Question on lift at speed

[razorman]

Cleaned my chim today and went for a celebration blast.
The sky was a bit black and so left the roof on and closed the windows while I was on the motorway. Going only slightly over the speed limit (honest officer) and noticed that I couldn't see out the back window. Realised that I had not closed the boot and I assume that the lift created at speed was opening it up, sure enough slowed down slightly and down it went again.

Obviously I know nothing about aerodynamics, but is this an example of why some cars have boot spoilers to counter this effect and if so although it would look naff does a chim need one?

[Glancs]

The same thing happened to my Griff on the Motorway a few years ago. Boot wasn't shut properly, it was night and I was on an un-lit part of the road. I didn't realise for about 15 mins. (Doh!) Thought everything was a bit too dark in my rear view!?!

The car drives along and in doing so pushes a hole in the surrounding air. The air that has been pushed out of the way by the front of the car then rushes around the back trying to re-fill the hole created by the car. Because of the relatively steep rear window (and other factors) the air crashes down over the boot area causing turbulence. This turbulent air is at a lower pressure than the surrounding air (like a partial vacuum) and the unsecured boot lid gets sucked up...... if you follow that?!!?

I don't think you need a spoiler on your car, just give the lid a good slam and make sure it's secure. I imagine you see them on other cars for visual effect more than function but someone else on here might know different :-)

[Nick_Chim]

don't slam it! - that can damage the catch - close it to, then press it until 'click' - job done.

[Cider Andy]

There's a lot of different forces at work there and I don't know nearly enough about it to explain even a few of them! But one of them is the 'ram effect', or air being forced into the cabin. The vents in the rear bulkhead are to exhaust this air. It then normally passes around the fuel tank and out through large vent holes in the tub below the tank. But with the boot open it has an easier path and just pushes the lid up. Rear spoilers are not fitted to hold the lid down, though (the catch is easily able to do that): they're fitted to force the back of the car into the road (make the back end artificially heavy), to improve grip and therefore traction.

[V8 Archie]

Boot lid spoilers are there to hold the rear wheels firmly on the road at high speed (you probably have to be going 100mph+ to notice the difference though). If the boot isn't shut properly the same effect that causes the car to lift will apply to the boot but because of the lightness of the boot lid will become obvious much earlier.

So yes, spoilers are there to hold the rear of the car down (or for cosmetic effect as they don't do much at road-legal speeds) and no, they aren't there to hold the boot closed.

[Boosted LS1]

In the good old days when I drove my TR8 soft top I used to smoke. Many a time I'd have a fag packet floating next to me along with all the dust and grit from the carpets.

Boosted.

[cdt]

Just get a spoiler that's so big and heavy that the boot will always click shut.

[Boosted LS1]

cdt said:

Just get a spoiler that's so big and heavy that the boot will always click shut.

I did just that

Boosted.