Presented by Nine Lives Racing
AClockworkGarage
AClockworkGarage Dork
9/27/23 2:13 a.m.

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?

 

Keith Tanner
Keith Tanner GRM+ Memberand MegaDork
9/27/23 2:47 a.m.

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?

VolvoHeretic
VolvoHeretic GRM+ Memberand Dork
9/27/23 10:42 p.m.

They look dangerous.

BoulderG
BoulderG GRM+ Memberand Reader
9/27/23 11:17 p.m.

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

iansane
iansane GRM+ Memberand Dork
9/28/23 12:53 p.m.

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.

Keith Tanner
Keith Tanner GRM+ Memberand MegaDork
9/28/23 1:10 p.m.

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

Driven5
Driven5 UberDork
9/29/23 11:45 a.m.

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?

Jesse Ransom
Jesse Ransom GRM+ Memberand UltimaDork
9/29/23 11:51 a.m.

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?

Slippery
Slippery GRM+ Memberand PowerDork
9/29/23 1:26 p.m.

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

Thomas
Thomas New Reader
9/29/23 4:26 p.m.

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

Keith Tanner
Keith Tanner GRM+ Memberand MegaDork
9/29/23 4:37 p.m.

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).

Thomas
Thomas New Reader
9/29/23 4:44 p.m.

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.

AClockworkGarage
AClockworkGarage Dork
9/29/23 7:31 p.m.
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.

Slippery
Slippery GRM+ Memberand PowerDork
9/29/23 7:33 p.m.

In reply to AClockworkGarage :

Never seen one of those. 

AClockworkGarage
AClockworkGarage Dork
9/29/23 7:45 p.m.
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.

stafford1500
stafford1500 GRM+ Memberand Dork
9/29/23 8:28 p.m.

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.

Keith Tanner
Keith Tanner GRM+ Memberand MegaDork
9/30/23 7:24 p.m.

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

Appleseed
Appleseed MegaDork
9/30/23 11:42 p.m.

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.

stafford1500
stafford1500 GRM+ Memberand Dork
10/1/23 11:00 a.m.

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.

Slippery
Slippery GRM+ Memberand PowerDork
10/1/23 6:59 p.m.
stafford1500 said:

The original BMW incarnation was likely a styling driven decision. 

Even the ones on the race cars?


 

 

Keith Tanner
Keith Tanner GRM+ Memberand MegaDork
10/1/23 7:01 p.m.

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

TurboFource
TurboFource HalfDork
10/2/23 7:22 a.m.

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

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