Air Dam / Spoiler effectiveness

There has been a number of posts recently on the MGB/GT forum on air dams/spoilesr. Moss offer 3 differnt varieties of these:

Urethane Air Dam @ $219.95
Special Tuning Spoiler @ $89.95
Limited Editon Spoiler @169.95

Does anyone know the relative effectiveness among these on the handling of the car? Are some just pretty or do they all do the job

Cross posted to MGB forum

Turbulence under the car isn't much to worry about until you're going pretty fast. The idea of an air dam is to get air to spill off the front of the car around the sides and over the top . The air that goes over the top is supposed to create some downforce and the area under the car is supposed to create some vacuum (low pressure area) to help keep you on the ground. All the little protrusions under a car can certainly cause some turbulence and the air dam will minimize some of that. I have no clue if any of Moss' air dams do squat. You'll probably lose more from adding frontal area than you would gain from under-car tuburlence abatement except at racing speeds. Lowering the car and adding a splitter would make more sense than an air dam but without wind tunnel tests, you're going to get anecdotal responses. Best of luck. Basil

I do know that adding the ST type dam can increase your gas mileage on long distance trips! I know that doesn't sound like much, but I've seen it happen and knowing that, it means it must be doing something productive.

I would not bother with the limited edition air dam, that's a cosmetic gee-gaw that will not help.

I have no experience with their urethane air dam.

Sean

I think what Basil said is the key, what mph are we talking about, street speeds, anything under the top speed of a street 4 cylinder MGB, and there's probably little to no effect, people use those air dams/spoilers because they like the cosmetic look of them on their cars, I sure it might add a bit of stabilty for say, running 80+ on interstate or a wide open road, but not that much effect elsewhere . I ran my SCCA H production 1275 Midget with and with out a splitter front air dam, I was playing around with some wild duct work that sat the oil cooler more inboard on the car, and the air dam on hot day wouldn't keep oil temps down to mark I wanted them to be, so I run the car without a air dam alot of the time as I played with different oil cooler duct work. A HP Midget probably has a top speed of no more than 120 mph, I notice no ill effect running with or with out a air dam on the the top speed straights, I'm sure if the speeds had been 20 mph or so higher, a air dam would have made a much bigger difference.

Thanks guys, this has been good information.

I'm asking because I do quite a bit on driving on the interstates out here. The cross winds out here can be brutal at times and with speeds running in excess of 75 MPH I am looking for a way to stick the front end down to the road better. I am in the resto stage on the B. I figure I'll add whatever helps.

Agree that it is pretty much a waste of time at normal highway speeds.

Disagree that they don't do much at racing speeds though.

When I was running the MGA as a stock class car with maybe 90 BHP out of a 1588 cc engine, every little bit helped. Wide tires - waste of time and potential added resistance on the straight. Spoiler - who needs downforce at only 110 MPH?

But a straight down air dam at the front got me 500 RPM at the end of the straight at Seattle (the longest straight we ran in the area) with no other changes to the car.

So what determines racing speed? Physics predicts that the pressure on the nose of the car would be the vector sum of the speed of the car plus any "breeze"

Last week I was going to Denver up I-25 at the speed limit of 75 MPH with a varying headwind of 45 MPH on or off angle of the nose. That looks like 120 MPH to me. Am I missing something here?

The Chinook winds in Colorado can be brutal. I've seen a number of tractor-trailers tipped onto their side this past year because of the winds.

Larry, I think that you'll feel a positive difference with either the special tuning or urethane air dam on your car under those conditions. From what I've heard the LE unit is more for looks, not very effective. I gather that you're feeling a bit of "lane wander" when at speed and getting hit by a side wind. The front spoiler will definitely help plant the front of the car and maintain a bit more steering pressure on the tires.

Larry, do you have the castor shims installed on the car? If you do you might consider removing them and living with the increased steering effort in trade for the increased straight line tracking.

I've got a fibreglass air-dam on the nose of the "B" and had it for a long time. It is the style that drops down basically flat with the hole in the middle for the oil cooler that isn't down there.
I noticed it made a positive difference at highway speeds in the wide-open Nevada/West highways (yeah, we get some wind). What used to be "interesting" handling in windy conditions became more relaxing.

Don't think it has helped diddly-squat on auto-x or in twisties but it looks nice (or did before I collected a few cones and cracked it).

Dunno about helping flow through the radiator, but I suppose it has helped that, too.

Here's a related question. My high speed MG work has been in MGAs (which seem more aerodynamically stable than MGBs) and MGCs (which are very stable without any aids - and no cracks about road hugging weight, please).

If anyone has experience with both CBB and RBB, maybe they can offer an opinion about high speed stability of the two. My impression of a CBB at speed (90 MPH +) is that it isn't bad. I have never had the opportunity to do anything similar with a RBB (and not just because that would exceed their top speed), so don't know if stability is much different from the CBB.

My off the cuff guess would be that the nose of the RBB looks better than the CBB but the raised ride height would probably head in the opposite direction, so maybe end up with a draw?

I figured I could get more informed responses as to handling on this forum,

As to questions on how the car is setup - currently it is unmodified RBB with 185R65-14 tires. It's in resto wth the following modification planned:

Targett dropped spindles and castor wedges
Moss camber adjustment
Lowered rear end
Panhard rod
Anti-tramp

And now it looks like a ST air dam

All with a "Dark Side" engine/drivetrain

It is important to note that it is theoretically provable that any "air-dam" type mechanism will have a positive effect on reducing air resistance at AIR speeds over about 60mph. This effectiveness will increase by the square with the increase in AIR speed.

You’ll increase your frontal area, and increase the force of drag, but the decrease in the drag caused under the car will be greater, so a net gain will occur.

This is all about air turbulence between the under carriage and the road.

I guess this how I understand it.

So if I mount what would essentailly be a skid plate from the bottom lip of the air dam to the rear of the tranny, I should see even better sticktoitivness??

Bill

Rear pointing scoops in bottom of the pan??

You planning on running at Bonneville? Otherwise more fuss and trouble than it is worth.

Bill

Your right - just looking at options. Passed Bonneville on my way out to Sacremento last month. Thought about stopping but had a meeting I needed to get to.

As Bill said and paul has surmised,it is controlling the air under the car so ,Bills straight down air dam picked him
up some speed by not letting so much air under the car if you put a splitter (ala nascar) or a forward facing lower
lip then you would pick up downforce at a slight drag penalty,if your car understeers at high speed you may want
the downforce no matter the drag,if you clean up the underside like the bottom of a sports racer or lmp car you
will get downforce via the Bernoulli effect. Unfortunately the rubber bumper cars were raised up in the air to meet
standards and did absolutely nothing for the aero.

Larry,

I have a modified Craddock front air dam on my street B... it does make a difference (butt dyno data only) We have the same desert cross winds that you have in Colorado. At highway+ speed cross winds easily pushed my B around. With this dam cross winds no longer move the car. The front end is very stable right up to 100+ MPH

I filled in the oil cooler and brake duct openings and added a reinforced flat plate across the bottom to expand the splitter surface area.

Steve

Steve did a nice job of narrowing our kit air dam and reshaping the ends,now this is an air dam that will give you downforce!

True, the effectiveness of any aerodynamic tools will depend on the speed of the car. Popular wisdom traditionally considered no noticeable effect below about 40 mph.
Proper design would have the bottom of the air dam lower than anything else under the car. The lower lip turned forward increases the effeciveness of this air flow control. A splitter is just another step up in that flow control. Extending the splitter back to the front axle line even more so. Any further back, probably not. The idea is to reduce the air flow and pressure under the car.
Lowering the rear suspension more than the front will cause a reverse wedge effect that will cause more high pressure under the car. Not what you want.
The Special Tunings front air dam and rear spoiler were developed in the M.I.R.A. wind tunnel specifically on the rubber bumper GTs. I remember the original ST Manual claiming a 5 mph increase in top speed for a stock MGB GT, allas, that is not in the reprints. It will reduce the aerodynamic lift of the body, improve the air flow under, around and over the car. Even through the radiator. When the S.T. department was shut down the front air dam mold was bought up, but aparently no one took the rear spoiler mold. I still have an NOS rear spoiler, but it never fit very well and have never fitted it to a car. Was supposed to be good for 30-40 lbs of decreased rear body lift at speed.
I have used a S.T. front air dam on my MGC GT since the early '80s and really like it. Made the car quieter instantly. I modified the air supply to the radiator and it has never overheated since. Combine the added weight of the 6-cylinder C engine, front and rear ADDCO anti-roll bars and I don't ever notice crosswinds of less than 40 mph. The car came into its own on the trip to INDY'96 where the car delivered 27 mpg at an average of 70 mph.
Ordered a new one from Moss for the restoration, but the catalog stating that it is for chrome bumper cars is wrong. They will WORK on the chrome bumper cars, but they were designed for the rubber bumper cars. The one they show in the catalog has been modified, not so the one I recieved.
A friend has the urethane air dam on his '74 hillclimb car and his '74 1/2 GT. Good shape and effective. Could work better with more of a lip or a splitter. Much more durable than the fiberglass ST dam.
Personally, I think the MGB LE air dam looks bad on a chrome bumper car. Aerodynamics are dubious - designed by the marketers in the US, not by engineers in a wind tunnel.

C Ya,
Mark