SRAM's EX1 groupset is the first attempt at an e-bike-specific groupset and there's some fairly radical changes that are well rooted in the specific needs for pedalling under power. As a system it's well designed and functionally excellent, but the decision to limit the cassette to eight speeds means that there's some big gaps between ratios which affect the overall experience a bit. On the whole it impresses though, with the crispest shifting of pretty much any groupset we've tried and the knowledge that it's specifically designed for the job.
SRAM have looked at the way e-bikes are used – and also the way standard transmissions fail on powered bikes – and used that as the basis for EX1. Derailleur transmissions rely on the chain's ability to move between sprockets, and that means the chain has to bend. Chains are incredibly strong when pulled in a straight line but when you introduce a lateral force it's not such a pretty picture. A lot of the EX1 innovations centre around minimising the lateral force on the chain, which are higher on e-MTBs not just because of the extra power but the way the power is applied: on a non-powered bike it's common to ease off the gas when shifting but on an e-MTB the motor force is being constantly applied.
SRAM have dropped the number of sprockets at the back to eight. Their reasoning is that with the extra power available from the motor it's not as crucial to have close ratios, and the wider spacing allows for a slightly wider chain. Bigger gaps also means less shifts. One of the main reasons for chain failure on e-MTBs, SRAM say, is that the extra acceleration makes multiple shifts more common, and moving the chain more than one cog at a time greatly increases lateral stress. The EX1 shifter doesn't support multiple shifts: it's one-up, one-down.
The biggest five sprockets of the EX1 cassette are all machined from a single steel billet; the remaining three sprockets are smaller and will wear more quickly, so those are independently replaceable. The resulting cassette is also narrower than a 10- or 11-speed one, which means the chainline at each end isn't as extreme.
SRAM have done a lot of work with the shifting profile of the cassette so that the chain is in contact with the sprockets as much as possible, and the shifts over the big gaps between the ratios are as smooth as possible. The EX1 sprockets are designed to accept the chain in just one position, a bit like a thick-thin chainring; if the chain isn't in the right location then a correction tooth helps to sync everything up again.
Once SRAM had done the work on the shifting and timing on the cassette they found that chain strength wasn't really the root of the issue, and the PC-EX1 chain isn't radically redesigned, with the inner and outer plates subtly altered to work with the cassette's shifting ramps.
The new EX1 derailleur uses SRAM's X-HORIZON™ design, but it's been optimised to work best with the high-torque environment of an e-MTB transmission. Like other SRAM mechs it has a clutch mechanism to reduce chain movement over rough ground and it also has a lockout button to keep the mech out of the way when removing the wheel. It's designed to work specifically with the 11-48 EX1 cassette, of course.
The theory of EX1 is all very sound. Having had a chance to use it on a couple of bikes now I'd have to say that my impressions of it are very positive, but not without some reservations.
On the one hand, all the things that SRAM have tried to do with the groupset they've basically achieved. The timing teeth on the sprocket and the ramps designed to work with the chain in one position mean that shifts at the rear really are incredibly crisp. Even under full power the groupset never mis-shifts. The fact that you can only shift one cog at a time takes a little bit of getting used to even if – like me – that's your default shifting behaviour. I tend to nudge up or down one at a time but there's occasions when you'll round a corner to be presented with a wall, or you suddenly drop into a descent, where dumping a few gears is necessary. It takes a bit longer with EX1, but it's not a big deal. Sometimes (it's not often) you'll feel the chain re-align at the back if it's got out of sync with the timing teeth. That's not really an big deal either.
With a huge 436% range on the 11-48T cassette and average gaps between ratios of about 30%, there's more occasions during a ride when you're wishing that there was a gear in between the two you've got. It's worst on fire road climbs, where you have a bit of speed and you're towards the smaller sprockets. Dropping to a lower gear means a big increase in cadence, and vice versa. Changing to a bigger sprocket seems to be more choppy on Bosch-equipped bikes, as the system is more likely to interpret your jump in leg speed as an attempt to try really hard, and it adds a bunch of extra assistance so you end up pedalling even faster.
It's a slightly different skill to managing gears on a 10- or 11-speed setup, and one you have to learn. On more technical off-road, when you're moving more slowly and you're concentrating on the trail and not your cadence, it's hardly ever an issue. Obviously the decision to go to eight speeds is based on many factors: how SRAM wanted the chain to cope with motor forces, the machining of the bigger sprockets from a single billet, how the shifts would work with the single-position chain. There's a certain trade-off for the high strength and the crisp shifting though.
The clutch mech and the chainring do an excellent job of holding the chain and minimising chain movement when you're tumbling over rough ground. I've used EX1 with a Bosch geared motor and a small chainring, and also a Yamaha direct drive motor with a standard (Praxis) thick-thin chainring. In both instances I never had any problem with the chain jumping or dropping, and the transmission is very quiet in use.
Ergonomically I liked the feel of the EX1 shifter. The quality of the shift is a bit more tactile than many other mountain bike shifters. It's not an especially light shift but it is very positive, and because you can't accidentally shift two sprockets you can give it a hearty shove with your thumb. Lever position is great for both going up and down the block.
Overall the advent of SRAM EX1 is good in many ways. It confirms that the big players are backing e-MTBs, and it also shows that they're prepared to throw some resources at the specific needs of the drivetrain for a powered mountain bike. The EX1 experience is positive, with the sometimes-jarring gaps, especially between higher gears, the only real drawback. Shift quality is excellent and the steel sprockets should mean a long service life. It's expensive (€490 for the cassette alone if you buy it aftermarket!) so you'll only see it on top-end bikes for now, but the technology is bound to trickle down to lower price points before long.