Hi folks, opinions please! (Yes i'm still ill and have nothing better to do than post speculations on here... )

Looking at that last post on aerodynamics, something else occured to me. Given that a builder decides he must have a windscreen (road car), would it not improve the airflow over the car to put in a rear screen too? I've seen folk put polycarbonate between the spars of their roll cages and I've seen that funky dutch 7-a-like manufacturer whose name i forget doing fully hard-topped cars. Something inbetween might be more what I'm thinking, so you're end up with a kind of T-bar top like the old mr2 convertible. My gut tells me this would reduce drag, no idea what it would do with regard to lift/downforce though. Would also make wet weather gear far simpler and easier, a good plus for those of us who'd like to use their cars every day. Weight increase should be minimal if you've already got a roll cage with supporting struts and you use polycarbonate instead of glass (SVA?).

What do you think?

you could even make a super light roof as a space frame with poly triangle window panes...but I think it would look pants

Would look like the lunar lander. I'll leave that one for you!

hmmm, not sure if will; the wind screen cuases drag at the front and air wants to curl round the top to get inside, then air will be hittin the rear screen.

the mr2 you mention has almost a completely enclosed high side windows and a much more racked front screen.

thge small mnesh screen you see on BWM, mec and others is to prevent the air curling round so as not to mess the trophy blonde's hair.

you could try simulating this on a small model, a joss stick and hair dryer?

but then maybe I;m wrong

Think there's a fair chance I am too, but hey!
I do follow you, you can never know till you test it, but my gut feeling is it would prevent more turbulence than it causes. It depends on wether the opening between windscreen and roll bar is small enough that the airflow won't curl round into the cabin once you're doing a decent lick.
And yes, it might save some money in hairspray.

I know where this is going...
Rescued attachment bat.jpg

Proof positive! I want that one.

Saw the batmobile outside HMV in cardiff once, some boyband had turned up to sign albums. Was a bit of a backfire, even those who knew who the band were would rather stand and oggle the batmobile...

sorry but the old style is just not my cup of tea, I have only eye's for this one...
Rescued attachment batmobile.jpg

Hhhmmm, think batman was compensating when he penned that one.
Will never be the icon the first one was, though it is less camp.

I think there's a bit of a trade-off between smoothing the flow over the passenger compartment and increasing the frontal area of the vehicle.

It really doesn't help that the cockpit is so far back in the body. There is no space for the air to recover after negotiating the blockage of the windscreen/hood/whatever before it leaves the rear of the car.

Ideally from an drag perspective you want the maximum cross-sectional area of the car somewhere around a quarter of the way along its length.

Overall for a 7-shape car i'd reckon you're better off with a minimal aeroscreen just to skip the air over the cockpit area and leave it at that.

theoreticaly a roof and a rear screen that gentaly tapers towards the ground as to prevent flow detachment would reduce turbulance, a more streamlined windcreen would help too, i was planning on setting the winscreen base further forward on the car and mounting it 45 degrees from vertical and have a rear screen that tapered towards the ground which would encourage air to stay in the slipstream over the car's bodywork

matt

have a look at this car, its the morgan aero 8, headlights are a bit on the ugly side, but it shows how to manage the airflow of a car with the cockpit set so far back

http://car-reviews.automobile.com/images/cars/ArtImages/17641/01.jpg

Hmm, yes, but having had that car in our wind tunnel at work i can tell you it doesn't actually do a very good job of managing that airflow.

The problem is that you need a *very* gentle curve to keep the flow fully attached - not much more than 20 degrees is realistically possible, and you simply can't package that in the tiny length available to you at the back of a 7. Sorry. The best you can do is minimise the cross-sectional area at the back of the car, though normally people opt for the mechanical benefits of a wider track and just live with the aero penalty.

[Edited on 11/1/07 by Rob Palin]

thanks for clearing that up for me, would you say that making the winscreen at a more shallow an angle would give a better drag coefficient?

cheers

Matt

That's pretty much what I had in mind. Would like to sweep the windscreen more too, not sure about 45 degrees, think that might be too much unless you use a custom curved windscreen, not even going to guess how much that would cost. I would have the rear window slope down from the roll bar to the lip of the boot, though I wouldn't slope the back panel as I want to make a boot box for tesco bags etc and that would lose that space. Again this is a day-to-day use consideration, which would be more the reason for doing this than aerodynamics, reduced drag being welcome though of course!
Another thought, if this will reduce turbulence in the cabin your heater will be more effective too. Janurary motoring!

For a full-height one, definitely yes. For an aeroscreen it's less clear-cut since a small but steep one can deflect the air as much as a big but angled one.

Steep screens also effectively increase front lift, since the drag they cause acts at a fair height and that causes a nose-up pitching moment. On a standard 7 it's worth around 25% of the total front lift.

I am going with a different arse end on mine - hoping to smooth out the rear wings as they will be molded to the back - its actually the rear of a mgb re done. A split and raked front screen will be going on as well.
Dale

Looks interesting, that's got to smooth things out a little better than the brick shaped back end, nice styling too.

That plus a rear screen blended into the rear shape could be interesting, I bet that would cut the drag plenty.