75 Rear diffuser project

[Mezevenf]

Now that I have molds for my bumpers etc, I'm looking at getting some more subtle aero happening. I'm doing up a splitter for the front and now designing a rear diffuser, plans are basically just to copy the APR guys stuff for the evo's and STi's and the like and make something similar for the 75.

Examples




Now I know a lot of the racing guys just cut out the rear bumper to stop it acting like a parachute but this is destined for my Evo rear bumper so I'm being a bit more vein here and looking to get some more downforce in the process. I'm not sure if I'll need some upward angle toward the windscreen to stop the parachute effect or not.

Any ideas or comments very welcome, I'm open to criticism.

[Duk]

Great idea Bernard . Aerodynamics play such a big roll at speeds, especially at the rear of the car.

Having the defuser rake upward like the STi 1 does in your picture is probably the ideal way, but how do they blend in with the underside of the car at the front, behind the diff/rear suspension?

Are you keeping the spare wheel well?

Probably not the most helpful, but I think if you're going to keep reduce the parachute effect at the back of the car, then I'd say remove the spare wheel well to make the boot (trunk, to the collonially challenged ) floor, have a large outlet (I know not what your after) and then try and fashion a defuser like the 1 in the pretty pictures.

Keen to see this unfold

[slyalfa]

I would also try to move some air over/away from the rear breaks/gearbox
as there is a (-) air pressure there you should get a lot of flow there so you might as well use some of the vacumed air to remove heat from the breaks.

[jazig.k]

a rear diffuser is a big task to get woking efficiently. and it wont be easy without a wind tunnel or some very very good aero infomation of the car your making it for [same height, body and exhaust placement].
im presuming you understand how downforce is generated on an aeroplane wing. a car can be looked at like an upside down plane wing. you need a lower air pressure under the car than over the top, pushing the car down to the road.

understanding a diffuser is the first step. the idea of it is to help the air flow more smoothly[minimal turbulance] to join the rear air flow [coming from the sides and top of the car]. smoother air flowing from the rear means the air flows under the car faster[shorter in the path of travel instead of bouncing off the underside parts like the diff, wheels and brakes].

but before the air reaches the diffuser it should be minimised [air getting under the car] at the front of the car and the side skirts [skirts arnt just rice, they do have a purpose you know].
now you cant just block all of it. you wouldnt be able to cool the engine or the transaxel, but the down force would be high. so you want what ever air that does enter, to enter smoothly, and neatly, but only enough to suffeciently cool the car. not living in a perfect world there will be excess so make it work with you.
ideally air only enters from the front and exits at the rear in a straight line, and side skirts stop spillage [air 'spilling' from the side of the car into our low pressure zone under the car].

so far what iv been TRYING to explain is that the whole car works together, and not any diffuser will work with any car. so changing the front bar [EVO, EVO with your little add-ons and stock] will change the flow under the car, effecting the way the diffuser works.
i can help you with more details if you want do it.
i highly recomend a book called 'competion car downforce'. accually relates to the 155 touring car, and how flaps in the vents control how much air enters and cools the car [close them on cold/rainy conditions for higher downforce].
and another 1960's 12pot alfa powered racer, very tricky team when it came to bending the rules for extra downforce [used a underside fan to suck up air to cool the engine AND reduce the air pressure under the car] because the class didnt allow mechanical devices that generate donforce [this one served another purpose so was passed, but only for one race].

anyway, hope you work out something.

[Mezevenf]

I wasn't planning on just making a basic copy and leaving it at that Jazig, but just as a general idea.

I have a sort of basic conception about how to make an effective diffusor and I'm planning on a lot of research and testing before I'm happy with it, and also going to be toying with a full undertray.

I would like to talk to you some more about it if you have time and any ideas on the design, but I wont be testing this on my car until my Evo kit is on. The little add-ons at the front I likely wont be using as it makes my car stupidly low and still lets a lot of wind through, but it would be more ideal for someone with perhaps an intercooler and a higher ride-height.

For the transaxle and rear brakes I was planning on using pipes to deliver nice cold air from the sides of the car, either just under the skirts or via the rear side doors. As for the front brakes, I was considering ducting air from the front bumper toward the front brakes, but perhaps I could use this for getting some airflow to the engine as well which would allow me to let less air through underneath, but these are all just ideas. Thats why this thread exists

Believe me, I don't just want some wanky crap that looks the part.

PS Get my email's regarding the splitter Jazig?

[Mezevenf]

Max 75 posted this up on the picture thread... I spot diffusor! Anyone got any more pics of this car?

[Ziggy]

No more pics, but I made the wings, front fender and the rear defuser.....
There are pictures of the car on here if you look at past races..
ARCA - alfaracer.com - Organisers of the UK Alfa Romeo Motor Racing Championship. The place for Alfa Romeo Motorsport News

[Mezevenf]

Quote:

Originally Posted by Ziggy No more pics, but I made the wings, front fender and the rear defuser.....

[gtv27]

cool idea.

I think the evo and sti stuff is more pose than real - ie how turbulent would the air that passes under the car be ? Also the F1 diffusers have a much higher angle on them. A full undertray (?) is probably going to be required for 'real' downforce, but maybe the diffuser could help clean it up a bit and reduce drag?

[sh0rtlife]

i agree imo to get any real effect from it your going to need to be scraping the ground or have a FULL belly pan

[Mezevenf]

Sorry, forgot to mention full underbody tray is also in the works, but it needs less 'design'.

[BigSwede]

Maybe I can have a go at this as well without sounding too much of a professor/toe stepper...

As stated previously the whole underside works as a unit, just slapping on a diffusor will probably not do anything other then retaining the heat around the gearbox and brakes.

What we have to understand though is that the diffusor itself does not produce any downforce, it's basically there to bring up the pressure back towards ambient so that there is less of a wake behind the car. The part that actually produce the downforce is the flat undertray. To be able to produce any downforce however, it is very important that there is a mass flow under the car or there is nothing to work with.
You do not want to seal off the front end, that will effectively delete any downforce that the flat bottom would produce. If you have seen a sportscar prototype doing backflips this is usually how it happens, front end bottoms out and suddenly there is no downforce at all.
The same applies to ride height, low but not too low. Maybe 50-60mm or so at the front with a 1* rake (higher at the rear) is a good starting point.

The diffusor should be about 6* additional kick up from the undertray/floor and have a very smooth transition from the floor to keep the flow attached. Concave corners in the diffusor shall have a generous radius (to the sides of the diffusor) but the lower longitudinal corners (if present) should be sharp.

Some kind of edge around the outer edges of the floor by the rockers is also good to keep the leakage from the sides to a minimum, really only the ground clearance rules set the limits here.

Anything more fancy then this should be considered too advanced to understand without massive experience and/or simulations + wind tunnels. Strakes in the diffusor may look cool but if you can't verify the result you may have done it worse...

[Mezevenf]

Would having a higher ride height make much of a negative effect? Would it make the whole prospect completely useless?

I'd be keen to see how the blue race car above has done it, and at the very least I will definately be putting a splitter on.

[Duk]

Is this intended to be a dedicated racecar or a road/track car?

[BigSwede]

It would probably produce less downforce but you would still reap the benefits of the lower drag. I would still definetly do it, will probably be like taking off the ski rack from the roof (but you wouldn't konow anything about that would you?)

If you think it's strange that I keep writing "probably" you should know that it's because it's not always logical when it comes to aerodynamics, sometimes you get the complete opposite result because there is something else in the system that comes into play.

Aerodynamics at this level is all about "Trial & Horror", very time consuming indeed.