When testing electric mountain bikes, a good rule of thumb is to ride first and ask questions later. It’s good to free your mind and feel the joy before you try to understand the complicated engineering that goes into these increasingly sophisticated beasts.

But halfway through my first test of the Slash+ 9.7 SLX/XT, my ride was so nimble, flowy, and quiet—so unlike the maiden voyages I’ve had on other e-MTBs I’ve tested—that I couldn’t wait to dive in. I needed to know all the technical details of how Trek managed to build an electric mountain bike that feels and sounds so much like an acoustic bike.

Flip Chip

Let’s start with the basics: Trek took the frame geometry and suspension of their popular Slash acoustic enduro bike and added a motor and battery. Then they offered the electric mountain bike version in two builds—the higher-end $12,000 Slash+ 9.9 XO AXS T-Type, and the $8,000 Slash+ 9.7 SLX/XT, which is the one I tested.

The build on the 9.7 has a carbon frame, a Fox suspension package with 170 mm of travel in both the front and rear, and a Shimano XT groupset. The bike is a mullet, with a 29-inch wheel in the front that rolls over big stuff and a 27.5-inch wheel in the back that makes it lighter and more maneuverable.

The whole 45-pound package (for a medium-size frame) is powered by a 250-watt HPR50 motor with 50 Nm of torque and a removable 580 watt-hour battery. The LED display has Bluetooth and ANT+ connectivity and sits discreetly flush on the top tube where it’s easy to see for the rider.

The push buttons controlling the three levels of power and an additional walk mode sit on the left handlebar near the shift levers. The bike’s slim frame and the placement of the e-components make it difficult for passing riders to tell that it really is an electric mountain bike.

Beyond that, the 9.7 offers a bunch of extras, like tubeless-ready Bontrager wheels, a battery-range extender that fits into the water bottle cage, and inserts, like a flip chip, to adjust angles so every rider can go from a plusher-feeling ride to one that won't bottom out when going big.

There’s also a companion app that tunes the motor, tracks activities, and fine-tunes the ride with navigation features and real-time range calculators. All of this adds up to a nice package, but none of it so far gives away what really sets the Slash+ apart. There are two things: a near-silent motor and high-pivot suspension.

Quiet Time

There’s nothing that kills the endorphin rush of mountain biking more quickly than the incessant whir of an electric motor. The Slash+’s HPR50 motor has eliminated that annoying earworm by using a pin-ring drive transmission.

In most ebikes, the electric motor spins at a much faster rate than the person pedaling, and a few cogs and belts are required to make pedaling feel more natural. However, a pin-ring drive takes all those whirring cogs and belts and replaces it with one inner ring with pins, rotating at different speeds within an outer ring with slightly different-size pins. Not only is this quieter and more efficient, it’s also a lot smaller and less obtrusive.

Because Trek designed the Slash+ to feel more like an acoustic ride, the motor has only 50 Nm of torque. That’s substantially less than powerful e-MTBs, whose motors have upwards of 100 newton meters. The latter can power up the hills but also make you feel like a bull in a china closet on tight and curvy downhills. Less torque also saves wear on the Shimano drivetrain, which, when paired with high-torque motors, can result in broken derailleurs or chains.

While Trek is not the first to include high-pivot suspension on an electric mountain bike, the company is the first to mitigate the problems with high-pivot suspension (high-pivot suspension is when the suspension is placed higher on the frame).

Bikes with high-pivot suspension tend to feel smoother and more luxurious and responsive to ride, but that increased distance between the cogs and the chain results in more slack on the chain, which is called chain growth. Chain growth causes pedal kickback, where the pedal rotates backwards abruptly, and gives you that distinctive mountain biker look where your shins have been clawed by wild cougars.

To take up the slack, Trek routed the chain through not one but two idler pulleys. The result looks a little like a three-ring circus near the chain ring, but that added idler pulley allows the derailleur to do what it’s supposed to do, which is keep the bike shifting well.

Love at First Ride

I loved this bike off the bat because it felt nimble and fast and gave me the confidence to sail over a few steep lines that I normally avoid. On trail, I never felt the need to ride beyond level-two power, even on climbs, which added to its acoustic feel. The only place I could have used more of a boost was when riding home straight uphill on city street with a 13 percent grade.

After multiple subsequent tests, I felt the same joy with one caveat: There was a consistent and nagging rattle that sounded like a loose wheel, but I could never isolate the source of the noise, which likely indicates that it was internal and perhaps coming from the battery or a cable rattling in the downtube.

To give the Slash+ a more robust test, I passed it on to Samuel Hayden, a former collegiate gravity rider at Fort Lewis College in Durango, Colorado. After a 20-mile ride in mostly turbo mode on the double blacks of Piedmont (stuff I never ride), Hayden returned with a huge grin and with more than 65 percent battery power left in the tank.

On its weight and handling, Hayden reported that the Slash+ melded substantial power with a surprising lightweight build, noting it had a solid and reassuring connection with the trail and maintained stable flight during jumps.

His only major gripe? The 9.7 lacks a SRAM transmission, which tends to be more reliable for e-MTBs. But that problem is easily solvable, if you have an extra $4,000 on hand, by upgrading to the Slash+ 9.9 XO AXS T-Type.