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Rear diffuser - Worth it?

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Hi,

So, i'm debating with myself if to get a rear diffuser or not for my megabusa.  Is it really giving any useful downforce at the relative low speeds our cars are cornering or is it mostly cosmetics?

I understand the theoretic behind them quiet well engineer as i am and like the idea, but.. something tells me it's more for +200km/h cars where you actually might get some use or aerodynamic at all.

Any pros or cons? 

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I've got the Factory fibreglass one on my FW rear and accept it's cosmetic.

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Definitely keeps the diff/rear suspension cleaner if nothing else!

I'm sure they must help with aerodynamics at any speed tbh,the rear panel with no diffuser fitted must surely act like a parachute to a certain degree with air going up into it?

@Davemk1 recently fitted one,he noticed improvements instantly with less noise :)

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Look at a formula 1 car and if you can create anything like it with the ability to drive it 5mm off the ground and you have flat boxed in undertray , frontal aerodynamics, wind tunnel tested then you should be OK. Otherwise, fitting a spoiler to a brick, still makes it a brick. As Kev says , it will keep the diff cleaner.

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about adding spoilers to the brick... please read what Tim seipel  found. I'm happy to ask him a few more things as the description of the post was brief...

 

Last time I spoke with him he confirmed that other than reducing drag avoiding the parachute effect of the rear pannel, the diffuser does nothing.

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I have one fitted on my GBS Zero.  It is purely cosmetic.  GBS took a Zero to the wind tunnel at Mira and said it made little or no different to the aerodynamics of the car. 

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Interestingly, after I fitted mine it got pulled down on the motorway and fell off. 

Not sure if it was the diffuser itself producing 'downforce' or just air coming down the transmission tunnel, but it definitely got sucked downwards...

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19 minutes ago, AdamR said:

Interestingly, after I fitted mine it got pulled down on the motorway and fell off. 

Not sure if it was the diffuser itself producing 'downforce' or just air coming down the transmission tunnel, but it definitely got sucked downwards...

you probably forgot to attach it to the body. dont lie... and of course, it fell down because gravity

**Editted**

Now, being serious... probably was positive pressure in the bootbox area due the air being channeled trough the transmission tunnel and not securely fastened, so it blew the weaker "wall"

However... to have positive pressure inside few bodywork seccions, it only means that the air is being trapped, nothing else.

When you owned the car (even yet now) the rear diffuser was way too high and the fins are way to short to have any other effect than act as a undertray... 

 

Very different is whith the front splitter... as it really limits the ammount of air that is going under the nose... not generating anything but reducing the ammount of lift of course.

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3 minutes ago, AdamR said:

Interestingly, after I fitted mine it got pulled down on the motorway and fell off. 

Not sure if it was the diffuser itself producing 'downforce' or just air coming down the transmission tunnel, but it definitely got sucked downwards...

Funnily enough, the same happened to me, first with an aluminium one of my own design and then a Playskool fibreglass one. It wasn't on the motorway, but on track, in sprint meetings. The last time was whilst accelerating down the back straight at Llandow doing between 80 and 100 mph. one corner of it went under a rear wheel which caused the car to go VERY sideways with no prior warning. A bit of a brown trouser moment to say the least.

If you want one for cosmetic reasons, then fine, but make sure it is securely fixed. If it is improved performance you are looking for, I just cut 3 or 4 holes in the rear panel, to reduce the parachute effect of the rear tub.

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I see that this thing of forgeting fixings is a widely extended issue within Kit Car owners...

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18 minutes ago, maurici said:

I see that this thing of forgeting fixings is a widely extended issue within Kit Car owners...

Weight saving.

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As mentioned above I fitted one and I noticed that there was less air noise from the rear of the car. The shape of the underside of the car without the diffuser is like a parachute and there is no way that air isn't being sucked up into the void around the diff and gas tank.....it's a huge open area and even if you don't look into it every day it's there and messing with the airflow.

I'd be shocked if any kind of diffuser or simple cover in the rear didn't lower drag....realistically how could it not reduce drag? Does it produce downforce? Maybe, maybe not. If it does anything it might reduce lift some and I'd be surprised if it produced any negative lift at all. Pretty hard to tell without a controlled test in a wind tunnel. That said I'm fine with the idea of the reduced drag that seems likely.

As for fussing with the aero on these cars - I hear many say that "once a brick, always a brick" or that doing aero work is like putting lipstick on a pig. While I get what they are saying I disagree. It's is of course correct that nothing we can do with these cars will make them slippery as a bar of soap or allow them to generate 3g's of downforce......but just because they can't be turned into F1 cars doesn't mean that there aren't real and substantial gains to be made with simple changes. The very fact that they are so bad aerodynamically means that small, but real, improvements are easy to come by. Low hanging fruit as they say. No one would say that taking off the windshield and fitting an aero screen doesn't have real benefits in terms of drag and top speed....why wouldn't similar simple changes such as running a tonneau cover or a rear diffuser also give marginal gains. If one does pragmatic and practical changes to reduce the HUGE drag these cars have then I'm sure that the "accumulation of marginal gains" thing kicks in and the car will be faster and easier to drive. As long as one doesn't add so much weight in the process that it negates any aero gains all should be good. I made a simple and small front splitter, enclosed the engine compartment with simple carbon panels and fitted a rear diffuser from Carbon NV which gives a flat and enclosed bottom from nose to tail and it added 6.2 pounds (2.8kg) to the weight of the car.....or the same as one gallon of petrol.

Since adding that weight I put the car on a further diet and have pulled the equivalent weight off of other areas so the final weight is unchanged from last year's race weight but I have a fully enclosed floor. Will it produce downforce? Probably not but I'll be shocked if it doesn't reduce lift and drag some and that my lap times will fall by just a little bit. It might still be a pig but it's got nice lipstick and it's a bit trimmer and that makes for a faster pig.

dave

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Its produced at least an extra 2.8kg of downforce :d

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@Davemk1 cant agree more with your comment. 

This is basically the same sort of stufd mine has done. The splitter in the front makes a very noticeabke difference (and i found it back to back when i crashed in anglesey and i sort of ruined my splitter) in front lift.

Csr style cicle wings... have a good effect to reducing the lift straightforward to the wheels.

Enclosing the bottom of the car (engine bay and differential area at least) have a substantial drag reduction gain.

This sort of rubber flaps that popular in the front cicle wings are a lift creator and should be deleted for any competition use (i recently learned that).

Canards in the front nose, deflecting the airflow away from the wishbones and suspension also makes a good difference, but the truth is that when the air has entered in contact with the car... unfortunately the turbulence is that big that there is nothing you can do to handle this air...

The right thing to to is manage the air before it hits the car. And that is why the f1 appart of having thw big wing in the front to create downforce... have loads and loads of appendix and canards and deflectors all around. They are not intended to create downforce... they are only so handle an manage where the air is being deflected.

 

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Look at a formula 1 car and if you can create anything like it with the ability to drive it 5mm off the ground and you have flat boxed in undertray , frontal aerodynamics, wind tunnel tested then you should be OK. Otherwise, fitting a spoiler to a brick, still makes it a brick. As Kev says , it will keep the diff cleaner.

Yes, these are my thoughts too, just need some opinions here:)

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