The wonderful feeling of steady speed cruising on the flats is subject to two factors: resistance and inertia. Resistance consists of two components: rolling and aerodynamic resistance. Rolling resistance is constant, while aero is progressive and ramps up as a function of the square of speed. The faster you go, the greater the role aero resistance plays. Inertia is simply the effect of your mass going down the road plus, to a much lesser extent, the rotational inertia of your spinning wheels.

Indoor training tools often tout their realistic road feel, for good reason. You want to train under the same conditions you race on. As it turns out, they have two components in their resistance as well, drivetrain and air, fluid, or magnetic resistance, and they correspond quite well to rolling and aero resistance. Inertia is usually replicated with flywheels. Here’s a look at how two trainers and two rollers stack up in terms of resistance and inertia.

Resistance

Here’s a mathematically generated resistance graph for a typical road rider with a CdA of .37 and Crr of .005. Resistance (Newtons) is plotted against speed (meters/second) squared. The y-intercept is the constant force of rolling resistance, while the up-sloped line represents the progressive force of aero resistance. You could divide the area under the line into the two separate components.

Here’s the LeMond Revolution compared to that road rider. This plot was created by riding for one minute intervals at a steady speed for a range of speeds. Note how the two slopes are very similar, indicating that aero resistance ramps up very similarly for the two. Since the LeMond’s drivetrain consists of a chain and a belt drive, both very efficient power transmitters, it has less rolling/drivetrain resistance than a road rider, who relies on tires on the road to deliver power.

Here’s the Kinetic added to that graph, using Kinetic’s own published data. While the road rider has 4 N of rolling resistance and the LeMond 2.5 N of drivetrain resistance, the Kinetic has a whopping 12 N of drivetrain resistance. A small drum biting into a tire is clearly a very inefficient way to transmit power. Even though its fluid resistance unit ramps up at the same rate as our road rider, the huge amount of drivetrain drag changes the total resistance dramatically. The net effect is more resistance at all speeds and a less realistic ramp up.

Here’s the same graph with TruTrainer and Kreitler rollers added to the mix. Most of the resistance from rollers come from the tires on the drum, and since the drums bite into the tires more than a flat road surface, rolling resistance is higher than our road rider. The TruTrainer adds a bit of resistance with an internal flywheel, so its plot is a bit higher than the Kreitler’s. Both have a very shallow upslope, probably from the air resistance of the spinning wheels. I used the same wheels for both tests, so that explains the same slope for both lines. As you can see, resistance doesn’t ramp up much, which is why you can easily ride at 40 mph on rollers.

Inertia

Inertia is much easier to test for. I simply measured the coastdown time for each from 20 mph. With its low drivetrain drag and big flywheel, the LeMond won easily with 65 seconds. TruTrainer, with its beautifully milled drums and internal flywheel, came in second at 38 seconds. The Kinetic has a pretty heavy flywheel, but with its inefficient drivetrain it only coasted for 14 seconds. Kreitlers, with almost as much rolling resistance as the Kinetic but no flywheel, came in last at 6 seconds.

So why does inertia matter? Who cares how long coastdown takes? Keep in mind that your pedal output isn’t constant – you have a power peak with each downstroke. If you have a lot of inertia, you’ll maintain more of your speed between power peaks, it’s that “staying on top of the gear” feeling. With less inertia, you’ll have to re-accelerate with each downstroke to maintain the same average speed, and you’ll work harder through the dead spots.

Put it all together…

The LeMond Revolution feels fantastic, and it’s interesting that the numbers bear out the sensation. You can crank out tons of power without abusing your knees. The free spinning slippery sensation adds to the fun factor. It’s really easy at endurance pace, really hard at threshold, just like road riding.

I’ve always found the Kurt Kinetic brutal to ride. Its extra drag and minimal coastdown hurt the backs of my knees. I always wanted to overcome this, thinking I’d be a better climber if I could just fight my way through it. But in the end the misery and the fear of damaging my knees always cut my workouts short.

Rollers are much more pleasant to ride. Even though they have much more rolling resistance than the road, they spin quite nicely since they don’t have much wind resistance. The drawback, of course, is the difficulty of putting out big watts on rollers. You can always resort to tricks like running less air in your tires, putting a towel under the drums, or getting a resistance attachment.