Splitters? Diffusers?

[collegekid01]

So after watching the newest episode of top gear (LOVE THAT SHOW) I thought... If they can throw a plywood splitter on the front of some crappy car and improve it's lap time isn't that improving it's efficiency? So has anyone ever considered adding some sort of home made front splitter or rear diffuser like the ones on high performance cars?

[Snax]

I haven't really seen the idea of a splitter discussed much here. Lots of air dam and belly pan stuff, but I think this warrants further consideration.

Many of us have made and installed lower grill blocks to reduce the amount of air going under the car and excessive flow through the radiator, but I think coupling it with a splitter could provide even greater benefit without even really going so far as extend it down much if any beyond the stock body work.

[lunarhighway]

i slitter will work at reduceing some air going under the car but mainly at generating downforce. while reduceing under car air might cause a slight reduction in drag this air will have to go elsewhere so that's either around or over the car, and perhaps a little more air trough the radiator, but that will all take energy.

downforce will improve high speed cornering, wich is very important for setting a good lap time. it won't however mean the car used less fuel.

most racecars never run at their top speed due to all the curves so they have power to spare to overcome additional drag without loosing speed. and if this additional drag means they can go trough a corner faster than more drag might actually mean a better lap time.

a rear diffuser won't add any drag, but if the rest of the underside is relatively rough (as it is on a lot of cars) you con't see much improvement in FE

autospeed recently did a test on a honda insight with a rear undertray, wich didn't yield as much improvement as they'd hoped for.

an airdam can be a good tool to reduce under car airflow, especially if it's curved backwards to the sides.... this way you don't offer any resistance in the direction you want the air to go (around the car) and you make it easyer for some air to make it around the car rather than below it.

the best, but not always easiest thing to do is to make the bottom of the car as smooth as possible... start from the front and work your way to the back.

an undertray is a good start as it block off the rough underside of the engine

if the underside is perfecly smooth it suddenly becomes a shortcut for the air and makeing the bumper/airdam slightly higher in the center actually invites more air undermeath.

in it's finest form this can even generate a ground effect where the low pressure underneath the car actually generates downfoce... wich in turn is good if you want to go fast.


in general however raceing aerodynamics are often not a good starting point for aero mods... there's a lot written about them and a lot of aftermaket stuff available, but generally none of these things will help you much in improveing your FE.

[theholycow]

The autospeed information was actually pretty easy to find.

http://autospeed.com/cms/A_110872/article.html

In it, they link to some other modifications they've tried. Also, they index DIY mods at http://autospeed.com/S_10/cms/section.html and include quite a few interesting ones.

[Snax]

Well here's how the theory behind aero improvement from a splitter breaks down regardless of downforce.

In essence the front of the car creates a pressure bubble based on the leading edges and form of it. That pressure bubble has a peak leading area relative to height of the leading edges of the car. A splitter can effectively lower the height of that peak peak pressure area, forcing more air over the top of the car and around the sides. So it essentially works like lowering the nose of the car without actually lowering it, vs. an air damn which really just broadens the pressure area by lowering the frontal surface, also reducing airflow underneath.

Combined, they work really well, but they both accomplish similar results.

[collegekid01]

Those are both good points, it would make more air go over the top of the car rather than under, but I could see how maybe an under tray would be more effective at adding mpg. I will say that I think the rear diffuser would be better for mpg because it helps smooth out the air coming off the back of the car. It's like vortex generators for under the back of the car! I think there could be something there.

[GasSavers_Pete]

Originally Posted by lunarhighway

downforce will improve high speed cornering, wich is very important for setting a good lap time. it won't however mean the car used less fuel.

most racecars never run at their top speed due to all the curves so they have power to spare to overcome additional drag without loosing speed. and if this additional drag means they can go trough a corner faster than more drag might actually mean a better lap time.

Dead right.

The Porsche 917 turbo had more downfoce devices than enough and 1 , 000 + horsepower BUT is also had the aerodynamics of a house brick.

Even VW humble Kombi scored better in the Cd stakes.

Pete.

[Snax]

To that last point, it is generally considered 'free' downforce in the racing world to use aerodynamic aids on the front of the car (so long as frontal area is not actually increased and turbulence is not increased). In other words, without changing the frontal area, you are merely altering the airflow over the various surfaces without increasing frontal pressure. So an airdam can technically increase frontal area and drag if it is too low, whereas a splitter generally does not.

[collegekid01]

yeah but what about the rear diffuser? that isn't creating downforce is it? it's just smoothing out the air flow coming from under the car.

[lunarhighway]

in theory a diffuser is a good instrument for decreasing drag but a recent autospeed article showed poor results with an add on diffuser on a honda insight

http://www.autospeed.com/cms/A_110872/article.html

does that mean a diffuser is not working, i don't think so, the insight is a very optimized car so maybe the airflow underneath the car is managed so that a diffuser doesn't make a significant impact.

but the fact they had much more effect with a front undertray seems to sugest you should treath some aero aspects front to back... once the air gets turbulent the damage is done and an undertray at the back seems to make little difference...

so my advice to improve under car aero on any car is to start at the front and work your way to the back...