DeZigner

If anyhting, I think it would be the opposite...meaning the fast moving windshield creates a monentary vacumm that would have the tendency to suck air, amd some rain, back into the cabin area. However, not enough rain to actually get you wet.

My guess.

jkwak922

interesting....

eskimo

I'm going to just wing this one and try to explain it, if only to make myself understand. Between peaking out in the garage at my dad's Z, and implementing photoshop in some 'interesting' ways I'm going to guess this distances and since metric is better, I'm going to convert them. So basically the idea is this, the windshield stops all the raindrops below its topmost point, the raindrops just above it will still fall into the car, but if you're going fast enough, the drops wont make it onto the top of your head before it moves out from under them, or at least your seats will make it out. Going fast enough (way way way faster than you should, or could go in the rain) and the car behind you (going the same speed) won't even know it's raining, except for those other indicators clouds, wet roads and whatnot. Going too slow, and the windsheild will only stop the rain right below it, and a short distance behind it. Now I'm going to explain all this, but you can just skip to the end if you want to know how fast you need to go to keep the passenger seat dry.

First things first, we need to know how fast a raindrop is falling. Anything falling from the sky falls at 9.8m/s². That being a measure of acceleration, we have to guess how long the raindrop has been falling, which depends on from how high up it fell, lots of calculations that will probablly end up inaccurate. So, thanks to Brian Yeaton of theweathernotebook.com, I don't have to figure out that that it's 565mph, but wait, why does it hurt a lot more to ride a roller coaster in the rain and get smacked by rain drops then it does to lie on your back and get pelted by these near supersonic raindrops— wind resistance. Well Brian says that after this effect: "A large raindrop measuring 5 millimeters across - about the size of a small housefly - falls at around 20 mph. " A.K.A. 8.94m/s at the time of impact with the ground. This being the result of many many calculations and averages as well as assumptions, or as we in the bussiness like to say, close enough.

So at what speed would you have to drive to keep your hair dry? Depends on how tall you are. Since I don't know how low your head is below the top of the windshield, we'll stick to keeping your nice, plush, freshly Armor-All'ed, empty (for simplicity) leather passenger seat dry. The headrest is ~6.3 inches (0.16m) below the top of the windshield and ~16 inches (0.41m) behind it at it's furthest back point. Once again approximate, I might measure in the morning.

So, the raindrop will cover that 0.16m from the top of windshield to the top of the headrest in 0.02 seconds (no there is no metric time ) So that means that the headrest needs to move forward 0.41m in 0.02s in order to have the windshield block the raindrops. Which translates to 46 miles per hour. Any slower and the raindrop will hit further and further forward points in the car up to a straight line down from the rear-most point of the windsheild. Faster and you might be able to keep the entire back of your car free from rain.

P.S. If you want to figure out how fast you need to go to keep your hair dry, you could just wear a hat OR hold a yardstick straight out from the top of the windshield, and use a ruler to measure how high from the top of your head to that yardstick. Take that number to google and type "[your number] inches to meters" this meter distance is your x in meters. x/8.94m/s = t your time to get out of there. Then use the distance from the windshield to the back of the headrest 0.41m/t which = sm/s. Then, since your spedometer isn't in meters per second go to google and type "s m/s to mph" and that's your speed. Can your Z go that fast? Unless you're short, I doubt it. 2 inches below the windshield translates to 160mph. So buy a hat, but don't worry about your seats. And if you're head is below the headrest you can save your eight bucks for new wiper blades.

LemanZ

Eskimo, is it all the raindrops from the sky are same speed? From your theory above, do you mean no matter how heavy the rain is, they will remind the same fall speed?

DeZigner

I got caught by a sudden cloud burst yesterday afternoon. I was fine until I had to stop at a light. Let me tell you, in a down pour, those 20 seconds felt like a half-hour!

Dry today.

eskimo

Well it would vary, but not by how 'heavy' the rainfall was. As the 'heaviness' is based on the distance from one falling drop to the next, the more dense (ie. the less space around each drop) the heavier the rain. Larger raindrops, however, would fall faster, or at least hit the ground at a higher speed, but being that the speed of rainfall was derived by Brian Yeaton, and not myself, I can't look into his equations, so I'm going to assume that the size of the raindrop vs. it's speed hitting the ground is going to be largely negligible in this scenario as the mass from a small raindrop to a large raindrop is also going to be largely negligible. Also, Yeaton says that the speed he offers is from a large raindrop, implying that the majority of raindrops are smaller than his example, meaning they would be going even slower when they hit your car, and you could technically be driving even slower.

So yes it makes a minute difference, but no, it doesn't really matter.

ohw

Eskimo,
Thank you for the insight. That is an excellent writeup and most entertaining.
I assume that the calculations did not take into account of the air turbulence and the water brought up by the cars in front of us. Since we cannot drive in a vacuum, in addition to buying a hat, I am thinking of some ugly old bed sheets (or shower curtains) to cover the interior to make sure it stays dry and clean. I bet not many people have to worry about this kind of problems when they drive in the rain.

eskimo

I can't see how water kicked up from a car in front would be a problem, nor air turbulence, other than a tail wind. Also, I don't really understand why you mention not being able to drive in a vacuum, as doing so would be disadvantagous as the wind resistance against the falling raindrops is the only thing that slows them enough to be able to keep your car dry. Just have to keep the windows up to keep the horizontal spray from other cars out.

elyliu

nice analysis eskimo. I was thinking the same theory but didnt have time to write it out....

maybe we should be less crazy and raise our tops when we see dark clouds in the sky or when our friendly weather man says "Heavy thunderstorm comin out way!"

eskimo

Actually got out there and measured and it being 26inches to the back of the headrest at its furthest back point, from the furthest forward point of the top of the windsheild instead of 16inches as I estimated, you actually have to go 82mph to keep everything dry. But the idea remains the same.

2004 Roadster

Hmm, driving my Z in the rain . . . never thought of that even with the top up. With just over 1000 miles on mine, I am still a bit ****.

ZRoadsterClub

Eskimo,

You still haven't accounted for the change in air density at different pressure altitudes.

Would you agree you'll stay dryer at sea level compared to rain at 10,000 feet based on the different dynamic air pressures?

eskimo

It really has to do with how fast the rain drops fall more than anything, and that's really hard to get a single number for because of the plethora of variables involved, the number I used was the one that I found the quickest, and while deviance from that is possible, I wouldnt imagine it would be much.

Anyway, any variance in drop size, altitude of clouds, pressure in the air, any of that will change the precise speed you'd have to go to keep certain parts of the car dry. Underneath all that and all the numbers in my original post is the simple idea that yes, you can in fact keep dry, at least somewhat in the rain, and this is how.

ZRoadsterClub

mwest300zx

My brother and family were in from LA a couple of weeks back. Visited other family, took 2 cars. My nephew (he is 15 and into cars) rode with me in the Z. Started raining with about 45 minutes to go. He sees very little rain in Southern Ca. Was amazed we could drive with top down and not get wet. He loved it.

Zcollector

Aerodynamics or Air Flow Dynamics

Chief F1 Fan

iTrader: (1)

Yesterday while driving to work in the POURING rain, some guy in a Saleen 'Stang went the other direction w/ his top down. I couldn't believe he wasn't getting soaked but he must not have been. Following about 20 car lengths back was a guy in a Z28 Convertible, same deal. Cracked me up and made me think about this thread. Obviously as long as you're moving fast, it doesn't seem to make a difference but you look like a complete idiot to those around you!

Ruhtra

LOL i love it i can so picture it u driving thinking ur too cool for school others are just staring

cschoolben

hey, anybody left their top town to go in a store and it rains while they are in there and don't realize (hear) it? does that totally ruin the power seats and carpeting?