Aero Questions, Especially for MGB racers

D

dcharnet

Member
For any of you racing MGB's, I have the following questions:

1. I am looking at ways to control undercar airflow so that negative lift is created, ideally at both ends. I am thinking about a panel behind the spoiler which directs air under the crossmember so that airflow does not break up there quite so much. Have any of you tried that, and what did you do?

2. I am concerned that it is relatively easy to create more negative lift at the front, but that this may move the center of pressure aft and create dynamic imbalance at higher speed. I need to create negative lift at the rear, which means speeding the underair through and doing a better job of extracting it. Right now, the only tools in the arsenal I am aware of are putting a slight rake on the car, and relieving the rear valance so it traps less air. Any other deas out there?

3. My limited understanding of fluid dynamics leads me to conclude that the installation of an original bumper on my '63 B (of course without the outriders) would lower Cd. The idea is that bumper would receive airflow and better disburse it in a way that is more aero-friendly than the perpendicular surface the air now meets. This might also send more air over the car and less through the grill, but as speed increases that would be a good thing.

Any thoughts on these issues?
 
Don,

Take a look at the photo of the Jensen on the transporter on the Huffaker/Slick thread over on MGE. It looks like they got creative with the "fuel cell container" most likely for that very purpose.
 
The front is easy. You're allowed to add an "air dam" which can be as long as from the front outline of the car, to the front of the front wheel opening

The rear is trickier, as there doesn't seem to be an allowance for anything other than replicas of factory bumpers. If you start getting tricky with oddly-mounted fuel cells (which could act like a diffuser) you run into this rule

Code: 
9.1.5.d.2
2. An authorized addition, modification, substitution or removal can not perform a prohibited function.
Since you're not explicitly allowed a diffuser, you can't make a fuel cell in the shape of a diffuser and claim ignorance. A fuel cell just needs to hold fuel. A fuel cell that changes how the air flows under the car seems very protestable.

Yes I know people have (legally )mounted their fuel cells in the trunk area right in the wind stream. I think you are pushing the rules to unecessarily cant it upwards at the back which would have the effect you're seeking. You can have the fuel cell there, but you can't use the fuel cell as a diffuser.

Here's the (front) Air Dam rule.

Code: 
Air Dams: An air dam can be fitted to the front of the
car. It must not protrude beyond the overall outline of the
car as viewed from above, or extend aft of the forward
most part of the front fender opening (cutout), and must
not be mounted more than four inches above the horizontal
centerline of the front wheel hubs. An intermediate
mounting device can be used on cars whose front bodywork
is above the four inch maximum. If the air dam covers
any portion of the stock grille, an opening must be created
in the air dam. The width of the opening must be equal
to or greater than the widest horizontal measurement of
the portion of the grille that would otherwise be covered.
The height of the opening must be equal to or greater
than the distance measured perpendicularly to the ground,
between the lowest and highest point of the portion of
the grille that would otherwise be covered. The opening in
the air dam must be symmetrically aligned in both planes
to the grille. Openings in the air dam are permitted for
the purpose of ducting air to the brakes, radiator, and/or
oil coolers. Openings can be cut in the front valance to
allow the passage of up to a three inch duct or a rectanGCR
- 509
9.1.5. Production Category Specifications
PCS
gular or square duct with a maximum area of seven square
inches leading to each front brake. These openings can
serve no other purpose. When bumpers are used or when
they are part of the bodywork, the air dam and bumper/
replica bumper must appear to be two (2) separate components.
The air dam can have no support or reinforcement
extending aft of the forward most part of the front fender
opening (cutout).
 
Everyone always says, "no other purpose", but it's really only if you beat someone. Sort of like the screen in front of the radiator. ;-)

James
 
Original Poster:

Is this an SCCA Production car? If not, what are your rules limitations, if any?

Those are really the key questions, as most of what you "could do" is likely to be rules constrained.

As several have said, under the SCCA prod rules it is relatively easy (conceptually anyway) to get front downforce, but there is little leeway to get rear downforce. Which - IMO - will render useless any project which creates mega front downforce.....
 
Right, thank you Fred and all for the input.

What about cutting some of the rear valance out? Is that an illegal body mod in SCCA now or was it in '72?
 
dcharnet":4bqrki9i said:
Right, thank you Fred and all for the input.

What about cutting some of the rear valance out? Is that an illegal body mod in SCCA now or was it in '72?
Click to expand...

Don, the rear valance is not even allowed in SCCA prod class racing. Technically not allowed in vintage either, but tolerated, much like head fairings.
 
If you really have too much free time, you can make a muffler that is big, flat, has little heatsinks on it etc, to fill up the rear panel area.
Look at the SM muffler size rules added recently.
The rear axle area has so much room ,it would be hard to get any good airflow behind it. IMHO. Flaring in under the axle, ducting the air smoothly out from under the car may pick you up a few 1/100.
That means skirts, pans, VG and fences,IMHO. Send it to me and I can spend a lot of your money for fancy, look good undertrays.. Will it go faster than the added mass and drag it produces?? maybe.

Edit, I would try this. Install a full length under tray of lexan , with 1in outer fences from the seat area to 4-6 in past the rear valence, Run some laps, remove it run more laps, put it back on.
tell us how it works. Thanks, MM
PS pS I would be surprised if some of the real Group 44 cars did not have some under car air control
 
Mike:

In your view, is some panelling right behind the front valance intended to channel air under the crossmember worthwhile?

Don
 
Don -

The devil is in the details as it were. The biggest thing you can do to help your aero will be to stop as much unneeded air as you can from coming through or under the nose of the car. This means blocking any air coming through grille or any other front openings that is not needed to feed coolers or air intake (as much as possible under your rules, which are likely fairly free as far as internal ducting etc as long as external appearance is not much affected). And also stopping as much air from going under the nose as possible. Not sure what if anything you can do on a B. A vintage instance of an air dam was the "Spook" on the BRE Datsun roadsters, which I understand was slid in as brake ducting.

If you can keep air away from your underbody crossmembers by blocking it at the front you won't need to think about fairings etc. If you can't and your group will tolerate underbody fairings they could help.

Hope that helps.

PS: But a good data logger system to help measure the effects of your changes! 8)

Al Seim
Race Technology USA
 
http://www.aerocivic.com/
aero testing stuff, really good IMHO. WTMFT ( way too much free time)*
I treat auto aero with the sq in. of involvement theory. The less square in. of parts that the air hits, the better.
Along that road; reducing piece count under the car is a major area for improvement . IMHO.
So yes, I think that a belly pan , deflecting the air off of ugly parts will go faster.
Keeping air from under the car turns out to be the best .
Some control over the car will show some improvement. The rules play the big part here. reality plays a big part. Will you have any major speed increase ? At long , high gear pull tracks, MAYBE. maybe not.
As far as your bumper goes. if it splits the air above it and under it It will go slower. If it is the splitter and very close top the ground, than maybe.
The flat front with a small splitter, very close to the ground , directing air around the nose and above the car will be the best,IMHO.
Taking the rules to the limit;. you should need a plumb bob as you go around the nose panel/airdam/splitter .
Take a look under the modern VW TDI or some other high fuel MPG car. If the crap did not work, they would not go thru the hassle of putting it on there.

Back to the rules tho, I still have the front end of the HP tapped up and cant tell if it is legal. I guess that the tape is legal but any screen is not. stupid stuff. IMHO.
 
The air dam on my #66 Civic was worth 5 mph in the speed trap at RA. The Gauper pistons were worth another 5 mph. The pistons cost ten times as much as the air dam.
We put the black plastic down to about 1 inch off the ground and let the track hone it to the "perfect" height and curvature.

James Rogerson
 
A vintage B is limited regarding air dams. It is inevitable that air will get underneath.

One possible device for dealing with this is hood vents like louvers or similar. If these are placed in the proper place, i.e a few lateral rows right behind the radiator, some of that disturbed air can be drawn out, which adds negative lift. And, under-hood temps lowered (11 degrees =1% power, allegedly).

Does anyone disagree with that?

The next issue is the certain negative effect of these vents/ louvers on Cd: Disturbance of laminar flow from the device itself and any hot air emerging into the air stream. Of course, this is more of an issue at higher-speed tracks like Road America.

Given the above, does anyone have an opinion on the net gain or loss of hood venting in a production race car, especially an MGB?

One direction I seeks to have two bonnets: A vented one for the bullrings like Blackhawk, another unvented one for Road America. Another is to somehow try to make the venting adjustable. This could involve use of flat vents (nothing protruding into the airstream) with some kind of attachable cap which could seal it.

Any observations would be appreciated.
 
dcharnet":53kgkfre said:
Given the above, does anyone have an opinion on the net gain or loss of hood venting in a production race car, especially an MGB?
Click to expand...

There is no rule allowing vents in a production car hood, unless those vents came with the base model, stock.
 
blamkin86":tbwfxu2l said:
dcharnet":tbwfxu2l said:
Given the above, does anyone have an opinion on the net gain or loss of hood venting in a production race car, especially an MGB?
Click to expand...

There is no rule allowing vents in a production car hood, unless those vents came with the base model, stock.
Click to expand...

Aha, thanks.

That fundamental point established, does anyone have any comment on the questions? Maybe for this dispositive reason, no one in production gets into these areas.
 
Don:

One problem... (Mr. Lamkin and I are on the same wavelength apparently...)

Per SVRA General Rules and Regulations:

A. Body: The body configuration should be as raced "in period". All body parts must be of the same material and design as those supplied by the manufacturer as standard or optional.

1. Undocumented supplemental aerodynamic devices such as spoilers, air dams and wings are not permitted.

Rick
 
Rick Starkweather":15pl2093 said:
Don:

One problem... (Mr. Lamkin and I are on the same wavelength apparently...)

Per SVRA General Rules and Regulations:

A. Body: The body configuration should be as raced "in period". All body parts must be of the same material and design as those supplied by the manufacturer as standard or optional.

1. Undocumented supplemental aerodynamic devices such as spoilers, air dams and wings are not permitted.

Rick
Click to expand...

Hi, Rick, I know. Sebring-style valances are OK with SVRA, and there are many cars racing vintage nationally with louvers. This is no issue ith VSCDA. I accept this regulatory framework, and am trying to get to the merits.

Hope you Flying Circus guys are coming to Grattan to show us What For.
 
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