NASCAR roof flow fences

[GasSavers_Otto]

NASCAR racers use those roof fences that are ~3/4" high and run front to back on either side of the roof, just above the windows.

What is the aerodynamic purpose and advantage of these mods?

Any point in adding such to a street car, operating at typical highway speeds?

Such could easily be made out of clear polycarbonate corner guard, as sold at Home Depot for protection of house drywall corners. (BTW, the plastic corner guard stuff makes great vortex generators, if cut into small segments and adhesively attached. Homebuilt airplane folks use them for enhanced wing lift.)

[cfg83]

Otto -

Ford Fusion
http://www.gassavers.org/showthread.php?p=68880

Originally Posted by Graeme

The fences on the roof of NASCAR cars is to reduce lift if they drift so the air is coming from the side. It "spoils" the lift. Those, coupled with roof flaps, make flying NASCARs the exception rather than the rule.

Soooooo, I don't think they help with your aerodynamic coefficient.

CarloSW2

[Drag Limited]

I dont think he was asking about the roof "flaps", rather the 3/4" "railing" running front to back on either side of the roof.

[Drag Limited]

here you can see both of them on the roof on the left and right.

[lunarhighway]

the old opel ascona ralley version had something similar on the hood...
i'd be surprised if it would aid FE. isn't this also something to spoil the lift in cornering to prevent the vehicle from flipping over?

[GasSavers_RoadWarrior]

If the side of the car is aerodynamically messy then it may make more sense to guide the air over the top of it, stop it spilling round the sides. Wonder if you could keep it out of the front wheel wells better by having a fence on the front.

I'm pondering some ideas for modifying the roof rack rails on Marvin. I want to keep the rack, has come in useful dozens of times. Kind of thinking about closing in the insides with a rubber flap.

[GasSavers_ALS]

They disrupt the boundary layer as the car is sliding sideways. All down force is lost when the air flow moves to side to side. The roof acts like an airplane wing and produces lift. If you have follow NASCAR for any length of time you will notice that the problem of cars going airborne while sliding sideways has all but disappeared. That problem has been greatly diminished since the roof flaps and the metal ribs running the length of the roof have appeared.
The roof flaps are there for the same reason. When the car is moving in reverse the spoilers are absolutely useless. The roof flaps deploy to break up the airflow over the roof. If left unchecked that air flow would produce lift passing over the roof like air over a wing. At the speeds they are traveling the air flow would literally lift the car off its wheels.

[donee]

Hi All,

So the NASCAR roof fences might then kill lift due to cross wind conditions as well. Killing that lift would probably help cross-wind drag as well.

[GasSavers_RoadWarrior]

I'm optimistic about small fences improving crosswind stability, due to them acting to virtually round the leeward corner of the vehicle and allow smoother flow off the edge of the roof there and less suction.

[GasSavers_Pete]

You may want to have a look at the roof design of the Renault 16 built during the late 1960's and early 1970's.
This five door hatchback used ribs on the roof forming "sails" toward the rear where the hatch was fitted.
Directional stability was superb and even at 100 MPH cross winds had virtually no effect on the stability of the car.

This link show the panels well:
http://www.renaultclassiccarclub.com/16TSnew012.jpg

Just as a side issue the Renault 16 has a number of aerodynamic features which are well worth a look.

Cheers , Pete.