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Ideas for wireless shoe heater

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Myke

Mon Jan 04 2010, 01:46AM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

kell wrote ...

I thought the 25 watt figure somebody threw out was way too high. Imagine putting a 25 watt bulb inside your sock -- it would burn you.

The 25W figure was for the induction heater method. The coupling between the plate and the coil would be pretty loose and also there are other metallic things close to the plate that might unintentionally heat up.

MinorityCarrier

Mon Jan 04 2010, 08:47AM

Registered Member #2123 Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312

I think foot-watts would be variable. Folks with diabetes would have lower wattage due to poor circulation. Also, as feet get cold, vaso-constriction sets in reducing heat being carried to the skin.

In my own personal experience, during winter back-country backpacking, I've had my feet get very cold and it takes hours for them to warm up in a -20F-rated goose-down sleeping bag.

25 watts would probably not cause a burn if widely distributed agains very cold skin and underlying tissue of a foot, for a short period of time.

Steve Conner

Mon Jan 04 2010, 10:43AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

95 watts seems a little low. The figure I saw for heat output was 180 watts when you're asleep or watching TV, but it goes all the way up to 1kW+ during strenuous exercise. I remember seeing a chart used by HVAC engineers that lists heat outputs for all manner of activities.

Maybe it's 95w of radiated heat and the rest is latent heat of the water vapour in your breath. (The table listed these separately, because from a HVAC guy's point of view they're two different problems.)

If you come in from a winter bike ride and put your frozen foot into a hot tub, I bet more than 25 watts of heat goes into it.

Bjørn

Mon Jan 04 2010, 10:55AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

The average intake of energy through food is something like 10MJ a day for men (in some parts of the world). If we divide that by (3600*24) we get 115 W on average so 95 W does not seem too bad.

Coronafix

Mon Jan 04 2010, 11:38AM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

What about FIR insoles.

It was one of the first links I found.

Nik

Tue Jan 05 2010, 02:41AM

Registered Member #53 Joined: Thu Feb 09 2006, 04:31AM
Location: Ontario, Canada
Posts: 638

I was going to suggest getting those hot packs that you just shake and put in your shoe...

Mattski

Tue Jan 05 2010, 03:43AM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

I think it is certainly do-able to make a wireless shoe heater. I see broadly two ways of doing it.

Option 1 - Transmitter in pedal with lossy material inside shoe. Should work on a fairly wide frequency range. One problem is what you put in your shoe - the usual flexible lossy materials (RAM) made of carbon or ferrite in a rubbery material are a little tricky to buy, and probably difficult to homebrew. Also they have a particular frequency range that they are lossy. You could put steel in but then you have comfort and weight issues to consider. You also have efficiency to consider. If you have steel parts in the bike near enough to the pedal then you could have significant loss.

Option 2 - Transmitter with resonant receiver in shoe driving a resistive heater. More complicated design of the receiver but the heater will be much easier. You can buy heating socks or as Steve linked, insoles, which should be able to hook up more or less directly.

It doesn't really strike me as practical, when there are battery-powered products you can buy, but could certainly be fun to work on. If I made one I'd try to make it powered by the motion of the bike itself :)

Note: just re-read your post. You say you want the transmitter on the leg. I think it would be much easier to attach it to the pedal, because then the coupling will be much better no matter what way you go. Also I think the transmitter would end up being an awkward shape on your leg, rather difficult to make conformal and have radiation going down to your feet.

Renesis

Tue Jan 05 2010, 02:54PM

Registered Member #2028 Joined: Mon Mar 16 2009, 08:13PM
Location: Norway
Posts: 319

Hmm, i wonder how the eskimos are able to survive...

The reason why your feet goes cold is mainly because your body restricts their blood circulation to preserve your body's core temperature. In a life or death situation your body will sacrifice your toes and fingers in order to prevent fatal hypothermia. If you teach yourself to regularly, if not constantly wiggle your toes then this will help keep the blood flowing. When im walking in the cold i work my toebending muscles for almost every step i take. Remember, when your limbs has gone cold, it is generally too late for this technique. This must be done prematurely.

Dont use cotton, use wool. Sheeps wool can keep you warm even if its wet, and is a much better insulator than cotton. Homemade wool is by far the best, because the sheeps wool contains special fats which are removed in spinning factories. These fats are water repellant and good for your skin. The downside is that wool cannot keep you warm in wind, so be shure to wear windtight and water repelling boots.

Wear big boots. No matter how many layers of socks you wear, they wont insulate much if your boots are to small. Remeber that it is not the socks fabrick that keeps you warm, it is the air trapped inside it. Without enough space, that air will dissapear.

It goes without saying that you should wear many layers on your feets and torso. If things start getting a bit too warm then you can remove some layers, you want to prevent sweating as much as possible. Wear the thinnest layers on the inside and the thickest layers on the outside.

Of course, downhill biking while wearing several kiloes of clothing is a challenge to say the least, but the old-fasioned clothes are still the best by my opinion.

I have a limited amount of winterclothing here at my dorm, most of it is at home. The black socks are my inner layer, they are tightly knitted commercial socks made of wool, they will absorb my sweat and still stay warm. The red socks are thick, heavvy home made socks (thank you grandma) used as the main insulation layer. Double up if its very cold. All this should fit into your boots and still give you plenty space to move your toes. Gore-tex boots are fine, especially if you wax'em. Leather and rubber boots are even better, but they need a lot more stuffing to stay warm.

I hope this was helpful

Steve Conner

Tue Jan 05 2010, 03:42PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

+1 on woollen socks! They keep your feet relatively warm even when they're wet.

You also get water-resistant socks called Seal Skinz, they're popular for winter biking here. (Like a posh version of kell's plastic bag trick.) Wear these with some thick wooly socks and it should warm things up a bit.

thedatastream

Tue Jan 05 2010, 03:57PM

Registered Member #505 Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329

Integrate a dynamo or generator into the axle of the pedal to provide a local power source. It would _obviously_ have to be well sealed to avoid getting crufted up but generating the powe where it is required would be much more efficient and simple

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