(T) Skywindow & Speed

I was cruising home a couple of days ago with the roof fully open and decided at one point to speed up to, say for example, 90.
I noticed that the steering seemed lighter and the car a bit less stable.

Is that my imagination playing tricks or does the angle of the panes of glass have some noticeable effect on the aerodynamics?

I would have thought that the open panes would have acted like spoilers if anything with a push down effect. Don't know whether the open roof has any effect - suppose it must interfere with the airflow - increase on drag? Just for example of course, I was on a motorway on the way to Brooklands a while back with the roof open and happily trundled along at 110 - can't say I particularly noticed any difference save for wind noise.

This calls for further testing and research - can I use that as an excuse if I am pulled over - after all the Police test their cars don't they?

Yeah go for it.

I expect the direction of the windows would act a bit like a spoiler but only the front one as it directs airflow above the rears when they are stacked.
If this does increase downforce I would expect it to affect the front wheels more though.

The general instability doesn't seem to get any worse - up to an indicated 120 (km/h of course officer).
Still not a particularly pleasant feeling though

Ok here is the technical reason for what you felt. The panes would act like a very poor spoiler as the angle of attack is not good and the glass is flat so reducing its effect, (proper spoilers have a distinct curve on them to increase downforce). Secondly you have a bloody great hole in the roof. The air going over the top of the car causes low pressure inside the car through the open roof. This causes the vehicle to lift, the slight increase of downforce at the rear may have corrected this at the back but increased the effect of lift on the front. A vehicle becomes unstable in these circumstances so I would avoid it in the future, alternatively you could open the front windows and this should reduce the lift by correctling the low pressure by enabling air into the vehicle from the front.

That actually makes a lot of sense!!
Good work Numan!!!

Thanx Numan - very clear info and advice. Is there any recommended max speed for open roof situation? Can't say I'd fancy the roof + the windows open though - need a Snoopy helmet for that!

How about having the air con on or is that far too little air????

Doesn't smell like FRESH air

I meant to keep the pressure stable in the car!!!!!!!!!!!!
Besides, I kinda like the air that comes from the aircon, so clean and fresh not like that muggy dirty air from the outside (mind you, i do live in Spain and it's bloody hot and dry!!!!)

Aircon on flat out cannot supply the amount of air required to equalise the pressure. Personally I would have thought 70 to 80mph would be maximum for speed with full window open to maintain stability. I dont have this problem as mine doesnt have a sky window.

Just in the interests of science I should probably admit to having driven at 120 leptons (on a private test track) with the roof open and although the car still feels a little unstable it doesn't really feel any worse than at 90 leptons.