Windsplitters... I think.

At a recent Radwood show I came across a Mk 1 rabbit with little vertical walls on either side of its hood. I've seen similar features on the older BMWs and first encountered them in Gran Turismo 3(?) on their very modified 3rd gen camaro.

In my google searches I came across the term 'windsplitter' but literally nothing else.

I love the look and would like to put them on my 3rd gen but I abhor fake scoops, vents and other extra nonsense. I even swapped my Z28 hood for a flat RS hood. 

So the question is... what are these and do they actually do anything?

Hmm. I know that there tends to be a low pressure area on the hood as the air moves diagonally towards the fenders. Maybe these prevent the air from spilling over and thus give some more pressure on top of the hood and thus less front end lift?

They look dangerous.

Had them on the hood of my 1981 Scirroco S. They balanced the louvers on the rear hatch. Looked awesome at the time...

I've been trying to get a pair of those for my e30 for awhile. They look cool. Don't know if they do squat though.

Didn't something like them come on an anniversary edition late 00s grad prix too? They were on the roof at least.

Now I'm thinking I need to make some for the CRX :)

I think they're called 'strakes'. The Group 5 Capri added similar, but also extended further back more like that digital Camaro.

My best guess would be they mildly increase pressure on the adjacent zone of the hood whenever airflow is not parallel with the centerline of the car... Like when turning. That's effectively why similar is used on the roof of NASCAR.

Alternatively, if the side of the car are even lower pressure than the hood adjacent... Perhaps it could be preventing flow spillage from the top of the car which is used to create downforce, to the sides of the car which isn't?

I like the "keep pressure on the hood" theory, but I suspect there's a reason they only existed for a little while in an era that didn't have great aero info.

 But i could be totally wrong! Maybe search for the FIA banning them as evidence of efficacy?

They look like the fences on top of the wing of a Cessna C21A

Based on Keith's NC hood pressure data, I'd think adding those fences would actually decrease pressure on top of the hood, since the data show low pressure right around the same area where the fences are on the BMW.  Adding fences would prevent high(er) pressure air outboard of the hood from flowing onto the hood.

My guesses:

1. vortex generators to reduce drag from the sides of the car
2. style

I'd assume the marketing people from the era would say #1 is true while the engineers would say #2 is true

In reply to Thomas :

I think it's low pressure there because the air is accelerating over the sides of the hood. If you can stop that, you cut down on the low pressure area. It would certainly be interesting to experiment with - very easy to control variables by testing left vs right side.

I agree that the BMW comes from a time when aero was a fairly young science, and of course the Camero is from a video game. Perhaps it's not seen anymore because aerodynamicists have more subtle ways of moving airflow, or they're using that low pressure area for extraction (take a look at the vents on a 90's Impreza or Ford Ford Econobox Cosworth).

In reply to Keith Tanner :

Ah ok, that makes sense.  I didn't consider flow in that direction.  Yeah, it would be easy to test with some yarn or paint.

iansane said:

Didn't something like them come on an anniversary edition late 00s grad prix too? They were on the roof at least.

The 40th Anniversary Grand Prix had fences on the roof, air extractors on the hood and a more low profile spoiler, unique paint and interior colour, white face guages, and unique wheels.

The W-Body guys call the fences "roof rails" and they show up for sale often enough. They generally run around $150 for a good set.

They were supposed mimic the appearance of the NASCAR models. No idea if they had any real effect on aero though.

I do know that they're awesome and I want one.

In reply to AClockworkGarage :

Never seen one of those. 

Keith Tanner said:

...and of course the Camero [sic] is from a video game. 

I first encountered it in a video game, but it IS a real car.

I am enjoying watching everyone ponder how these devices might work. Keep going i need a few more chuckles.

I will post my guess later.

C'mon, spill! I was hoping you'd join in :)

In reply to Driven5 :

I'd like to believe its of the same principles of the wing fences on MiGs. To stop the spanwise flow of air. In this case it would help keep the high pressure on top of the hood.

For Keith and Appleseed, yes the fences generally act to keep the lateral flow constrained and generates higher pressure at the cowl. A side effect is a very strong vortex shed on the outside of the fence/wind splitter that runs down the side of the car, likely under the rearview mirror and along the door. The down side is the vortex is draggy.

The original BMW incarnation was likely a styling driven decision. The Pontiac roof rails are a nod to the then current NASCAR bodies that required roof rails to meet rules for lift-off when spinning (we still have them on the current cars and they are still an area we mess with) Probably not too far from the MonteCarlo NASCAR sort-of homologation edition thing. The Camaro fences, like the BMW are probably a styling thing. I pose the idea that the hood fences are possibly some sort of rules/homologation thing, but I don't have any supporting references.

stafford1500 said:

The original BMW incarnation was likely a styling driven decision. 

Even the ones on the race cars?

BMW may not have been aware of the high drag vortex at the time given the tools available to them. 

Closer view of the ones on our Pontiac....