Possible rear bumper mod

[Staska]

Possible rear bumper mod.

If one could remove trunk for spare wheel and bottom part of rear bumper and connect it with bottom cowering plate - it should be efficient and less costly than roof chopping. It would be ideal for sedans with long rear.

[Frank Lee]

Can't have much rise before flow separates there, rendering the effort useless.

[Staska]

Of course you are right. 14-15 " wheel is about 20 in diameter with tyre. So we could have bottom partial kamback 20" length. It is more in length than more of top kamback..

[Frank Lee]

That red curve is too steep and I'm quite sure wouldn't represent an aero improvement over stock.

[Staska]

Ok. That kind of slope in degree we would use ? 10-15 or less ?

[brucey]

4 degrees is what the number is around here for the bottom.

I think it's 10-15 on top.

[Piwoslaw]

Originally Posted by brucey

4 degrees is what the number is around here for the bottom.

That depends on the chassis shape: A fastback or sedan has lowest aerodrag (Cx) around 3°-4°, but a squareback (like the Fiat Idea you used as an example) is better off with a flat rear(*).

(*) If the car already has a Kammback, then it is no longer a typical squareback and a slight diffuser angle may be OK.

On the other hand, race and performance cars have more aerodrag Cx and less Cz (= more downforce) when the diffuser angle is greater than 5°.

Either way, getting rid of, or covering up, a spare that hangs down in the air flow is something to think seriously about.

[Tango Charlie]

That's usually where the muffler is hanging out, too.