Downforce pressure

What is the downforce pressure on MKII GT40 and what is the downforce difference between MKI and MKII?
 
I think what you're really asking is "what is the downforce" not what is the "downforce pressure". I would like to know myself. I do know it was minimal in both cars, certainly not as much as today's performance cars like the Carrera GT or F430 with their aerodynamically tuned under trays.
 
I cannot vouch for the complete accuracy of these values, but:

1966 MkII

Lift:
100 lbs. @ 150 mph, with 525 lbs. of drag
144 lbs. @ 180 mph, with 756 lbs. of drag
177 lbs. @ 200 mph, with 933 lbs. of drag

1967 MkIV

Lift:
148 lbs. @ 150 mph, with 365 lbs. of drag
213 lbs. @ 180 mph, with 554 lbs. of drag
263 lbs. @ 200 mph, with 648 lbs. of drag
318 lbs. @ 220 mph, with 828 lbs. of drag

pulled from Ian Bamsey's The Anatomy and Development of the Sports
Prototype Racing Car.

I cannot find any MkI/MkIB info, but I suspect it would be pretty close
to the MkII info. And note - that's "Lift", not "Downforce".

Ian
 

Jim Rosenthal

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I didn't think it was a daft question. I don't know the answer, either, but isn't this the kind of place where you're supposed to ask things like that?
 

Randy V

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I agree Jim - and I don't have the answers either...

Welcome to GT40s Gene...

I think you can see from the charts that Ian has posted that the most downforce either of them would have would be when they are sitting still...

I don't think Ford planned it that way - but I don't think that they were real big on doing wind-tunnel testing back in the day..
 
Actually, in digging up that info, I read a little about what was going on.
They were interested in wind tunnel testing (and, as we know, they did
a bunch of it for the MkI, MkII and MkIV), but they didn't understand
lift vs. downforce vs. drag too well at the time. It would be interesting to
dig up the MkIB numbers as compared to the MkIs and MkIIs, just to see
if JW Racing's inklings about aero were correct. Maybe even some Mirage
data too, to see how it compares.

Ian
 
Be interesting to know at which stage in development those readings were taken, both Mk2/IV had rear spoilers of various designs during competitive lives.
 

Randy V

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Be interesting to know at which stage in development those readings were taken, both Mk2/IV had rear spoilers of various designs during competitive lives.

As did the MK1...

Gulf cars with the Alloy adjustable rear spoiler..
 
Would probably also depend on whether the car had front canards installed or not...

Would be interesting to know where that "cg" of the lift is at those speeds - more towards the front or the rear?
 
I think this is the sort of complexity David was alluding to when he called it a daft question. Handling and all sorts of characteristics are affected and interactive. For a given "lift" force, what percentage is front, and what is rear? What is the rate of change of lift as the speed changes, or as the car pitch changes as it accelerates hard then brakes hard, or in handling tight bends, and so on? It is a dynamic condition that is very hard to describe in numbers which can then be compared, due to the changing distributions of the vertical and horizontal aerodynamic forces.
 

Randy V

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Dalton - Are none of those dynamic complexities involved in current racing cars that are well documented in their aerodynamics ??

Yes - it's an easy question to ask - yet it's a hard question to answer.

I'm not a Brit as you know - but my understanding of he word daft is not complimentary by any stretch and quite frankly I thought it to be out of character from what I've grown to know David by..
 
Thanks for the information and the support fellas.

Does one with wisdom truly believe that there is such a thing as a daft question?
 
As far as I know, these were MkIIA and MkIV as raced. There was some
mention of the J Car having reduced drag - at the time, reducing drag
was the priority, reducing lift secondary, and no real thought into actually
significantly increasing downforce.

Broadley learned a bit from the GT40 experience - instead of ~300kg lift
that the GT40 experienced, the T70 IIIB coupe had ~200 kg downforce
(Derek Bell's site has some good info WRT the T70 and a brief history of
aerodynamics in racing in general).

Anywhere, here is a pretty nifty site with a few cars and their lift/downforce
and drag values:

Mulsanne's Corner: Race Car Aerodynamics Database

Ian
 
Thanks for the information and the support fellas.

Does one with wisdom truly believe that there is such a thing as a daft question?

No, just daft people asking questions. :laugh:

Seriously, it's a good question. To shed some light along with the previous heat, here is an interesting comparison. When Ford was jointly developing the Pantera with De Tomaso, they conducted wind-tunnel tests, and used a Boss 302 Mustang and a GT40 (presumably a Mk 1?) for comparison purposes. Here is a link to the results:

Pantera/GT40/Boss 302 Aero tests

The image initially opens in a reduced size; you can select the full-size version in the upper right corner, and that enables a very good look at the three graphs (for drag, total lift, and front/rear lift (negative lift is what we call 'downforce' but it was a rare commodity back in those days).

The graphs are a bit confusing as they are unlabelled, but by analyzing the data in the adjoining chart, it's possible to determine that the Mustang results are indicated with squares, the Pantera with circles, and the GT40 with triangles.

As you can see, the aerodynamics of the GT40 weren't particularly good, certainly by today's standards. There was still a lot left to learn about automotive aerodynamics in the 1960s and 1970s....
 
As an interesting side note about aero in that era, when Lotus started bringing cars to Indy they set the cars up differently then when running F1.
They thought the g-force generated in the banked turns would compress the cars suspension down to the stops and set up the alignment based on that assumption.
When they finally got around to measuring what was actually happening they discovered that the "round tub" cars were generating lift at those speeds and the cars ride height was actually 1.5" higher! (from "Lotus, the Indy Years") than static.
That led to wings and other methods of pushing the cars down.
Dave
 
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