Flat undertrays and diffusers - an undertray's beneficial from the point of drag at pretty much any ride height AFAIK, but is there any potential to reduce lift in a road car thats a) at a normal fast road use type of height and b) not going horrifically fast? I suspect the answer may lie in looking at the bottom of a tarmac spec rally car.
Teh effect of a flat underside is twofold: first effect is that the air is less perturbated and and such you get higher mileage and a higher topspeed.
The second effect which on normal raod cars does not apply (OK: some of the latest Ferrari's have it) is the venturi-effect. The flat undertray is at an angle between 4-7 degrees to the road surface. As such the volume increases from the entry point (the front) to the rear, this created a slight vacuum ( a few millibar below atmosferic pressure which in turn "sucks the car to the ground (downforce = surface undertray x pressure drop)
It doesn't work on a road car because of the free height of the car which allows air to be sucted in from the sides. It worked very good on the side-skirted Formula 1 (the so-called ground effect cars). Circuit racer (free heigth less than 8 cm have some gains too)
Now the rallye-car: they have a flat undertray too and they benefit a little from less turbulence under the car. There is no groundeffect: the rallye car just stand too high.
Their undertray serves two other -specific to rallye- aims: protecting the underside and the mechanical parts. Last WRC I saw had even moveable protection around the triangles but then again the abuse throw at those cars is horrendous. Secondely the flat undertray (4 mm steel or 8 mm aluminium) lowers the centre of gravity as much as possible and is a solid thing to attach ballast on.
Tom De Moor
errr yeah what tommy boy said :)
"Doki" wrote in message news:467423f6$0$27859$ snipped-for-privacy@news.zen.co.uk...
How fast are you planning on going? What car is it for? What do you actually want to achieve?
Fitting a flat floor will almost defiantly cut drag but not necessarily improve downforce/cut lift. there are a few road cars that run flat floors with diffusers but these are really quite rare and held for supercars (Think Porsche Carrera GT and Veyron) & track day stuff. The problem being that to act in a way to produce downforce they need to run lower pressure under the car than over it. Now the air flowing over the car has to travel faster than the air under the car as it has further to travel. Faster air flow causes a drop in pressure just like in carbs, this is party countered by the tendency for air to come in and help to equalise the pressure difference and areas where there is a slower flow (where things like windscreens come down to the bonnet). Lowering the pressure under the car can be achieved by sloping the flat floor so that it is higher at the back than the front but you still have the problem in a car that is at normal road height of air flowing in from the sides of the vehicle to fill the "gap". This is of course why skirts were introduced but they wouldn't be practical on a road car, there is another option and that is to use dive planes as vortex generates on the front bumper, you can then create a virtual skirt made from the tip vortex (if you think of the left side of your car and a vortex like @ travelling down the gap between where the car stops and the road the right side would have the vortex going the other way). You can add to the effect and improve front end downforce by reducing the amount of air that can get in under the front bumper, fitting an air dam (extends the bumper vertically down) is effective but again impractical on a road car but you could fit a splitter (a horizontal extension at the bottom of the bumper) as this traps the high pressure air that the snub nose of the car creates as it travels through the air and stops it from spilling under the car, this can also help the dive planes to function more efficiently. Splitters have a very low drag effect and if configured correctly when combined with a flat floor the dive planes can actually improve drag as well as increase downforce. Though you will look a bit of a plank unless you have some sort of race car. You would do better on a cost/benefit basis with a roll of race tape and go round all the panel gaps with it, this is also instantly removable so insurance frendly.
If you're going to have a flat undertray make sure you have enough downforce to keep it on the ground ;-)
Or of course, a Midas Gold.........
Or a Rossa K3. like my old one, smoothhhhhhhhhhhhh underneath.
No diffuser though. But mine didn't have Gordon Murray's input like the Midas.
1) Dunno. 2) My golf. 3) A noticeable reduction in drag would be nice.
It's all a bit theoretical and unlikely for the time being. Really wanted to know whether it's a worthwhile idea *before* doing a load of research.
Ta.
The drivers must have had some stones to drive *them*, if the side of the car lifted for any reason, it was bye bye ground effect, bye bye grip, bye bye world lol
That's indeed why the skirts were banned, why a minimum ride height was issued and why now the high tech, multi-million dollar Formula 1-cars have a low budget piece of plywood bolted under the undertray.
If the plywood is scared or used up, the car is question is disqualified. SChumacher was on the recieving end for that reason (iirc at Spa).
Tom De Moor
Exhaust in sills to give flat floor.
McLaren F1 GTR had full venturi underbody ground effects including a fan.
These days every man his dog has flat underbody. Honda Insight. VW Golf has plastic underbody protection that makes a large part of the underbody flat. Even agricultural vehicles.
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