Friday, January 14, 2011

nice primer on air resistance. stationary fan.

D: this chart is a bit misleading. Sure the air resistance does increase as the cube of speed.
But the other aspects of a bike matter still.
Linear so less and less a part of the total energy required.
But the chart exaggerates the use of a recumbent with fairing.
Particularly when weight is considered!
Let alone other practical considerations.

D: strangely I could not find a decent front profile pic of a bent online. At least not easily. So I took one of my Cruzbike.
Keep in mind my seat position is very upright for a bent.
But is sooo comfortable.
And I improve on the 'aero tuck' cross section in 'lazyboy' comfort!
I once tried to demonstrate the aero tuck to a co-worker. And promptly sprained my neck! WHY?!

But because the power to overcome rolling resistance is linear with speed, and the power consumed by air resistance is a cubic function with speed…by the time you get to 20+ MPH typically over 70% of the overall power you deliver to the pedals will be consumed by air drag!

For example, let's say that the tires rolling resistance power/wattage equals the aero drag power at 12 MPH, which is pretty typical. So at 12 MPH, we have one unit of power going to overcome rolling resistance (RR), and one unit of power going to overcome aero drag at let's say a total of 40 watts at 12 MPH. That would be 20 watts for aero drag and 20 watts for rolling resistance at 12 MPH.

When we double our speed to 24MPH, we now need 40 watts of power for RR because it is linear with speed, or 2 X 20 watts. Since the aero power is proportional to the cube of the speed, when we double our speed the aero drag component becomes (2^3) x20 watts, or 8 X 20 watts= 160 watts.

When we add the aero and RR components together at 24MPH, we now have 40 watts + 160 watts= 200 watts @ 24 MPH.

We only doubled our speed from 12 to 24 MPH, but the power required went up by a factor of 5 (40 to 200 watts) because of the dominance of the aero drag at higher speeds.

Mechanical chain drive for a fan for a stationary bike resistance unit.

D: I have a nifty 200 bux on sale magnetic stationary unit.
It works. I guess.
But it is just not the same as the wind whistling past me.
I'd say ruffling my hair but I just went bald LOL.
Anyway, there are plenty of electricity generator schemes online.
But I just want to run a long chain forward to an industrial fan.
Shouldn't be too hard to get a decent match to air resistance.
Plus it cools you.
And psychologically I MISS that air movement!!!

I'm just thinking thru wais required to DIY.
A long chain. I imagine a bent owner already has one for RWD.
A floor guide for components. 2x4s and nails should work, with some plywood.
An additional chainring or two.
An adapter to switch movement direction at the fan back.
Tear the guts out of the electric motor.
A busted unit will suffice, so long as the bearings are fine.
A coupla lengths of chain 'n rings to get that power there.
I imagine welding guides for all that to the stand on an industrial fan is simplest. Or bolting/strapping.

Hmm. Now I wanna build it...

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