The benefits of which could be less weight and a much better centre of gravity for improved handling


Some excellent sleuthing by our colleagues at sister site has uncovered a patent application by SRAM for a new and radical e-bike motor that could, and we stress could, deliver big gains in weight and handling.

As we know, the best e-bikes are trying to juggle several opposing requirements: power and range with weight and handling. The more powerful the motor, the more energy it uses. The more energy it uses, the bigger the battery it needs. But weight is the enemy of agility and dynamic handling, and bigger batteries and motors are heavier and are more difficult to package. Which is part of the reason why lightweight, mid-power e-bikes are trending at the moment. But as we found out when we tested the latest models – including the Forestal Cyon, Trek Fuel Ex-E, Pivot Shuttle SL and Specialized Levo SL – even the most modern, ultra-expensive e-bikes are not without problems. Chiefly range, owing to the smaller batteries used to save weight. Experience has also taught us that full power e-bikes with huge batteries (such as the Canyon Spectral:ON with 900Wh battery) can deliver remarkably agile handling with the right geometry, suspension and packaging. 

SRAM e-bike motor patent

The concentric motor design visible here is compact, freeing up space in front of the motor to fit the battery, held by a cage that pivots towards the front wheel for access

SRAM’s patent looks to exploit the inefficiency of current motor and battery packaging to move the bike’s centre of gravity both lower and further back. Lowering the centre of gravity makes the bike more stable, particularly on steep descents, and helps with cornering. Bringing the centre of gravity further back also helps prevent the bike wanting to tip forward under braking, but additionally makes it easier to lift the front wheel, which is critical when it comes to making a bike feel dynamic to ride. 

Currently big batteries are stuffed inside the down tube to bring power to the motor, but this puts the heaviest part of the system well in front of the BB and high in the frame. In SRAM’s patent, various configurations are shown, but all revolve around a similar theme; mounting the battery around the front of the BB at the base of the down tube. Instead of a cylindrical shape, SRAM proposes a battery that almost wraps around the front of the motor. It’s held by a protective cradle that swings forward to allow the battery to be removed for charging or exchange. 

SRAM e-bike motor patent

This side elevation shows the position of the motor and the arrangement of the battery cells around the front of the motor and up into the base of the down tube. A 500Wh Bosch e-bike battery has 40 cells, and there are 39 visible here, although we don’t know their capacity

This leaves the down tube empty, which would allow, for example, in-frame tool storage, or, perhaps an internal range extender. Rather than cutting a hole in the down tube, it can be consistent in cross-section, which is the strongest and lightest solution. 

SRAM e-bike motor patent

Here we can see how the protective cage/door swings forward to allow the battery to be removed. We can also see the direct connection between the battery and the motor (no wires)

But how does SRAM find room for a battery when most motors take up all of that space in front of the BB with a planetary drive? Good question. Well the answer is that SRAM’s patent shows a concentric motor, known as a harmonic drive, similar to TQ’s latest HPR50 motor found in the Trek Fuel EXe. This compact unit leaves space for approximately 40 cells in front of the motor. Depending on the size of the cell, that could give as much as 500Wh – more than even Fazua offers with its latest Ride 60 system. 

Further weight savings, packaging improvements and reliability benefits could come from a wireless control system. SRAM already has the technology, and its patent application mentions wireless control between the rider and the motor/battery, presumably over power modes. Fewer wires mean less chance of water getting into the electrics and causing issues, it also means less weight, and designers don’t need to work out how to route and secure the wiring in the frame. This will also make assembling bikes in the factory a faster, simpler and cheaper job. 

The principle drawback we can see, providing enough protection can be built around the battery, is overheating. Keeping the motor and battery cool when they are in such close proximity, with reduced space for air circulation, will be a major challenge. 

While patent-spotting has become a popular pastime for tech nerds in recent years, we have to point out that applying for legal protection does not indicate that a product will ever reach the market. That said, we know SRAM is working on a motor (a prototype was spotted being ridden at the EWS Tweed Valley in June 2022), so if it does make it to market, the potential benefits could herald a massive leap in e-bike performance.