Monday, January 4, 2010

Cycling In Heat & Helmet Cooling Power

Burrr. Its a chilly, white winter here where I live and in want of some heat, I decided to bring some 'heat' into today's post! I'll talk a little about the cyclist's elements during hot weather cycling and will seek to understand what role certain elements of our kit, like helmets, have on cooling.


When a cyclist gets out for a ride during the pleasant summer months, the modes of heat transfer seen between them and the environment is of the following order. You might think it is easy to fathom, but it really isn't.

There's many packets of energy entering and exiting and bouncing off here and there, from the thermal radiation from the sun to the heat flux in the skin. Overall, what we see is a complex thermodynamic system in action that tries to approach homeostasis. Here's a diagram :

Some of these elements may be familiar to us. Taking a break after riding, you can feel the hot sun on your skin. Comfort and discomfort are associated with humidity. When the relative humidity and air temperature are both high, the rate of sweat evaporation happens more slowly and we sense a higher temperature than the actual temperature of the air. The temperature we ultimately feel is governed by the temperature-humidity index (THI), also called discomfort index.

Now there happens to be an easy formula (though there are several more complicated ones) you can use to calculate THI before your rides.

THI formula developed by Clint Brookhart, P.E

where H = % humidity
Tf = Temperature in Fahrenheit

As the temperature gets higher, low %'s of humidity can result in an index that is much higher than the actual temperature. But the above formula does not take into account the reverse air cooling taking place when you ride your bike in a breeze.

Another aspect of the image above is the thermal radiation off the ground. Because of its dark color, roads happen to be good emitters and this infrared radiation varies proportionally as the fourth power of road surface temperature. You can get an idea of the energy emitted per unit surface area, or emissive power, by using the Stefan-Boltzmann law.


With regard to the emission properties of colors and surfaces, you and I were taught from early years in school that light colors are always good in sunny weather for better reflection. Yet our sport is one of multitudes of color, dark color.

It always struck me as odd how we riders have managed to live with dark colors in hot weather. The saying has always been to choose fabrics with wicking properties. These days, companies like Campagnolo are trying to capitalize on the apparent discomfort of dark clothing by incorporating fibers with UV ray protection. It seems to me a majority of us don't really think about apparel's effect on heat transfer and prefer more to wear matching kits, including helmets. The predominant color for shorts among men and women is black, but that seems to be for obvious reasons.


Which brings me to the question of helmet color. Are light helmets better than others in terms of thermal behavior? And how many cooling vents are optimal? This must be an important question, after all the helmet sits on a cyclist's head and part of the body's internal temperature regulatory mechanism is contained inside the brain.

The question posed above cannot be easily answered. In 2006, a paper in the Journal of Sports Sciences from Bogerd noted :
"Given the large variation in designs on the market, there is no widely adopted systematic approach to designing bicycle helmets for optimal ventilation. A quantitative survey of a large number of modern helmets is required to understand the role of "common sense" parameters, such as the number of holes in the helmet, since the helmet geometries are complex. In particular, the importance of forced, rather than natural, convection complicates a qualitative a priori analysis."

The paper referenced above isn't a bad read. It reports on the heat transfer, or forced convection cooling power, of an ensemble of 24 helmets. The study was done using an artificial headform. Apart from vent cross-sectional area and helmet channels, even the auxiliary elements of helmet design, such as the straps, and biological parameters like facial measurements and hair thickness have significant influence on ventilation properties, it seems. In the end, they concluded that "the wide variation in ventilation performance in the present ensemble serves to emphasize the lack of systematic understanding of the principles behind bicycle helmet ventilation."

As difficult the task above of studying helmet ventilation was, it didn't really seek to answer the question of helmet color and its effect on heat transfer.

A simplistic thermal test between a black and a white helmet was done back in 2000 by Terry Morse of California. The test aimed to answer the question of whether or not a black helmet is hotter than a white one of same design when worn in direct sunlight, both while at rest and while moving.

Terry placed a temperature probe at the crown of a Styrofoam head form, and put the helmet on the head. He then hung a halogen lamp 5" above the helmet, turned a household fan on high speed (6.5 mph), & recorded the temperature every minute until it stopped changing. He then carried on the same test with the fan on low speed (5.0 mph), and finally with the fan off. He did these three tests thrice for a black and a white TREK "Vapor" helmet and a bare head situation.

What he found was interesting :

1. In the air cooled phase of the experiment, the bare head form showed the least temperature change (delta F) from ambient temperature while the head form with a black helmet showed the highest delta F. In fact as the airspeed was decreased, the black helmet's delta F kept increasing before it stopped showing any changes.

2. About 16 minutes after turning off the fan, the delta F in the base head form was fastest and highest. It was the most reluctant to stop gaining heat while the black helmet was the best emitter of heat, in other words, there was more cooling with a black helmet on. But Morse maintains that the heating rate was very close between both black and white helmets.

Graphs showing the temperature change (dF/dt) in high, low and zero fan airspeed. About 16 minutes after turning off the fan, dF/dt in the base head form was fastest and highest (green line). Click to zoom.

Studying the effects of helmet on head cooling is no easy matter. While the test above is simple in nature and a Styrofoam is certainly no human head, do you think it makes a reasonable conclusion that you're better off with something covering your head in conditions of little wind speed? What else would you consider when designing & selecting a helmet for cooling? Please feel welcome to discuss with comments.

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