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The Merkur Club of America Forums • XR v Cossie (Cosworth) Cd
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XR v Cossie (Cosworth) Cd

Posted: Mon Dec 17, 2007 4:57 pm
by CV12Steve
With photos and NACS, etc. info flying around, an old question worked its way to the front of the gray matter: I seem to remember period data that indicated the US-market XR with its deeper front bumper cover had a lower Cd than its European cousin (.29 v .31 comes to mind).
So here's the question(s):
How much of a factor would the longer wheelbase of the US XR be, as opposed to its less blunt nose?
Did the European 3-door have a bi-wing spoiler?

Re: XR v Cossie Cd

Posted: Mon Dec 17, 2007 5:39 pm
by janvanv
CV12Steve wrote:With photos and NACS, etc. info flying around, an old question worked its way to the front of the gray matter: I seem to remember period data that indicated the US-market XR with its deeper front bumper cover had a lower Cd than its European cousin (.29 v .31 comes to mind).
So here's the question(s):
How much of a factor would the longer wheelbase of the US XR be, as opposed to its less blunt nose?
Did the European 3-door have a bi-wing spoiler?
None because 3 door Cossie, and 4 door Cossies both 2wd and 4wd for that matter ALL Sierras have 102.7" wheelbase.

Escort cossie was 100.5" wheelbase but the only thing different was the length of the exhaust pipes in the tunnel and the rear half of the propshaft.

The Euro 3 door Cosworth had a single big wing, RS500s has a vestigal stump of a lower wing, plain 3 door base cars had no wing.

Posted: Mon Dec 17, 2007 5:40 pm
by MerkXRTurbo
The wheelbase is the same (wheelbase meaning the distance from one hub to the other, looking from the side of the vehicle) between all Sierras.

Every coefficient of drag number I've ever seen for the XR has always been .32. I would assume that this is very close, if not exactly, the same as the XR4i, which the body is identical, besides the bumpers (I doubt that a slightly fatter lip makes a real-world difference). The Sierra was designed for aerodynamics, as it was an economy car. The XR is a performance twist on the Sierra design, but it still shares the same body with a few very minor changes, so it convienently shares the stellar aerodynamics of the economy version.

The Cossie, however, is a totally different animal. You can't compare its aerodynamics to an XR, as that is not the point in the design of the car. When it was designed I think it's safe to say that they pretty much didn't care about aerodynamics... the project had complete focus on performance. It has a different front bumper and side skirts, and then a whale tail which creates a significant increase in CD in favor of down force to help keep the car from lifting at high speed (down force and aerodynamics are always a trade off, as far as I'm aware). The RS Cosworth has a CD of .35, from the data I've seen. The RS500, however, has a dip in the front bumper (don't know if this makes a significant difference in CD), front splitter at the bottom of the bumper (this probably makes a slight difference), with the most notable change being the lower wing, and the more extreme angle on the whale tail for extra down force. From what I have gathered, it has a CD of .38.

The XR is much more aerodynamic than the Cossie, but probably similarly aerodynamic to the base Sierra and XR4i.

Posted: Mon Dec 17, 2007 5:54 pm
by bw_krupp
MerkXRTurbo wrote:(down force and aerodynamics are always a trade off, as far as I'm aware).
Not necessarily true. An absolute brick of a car can have horrible drag and lift, and a car with sci-fi aerodynamics for reduced drag can have downforce as well. I remember reading somewhere (probably on the internet, so it has to be true) that the hood vents on the cossies were more to reduce lift than anything else. I can't see how those hood vents impacted the cd much at all.

Though there is a difference between reducing lift and downforce. You can put a big wing on a car that has lots of lift and it will have downforce, but the forces that created lift are still there. So in the end you could have two cars of roughly the same weight, size and cd, but still have drastically different downforce just by the way the air is utilized.

That is my random thought of the day, enjoy.

Posted: Mon Dec 17, 2007 8:25 pm
by CV12Steve
MerkXRTurbo wrote:The wheelbase is the same (wheelbase meaning the distance from one hub to the other, looking from the side of the vehicle) between all Sierras.
I never actually looked at the numbers but just assumed (yeah, I know . . .) there was a difference based on the shorter Cossie sill covers. As an aside, since it looks like the cladding breaks at the same points relative to the door opening (in photos anyway), why are the Cossie sill covers shorter, or am I misinterpreting something (never seen Cossie cladding up-close and personal)?
Every coefficient of drag number I've ever seen for the XR has always been .32. I would assume that this is very close, if not exactly, the same as the XR4i, which the body is identical, besides the bumpers (I doubt that a slightly fatter lip makes a real-world difference).
Trying to peer through the fog of time I'm pretty sure it (the difference) was referred to in at least one automotive magazine (maybe even AutoCar or Motor, which I was getting at the time) and not any Ford literature I saw. I don't recall if it was supposedly sourced from Ford research.

Posted: Mon Dec 17, 2007 9:06 pm
by CV12Steve
MerkXRTurbo wrote:The Cossie, however, is a totally different animal. . . . It has a different front bumper . . . The RS Cosworth has a CD of .35, from the data I've seen. The RS500, however, has a dip in the front bumper (don't know if this makes a significant difference in CD) . . . it has a CD of .38.
In this incarnation of my XR I'm favoring the down-force side of the equation. It will now be more of a toy so economy of operation isn't really a consideration (no more 30~40,000 mile years).

The thing keeping me away from a Cossie front cover is the thought of not have a bumper structure under it. I once made a right turn in town (≤ 15MPH) only to be met ¾ of the way round by a Cadillac (way before they even started to think about being cool) coming at me, pulling away from my side of the street. The cop had my rear wheel less than 6’ from the curb but the guy still hit me pass. headlight to pass. headlight. After seeing the damage that caused . . .

Posted: Mon Dec 17, 2007 10:24 pm
by milehighXR
I seem to recall Graham Robsons The Sporting Fords Vol 4 as saying that the Cossies were .34. I'm gonna have to spend more time in the book again, beacuse I can't find the page in 30secs. I don't think he specifed which Cossie(RS or RS500). But pne of my brouhcures for the XR states it has a .33CD.

Posted: Mon Dec 17, 2007 11:02 pm
by anglin
Straight from the mouth/fingers of Rick Byrnes:
Rick Byrnes wrote:In a message dated 97-10-28 06:14:00 EST, you write:

I misplaced the original message addressing the differences in wings, and can talk to it a little. While I am not an Aero Engineer, one of my staff is, and made me privy to some actual data, performed on a XR4, with Bi Plane, Single (89) and the Cosworth RS 500 wing. AT 150 MPH:

- Bi Plane Lowest drag highest negative pressure.(lift) .328Cd
- Single wing Increased drag, a little less negative pressure .35?Cd
- RS 500 45 # positive pressure (downforce) and .39? Cd.

I have looked, and cannot now find the data provided, nor do I precisely remember all the numbers (remember CRS syndrom) but all run at production ride heights, and in the same wind tunnel.

Interestingly enough, the bi plane wing is quite effecient at reducing drag without dramatically increasing lift. The number extrapolated to 200 MPH is 315 # negative pressure.

This wing was (in fact the whole car) was copied from the European PROBE aerodynamic research vehicle produced in the very early 80's I have a photo of that original and it was uncanny how much the XR4 looks like it. I have a friend who is a professor at Texas Tech Univ, (PHD in fluids), that when he first walked around my car, was amazed that so much was done right. (he has 2 SVO Mustangs so we forgive the egg head discipline).
...
It sounds to me that the whale tail @ 17 # downforce would be about right since the Wickerbill on the RS500 wing makes a dramatic difference.
...
Incidently, I spun my XR4 at about 170 MPH and the "load of plastic" bi plane wing stayed put, even with just #10 machine screws holding it down. Spinning is such great fun. On the salt they call it doing piroettes. (sp)

Posted: Tue Dec 18, 2007 2:15 am
by Colin
anglin wrote:
Rick Byrnes wrote:I spun my XR4 at about 170 MPH and the "load of plastic" bi plane wing stayed put, even with just #10 machine screws holding it down. Spinning is such great fun. On the salt they call it doing piroettes. (sp)
:shock: gulp

Posted: Tue Dec 18, 2007 10:33 am
by MerlinMerkur
Ive spun 3-360's at 60mph ish... I cant imagine 170 lol... Holy moly.
You know its funny, now that I think about it.. There's a new way you can tell that your rear dampers are wearing!!!
When mine were going out and I got on the highway 75-85ish.. the vibrations visible on the wing and felt in the car reduced lol.. durn bi-wing took weight off the rear wheels I think lol.
HAHA.. maybe we should put "hood" pins on teh hatch too!!! Pimp (rice out) that lol... 8)

Posted: Tue Dec 18, 2007 2:50 pm
by Snake
But what about the rubber flap/wing on the side of the rear side glass on the '88-'89s, maybe they added drag? Anyone know why only 2 model years had them?

Posted: Tue Dec 18, 2007 4:56 pm
by CV12Steve
Snake wrote:But what about the rubber flap/wing on the side of the rear side glass on the '88-'89s, maybe they added drag? Anyone know why only 2 model years had them?
I remember reading at the time they were for cross-wind stability. They would trip airflow trying to go across the hatch to reduce rear lift.
No one responded my other thread asking if they actually made a difference.

Posted: Wed Dec 19, 2007 1:21 am
by John Brennan
I always guessed they put them in there because the single wing was less effective at controlling such things than the biwing.

Posted: Wed Dec 19, 2007 7:19 pm
by milehighXR
John Brennan wrote:I always guessed they put them in there because the single wing was less effective at controlling such things than the biwing.
Probably because the biwing did a fantastic job of keeping rain from landing on the rear window at 50+MPH, and the single wing didn't. Hell the Scorpio does a great job keeping the rear window free of rain at 50+MPH, with it's little dohickeys.

Posted: Thu Jan 31, 2008 12:39 am
by CV12Steve
CV12Steve wrote:
MerkXRTurbo wrote:Every coefficient of drag number I've ever seen for the XR has always been .32. I would assume that this is very close, if not exactly, the same as the XR4i, which the body is identical, besides the bumpers (I doubt that a slightly fatter lip makes a real-world difference).
Trying to peer through the fog of time I'm pretty sure it (the difference) was referred to in at least one automotive magazine (maybe even AutoCar or Motor, which I was getting at the time) and not any Ford literature I saw. I don't recall if it was supposedly sourced from Ford research.
I didn't find the magazine article I was thinking of, but I did find the following references in XR4Ti Genesis, a 6-part series in MOCA's MERKUR MOTORING (beginning Jan '88 vol. 1 no. 6):
"The car would be almost three inches longer than the European XR4, all of it in the carefully restyled bumpers. Later, the wind tunnel would confirm an excellent translation, so good in fact that the North American car improved slightly on the European car's coefficient of drag, of 0.32."
(part 3 pg. 17 vol. 2 no. 2)
"Consequently, Merkur XR4Ti versions have deeper and wider polyurethane pieces, which the wind tunnel confirmed have slightly better performance than the Sierra XR4i. The rear bumper of the Merkur exhausts under-car airflow marginally better . . ."
(part 5 pg. 19 vol. 2 no. 4)
I'm positive there was at least one period, mass-market publication that included diagrams in an article makling the same claims.