This Kit Hides A Secret Electric Motor In Your Own Bike

The Vivax Assist is a tiny, four-pound motor that no one will ever know about as you race by them.

Early electric bikes were the opposite of cool, with clunky designs that painfully advertised that you might not be strong enough to pedal up a steep hill on your own. That’s changing with bikes like the beautiful Faraday Porteur, which manages to almost completely disguise the fact that it has a battery. Still, even the Faraday looks special. If you want an entirely stealthy e-bike, here’s another option: The Vivax Assist, a tiny, lightweight motor that your bike shop retrofits directly into the seat post of a bike you already own.


“We hid the motor in the seat tube so the bike looks better,” says Ulrike Treichl from Austria-based Gruber Antrieb GmbH & Co., which manufactures the kit.”But it also helps with stability–the bike is streamlined and the motor is in the right position to be balanced.”

The battery hangs either under the seat in a saddlebag or, for those who want the ultimate disguise, also comes in an option that looks like an ordinary water bottle stored on the frame. A wireless button hidden in the handlebars turns the drive on and off.

With both motor and battery, the kit weighs just under four pounds, which the designers say is lighter than any other electric system. It’s light enough that, with the motor off, the bike feels basically the same as it did before.

“You have a very authentic feeling while riding,” Treichl says. “You don’t feel like you’re on an e-bike.” With the push of a button, the motor quietly kicks in, though the rider always keeps pedaling–it doesn’t work like a scooter, but more like the push of an invisible hand behind you as you ride. The intelligent power drive responds and adjusts depending on how hard you pedal.

It’s intended for riders who are already in fairly good shape, as it provides a smaller boost (110 watts) than the typical e-bike. Still, even an inexperienced rider can use the system. “If you use the motor in the right way along with gears, you can climb a hill with a 10% grade even if you’re not very athletic,” Treichl explains. “We tested this here in the mountains.”

For someone who wants to make it to work on a hilly commute without sweating, or for riders who want a little help keeping up with a group on a century ride, it’s an interesting option. It could also be helpful for cyclists recovering from injury, or anyone who wants to take longer rides without pushing themselves too far.


“Healthy exercise and training is not about taking your body to the absolute limits,” says Monica Schweitzer, CEO of Gruber Antrieb GmbH & Co. “It’s more about using your own judgment to discover just when less is more, finding your ideal pulse rate and optimizing your training. You should be smiling all the time you’re on your bike.”

The only catch: The kit is definitely not cheap, at around $3,400. Vivax also sells a carbon-fiber road bike, with the invisible kit built in, for over $8,000. And the price is unlikely to come down anytime soon. “To make the this motor so small and lightweight was a lot of work, and we need the best materials,” Treichl says.


About the author

Adele Peters is a staff writer at Fast Company who focuses on solutions to some of the world's largest problems, from climate change to homelessness. Previously, she worked with GOOD, BioLite, and the Sustainable Products and Solutions program at UC Berkeley.