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MG MGA - Drag coefficient - MGA

Does anyone know the drag coefficient of an A roadster and a coupe? Wanted for bragging rights only!
I noticed in a recent Porsche pamphlet that a Boxster is 0.29 and a Boxster S is 0.30
Mike
Mike Ellsmore

Good question Mike if the figure isnt out there easily available would be interesting to put on into wind tunnel to get some figures. But someone out there would have already done it Im sure
Shane Rossetto

In cases like these Barney always has something to offer and the photo I was thinking about is on his site.

http://mgaguru.com/mgtech/history/hs130.htm

Plus also this page which indicates
drag coefficient 0.70
But later
total drag coef. = 0.025591936 lbf/MPH2

http://mgaguru.com/uta_mga/performance1.htm


So not sure which is the correct figure.

Hope you've got a beer riding on this !!!

Mark.
Mark Hester

Refering to the numbers here: http://mgaguru.com/uta_mga/performance1.htm,
I believe those numbers are all correct, just a different way of looking at the same issue. The Cd=0.70 is a ratio compared to a cube moving with flat face forward into air flow. The MGA is not so good in this respect, as most new cars now will be well under 0.4 with some under 0.3. This makes a HUGE difference in the ability to make top speed.

The biggest problem with the MGA is the steep windscreen which looks almost like a brick wall for the lower half where it blocks up a near stationary air pillow. At road speed this forces air near the body cowling to make an abrupt turn to travel latterally around the sides of the windsrceen rather than flowing over the top. You can see this clearly in the picture here: http://mgaguru.com/mgtech/history/hs130.htm
High pressure in front of the windscreen makes for a lot of drag. Recirculating air vortex and turbulance around the passengers' shoulders causes some negative pressure (partial vacuum) aft of the windscreen, which is also a drag problem. For a seriour racer, the first thing you do is to get rid of the stock windscreen.

The other factor, total drag coef. = 0.025591936 lbf/MPH^2 gives forward force (in pounds) required to overcome wind resistance when multiplied by square of the speed (in MPH). Mutliply that by velocity in ft/sec and divide by 750 to get the HP used to overcome aerodymanic drag. At 60 MPH this works out to 92 pounds force, and 10.8 HP, not terribly bad for fuel economy. At 100 MPH it's 256 pounds force and 50 HP. Yikes, that's gonna suck up a lot of fuel. Force increases with square of speed, and power increases with cube of speed, so you can see why the steep windscreen is a performance killer at high speed. At 120 MPH it looks like 368 pounds force and 86 HP.

Add to this the force required to overcome rolling resistance. That factor is 1.67% (with modern radial tires in this case). If the car weighs 2250 pounds with fuel and one passenger it will require 37.6 pounds force to overcome rolling resistance and push it forward at a constant speed (any speed). This works out to 4.4 HP at 60 MPH, 7.4 HP at 100 MPH, and 8.8 HP at 120 MPH. This is still not accounting for frictional losses in the gear train, or any force required for accelleration.

When the power requirement for wind resistance, rolling resistance, and internal friction equals full engine power output, it stops accellerating and you have top speed (unless you get a strong tail wind or a downhille grade to help).
Barney Gaylord

Just to add a point on form drag, Lotus enthusiasts have long known that the lovely shaped Lotus Elite of the 1960's has the lowest drag of any car built then or since. The MGA coupe wirh curved windscreen probably does a little better than the roadster and this may explain its marginally higher top speed.
J H Cole

I can vouch for the slab windscreen effect. Very noticable on an MGTC and the like in which you can fold the windscreen flat forwards. The difference in performance is very very noticable.

Steve
Steve Gyles

OOPS !!! Excuse my error on Horsepower. It is NOT 750 ft-lb/sec, it is 550. (I was momentarily thinking of electrical conversion). So change that "750" in the equations above to 550, and increase the HP requirements x 1.36 (Twin Cam or race engine territory for 120 MPH).
Barney Gaylord

All good stuff as always Barney.

Are there any figures on the drag coefficient with the windscreen removed?

Or perhaps with the competition "Different Angle" windscreen uprights? From memory the last set someone tried to sell me were only a 1/2 screen though.

I know from my own experience, a stock MGA can go 4 seconds a lap quicker on our local track with the windscreen removed.

Just for my own interest, as I might use my MGA for the local MGCC economy runs, is there much else I could do to reduce the drag?

Does a MK II grill increase drag?

Cheers Mark.
Mark Hester

To my simple (and probably very flawed !!) logic the drag must be less on a Coupe as despite being heavier than the Roadster (the difference in doors alone is amazing), it is booked with a higher top speed.

Just my 2p !

Steve
Steve Owen

My 2p now. Haven't got any figures, but back around 1963 when I had my first 1600 roadster and there were no speed limits here, my "A" would consistantly register 103mph flat out on the level with the hood down and 110mph with the hood up and red lined!

So the screen alone is definetly the worst case, and the coupe has to be better than a hood due to the smoother curved lines.

Pete
Pete Tipping

The normal drag formula is Fd=1/2 Cd A V^2 and power = F V. If you use 78 hp, 103 mph, 2138 in^2 frontal area, .85 mechanical eff. and 7.4 hp for the tires, Cd come out to .71. That is pretty close to .70. So if I did my math correctly, it is that bad.

P.S. my email has been down, any northern California people who would like to get in touch now can.
Gordon Masor

Gordon-I guess it's how you look at it. Is .70 bad as compared to a '57 Buick? a '59 Edsel? Can't compare to todays cars,it would be like comparing a Commodore 64 to the latest laptop. I'm sure a modern F1 car has a higher cd than a Honda Insight - I know which one I'd like to drive
gary starr

Chuckle. Bad comparison. A modern F1 car will have a very high Cd. Those things are like a brick going through air with the open wheels, and designed to generate a lot of down force, not to minimize drag. I haven't seen it done yet, but apparently am F1 car could be driven on the ceiling at full speed, as they generate downforce in excess of the weight of the vehicle.
Barney Gaylord

Barney. That's the theory of flight for you! To all intent an F1 car is an aircraft with the wings bolted on upside down.

Steve
Steve Gyles

I was told by the BMW Sauber guys that the reverse aerofoil profile to create downward force has been prohibited in the F1 regulations.

It was great to be able to go for a tour of the complete facility, even into the garage at the factory, and sit in the current season's car (apparently I am the same size as a "standard" F1 driver - my colleagues (all larger) were as jealous as hell)!
dominic clancy

Dominic

In that case what is the purpose of the wings?

Steve
Steve Gyles

Billboards for sponsors?

Derek Nicholson

The aerofoil section I was referring to is the shape of the body of the car: At one stage the underfloor and body was being designed to act as a reverse aerofoli that sucked the car down onto the track - this is apparently no longer allowed.

The airflow calculations are done on an array of computers that is like a supercomputer. The computer graphics generated were amazing to see. The tyres are indeed like a brick wall, and the trick is to direct the airflow into the cooling ducts, and otherwise to maximise downforce where allowed and othewise reduce air resisteance to the minimum possible in a VERY unaerodynamic shape. If I remember correctly the front wings are to get enough force on the steering to make it effective. There's probably a website on it all if someone looked.
dominic clancy

Ah yes. I agree with you there Dominic. They banned the 'ground effect', whereby the cars were running with the chassis almost touching the ground. To make sure no one cheated, they forced every car to be fitted with the wooden plank on the underside.

Steve
Steve Gyles

I remember the plank thing. When they were first introducted, they would just let the wear out and they would be back to square 1. Then that started measuring them at the end of each race.
All that Parc Ferme stuff... always wondered what goes on there.

Anyway back to the Drag thing....
Approx what drag would an MGA without windscreen get?

Mark.
Mark Hester

FYI, the coupe has lower drag than the roadster, the roadster shows lower drag with top up than down and the best of all is the roadster with racing screen.

I used to race my roadster in a production class that limited modifications. They allowed a racing windscreen with plexi screen, but had omitted to specify what size, I suppose figuring that people would use whatever made sense.

They bitched and complained when I mounted a piece of alloy with 1/4" of plexi sticking up, but it was technically within the rule - which they changed the next season to prevent such creative 'abuse'. Worked, though.

I've had my racing MGA to a Radar clocked 130 MPH on a local track - it would have a higher top speed with a longer straight.
Bill Spohn

This has long been one of my "dream machines":-

http://www.mgcars.org.uk/cgi-bin/gen5?runprog=mgoc&p=emg/mgaspo.htm

Dream on!

Derek Nicholson

Are the short plexi screens and supports (similar to that shown in Derek's URL) available commercially? Or is it a case of modifying an existing frame and fitting with suitably cut plexi?

Bearing in mind I rarely put the hood up I have often thought about putting this style of screen on. It would also allow me to wear my WW2 tank driver's goggles without looking a pillock!

Steve
Steve Gyles

Steve

You do let yourself in for it don't you..
Got a tooth pick for the flies too?

Pete
Pete Tipping

These guys make cut down windscreens ready to go.

http://www.ragtops.com
JohnB

The competition windscreen glass is also available from Anglo Parts I think.

They also sell the shortened side bars and pillars, but at amazingly expensinve prices. I have no collected all the chrome parts by being patient (three years) and buying odd bits on ebay. So far I have spent around 100 US.

Now I need to get cutting and polishing ready to get them rechromed. I did speak to Metal Mickey in the UK, and his prices for doing the bits (even supplying them) was significantly cheaper than Anglo Parts, so I'd suggest him. As I now have the bits, I'll use the chrome place around the corner, which gets good crits from others locally.

I think this will be a job for next winter. I plan to have made up special nuts with a thumscrew top (a bit like the 1600 sidescreen nuts on the end) and put holes through the trim panels so that I can change the screen for winter and summer driving without having to remove interior trim.
dominic clancy

This thread was discussed between 30/10/2006 and 06/11/2006

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