stick caught in the derailleur because of the noise. I guess there is less torque involved, so shifting sounds less crisp than it does when the chain is under more load.
The Powertrain’s high-resolution speed sensor likely has a lot to do with the efficiency of the Coast-Shift function. Instead of taking a speed measurement once per wheel revolution, it takes a measurement 6 times per revolution. That’s because there are 6 magnets on the disc rotor adapter that forms part of the speed sensor hardware.
The SRAM Powertrain offers a third mode that delivers assistance while you’re pushing, up to a maximum speed of 6 km/h. I ended up testing this feature quite a bit, thanks to the surprise super steep sections.
To initiate it, you have to press and hold the top left Pod Controller button. To engage it, you either start pushing the bike forward, or you can roll it back a little. The motor will engage to prevent the bike rolling back further. That’s a pretty nice safety feature, especially when you are forced to make a last minute dismount on a steep climb. You don’t need to roll the bike back far for it to engage. That’s thanks to the six magnets that are used for the speed sensor.
I will say, it was pretty easy to accidentally release the Pod Controller button, disengaging Push Mode. That was a bit of a pain, as I then had to wait a bit for it to re-engage. Due to regulations, you can’t change which button is used for Push Mode. You can remove it entirely, though.
If you have the dexterity, you can actually shift while using Push Mode (we did not test this). Theoretically, you should be able to find the right gear sooner, ready for when you get back on the bike.
SRAM Eagle Powertrain | In Summary
- Power delivery is smooth, controlled and highly adjustable
- Cockpit is clean and clutter-free
- Auto-Shift is impressive and reliable over terrain where gradient does not change dramatically
- Coast-Shift ensures you’re always in a sensible gear
- Auto-Shift can be over-ridden at any time
- Auto-Shift and Coast-Shift are optional
- Easy to accidentally release Pod Controller button when using Push Mode
- Transmission isn’t able to shift fast enough when gradient tips up sharply
The SRAM Eagle Powertrain is winging its way onto the market very soon. More details to follow!