Generating heat while cycling. The art of Venting


When we understand what is happening within our body under the load and stress of cycling, we can dress better for our sport and increase the daily enjoyment of saddle time. 

You walk up four flights of stairs and develop an instant red face and feel your shirt sticking to your back. What is happening? 

Stairs are harder than walking on flat ground as you must overcome gravity – your body must work harder. Generating this extra demand on your muscles, walking up stairs and cycling up hills, increases your heart rate, blood flow and muscle activity. The body is working harder and this ‘work’ generates heat. Lots of it.

Your body wants to be at the optimal range 36.4°C to 37.2°C, On average. Fat people are warmer than thin. Young people are warmer than old. Small people are warmer than tall but an average of 1.5 degrees, but for this…we will go with the human average. As soon as your body’s internal temperature starts rising, with this increased work rate, your hypothalamus region in your brain tells  glands distributed all over your body to start cooling you down by producing sweat and radiating heat.

So when you plan to go for a spin you look at the outside temperature on your phone and put on a jacket and head out; job done?

In a syllable- NO

Smart people have worked out that a cyclist has a metabolic efficiency of roughly 25% cycling on the flat.  (Permission to ignore the next sentence granted). 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 pedalling watts and Q is waste heat to the surroundings.  Plug in 586 watts for Q, and W = 195 pedalling 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. So how can we integrate this information into our clothing choices.

Let’s start with a ‘control’ cycling example. Turbo training inside, where wind, outside temperature variations and road profile are removed.

This means that a rider pedalling at these random 195 watts can usually do so in a 12degreeC ambient temperature room without an overheating problem. Most of us know this from being on a turbo trainer with the window open. If you apply additional watts,  you will likely start to steam up the kitchen windows as the energy you’re releasing in heat rises. Working harder, calls on more muscles being called to action, your heart has to work harder to supply oxygen and nutrient-rich blood to where it is needed. All that work also creates heat.

Now move the cycling to the road.  You are in the saddle pedalling at 195 watts pushing just past 20kph. Your body isn’t working so hard, you aren’t sweating lots and the passing air is cooling the skin and garment surfaces. 

You come to a hill and start to climb. Immediately you slow (less cooling effect from passing air). You are out of the saddle- which means more work for your heart as more muscle groups are engaged. Your heart rate begins to climb as your body works to overcome gravity also adding yet more heat. As you push on your heart rate rises further, 30, 40 -50 bpm and you are breathing harder to introduce more required oxygen and your net heat generation rises further.

Do you even touch the zip on your jacket?

You body is now generating 300watts just in heat. The sweating is now excessive in an over attempt to regulate body temperature and blood is now diverted to the skin by the body in an attempt to cool off, away from the muscles losing much needed power. Do you touch the zip on your jacket?

We plough on sweating with a big red face, inefficiently grinding to the top a climb like a car driving through a desert with all 4 windows closed and the AC off.

Did you lower the zip on your jacket? Did you vent some extra fresh air into your core to cool the body? You did not.

There is an Art to ‘venting’, to work with your clothing during the spin and change as the temperature and conditions change.

 For 12 years I have stared at the Specification tables that come with fabrics we use to create clothing. “Technical ‘ is a group of fabrics that had some thought behind their creation regarding activity and not catwalks.  The term ‘Technical fabrics’ is overused and poker style underexplained by marketing people, to separate your cash from its walleted home. Some are better: more breathable, windproof and in general technology is improving every season but they are not magic. The new membrane inner fabric in our Mistral jacket has a breathability rating of 12K, over double the breathability rating of the first Mistral jacket we launched in 2013, and will process moisture away from the body,  if for example, you’re are riding along a flat road at 23kph.  Come to a hill and race up the hill at 400 watts where your body’s gross heat generation could be 1200 watts, and you will be sweating in the micro climate between the body and the fabric. The heat generated will be even too much for a highly advanced fabric to process. Just unzipping the front zip by 15cm could be enough to level the insulation and reduce the heat exchange. 

We must become more interactive with our garments. Understanding the body’s ability to generate massive amounts of energy AND heat in definitive circumstances, should alter how we take control of the amount of heat created. Coming to the bottom of a two minute climb, your jacket zip could be vented down 20cm at the bottom of the hill just after you changed gear to a bigger sprocket. The two should go hand in hand.

Setting off from home you are generating zero heat. In 3 minutes you will be generating 800 watts, so by self educated trial and error, you can work out it is OK to feel a chill for the first few minutes as you will warm into your garment choice, rather than be too hot for all of the spin. 

Of course you are going to be cold when you stop for a puncture as your body returns to ’normal’ and stops generating the heat. Once the wheel is off and the tube is unrolled; the next step is to put the layer on that you have rolled in the middle back pocket of ALL rides, winter and summer.

If we know we are going to face a climb or rain or the sun will come out mid spin, the extra layer, added or removed is vital to the full enjoyment of the spin.

Of course you are going to be cold leaving the winter coffee stop. Once you stop generating heat and the base layer is damp the chill will rise quicker. I’m not pro enough to bring a second base layer but hang my main garment on the back of the chair immediately to permit the base layer to dry off before I get shivers and put in on, in the coffee shop when the body cools.

-Top of the climb, gilet/jacket on. You generate little heat descending and the increased windspeed strips body heat.

-Finish of an event – extra layer on before you cool, even before a performance excuse is generated.

-Riders carrying extra weight (like Me) have a built in extra layer of insulation, so when you are riding beside the 55kgs kid with the arctic Parker on the club ride, judge not. Copy not. What works for Wout or your buddy might not work for you. Dress for yourself. Learn what suits you and use the zip.

So when we plan to go for a spin, look at the outside temperature now and at the end of the spin.

Think of the route you are going to take and who might you be riding with….and decide if the extra layer is on the pocket or on the shoulders.

2 thoughts on “Generating heat while cycling. The art of Venting”

  1. I have been riding on and off for nearly 60 years. This short article is the best I have read in terms of advice. A very welcome, good, honest, energy and experience informed article. Now, where are those zips?

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