The drive unit of E-MTBs is one of the most stressed components. It not only has to transmit the rider's power, but also the entire motor power to the rear wheel. Heavy chain wear and a high defect rate were an annoying but unavoidable problem with the classic components of motorless mountain bikes.
Less wear - longer service life
That wants Sram with its new EX1 drivetrain change. "The groupset is precisely tailored to the needs of e-mountain bikers," explains Markus Schneider, the responsible product manager at Sram. A wider, more stable chain alone is not a solution to increase durability, which is why other approaches were sought. "Only if the shifting process is defined and accurate can the drivetrain withstand the enormous forces on the e-mountainbike in the long term and the chain is protected," explains Schneider.
Fewer gears - wider spread
In addition, Sram has enormously extended the gear spread once again with a large 48 and small 11 sprocket on the rear wheel: 436 per cent are available. However, the number of gears has been reduced to eight. This is surprising at first glance, but proves to be consistent in the riding test. Due to the high performance of the E-MTB, double gear changes are the rule rather than the exception. With previous systems, many gear changes involved shifting through several gears with the press of a thumb. The EX1 categorically rules out this possibility. Each gear change is triggered by a separate thumb click. This is the only way the chain can move over the cassette in a defined manner. In the riding test, the EX1 proved that the engineers' plan seems to work. The gear changes are extremely smooth - even under full load. It takes a little getting used to the fact that only one gear can be changed per lever press.
New four-piston brake system
To curb the enormous propulsion, Sram has also introduced the Guide RE four-piston brake system, which is designed to reliably keep the additional weight of an e-bike in check. The brake lever has been adopted from the well-known Guide R, and the lever width can be adjusted without tools. The four-piston brake calliper, on the other hand, is borrowed from the downhill-proven Code. Sintered pads are intended to keep durability high despite the tough conditions.
Why do you actually need a special e-mountainbike groupset?
We asked EX1 developer Henrik Braedt about it.
EMTB: Why a huge 48 sprocket?
Henrik Braedt: E-MTBs should get by with one chainring at the front. This improves the running of the chain and provides better guidance. In addition, shifting gears at the bottom bracket harbours a high risk of failure. That's why we moved the entire gear spread to the cassette.
There are problems with the fatigue strength of conventional drive components. Most drives therefore reduce their performance during the switching process.
The loads on the drive are up to three times higher on an e-MTB than without a motor. Robustness was therefore at the top of the list of requirements alongside riding enjoyment. The loads on the drivetrain are significantly lower with large sprockets. With the EX1, you can change gears at any time - even under full load - without the gearbox cracking. This is not possible with any existing gearstick.
Eleven gears are the norm on a mountain bike, so why is there only eight on the EX1?
We realised that e-bikers almost always shift two gears at once, as acceleration is much higher with the motor. We had no limit during development. We chose eight gears because they were the most successful in our tests.
The Sram EX1 in detail
Crank and chainrings
The developers have designed a special crank to cope with the high torques on the e-bike. It should be particularly stiff and durable, yet weigh very little. The aluminium crank should be compatible with Bosch, Yamaha and Brose drives and will be available with three chainring sizes - 14, 16 and 18 teeth with the typical Sram X-Sync design.
EX1 rear derailleur
High torque and low cadence - these are the challenges that a rear derailleur on an e-bike has to face. The aluminium rear derailleur of the EX1 weighs 289 grams and is equipped with the familiar technologies - X-Horizon, X-Sync derailleur pulleys and the Roller Bearing Clutch friction brake. It can manage a maximum of 48 teeth and of course only the eight gears of the EX1 cassette.
EX1 gear lever
In principle, the EX1 shift lever works like any Sram lever, with the difference that it can only shift one gear per shift. This is primarily intended to increase the necessary shifting precision on the e-bike and prevent over-shifting of gears. The lever is Matchmaker-compatible and weighs 122 grams.
The cassette
We have now reached 12 sprockets at the rear on mountain bikes. Sram has reduced the number of sprockets on the e-bike drivetrain to 8, and the cassette is made entirely of steel to withstand the high forces generated by the drive. A step backwards? Not according to Sram. Eight sprockets and the sometimes very large gear steps should correspond exactly to the shifting behaviour on the e-bike. In the EX1, 11 - 48 teeth are distributed across eight gears with a gear spread of 436 per cent. The gradation in detail: 11-13-15-18-24-32-40-48 teeth.
Wider chain
Of course, the chain has also been adapted to the requirements of the e-bike and is wider than a conventional mountain bike chain. As a result, it should also be much more stable and glide more quietly and smoothly over the sprockets.
The prices of the Sram EX1 at a glance
Stefan Frey
Editor
