Any Benefit To Doing This?

[Bill D]

The RCR monocoque chassis has a smooth bottom. I've added a panel to the front structure to make the bottom smooth, and I've added a rear belly pan to the steel frame as well. I would like to install a louvered panel under the engine bay.

Would this add in smoothing the air flow underneath the car? The louvers would be facing towards teh rear of the car and the air flowing past them would (should) draw air from the engine compartment.

Thanks
Bill D

 

[Ron Earp]

Chassis topic.

 

[40 Bud]

I am no aero genius, but it seems to me that a fully flat bottomed car without some diffusers, splitters etc. woul fly like a bird when reaching high speeds.

 

[Mark Clapp]

I was thinking that the engine bay bottom would need to be open to let out all the air that is directed in by the side and top scoops. I don't know if the rear vents alone would relieve the pressure. Just thinking out loud.

 

[Bill D]

That's why I would add louvers to the bottom. Air rushing past would create a vacuum effect drawing engine air out.

 

[Bill Musarra]

Bill,
The original road cars had a smooth bottom with only a break for the bottom of the engine. They didn't have any issues with heat or air flow. I guess the louvers wouldn't hurt, but seems like more work to me. I think the splitters like on this Ford GT would be more effective in stabilizing the car. (Notice its bottom is smooth also) If set up correctly, the nose is lower than the tail end of the car, which should create enough negative pressure on its own(at speed).

Bill

 

[Mike from Vegas]

Generally, flat-bottomed cars create downforce, unless there is a pressure build-up under the car. This works, because it creates a laminar flow under the car, like an inverted wing (similar to a bertuli effect, meaning a low-pressure zone is created under the car, which "sucks" the car onto the road). Also, a venturi effect on the louvers under the engine bay may help cooling of the engine compartment. Just my two cents.

 

[John W]

original cars are not actually flat bottomed, the floor pan is ribbed from side to side for strength...

 

[Chris Duncan]

Louvers reduce static flow area, is this outweighed by the increase of CFM flow from the vacuum effect? By enough to make it worth the extra time/cost? And what about the heat when not moving?

The picture from underneath the rear of the Ford GT is of the diffusers, not splitters.

 

[CliffBeer]

Bill, if you're going to make the bottom smooth from stem to stern then make sure the nose section is 1" or more lower than the tail section (assuming you have typical-rate springs). Even with this delta, with some modest pitching you could get a situation at speed where the nose is higher than the tail and thereby create a positive pressure zone under the car. Don't worry about a LeMans style Mercedes flip unless you're doing 180mph+, however, you could lose most of the steering at 120+ in such a situation - not fun.

 

[Bruce B]

Ehem, it's actuallly the Bernoulli effect. And for the best non-technical/technical discussion of all this. Check out:

Mulsanne's Corner, technical analysis of contemporary sports prototype racing cars

Wonderful stuff on all things racey. Classic racers, aero, engines.

Almost forgot. Thought about a diffuser the first time I saw the RCR flat plate for the backend at Carlisle. So Bill, you're thinking just what I did.

 

[Lynn Larsen]

Bill,

Following the angle that Fran's chassis have from the bottom of the car, at the back of the frame, up to where the pivot point for the rear clip, you are going to be very close to the optimum angle for a diffuser. Hopefully the webpage that Dreamin' gave you will provide this optimum angle, which allows the air flow to remain laminar on the diffuser. What you probably should do to make this actually work is to put strakes (fences) on either side of this panel, the bottoms of which remain in line with the rest of the the bottom of the car. The picture of the Ford GT shows this pretty well; although, I don't think you'll need the channels to the sides (left two shown), just the inner most strake and, of course, its twin on the other side. From what I recall on Ron E's chassis, this panel is pretty much the same width all the way up. If it were to widen appreciably, you might want to put interior strakes in it so that air doesn't diverge outwards too rapidly as well (take a look at the diffusers on the ALMS or GrandAm Vettes to see what I mean.)

The idea is that, with the air remaining laminar to the diffuser (not breaking away and becoming turbulent) there is a speed gradient with the air near the ground going slower and the air nearer the diffuser going faster. This faster air causes a low pressure boundry (bernoulli's effect) along the diffuser, thus creating down force. Given the proximity of the engine bay to where the diffuser starts to angle upward, it is possible that louvers under the engine tray could cause the air to become turbulent and ruin the effect of the diffuser. (To be honest, none of this is likely to happen at the fastest speeds you'll be going on a highway; as mentioned, it would only be at very high speeds on a track when this would all come into play.) On top of this, you want to push as much air as possible out of the tail grills to fill in the turbulent air there from the spoiler (does the term "drafting" ring a bell.) The down force off of the spoiler comes at the cost of drag cause by this turbulence, so if you can fill this area with air from inside the rear clip, it will push the turbulence back away from the car and reduce drag. (I think this is one reason Mk IIs and the later Mk Is had larger grills in the back of the car. Of course, I could be wrong as I am speculating here.)

Lynn

 

[Bill D]

Thanks Lynn.

I'll do some mockups, post the pictures, and ask fro some feedback.

Cheers
Bill D