Rear Diffuser

[rallyeash 155]

im thinking of making/having made a nice small smart rear diffuser for the 205, ive searced on here for a while but all i can see is full on ones that nearly touch the floor.

what im looking at doing is having one between the rear beam area and join up to the bottom of the rear valance somehow. nothing to serious and obvious but somthing that would help the airflow, as im sure when theres no spare wheel etc under the boot floor the rear bumper has got to have a small parachute effect. im also thinking of moving my bosch fuel pump, swirl pot etc up under the boot floor as the car still has a smell of fuel on the inside (all internal atm) and a diffuser should keep it all out the way.

 

any pics, ideas etc all welcome

 

thanks!

[tri_longer 25]

im thinking of making/having made a nice small smart rear diffuser for the 205, ive searced on here for a while but all i can see is full on ones that nearly touch the floor.

what im looking at doing is having one between the rear beam area and join up to the bottom of the rear valance somehow. nothing to serious and obvious but somthing that would help the airflow, as im sure when theres no spare wheel etc under the boot floor the rear bumper has got to have a small parachute effect. im also thinking of moving my bosch fuel pump, swirl pot etc up under the boot floor as the car still has a smell of fuel on the inside (all internal atm) and a diffuser should keep it all out the way.

 

any pics, ideas etc all welcome

 

thanks!

 

 

Have a read through Simon's project thread as he has done a full diffuser and undertray. Proper job.

 

http://forum.205gtidrivers.com/index.php?showtopic=111533

[Tom Fenton 1,541]

A diffuser needs to have sides and a certain rake to be effective, what you describe is really more of an undertray, but certainly a good idea in my opinion anyhow.

[rallyeash 155]

ive had a look at his other 205 rear diffuser project and its a bit full on for what i want

 

Tom, what im thinking is it will have sides which will tye in with the bottom of the rear valance if that makes sense?

[rallyeash 155]

what sort of angle, side height is needed to actually achieve anything?

[rallyeash 155]

somthing along these lines

[Cameron 16]

A general rule is 7 degrees for the angle, but you can use steeper angles with a good radius on the transition from flat floor to diffuser.

 

Another general rule is to start the diffuser 0.7 * length of underside if I remember my uni notes right! So if the distance from front edge of splitter to rearmost edge of diffuser is 3m, the diffuser should start 0.9m back from the rearmost edge - this ends up being just a little way in front of the rear wheels.

 

Ps.. contrary to popular belief the diffuser isn't the part that sucks the vehicle down to the road, that is pretty much all done by the floor! The diffuser (as the name suggests) allows the airflow from under the car to re-join the airflow around the car more smoothly, reducing drag and improving the efficiency of the floor. This is what gives you downforce!

[swordfish210 20]

If you're looking for a slightly quicker solution to limiting the "parachute" effect you can just remove part of the rear valance in the middle. It should create less of an air dam in there but it wont be as effective as a properly cut undertray.