Review: Tern Castro Duo

By Stephen Haynes

The aluminum-framed Castro Duo folding bike from Tern takes urban transportation and adds style and functionality for a nice, simple bike anyone can feel comfortable riding. Part of Tern’s mission is to provide utilitarian urban transportation and to promote sustainability. Each year the company donates one percent of its net profits to social and environmental causes.

As the name “Duo” implies, the Castro Duo has an automatic, two-speed hub (more on that later) with a coaster brake. The coaster brake gives a clutter-free cockpit, with no brake levers or cables. The bike’s 24” wheels mean a smoother ride but also add bulk to the folded-bike package. The swooping, one-size-fits-most frame is complemented by a long seat post and adjustable stem, so it’s easy to size up just about anyone. I found the adjustable “Andros” stem to be a great way to dial in the ride feel, and it’s so easy to use, you can micro-adjust at any point.

The Castro Duo employs what’s known as “N-fold” technology to fold down. A joint near the center of the frame and another on the stem create what looks vaguely like an “N” when folding. Once the two halves are unfolded for riding, a large lever locks the joint in place. This sounds complicated, but is actually very intuitive. The whole folding procedure takes about 10 seconds.

The Castro Duo comes equipped with full fenders and a chainguard as well as an integrated rear rack. Folding pedals are a nice touch as well and decrease the Castro Duo’s girth when folded, and a center kickstand can hold the bike upright either folded or unfolded.

While the Castro Duo may seem rather Spartan at first glance, there is room to grow. Cable routing and linear-pull brake bosses on the fork give you the ability to add a front brake and increase your stopping power. Also, Tern has worked with the German company Klickfix, specializing in quick-release adaptors, to install its integrated rear rack and luggage socket on the head tube of all their bikes, so that one can easily add bags and baskets that employ the Klikfix attachment system.

Optional front and rear battery-powered lights ensure your ability to see and be seen for an extra $20 each. The front light nestles snugly between the adjustable stem and swivels around the axis of the handlebars so you can point it where you need to. I’m told that the front light will also be offered in a dynamo version.

The SRAM automatic two-speed hub is an interesting little gadget that detects your speed and shifts into a higher gear for you. The shift point for the Castro Duo is set at 12mph, and once you reach this speed, the second gear engages and suddenly you’re going faster than you were before. Pretty cool, but also kind of weird.

There were a few times I wasn’t anticipating the shift and my knees paid for it. Also, when building speed to go up hills, the hub will shift at what seems like the most in- opportune time. You can disengage the second gear by quickly back-pedaling; once I realized I could do that, hills became less of an issue. Still, if you live in a very hilly region like I do, you may want to check out the Castro Duo’s older sibling, the Castro P7i, which features a Shimano Nexus 7-speed hub.

My average ride on the Castro Duo consisted of going to and from the store a few miles away. This seems to be the bikes’ bread and butter, if you’ll pardon the pun. The bike is great for short easy trips to the office, school, store, or destinations less than five miles away and is ideal for flat-ish terrain where the ease of use and the second gear really shine.

All in all, the Castro Duo is a great little bike. Although not ideal for the area in which I live, I think this would be the perfect solution for a college kid in a dormitory or working professional living in a small urban apartment. It folds down small enough to be inconspicuous in a small place and can alleviate potential theft by giving one the ability to take it inside with them.

As a father looking at the prospect of sending a teenager to college next year, I’m definitely looking at the Castro Duo as her primary means of transportation.

Tester stats

  • Age: 34
  • Height: 5’11”
  • Weight: 225lbs.
  • Inseam: 30”

Bike stats

  • Country of origin: Taiwan
  • Price: $800
  • Weight: 31.5lbs.
  • Sizes available: one size
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