The Bicycle Jersey Problem

[JStephen]

My hobby and sport is bicycling. I recently ran across a discussion of bicycle clothing that presented an interesting problem. If any of you are in academia, this might be a good lab project for someone.

The issue was whether you can actually stay cooler in hot weather with an additional layer of fabric- a base layer. Evidently, cycling clothes are sold for this purpose. The idea is this: If you sweat on bare skin, the sweat is in droplets, not a film. You can put a layer of wicking fabric over the skin, spread the sweat out, evaporate more of it, and actually keep cooler. So we kicked it back and forth in the cycling forum with no definitive answer. What do you all think? Smart idea or snake oil?

(For what it's worth, the proponents of this idea claimed it worked better in drier climates than in humid.)

 

[Clyde38]

That's interesting. I don't have the answer, however if you have been watching the Tour de France you'll notice that as the riders get hot they unzip their jersey's. One of the advertisments for men's cycling wear claims:

Variable-thickness, antibacterial pad provides excellent moisture transfer, heat dissipation and shock absorption

[link

http://www.linkedin.com/in/clydehancock

]

[/url]

 

[chicopee]

The principle that you are discussing is called "evaporative cooling".
Anyone who has travelled to North Africa and probably Asia use that principle instead of refrigeration to keep water cooler than ambient temperature.
This is the way that I observed it. Water is stored in a leather pouch. This water will ooze out of the leather after a period of time. The leather pouch with water is mounted on the front bumper of a vehicle or some other means of transportation. As the oozing water evaporates, it will absorbe heat from the stored water and reduce the bulk water temperature. The speed of the transportation accelerates the evaporation.
The evaporative cooling works relatively well in dry climates but if the relative humidity is high, evaporation will be much slower which in turn will not effectively cool the bulk of the water stored in the pouch.

 

[zdas04]

The enthalpy of water vapor is considerably less than the enthalpy of liquid water. This difference is the "latent heat of vaporization" and is very quantifiable. The trick is accelerating the evaporation so that you get useful cooling. According to the guys that engineer irrigation systems (where minimizing evaporation is very important), for a droplet bigger than 50 microns in largest diameter evaporation is a surface function. For droplets smaller than about 50 microns evaporation becomes a body function and tends to be 1.5 times faster than a big drop.

The theory that a fabric like polypropylene that wicks moisture in one direction will take big drops of sweat and disperse it throughout the fabric is sound--the drops of sweat are large enough to evaporate via a surface function, the dispersed moisture is small enough to be a body function. This is probably important, but I would be surprised if it is the biggest benefit.

Sweat contains considerable dissolved solids. If it evaporates on your skin those solids will stay on your skin and form an insulating layer that inhibits perspiration and reduces heat transfer rate. If the sweat evaporates in the fabric then that effect is delayed a considerable time. I would expect that if you could quantify the two effects, the solids removal would be bigger, but I'm not sure.

David

 

[Compositepro]

Yes it can help cooling. To work the fabric must wick water well, be thin, and be in contact with the skin (stretchy fabric). Sweat that drips off does not contribute to cooling.

 

[btrueblood]

I've worn those shirts for several years now when visiting hot climates. They work well, and perform as advertised.

 

[racookpe1978]

I've got an unrelated recommendation on bicycle wheel wind resistance, if you wish to start a new thread on that subject.

 

[JStephen]

There actually is a Bicycle Engineering forum, althought it is mostly dead.

The intriguing thing about the jersey problem is the concept of adding a layer of insulation to keep cool when it's hot, which just seems wrong.

 

[Compositepro]

The jersey provides insulation against convective cooling when dry, but when it gets wet it loses its insulation capability and promotes evaporative cooling, this is just what you want to happen.