STAC Zero – First Look

A few months ago we received a very interesting email at Were we interested in checking out a new type of bike trainer? The sender promised that it would be one unlike any other we have ever tested before. As they put it:

We have a unique resistance mechanism, which generates the required resistance without ever touching your wheel.  This means that there is no noise generated, and there is no tire wear, meaning you will never need to buy another training tire again.  You also aren’t required to remove your rear wheel, as with the direct drive trainers.  Furthermore, the light weight and small size makes it perfect to bring along to races for warmup, without any concerns about wearing down your race rubber.

To say our interest was piqued is putting it mildly. We were on the phone just a short time later learning about what would become the STAC Zero.



But before we can fully jump into the STAC Zero, it’s important that we first cover eddy currents. Eddy currents as defined by Wikipedia:

Eddy currents (also called Foucault currents) are loops of electrical current induced within conductors by a changing magnetic field in the conductor, due to Faraday’s law of induction. Eddy currents flow in closed loops within conductors, in planes perpendicular to the magnetic field. They can be induced within nearby stationary conductors by a time-varying magnetic field created by an AC electromagnet or transformer, for example, or by relative motion between a magnet and a nearby conductor. The magnitude of the current in a given loop is proportional to the strength of the magnetic field, the area of the loop, and the rate of change of flux, and inversely proportional to the resistivity of the material.

Or for those looking for the slightly simpler version – here is Andrew from STAC explaining them.

Today, this technology is already used worldwide by trains (disk eddy current brakes) and roller coasters (linear eddy current brakes)—both providing contact-free braking creating no odor or noise.

So what does this have to do with a bike trainer? Well, quite a lot, actually. By employing eddy currents generated by a wheel constantly passing over a pair of magnets, we can create contact free resistance. The faster you spin the wheel, the more resistance you generate. The closer you move the magnets, or the more magnets you use, the more resistance you generate.

But unlike other trainers out there, a trainer utilizing eddy currents can give you a solid workout without having to press a roller onto a tire or having to replace your rear wheel with a direct drive cassette mechanism. And this was the concept that the STAC Zero was created from.


The STAC Zero utilizes a pair of magnets that are positioned via a closing caliper that is tightened with a quick release lever. STAC tells us that the system is capable of creating over 2000 watts of resistance. While they couldn’t tell us the specific power curve it generates (since the exact resistance depends on the exact rim construction and magnet placement), they did say the unit tends to have a linear progression. According to Andrew: “We’ve recorded numbers well over 1000W, and that was mostly a personal limitation, as opposed to a limitation of the technology.  One of the co-founders has done 20min intervals @ 360W+ without a problem, and I’ve done shorter intervals well into the 500-600W range.  If you want a general picture, the way I typically have it set up for my own training, it will peak out at around 900W @ 95rpm on a 52×12 cog set, and that increases roughly linearly from 0rpm.  I could get more wattage by spinning faster.”


For those looking for power measurements in their trainers, STAC has an option for you. There will be two versions—one with power and one without. The power version will transmit both ANT+ and BLE and is powered by a simple pair of AA batteries.

Since there is no large bulky flywheel, mag unit, or fan, the Zero can fold down incredible small. The folded height of the production unit is less than 3 inches and is designed to easily fit under a couch, into the closet, or in your trunk for a race day warmup.


There seem to be only two real negatives to using eddy currents. First is that while a heavy wheel will keep spinning once you stop pedaling, a light wheel (which would describe most current bicycle wheels) tends to quickly come to a stop – much quicker than what you’d experience on a more standard trainer (or out on the road). STACs solution for a more appropriate road feel is adding two relatively small weights to the wheel. The weights are secured in between the spokes, held in place by the wheel’s own speed. The faster you pedal, the more secure the weights are held in. If you have a carbon fairing on your wheel, they thought of that too. An optional weight unit fastens onto the hub, preventing any contact with the fairing and eliminating the chances of cracking a carbon fairing.

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Second, this only works on wheels with an electrically conductive construction. Unfortunately, the construction of some carbon wheels make it impossible to generate enough force this way. If you have a full carbon wheel (no alloy braking surface), the trainer will not work on your wheel. If you have aluminum wheels, carbon wheels with an alloy braking surface, or aluminum wheels with a carbon fairing, it is fully compatible with your wheel.

Our Thoughts

While this is just a First Look article, we actually have had the opportunity to spend some time a preproduction prototype version. Ours was one of the first units the team built, and was far from the finished version we expect to see in homes in just a few months. But it did give us more than enough opportunity to try it out and experience eddy current resistance for ourselves.

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We tried it in stages. First, without the magnets in place. We wanted to see what an owner would experience if they just plopped their bike on the trainer and didn’t bother with the weights. At high speeds the trainer felt just like any other trainer – you had a consistent spinning feel and the harder you pushed the harder it felt. The problem was getting up to speed. As you first start pedaling you feel the wheel want to stop every time you are at the top and bottom of your pedaling motion (when you have no power going to the wheel). It’s a very unnatural feeling – almost like you were pedaling through molasses.

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The weights we mentioned earlier would combat this. And in our short timeframe with the trainer, this was our one challenge. It’s important to note that, like the trainer itself, the weights were preproduction with lots of edges and a lack of polish. We were definitely paranoid that they might somehow get loose and damage something. But they did perform as promised, making the starting and stopping experience much more road-like. In other words, when you stop pedaling, the wheel keeps on spinning.

Regarding noise, the trainer is only as loud as your bike is. Have a rubbing brake or unaligned derailleur? When you use the Zero, you will definitely notice it. In fact, our derailleur was not properly aligned during one of our training sessions, making enough of a racket that one of our fellow editors actually thought the trainer was doing it. But after quick inspection and derailleur adjustment, the noise was gone.

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Unfortunately, our test unit did not have the aforementioned power meter, so we didn’t get a chance to compare it to any other meters on hand. STAC did tell us that their own testing shows their unit is consistent with other power meters on the market, but we were unable to validate that.

Wrapping Up

The Zero is currently available on Kickstarter at Early adopters can get a non-power meter version for $269 CAD ($130 less than MSRP). Or a power meter version for $329 CAD ($170 less than MSRP). For those that have full aluminum or aluminum wheels with a carbon fairing and want an ultra-quiet trainer, the STAC Zero is going to be worth taking a look at. STAC expects to ship units in August. And until then, stay tuned for a full review on a final production unit with full video of our tests in progress.

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