![]() however, if you are moving slowly (and there is no wind) or stop sweating or the ambient temperature is higher than skin temperature then the impact of the black lycra will start to be felt and it WILL be hotter. So why do I wear black lycra and not die a horrible death - well basically because when you are moving the additional heat transfer rate in from wearing black is offset, or more than offset, by the convective heat transfer out and the impact of evaporative cooling. So all up the impact of colour of lycra is only significant when it comes to radiant heat from the sun which is increased for black material. ![]() ![]() There will be a very small amount transmitted by radiation but that heat flow will be minimal since the temperature of the two surfaces is pretty low. Heat transfer between the lycra and the body underneath - heat is mainly transferred between the skin and lycra by either conduction (where the skin is in contact) or natural convection (airflow caused by the temperature differences between air close to the hot surface and air close to the cool surface) - neither of these is impacted by colour of lycra. Evaporative heat flow - this isn't really a pure form of heat transfer but is related to the rate at which the sweat is evaporated from the surface of the lycra clad body due to the partial pressure of water vapour un the boundary layer of air (humidity) which will be lower and closer to the ambient humidity with higher wind speed (hence you get cold when moving fast if you are sweaty) - this heat transfer rate will be unaffected by the colour of the lycra since black lycra is no better wicking than light lycra. Convective heat flow out - as above it is a function of the airflow speed and the temperature difference of the air with the colour having no effect. Radiant heat from the body - see above of the relationship however the temperature of the body is very low (compared to say the sun) and therefore the heat transfer rate is very low (even though there will be a slightly almost unmeasurable increase in heat radiating from the body clad in black it is insignificant because the heat flow in will swamp the effect - sun being 5,500K versus body at 300K). Conductive heat flow - well lets hope not since conductive heat transfer is from two contacting solid bodies and that is one thing you hope not to have on a bike. Convective heat in (this is the heat being transferred through the movement of the air across the body) - this is only a heat flow in if the air temp is higher than the body temp (assuming that the outer surface of the lycra is in thermal equilibrium with the person inside it), if this is the case the the heat transfer rate will be proportional to the temperature difference and the velocity of the air, if the cyclist is moving slow or not moving then the heat transfer rate from convection is essentially zero (well very small due to natural convection or natural airflow due to different temperature air). ![]() Radiant heat from the sun being absorbed by the body - this heat flow is proportional to the temperature of the emitter to the fourth power (sun, therefore very hot) and the emissivity of the surface which is high for black and low for light colours, therefore a massive increase for a black clad body over a white (as proven by the thermometer in the case experiment and backed up by hard science) So for the education of Oxford (not enough work on at the moment) I thought I would go through the various forms of heat transfer going on for a body in lycra in the sun (presumably riding a bike and working hard). The other thread (which was about an anti-cycling Facebook page for goodness sake) has a lot of half science and poorly used references being used to try and prove that black lycra is cooler than light/white lycra for riding - actually it is only Oxford that seems to have that opinion. Ok since I don't want to continue to participate in the most epic of thread derails I thought it best to start a new thread where the science of black (dark) and white (light) cycling kit could be discussed.
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