rear wheel skirts...

[Nautilus]

Originally Posted by Daveedo

As for the undertray it seems as though the amount of benefit is dependent upon how high your car is and how rough the underbody is. If your car sits low to the ground already then the undertray may not be that effective. However, the higher it sits and the rougher the underside then it may have more of an effect. That's what I gleaned from reading a VERY long thread on the subject. If I misinterpreted it then by all means let me (and everyone else) know.

PS When doing your aeromods try to do the ABA testing so you have before and after mod baselines for comparisons. Thanks.

I've tried to cover as much as possible from the rough parts of the underside with a tray on my car. Less for mileage improvements, although they are, but for a better handling at speed.

Original: small undertray before engine, entire lower side of the engine bay open to air, floor mostly flat from behind the front axle subframe to rear axle, save for the exhaust tunnel.

Afterwards: full undertray from a Seat Leon 1.9TDI, from front bumper lip to behind front axle, two Coroplast additions to seal entirely the floor before front wheels, aero shaped deflectors in front of the wheels patterned on those from a SLR McLaren, rear undertray from rear axle to the middle of the spare wheel well, two small undertrays behind rear wheels to connect the bumper lip to spare wheel well, two boat-tail fins behind rear wheels.

Patterned everything on factory undertrays from economic version of Skoda Octavia - green in the picture. Did not use the undertrays in the middle of the car, between axles, as the floor is mostly flat there.

Most aero mods have minor effects, but they can be felt. First, fuel mileage improved visibly when running long distances at steady speeds. Then, wind noise diminished. Third, handling was slightly better, car is more steady in a turn or at speed.

[freebeard]

Autospeed report reduced stability with the McLaren-style wheel spat.

Most aero mods.... Third, handling was slightly better

Better overall, or specifically with the wheel spats?

...two boat-tail fins behind rear wheels.

May we see this?

[Nautilus]

1. There is some small amount of front end lift. Moving non-essential parts from front to rear also lightened the front and contributed to lift.

But, as Dick Shine proved with his track Golf Mk 4, some front lift balances better the nose-heavy chassis of the Golf Mk 4 / Seat Leon Mk 1 / Skoda Octavia Mk 1 / Audi TT Mk 1. In fact, Shine Golf Mk 4 is raised by the front a ridiculous amount, maybe 2 inches taller than rear. Something like this can't be done just with aero mods.

2. The more aerodynamic a car is, the more vulnerable to side winds at speed it will be. Aero vortices around "brake" the car in a straight line, but they also "brake" the side to side movement in the wind. Track cars have miserable drag, Cx over 1.0, they are designed for downforce, not straight line runs.

In a road car with just undertrays, spats before wheels, sealed body gaps and sealed non-functional holes in the front, instability in side winds is too small to matter. It takes a bodywork like a Tatra T77 to become dangerous.

[gumby79]

You said track days ... what form? Makes a difference on what is good V bad. Like everyone has stated test test then test some more. Tuft tests to determine if it is a spoiler or a factory aero mod. If your into drag racing then converting to a Bonneville spoiler could be an option for down force and aero+. I saw many last time to the drags in Utah.
If road corse then any down force at any cost may be worth the inproved times look at time attack cars they use this principle.
Aerodynamic visualisation fluid( clay disolved in water) will also show the points were the air separation is happening and then were reattachment occurs... like how after a snow storm your car is covered in road spray that almost always has the same patterns .
Aba on a set corce preferably 3gal or more to make the filling discrepancies (.001-.01) a smaller % value (makes for less noise in your results). If posable data log through a Obd2 dongle device. Useful at the track too.

[aerohead]

Originally Posted by freebeard

Autospeed report reduced stability with the McLaren-style wheel spat.



Better overall, or specifically with the wheel spats?



May we see this?

The 1992-98 McLaren F1 had intentional 'lift' designed in.It was only to be driven by veteran race drivers and had straight-line speed advantage.
A long-tail version of the F1 was also raced and it produced downforce as a concession to speed.
The Kamm-designed 1953 Cunningham C-5RK couldn't be matched for speed at the 1953 24-Hours of LeMans.It led the entire field on the first lap but it didn't necessarily handle the best through the curves and didn't finish the race.
Stability is a quirky subject,with lots of conditions and caveats.
Today,all super-high-performance cars are 'computer-controlled' with active aerodynamics engineered in to balance speed vs stability.

[aerohead]

There's probably some data in the seminars.
The 1938 Adler-Jaray was tested for skirts:
* no mods = Cd 0.38
* front skirts only = 0.37
*rear skirts only = 0.34
*front and rear = 0.33
-------------------------------------------------------------------------
At MIRA,the Audi A2 was tested with rear skirts.showed a delta-0.001 drag reduction ( on a car with normal Cd 0.288)
--------------------------------------------------------------------------
Hucho says that the better the flow leading up to the rear skirts,the more effective they become.No truly-low-drag cars are without them.
Wheelhouse volume is going to affect them.
Underfloor condition will affect them.
Presence of 'taco' fairings will affect them.
Ditto actual wheel fairings.
Or combination of any and all.
Very conditional !

[Nautilus]

Stability has also been improved by fitting Yamaha Body Dampers front and rear. They take away the vibration which escapes filtering by suspension.

[JulianEdgar]

Originally Posted by aerohead

The 1992-98 McLaren F1 had intentional 'lift' designed in.

None of my references support that assertion. In fact, the car had measurable downforce - 136kg at 240 km/h.

I do wish people wouldn't post stuff that isn't true.

[aerohead]

Originally Posted by JulianEdgar

None of my references support that assertion. In fact, the car had measurable downforce - 136kg at 240 km/h.

I do wish people wouldn't post stuff that isn't true.

Take it up with McLAREN.

[JulianEdgar]

Happy to quote my references. What are yours?

But honestly, to believe that the McLaren F1 had 'intentional lift designed into it' is absolutely absurd. As soon as I saw that had been said, I knew it was wrong - and it took less than 5 minutes in my library to prove it.