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.

	Air Temp (deg F)	Power (watts)	Weight (lbs)	CdA	Crr	Air Density (kg/m^3)	Speed (mph)	Time	Result
Rider 1	80	200	150	0.30	0.0040	1.165	21.0	28:30
	50	200	150	0.30	0.0049	1.234	20.5	29:18	48 sec slower
Rider 2	80	300	200	0.35	0.0040	1.165	23.0	26:01
	50	300	200	0.35	0.0049	1.234	22.4	26:43	42 sec slower

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.