Does Your Power Decline When Riding Indoors?

Does Your Power Decline When Riding Indoors?

By Dean Phillips, Fit Werx

Many cyclists struggle to hold the same power output on a trainer that they maintain outside. Lack of motivation and scenery are often blamed, but the true answers are locked under the dust of your old physics textbooks from high school. I’ll save you the bad memories and explain…

The real reasons you struggle to hold power indoors are that there is less cooling effect and less flywheel effect indoors.

Less Cooling Effect

A cyclist cruising at 20mph outside experiences the equivalent of a big 20mph cooling fan continually blowing air over them. When you are riding a trainer, that massive 20mph headwind is gone. Without adequate cooling (just like any engine), your body temperature rises faster, your perceived effort goes up, and eventually your power output declines as you overheat. From a thermodynamics standpoint, this is an example of how forced convection from blowing air provides far more cooling than free convection in still air.

Like most cyclists riding indoors, I start my workouts with a warm-up that gradually builds power over time. My basement is around 55 degrees and I often found that once I reached about 200 watts of resistance I hop off the bike and turn the fan on. I wondered why this seemed to happen at around 200 watts and the answer became apparent when I revisited my HVAC design engineering days.

In HVAC design, when a building contained a room dedicated for heavy work, standards had us design that room at 55 degrees Fahrenheit with a cooling load designed to remove 2000 BTU/hr, or 586 watts, of waste heat per person. Studies show that a cyclist has a metabolic efficiency of roughly 25% and, based on this, we can calculate how much power goes to the pedals and how much is lost as heat by using the formula 0.25 = W / (Q+W), where W is pedaling watts and Q is waste heat to the surroundings. Plug in 586 watts for Q, and W = 195 pedaling watts and the answer is 586/195 ≈ 3. Cyclists release 3 watts of waste heat for every 1 watt that’s applied to the pedals. This means that a rider pedaling at 195 watts can usually do so in a 55 degree ambient temperature room without an overheating problem, but if they apply additional watts, they will likely start to overheat.

Doesn’t the rider’s size have something to do with this? Yes and no. Overheating has more to do with the absolute power you ride at, and less to do with the power relative to your Functional Threshold Power (FTP). In other words, regardless of your size and weight, hitting 195 watts in a 55 degree room is what matters. A larger stronger cyclist riding at 16 mph and producing 195 watts may start overheating during warm-up, while a smaller cyclist may be going 18mph at 160 watts and may never overheat in the 55 degree room as they never cross that magic 195 watt threshold.

Overheating occurs when riding at a high enough power level for a long enough time overtaxes the body’s cooling system. The most challenging indoor cycling workouts are those that stress both power and duration – the “double whammy” of overheating. Long intervals such as 2×20 or 4×10 minute intervals at FTP are common staples for cyclists and triathletes, but these are among the toughest to perform indoors without suffering miserably and/or experiencing power loss. A power loss compared to riding these intervals outdoors at the same wattage is inevitable without substantial cooling assistance.

Shorter higher intensity intervals are more reasonable to perform – such as 5 x 2 minute VO2 max intervals. While the intensity is higher and the effort is hard, there isn’t enough time for lack of cooling to negatively impact results the way it does in longer intervals. Longer lower intensity workouts are also less prone to power loss.

Based on this, some potential solutions to the increased heating demands of riding indoors are as follows:

  1. Keep the room as cool as possible, preferably under 60 degrees. You likely will not win any points with your family or roommate(s) by opening the windows in the living room in January, so consider moving training to a cool basement. Remember, training must be tolerated by those around us first, and comfortable conditions come second. J
  2. Use a large fan for cooling; the more air it blows across your body the better. You can increase the fan speed for higher intensity workouts.
  3. When all else fails, restructure your indoor workouts into shorter higher intensity intervals and longer easier rides.

Flywheel Effect

When you stop pedaling on a flat road outside, you can coast up to a minute before coming to a complete stop, but when you stop pedaling on your indoor trainer, your rear wheel stops spinning in seconds. Why? Outside you have the momentum of the bike, rider, and spinning wheels; that’s a lot of momentum working in your favor. An indoor trainer replicates some of this momentum through the flywheel attached to the resistance unit – the larger the flywheel the longer it can hold its momentum. While a larger flywheel helps slow the deceleration of the rear wheel, it does not replicate the momentum you experience outdoors and thus you have to work harder to maintain momentum on a stationary trainer.

Because of the flywheel effect, the way you apply force throughout the pedal stroke also changes compared to riding outdoors. Outdoors, riding on flat terrain, we may think we’re pedaling in perfect circles at 90rpm from all those one-legged drills we did the prior winter, but the reality is the pedal force of the down stroke leg helps move the recovering leg through the back and top of the stroke and into position for the next down stroke. However, when you start to climb a hill, your pedaling becomes more similar to what happens on a trainer – if you ease off the pedals, you lose momentum quickly. Your cadence typically drops climbing hills or riding a trainer as smaller hip flexor muscles prefer to have more time to activate as they are lifting on the upstroke and pushing forward over the top of the pedal stroke. It’s quite common for a cyclist’s cadence to drop 20-30 rpm when they’re climbing a hill and riding at a lower cadence on an indoor trainer makes the effort feel more in line with that same perceived effort outside.

Try these solutions to give yourself more wheel speed and a larger “flywheel effect” riding indoors:

  1. Only use enough press-on force on the roller drum to prevent the tire from slipping.
  2. Use a trainer with a larger/heavier flywheel. Some trainers, like the Kinetic by Kurt, offer optional super heavy flywheel attachments.
  3. Use a lower rolling resistance tire on the trainer. Many cyclists use big thick tires that wear a long time on the trainer, but those tires typically come with a rolling resistance penalty and rolling resistance is amplified on the trainer.
  4. Take on the challenge of learning to ride rollers. The benefit of two wheels spinning instead of one improves road-like feel. If you can’t get the resistance you want on rollers, try higher resistance tires, smaller diameter roller drums, or – my favorite – letting air from the tires until you run out of gears right at the interval power you are targeting.

Once you’ve taken measures to improve flywheel effect riding indoors, the best solution is simply allowing your cadence to drop to a level that feels comfortable. Depending on the stationary trainer, I’ll let my cadence drop 5-15 rpm riding indoors. If you work with a coach, send them a link to this article and beg to ride at 85 rpm instead of 95 rpm for the workout you’ve been prescribed.

So there you have it. The keys to riding indoors at the same power level and perceived effort as you can ride at outside are: 1) Riding in a cool environment. 2) Find the largest fan possible. 3) Riding at a lower cadence to make up for the lesser flywheel effect.

Happy indoor training! J


Dean Phillips is a co-owner of Fit Werx² in Peabody, MA. Dean frequently writes tech articles for Begginertriathlete.com and is humble enough that he would likely never tell you (so we’ll tell you for him) just how fast he is on a bike. Dean holds multiple TT course records in New England, having broken records previously held by some of America’s best pro cyclists and he set these while being a father of three children and owning his own business. Dean knows speed and how to get the most out of his training time.

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