# CFM outdoor vs indoor fans

I’ve searched but cannot find it- and I am sure it was here I read it!

Someone out some CFM figures for a headwind vs what some cheap fans put out for indoor rides. Anyone either got the figures or point me in the right direction?

Thanks

Sam

It would be LFM, a measure of airspeed, if you want to compare with outside - the CFM of riding outside is infinite, the entire planet’s atmosphere is rushing back at you… so a rider outside in still air is exposed to a flow with an LFM equal and opposite to the ground speed. Some fans provide specs for air speed vs distance; these tend to be in the 1-4 m/s at 1m from the fan, so 4-15 km/h… I doubt you will be able to attain anything close to 25-30 km/h equivalent, this is getting into wind tunnel territory.

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No. The CFM of riding outside is certainly not “infinite” and the “entire planet’s atmosphere” is not rushing back at you.

The only part of the atmosphere of any relevance here is that part with which one is directly interacting. It is roughly that volume of air which is displaced by a cyclist as the cyclist moves through it. In terms of rate (Cubic Feet per Minute) it will be more or less equal to the cross-sectional area of the cyclist multiplied by the cyclist’s speed (relative to the air).

Even if the entire atmosphere was someone relevant it still wouldn’t lead to any conclusions about infinite quantities. This might only be so were the atmosphere in question itself of infinite volume - which it is not.

I don’t think you can really compare the two because of the size and shape of the fan blowing the air.

I have two fans, a large blade shop fan and a smaller blower style fan. Even when they put out the same CFM, they feel very different and have a different effect on cooling because the blower fan is moving all of those feet/min through a small opening vs the very large output of the shop fan. Another example of this is exhale a full breath onto your hand through an open mouth vs pursed lips. Feels totally different even at the same CFM.

So even if you figured that X feet of air are moving over your body on an outdoor ride, that same CFM would not have the same effect inside unless you can get a fan that has an output that is shaped exactly like you are.

You would have to take into account all the air volume that is affected by the body moving through it - hence CFM is only used in constricted flow scenarios (ductwork, fans, etc). Free-body aerodynamics refer to airspeed “at infinity”, i.e. outside any interference from the body.