Diffuser question

[NigelO]

I'm considering one of Carbon NV's rather lovely carbon diffusers for my SEiW, purely in the interests of aerodynamics some more carbon bling....

I know that the main purpose is to reduce the "parachute" effect of the rear section, but I was wondering if it will simply create a "balloon" effect with the airflow down the transmission tunnel entering the rear body section above the diffuser and then not being able to escape as easily as it would if there was no diffuser there

Any thoughts?

[Dave Eastwood (Gadgetman) - Club Chairman]

It can.

The main approach people seem to take, if they do anything, is to either drill Speed holes () with a good size hole cutter in a row across the back of the tub, above the diffuser. (Mainly track nails, and outright competition cars, to be fair).

Or, the really neat alternative I’ve seen a few do, is to space the rear of the diffuser down a little, so that there’s say a half inch gap between the bottom of the tub and the top of the diffuser. It looks discrete and neat, that way.

To my mind, the better way, would be to give the under floor area the proper treatment, and panel in the tunnel from below, along with as much of the engine bay as possible, aiming for as flat a floor as the lowered floor pans, if you have them, allows.

At the same time, you could borrow Caterham approach to heat managmenent in the tunnel, with the big Duratec engined CSR’s, and fit a closing plate either side of the tunnel entrance in the engine bay, that closed off much of the gap between gearbox and tunnel - they don’t have to be anything like a perfect fit to the box/bellhousing etc. you’re just aiming to limit the air getting past.

That way, you avoid the balloon effect completely, and at the same time feed somewhat smoother airflow into the diffuser.

One point I’d make though, is that the Seven shape is very prone to front end lift, in excess of rear end lift. (I’m not for one second saying with simple mods alone, you will achieve downforce, but you may well reduce lift!!) just be careful that you don’t make the proportion of lift front to rear worse than it already tends to be.

Personally, I’d always want to add the front end splitter to the nose cone, before adding a rear diffuser, in the vague hope it might keep the balance, somewhat! 

[IanD]

I have recently fitted one of these all be it on a SE Narrow.

Lovely bit of kit can recommend it.

As for its effectiveness, Early days but will be tidying up the air flow at high speeds i am sure so in my eyes must at least reduce a bit of drag.  looks good too. 

I have fitted mine with a gap as Dave describes for the very reason that you are concerned about and so far seems ok.

[Terry Everall]

M<ine is also fitted with spacers like Ian D

[Simon Waterfall Syman84]

1 hour ago, IanD said:

I have recently fitted one of these all be it on a SE Narrow.

Lovely bit of kit can recommend it.

As for its effectiveness, Early days but will be tidying up the air flow at high speeds i am sure so in my eyes must at least reduce a bit of drag.  looks good too. 

I have fitted mine with a gap as Dave describes for the very reason that you are concerned about and so far seems ok.

Looks great @IanD might have to get a close look at Anglesey 

[maurici]

That is a quite repeating topic… that to be hones was solved in an experimental way once for all several years ago when a former sprint competitor and engineer, well known for some of us, took his caterham to MIRA’s wind tunnel.

The session wasn’t quite enough to actually find the magical receipt for a seven shaped car, but drawn quite a lot the lines to follow in the future.

To add up to Dave’ comments, From memory (I’m too lazy to search the link) those were the outcomes:

 

There is only two ways to improve the aero of seven shaped car:

 

Reduce front lift.

Reduce overall drag.

 

Guidelines for front lift limit:

Reduce AS MUCH AS POSSIBLE the amount of air going straight under the nose: IE Splitter.

CSR style front cycle wings were beneficial to reduce lift straight from the wheel.

Front canards around the nose. In fact, some somehow big front wings were done… and at the same time were diverting the air off the turbulent area (wishbones and shocks) so it helped with the drag.

Mudflaps should be avoided by all means to avoid front lift

 

Gidelines to reduce Drag.

Front cyclye wings as near as possible to the tyre.

Side doors/half doors (depends if you are in windscreen or not)

Windscreen off

Reduce positive pressure in engine bay, wich it means flared sides, limited underside air entrance and NO V8 scoops…

Full flat floor (includes sealing transmission tunnel and engine bay and rear tray)

 

From memory too, I remember that a rear finn difusser didn’t make ANY difference on lift. It was tested in loads of different positions, heights and car rake… It was a reason for it that I can’t recall it… Probably the nature of the full turbulent air going through the car, doesn’t helps to have good quality air in the rear undertry. And if placed there with the wrong slope, or without being spaced, yes, it creates a parachute/balloon

 

Then… going back to the original topic, Just put it there. You won’t notice it anyway… and carbon looks soooo sooo nice!!

[Davemk1]

I mounted mine with a 15mm gap at the rear end no so much to allow air to escape the fuel tank area but to bring the roof angle of the diffuser down a bit more into the acceptable range to maintain attached air flow.

dave