By Dean Phillips, Fit Werx

Have you ever noticed that your average bike speed is slower when it’s cold out? Look no further than any local 10-mile weekly time trial and some riders seem to be up to a minute slower on the coldest days compared to their PR times on warm mid summer days. The opposite tends to happen in running road races – hot temperatures always seem to result in slower run times. So why do cyclists go faster in these conditions?

Fortunately for cyclists, warm days provide two distinct benefits that runners don’t capitalize on: 1) Lower Air Density and 2) Lower Tire Rolling Resistance.

Lower Air Density

Air density is a measurement of how dense or thick the outdoor air is. Outdoor air temperature is inversely proportional to air density, so that means when the temperature gets warmer air density gets lower. Lower air density results in lower aerodynamic drag on a cyclist, so that cyclist will go faster for a given effort. How much faster? Let’s take an example of a cyclist that can ride a 10-mile bike loop in 28:30 or about 21mph on a day when the temperature is 80 deg F. This same cyclist rides this same 10-mile loop again on a day when the temperature is 50 deg F. The change in air density alone causes the time to increase to 29:00 and average speed drops to 20.7mph. Unfortunately for this colder weather rider, the losses don’t stop there, as increased tire rolling resistance will further reduce speeds.

Lower Tire Rolling Resistance

Tire rolling resistance measures how much energy it takes to roll your tires over the pavement. When a tire contacts the ground it deforms and then springs back to its original shape. Every time this happens energy is lost in the form of heat. A warmer tire has improved elasticity and will lose less energy during this contact with the pavement. Many riders have noticed that it’s easier to change a flat tire on a warm summer day when the tire seems to push back onto the rim with little effort compared to a cold day. While we know warmer tires have lower rolling resistance putting an exact value on that change is challenging. A rolling tire is continuously generating heat and while a portion of that heat is lost to the surrounding air, there is also a portion that heats up the tire itself. The faster the tire rolls the faster it heats up. A sunny day will keep the pavement hotter, which also impacts tire temperature. Different tire models will heat up at different rates as well. The exact change can vary, but you can expect to see tire rolling resistance increase approximately 6% for every 10 degree F reduction in outside temperature. Now let’s get back to our rider who’s already been reduced to 20.7mph because of increased air density on the colder day. Adjusting for the increased rolling resistance this rider will average less than 20.5mph and see the time increase to 29:18! This poor rider is going to go home thinking he/she needs to train more when the reality is the outdoor temperature alone caused the 50 second increase in time and 0.5mph reduction in average speed.

What About Overheating?

Will the warmer temperature reduce power output? If it gets hot enough and you ride long enough you will eventually see a decrease in power output on the bike from hot conditions. It’s unlikely that you’ll see a reduction in cycling power for events shorter than 20 minutes simply because they’re over before your body has a chance to overheat. Unlike running in the heat, cyclists still have a 20+ mph fan blowing over them that provides significant cooling even in hot temperatures. While your heart rate will likely approach record highs in bike events in the heat, when push comes to shove cyclists can still generate their normal power over these shorter durations. Even if there’s a small power reduction from your body overheating, it’s more than offset by the very fast conditions created by the warm temperatures so most riders won’t notice. In the end everybody rides the bike faster when it’s hot out.

The chart below summarizes the changes in speed and time to complete a 10-mile cycling event in these two temperature conditions. We’ll use examples of two different riders to show that these time reductions impact everybody similarly. Values for rider weight, power, aerodynamic drag coefficient (CdA), coefficient of rolling resistance (Crr), and air density are provided for comparison in each condition.

The speed reductions don’t always end there either. In addition to the colder weather slowing you down, cyclists tend to bundle up in extra layers, booties, gloves, tights and jackets when it gets cold out. All these extra layers simply compound the already slower conditions by increasing aerodynamic drag further.

Keep all this in mind next time you blame those slower winter and early season speeds on lack of fitness!

DP

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 your children and owning his own business. Dean knows speed and how to get the most out of his training time.

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

DP

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.