panelling under axle/ rear diffuser

[Not Anumber]

Ive read claims elsewhere that fitting a rear diffuser reduces drag and noticeably increases top speed and ecconomy by cleaning the airflow under the car.
As far as I can make out these diffusers are a sheet of metal or fibreglass with fins.
Does anyone have any views on whether this is the case and if so could the same be achieved by fitting a flat ally panel under the rear axle and tank so that the floor is effectively extended right to the back panel ?

[Oscar]

Adding an ally panel would definately improve the performance because otherwise the rear tub acts like a parachute!

My understanding (from a university project to design a track car) is that a diffuser has fins to create vortices and actually sucks the car down to road. If these aren't designed properly using a vastly overpriced flow analysis program you can end up with crazy results: acting like an aerofoil!

If you are going to put a panel under the back end (I'm going to) then i believe you need to have the filler pipe opening for the fuel tank external (not contained in the bodywork) to prevent gasses building up and, even worse, failing the IVA!

Hope this helps, no doubt someone will be along soon to give another, more experienced, viewpoint.

[alga]

The tub bottom will not be fully sealed anyway, there will be large holes for the bottom wishbones.

[Oscar]

Sure I saw someone on here a while back saying how the IVA men didn't like a filled bottom,

Seem to remember that if there was a plate, even with gaps, then they want an external filler point. Probably depends on tester if they deem the wishbone gaps enough for their requirements.

I'm going with external filler anyway so won't be a problem for me

[Davidbolam]

Saturn did a rear diffuser. When Andy fitted it to his demonstrator he said the results were very good.

the diffuser would act like an aerofoil but if anything you would get downforce and not lift.

They also look good and finish the rear of the car nicely.

[snapper]

The diff gets suprisingly hot with spirited driving or long distances, not enough for and emectric pump and diff cooler however I would suggest looking at air flow around the diff before you box it in completely.
Airflow down the tunnel tends to be hot engine bay air

[Not Anumber]

Thanks, some really good information there. Thats convinced me to do it come the warmer weather as i dont have to steer round the IVA requirements. I'll look to build in duct to flow some air in front of the diff.

[vmax1974]

The rear defuser on the saturn car was only designed to stop the rear panel acting like an airbrake not to give any downforce

If you wanted downforce you would have to taper the fins from narrow at the front and wide at the rear with no lip between the defuser and the rear panel

You would also need to add a lip to the front of the defuser to start the airflow circulating towards the defuser

The fins then speed up the air passing through them creating a vacuum which pulls the car towards the floor
But if you are going to go this far you should also think about unloading the pressure build up in the wings by venting

And maybe disturbing the airflow going over and through the front wings to help reduce the oversteer

Just my thoughts