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PEZ-Tech: Wattage Trainers x 3
Training with power is the best way to get better at whatever you do on the bike: racing, triathlons, mountain biking, century rides – you name it. So with winter training in full swing, we checked in with the guys at for the lowdown on the Tacx I-Magic, Tacx Cosmos, and the Kurt Kinetic Road Machine & Power Computer wattage trainers.

– Submitted by Craig Griffin & Eric Fletcher –
With the growing interest in training with power, a number of indoor trainers have come arrived on the market to give a cyclist the essential feedback of wattage. A power meter gives you the precise feedback that is necessary to plan and track your progress forward until your goals are met. Outdoors, power meters use a few different methods to measure force or torque, and use that as a basis for calculating wattage. On an indoor trainer, watts are a little bit easier to calculate since there are no external factors to consider: no wind, no elevation changes, no rider drag considerations. Hence, wattage can be calculated by measuring the resistance to the roller at the rear wheel, and the speed that the roller is turning. That’s it. And because there are fewer variables, a fluid trainer with wattage can be had for just a couple hundred bucks more than a regular fluid trainer, and far less than the hundreds or thousands of dollars for an “outdoor” power meter.

But what about the accuracy, consistency and usefulness of these trainers? We’ll take a look at trainers from Tacx (their I-Magic & Cosmos) and Kurt Kinetic, and compare their wattage readings to an SRM Professional power meter for reference.

Each of the trainers takes a different tack on how to measure and display the power information. The Tacx I-Magic ($799 plus $239 for the steering frame) requires a high end PC to display real time virtual reality terrain and has an electromagnetic resistance system, similar to a CompuTrainer. The Tacx Cosmos ($1399) mounts a nice full color LCD computer screen on your handlebars and uses an electric motor for resistance, providing up to 1200 watts of pedaling resistance, which takes care of most everyone this side of Tom Boonen. Finally, the Kurt Kinetic Power Computer ($50) is an add on to their popular Road Machine fluid trainer ($330), that pretty much looks like any other regular bike computer you’ve seen. Now that the introductions are complete, let’s see how they performed.


1. Opening The Box: The Tacx Cosmos trainer comes pretty much disassembled with minimal written instructions and IKEA style pictograms provided to help you build the trainers. It’s not hard work, but there are a fair number of nuts and bolts to deal with. The trainer itself is all metal, hefty but not massive.

2. Initial Set-up and Impressions: The Cosmos makes you fight to get the bike properly mounted on the trainer, although with time it may become easier – but for us it was a bit frustrating. The Cosmos handlebar mounted computer is rather large, crowding out some hand positions, but mounting it up is simple. The initial set-up and calibration of the Cosmos computer is quite easy. Prior to using the Cosmos for the first time, it is highly recommended that you visit the Tacx Forum ( and ensure that you are running the latest firmware and software versions designed for the Cosmos, and download/update as necessary prior to using the unit for the first time. The Cosmos includes the Tacx Catalyst training software, which you use to design workouts and analyze power, heart rate, and speed data. Because most of the software is contained within the head unit, there is no installation needed from a PC to get the Cosmos up and running. Navigation on the unit is easy and menus are bright and clear on the LCD display. One thing to note is that neither Tacx trainer includes a chest strap to send heart rate data to the head unit, so we used a Polar T31 non-coded strap for that duty.

The Tacx Cosmos head dishes out loads of data.

3. Using the Product: The Tacx Cosmos was impressive. Its ease of set-up, calibration and long list of features made it easy to love out of the box, and it’s much better in real life than it looks in the promotional materials. The pictures make the computer head unit seem rather clunky, but in person it’s well thought out and rather clever. The Cosmos computer was easy to set-up, calibrate and run. With a number of preprogrammed workouts (you can hook the head unit up to PC and create infinite workouts based on heart rate, power or slope), we were training in minutes.

4. Cosmos VS SRM Data: Training on the Cosmos with a SRM powermeter equipped bike it was clear that the wattage numbers were usually a little off, and tended to lag under and behind the SRM. Efforts up to 500 watts were within 5% of the SRM. At over 500 watts, the Cosmos’ results varied by 10%, but never more than that. That said, the Cosmos was pretty consistent wattage-wise and would be quite suitable for winter wattage based training. Also, with infinite numbers of variations in workouts, combined with a massive range of resistance, you could step out of your basement in the springtime with the fitness of your life. However, given its need for outside power, the Cosmos is not really well suited for race day use.


1. Opening The Box
Like the Cosmos, the I-Magic requires various little baggies of bolts and nuts. The Steering Frame was a similar story, but again neither assembly was brain surgery, just take your time and all is well.

2. Initial Set-up and Impressions
The Tacx I-Magic’s set-up is a bit labor intensive. We first installed the software on a Dell computer that was about 9 months old, and required us to visit the Tacx Forum ( to research a balky driver installation. We found that information readily, and also learned that the Tacx firmware and software updates were very important and highly recommended. So we updated the Fortius software and firmware for the trainer.

After all of that, we were good to go. The Tacx software that came with our unit was dated June of 2006, so the company is releasing steady updates for its software – the sign of a company on top of product development. Also note that for S & G’s, we installed the Fortius software on four other computers, from a brand new Windows dual booting Mac notebook (Tacx software is Windows only) to a kludgy Franken-computer put together with used parts from Ebay. All of them ultimately ran the software, but with different install experiences, from a straight install all the way to trolling the forums for a solution. Ultimately, all of these machines ran the software without issue. Also note that the biggest indicator for top notch performance is having heaps of memory, like one to two gigs, and a dedicated video card helps a lot. The software “auto-detects” the amount of headroom and scales the graphics accordingly. At full power, the graphics are impressive.

The Tacx I-Magic control head is easy to use.

The Tacx software is broken down into three categories:
a. I-Magic, which is the virtual world to ride and race in. You can race other simulated riders on road or mountain bike routes, do free cycling, time trials, etc. As the terrain turns to a climb, the Tacx braking system kicks in and you are struggling out of the saddle, sweating and swearing, just like in real life.
b. Catalyst sessions are user designed workouts based on slope, heart rate, or wattage. Any combination of intervals and workout length are possible.
c. Real Video requires the purchase of one of 40 Tacx DVD’s of famous rides and races like the Tour of Flanders or one of the fabled Pyrenees climbs. With the Real Video sessions you pop the DVD into your computer’s DVD drive and get to train to full screen video while the trainers provides the appropriate resistance matching the video. Both the I-Magic and Cosmos trainers come with the Fortius software, the difference being in that the Cosmos head shows Catalyst only (you need a PC for the other two modes) and the I-Magic requires a PC for everything.

3. Using the Product: The Tacx I-Magic is the wattage based trainer that looks like a toy, but kicks your butt like an angry coach with a parking ticket. Let me say this up front, the computer installation and set-up are not for the faint of heart. The software likes a much more powerful computer then the instructions let on. The Steering Frame (another $239 on top of the trainer price) is stated to be optional even though it is not nearly as fun without it.

With all that said- everyone LOVED the I-Magic once it was working. By the time I got to the shop, most of the other testers were jelly-legged with fatigue from riding the thing- yet they would not stop. Honestly, I think one or two had actually developed grudges against the other computerized riders and had even shattered themselves in informal time trials against a certain cyber hardman who was dishing out the beatings like an Aussie sprinter the day after being sent to the back of the pack for irregular sprinting. With tons of different features, courses and even a video based training sessions- there was a lot of fun happening in the course of a lot of watts being burned.

4. I-Magic VS SRM Data: The wattage was pretty consistent across the various wattage ranges we tested, using two separate SRM equipped bikes. Another nice feature of both the Tacx trainers is that you can adjust your Tacx trainer with the “scale” setting to reflect your comparison to another power meter (like our SRM’s). But you need a computer and an outlet to use it, so the I-Magic is limited to indoor use only (no race day warm-ups).

One minor drawback to the I-Magic is that it is only rated for 800 watts, so the trainer’s resistance starts to give up the ghost around 900+ watts. Still, other then burning quads, 1000 watt efforts did not cause too many problems overall. The resistance started to fade a little, but how much time are you going to spend over 1000 watts?


1. Opening The Box: Simplest of all the solutions tested was the Kurt Kinetic Road Machine + Power Computer. Assembly is a one bolt process that is a snap to put together, and is easily the heaviest, most solid trainer I’ve ever used. The 6+ lb flywheel makes everyone else’s trainer look like a toy.

2. Initial Set-up and Impressions: Set up of the massive trainer is easy, and installation of the computer is not unlike any other rear wheel computer. Put the bizarrely oversized industrial magnet on a spoke (or use any better one you have lying around- as the computer found my older Cateye magnet just fine), run the sensor wires to the handlebar, do some fine print reading of the instruction page, and you are up and running. Interesting to note is that the Kurt Power Computer is compatible with several other fluid trainers on the market, including Cycleops, Blackburn and others. Reprogramming the computer is required, but that just takes some reading and button pushing. From Kurt’s website, they state that fluid trainers have a fairly predictable resistance at a given speed (a power curve), hence the computer does some fancy math, and figures your wattage from there.

The Kurt Power Computer fits neatly on pretty much any set of bars.

3. Using the Product: The Road Machine’s feel is summed up in three words: solid, smooth, steady. Massive amounts are resistance lies in wait to burn up your legs. It was rock solid even under heavy efforts with choppy pedaling. The trainer itself is a no nonsense piece of training gear. But does the computer do its job?

4. Kinetic VS SRM Data: Tested against the SRM, the accuracy of the Kurt computer was excellent up to about 400 watts, and then began to diverge (lower) above that level. At approximately 500 watts, the diversion was around 10%, and stayed there up to 1100 watts (and gee that was fun to do three times). Overall the accuracy was not quite as accurate as the Tacx trainers, but for $50, it was by far the least expensive useful power tool out there. And keep in mind, if you don’t have the coin to spend on a SRM, you can use this computer/trainer combo throughout the season to measure your progress from road training – simply schedule a trainer session once a month for a step test or threshold test, and write down the results. That’s the essence of power training.

Conclusion: After testing all three trainers, the I-Magic was the most popular with our testers – despite the cost, set-up and required options and computer, everyone who used I-Magic loved it and could easily see themselves using it quite a bit over the winter. With the I-Magic, Tacx has done the seemingly impossible feat of making winter trainer time almost seem like fun while providing you with a powerful set of training tools at the same time.

Those who want to do some Boonen style training without the option of a PC in the trainer room or the interest in a VR system will like the Cosmos.

We found the Kurt Kinetic option to be the best bang for the buck. It doesn’t shoot for the moon, but the computer delivers reliable power data for the cost of just the SRM handlebar clamp. It also is a great trainer for general use and for race day warm-ups.

Regardless of what the best option for you is, there has never been more choice available to spend some quality time on a wattage based trainer this winter.

About the Authors: Craig Griffin is a 15 year bike racer and the owner of, an online retailer of cycling electronics. Eric Fletcher is a Masters racer and a connoisseur of finer bicycles. These guys love training with power and have extensive experience with many different power meters. sells a large variety of electronic training devices for cyclists, including bike computers, power meters, hrms and electronic trainers.


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