Why do caps go forward at high speeds in a open top car

big ears · 15 Nov 2004, 18:47 (Ref:1154284)

Please could some try and explain why, if you are driving an open top car, and your hat blows off, why does it go forward and not backwards.

**dtype38** · 15 Nov 2004, 18:52 (Ref:1154292)

Swirly vortex thingy comes off top edge of windscreen and curles down into passenger area making tubular eddies that make the air seem to move forward at head hight. Posh open sports car have little screen thingy that pops up behind passengers to keep cap/hair etc, in proper place.

big ears · 15 Nov 2004, 20:36 (Ref:1154445)

Thanks for that, i saw it on apast Top Gear Show, and i wanted to know more about it.

**Tim Falce** · 15 Nov 2004, 21:36 (Ref:1154509)

Last time I lost a cap while driving my car it went backwards but then it only has a half height screen. One thing I have noticed though, if I empty a cup of coffee out of my truck or land rover window at speed, the liquid goes forward before spraying back in my face, it never goes backwards and onto the road.

StephenRae · 16 Nov 2004, 09:20 (Ref:1154776)

Quote:

Originally posted by falcemob
Last time I lost a cap while driving my car it went backwards but then it only has a half height screen. One thing I have noticed though, if I empty a cup of coffee out of my truck or land rover window at speed, the liquid goes forward before spraying back in my face, it never goes backwards and onto the road.

How many times did you do this before you figured it out

**dtype38** · 16 Nov 2004, 18:03 (Ref:1155198)

Five caps and twenty seven coffee soaked shirts....

Some people can be very fixed in their driving style

imull · 16 Nov 2004, 22:10 (Ref:1155455)

he he. classic

R59 · 27 Nov 2004, 19:34 (Ref:1165777)

Better buy a new Merc SL - I drove a SL500 not long back, and there was very little turbulence in the cockpit, and very little wind noise - less than in a Alfa 164GTA, and that was a closed car!

The worst modern open top car I have ever driven was a Z3M Beemer, for noise and cabin turbulence (horrible throbbing headache inducing noise!), not for performance and handling

But then again, I don't wear a hat when driving, other than a crash helmet when regulations require it!

Rob.

Ntrprise · 2 Dec 2004, 13:04 (Ref:1169792)

In a word drag. It is easiest to illustrate if you imagine a massive room (say size of a football pitch) filled with ping pong balls. Drive a car through this room at 5 MPH and the ping pong balls will be hugging all 4 sides of the car.

Now imagine driving it at 80 MPH. You can imagine that the area directly behind the car is now an empty vacuum – no ping pong balls. The drag effect is in action – literally dragging and sucking the car back into this vacuum.

So the vacuum behind the car is making air rush forwards to fill the gap. This is happening at the rear of the car. The same happens behind the windscreen on an open top car. The air is simply rushing forwards to fill the gap.

Obviously, the ping pong balls represent air molecules. So when looking at aerodynamics – just imagine driving through a room with ping pong’s – makes everything a little easier to understand

With this illustration, it is arguably easy to see why the rear of the car is just as or even more important than the front of the car.

Same illustration can be used for a speed boat passing through water.

DaveStyles · 2 Dec 2004, 13:09 (Ref:1169799)

hmm..

thanks for the visualisation.. I cut some holes in my bonnet last season in what turned out to be a stupid place.

If I'd have known about the ping pong balls I *might* not have done it !

What a useful tool !

**dtype38** · 2 Dec 2004, 13:35 (Ref:1169834)

Hmmm, I also cut some holes in my bonnet last season. Very big ones. It made everything run nice and cool, and also stopped the back edge trying to part company with the bodywork at anything over 80mph. Didn't seem to make any difference to anything else though. Maybe its because I put some wire mesh over the holes to stop the ping-pong balls escaping!

**Tim Falce** · 6 Dec 2004, 15:25 (Ref:1172471)

Quote:

Originally Posted by dtype38

Five caps and twenty seven coffee soaked shirts....

Some people can be very fixed in their driving style

Well, I do remember a long trip back from Donington this year and drinking too much coffee on the way back, miles from the services on the M11, nowhere to stop and I only had an old cup spare and well, open window and all that, do i need say more. Mrs mob wasn't pleased with what soaked shirt this time.

**dtype38** · 6 Dec 2004, 21:54 (Ref:1172870)

Falcy, your avatar makes me LOL.... about every 12 seconds!!

StephenRae · 7 Dec 2004, 09:16 (Ref:1173184)

Quote:

Originally Posted by DaveStyles

hmm..

thanks for the visualisation.. I cut some holes in my bonnet last season in what turned out to be a stupid place.

If I'd have known about the ping pong balls I *might* not have done it !

What a useful tool !

Are we talking about the 'Balaclava' type or the 'Easter Parade' type bonnet?

**dtype38** · 7 Dec 2004, 09:58 (Ref:1173241)

Quote:

Originally Posted by StephenRae

Are we talking about the 'Balaclava' type or the 'Easter Parade' type bonnet?

With comments like that, its just as well he wasn't cutting holes in his helmet!

RWC · 11 Dec 2004, 05:33 (Ref:1176795)

Here's a real mystery you might like to try!

Get a balloon on a string and tie it to the ceiling of the car (or just get your girlfriend to hold it there you tightwads that can't afford roofs,LOL)
As you accellerate it will tilt backwards ...of course

Now get a kids helium balloon and tie it to the floor of the car
when you accellerate it tilts...forward!!Hit the brakes and it tilts back...WHA???!!

**dtype38** · 11 Dec 2004, 12:09 (Ref:1176936)

Even more strangely... if you fill a balloon with coffee, when you accelerate if tips backwards and spills coffee on your shirt, but if you brake it still tips backwards and spills coffee on your shirt.... isn't that right falcemob

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