NASCAR roof flow fences
 

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.)

Otto -

Ford Fusion
https://www.gassavers.org/showthread.php?p=68880
Soooooo, I don't think they help with your aerodynamic coefficient.

CarloSW2

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. :)

https://danspitstopracing.com/yahoo_s...494140_std.jpg

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

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?

https://www.fuelly.com/attachments/fo...767f78dbd7.jpg

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.

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.

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.

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.

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:
https://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.

Hi clench..,

Well, heading on down the highway, the cross wind vector sums with the movement of the car. So the resulting air flow direction relative to the car is what, less than 20 degrees at 60 mph, for most situation. So, what is going on is these fences actually act as fences to straighten out the flow over the roof, rather than as side-ways spoilers.

But, even if they did act as side-ways spoilers, turbulence can be good or bad. If you did not have them, your going to get detachement of flow on the downwind rear corner of the car. The more side wind, the longer the zone of detachment from the rear forward. The fence could limit this detachment from the rear forward to the C pillar, or cause a rapid turbulence that causes the cross section of turbulence to be small, and having a minimul impact on the axial flow along the down wind side of the car.

So, that is the jist of the guestimation. From here out takes engineering.

Yep, that's what I was figuring.

I'm not sure how the NASCAR fence affects cD, but the VW 1 liter prototype had some sort of fin on the tail end. I assumed it was there to maintain attached flow.

that's a 30 degree vector at 60 mph w/ a 30 mph crosswind (or is it 22.5? been a while since physics). wind speed is half your speed so you don't split the difference equally

Hi All,

Lets see. 60 mph, 30 mph 90 degree cross wind would be for every foot you move forward, the wind moves 1/2 a foot cross wise. A 45 degree angle would require a 60 mph cross wind. Back to the original problem, Arc Tan of 1/2 is 26.6 degrees relative air flow direction across the car. Kames..., I think you were confusing the Sin of 30 degrees (opposite over hypotenues) which is .5.

Just realised something else related to these.

When an aircraft wing is in ground effect the tip vortices that create downwash are blocked by the ground within a span width of the ground....

So any vehicle shorter than it is wide will not be affected much by lift vortices...

But any vehicle taller than it is wide, i.e. it's lifting surface is a span or more off the ground, is likely to make "tip vortices" off the side of it's roof. These will create a downwash effect that alters the lift vector such that it is tilted backwards from the vertical, inducing drag...

So for taller vehicles, i.e. my minivan, some "tip vortex" control strategem may be beneficial, the equivalent of tip plates or winglets on an aircraft. Therefore a form of fence at the sides of the roof on a taller vehicle may lessen the effects of vortices forming downwash, and may thus improve the induced drag by tilting the lift vector forward of the vertical.

i have inverse railing on my chevette on the roof. i think it may be more of a rain gutter than anything.

Hi All,

The vortex forms when the air from a high pressured area flows sideways to a place where air is lower pressure, in an on-axis general flow.

So, at C pillar area of a car is one of those places. The air on top of the car is high pressure, and the air on the side of the car is stalling, sucking the air in from the roof. As the car slopes away going behind the C pillar, the vortex has a zone to complete the first loop.

Which is why Ernie Rodgers put thos big plates on the Beetle. They interupt the first loop of the vortex.