Automotive Question

Because we don't want wires in our forward field of view as well as our backward field?

Maybe you could get around this by making the heating wires out of ITO or some other transparent conductor. Hmm...

Simple answer: they do. My car, an old Ford Mondeo, has one. The wires are visible, if you look carefully, but they don't interfere with the view at all. Quite why they're not more common, I have no idea.

I haven't found that the electric defrosters work much better than the blown air defrosters. I always figured they put the wires in the back window mostly because they didn't want to run air ducts all the way back there.

I'll agree with Mike at #3 - see, the front isn't just blowing air, it's blowing air that has been dehumidified by the air conditioner system. Glass heaters might help, but then there's the cost factor.

I used to have a Ford Taurus that had a windshield with just enough metal of some sort impregnated in the glass to do just that. There were no visible wires, and the windshield had a dark, metallic shine throughout. It really was nice.

Until a rock hit it and we found out how much it would cost to replace.

I've never seen a car with a windshield defrost system, but I've always found the rear-window defrost to be significantly faster than the air blowers. In the time that it takes the windshield defrost to melt a tiny little patch immediately above the vents, the rear window will be almost completely clear-- no scraping required.

In the winter, humidity is not usually an issue. In fact, the defrost isn't even run through the AC in the winter, because what would be the point?

I have no idea about the cost issue, but I would imagine that part of the problem is that it's rare. If heated windshields were as common as heated rear windows, I doubt there'd be a huge price jump. I can't imagine that the materials are actually all that much more expensive, otherwise why would they be standard equipment on the rear window?

http://en.wikipedia.org/wiki/Quickclear

@#6: the point of putting the air through the a/c is that not everyone uses fresh air. Some people recirculate the air, and people make pretty good humidifiers in small spaces.

I think that anything that interferes with the drivers forward view is a bad idea, even if it is only a 1% effect. You never know if you'll be distracted by something in you line of sight at a critical moment.

I leave the AC running all year round (the compressor is electric, not a belt, and has no impact on mileage or power output, and yes, I have tested this), but I live in a much more humid area than you do.

You should run your AC at least once a month for at least an hour. If you don't, the seals and hoses will rot and you'll be looking at expensive repair and coolant recharge bills. I drove my SC2 for 8 years and 120k miles, and never once had the AC worked on by doing this. Your manual in the glove box will tell you the same thing if you read it.

The front defrost/defog is meant mostly to deal with the interior surface. The exterior surface should be scraped free of ice and/or snow before engaging the wipers to keep the surface clear, and in fog, mist, etc, the wipers are used at need to clear the exterior surface.

The toaster over defroster on your rear window would violate just about every state's laws regarding obstruction of driver view. When the inevitable filament break happens, you'd also have a nice horizontal strip obstructing driver view.

Maybe because the automobile makers are afraid of being sued when somebody gets into an accident and blames it on not being able to see (or concentrate on) the road properly because of the heating wires in his windshield?

Chad:

In fact, the defrost isn't even run through the AC in the winter...

You sure about this? Our car automatically turns the AC on when we turn on the defroster. The light saying the AC is on is lit, but I'm pretty sure that in at least some cars the AC is turned on without turning on the light. You can still tell because the engine lugs (if you have a wimpy enough engine to notice).

I agree with Ahcuah #5. It seems to me that windshields get broken rather more and so the cost of replacing one with tiny wires in it would be pretty painful, given that you could lose a windshield every few years.

Before we had a garage that we could put our car into, every other day in Winter our car was like a popsicle. Even getting into it wasn't always trivial (breathing onto the lock to try and defrost it a) won't work and b) will make you look like an idiot). Praise Gore for garages (preferably ones with remote control openers).

What Aaron said. Most cars turn on the AC automatically when defrost is selected, regardless of whether any AC button has been pressed or any indicator has lit.

You can test this objectively easily enough. Pop your hood, and have someone inside the car activate the AC in a mode other than Defrost, so you know what it sounds like. Have them turn it off, then have them select Defrost. Odds are you'll hear the AC kick in.

Of course AC has a large effect on mileage. To see the effect, you have to compare with and without AC with your car in the same state. That means the battery fully charged, etc. If you compare with your battery discharged, you will see no difference.

If I'm in a hurry, I wipe the inside window glass with newspaper. I learned this secret from running pinball machines, where one spends a spectacular percentage of one's time cleaning glass. Newspaper has certain oils, I'm told.

On the question of the post, it is probably because car manufacturers fear that drivers' eyes will focus on the wires and not so much on the road in front of them, particularly if they are tired. They may fear some liability if the trial lawyers get ahold of that.

A secondary issue may be the fact that windshields are "safty glass"--two panes of glass sandwiched over a sheet of clear plastic--to prevent shattering, and installing the heating wires might cause a problem with construction of the safety glass.

Heated glass is standard equipment in airliner cockpit windows. The requirements for visibility and impact resistance in this application are rather more severe than those required for automotive use.

All the 'it won't work' problems raised here by commenters were resolved decades ago. The only outstanding issue for automotive use is cost: a complete set of cockpit windows on a 777 cost more than a typical car.

Yes.. I agree with Ahcuah #5. the cost of replacing a front widshield is costly enough without an additional expense of a defroster.. also the visability issue

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