AUDI R18 at Sebring

[Jon King]

Audi R18 testing at Sebring this looks a lot lower to ground than the r15plus, plus i wonder how quick this will be in the future especially.

[nopanic - neil]

Audi R18 testing at Sebring this looks a lot lower to ground than the r15plus, plus i wonder how quick this will be in the future especially.

Still looks like a Batmobile - but i still like it 

[mgmark]

That dorsal fin just doesn't work for me - without it, there are lots of Toyota GT One cues, which just shows how far ahead of its time that shape was....

MG Mark

[Stu]

That dorsal fin just doesn't work for me - without it, there are lots of Toyota GT One cues, which just shows how far ahead of its time that shape was....

MG Mark

As been mentioned on other forums, is it not some sort of F-Duct which would serve well on Mulsanne?

[mgmark]

Looking at a back end view, the fin is very thin over the whole cross-section, so it wouldn't allow any repacking of internals (not that you would want additional weight that high above the CofG anyway, and there are no ducts etc on its surface so it's not about repositioning cooling radiators like McLaren have used their fin for in F1.

That leaves using it to improve aerodynamics.  The faster a car goes, the more critical the location of its Centre of Pressure becomes.  Streamlining to reduce drag (the enemy of speed and fuel consumption) naturally lifts the back of a car as it goes faster, reducing the downforce on its back wheels relative to the front wheels, which results in oversteer and, as the back lifts, the Centre of Pressure moves forward which causes directional instability, particularly in cross winds.  Aerodynamic oversteer can be countered by setting up the suspension to produce understeer, but then that becomes excessive at lower speeds.  Aerodynamic add-ons to counter this have been used over the years, such as tail fins to move the Centre of Pressure back, the Kamm tail or a spoiler to reduce lift by interrupting the airflow, or a rear wing to generate downforce to counteract the lift.  These improve stability but at the expense of increased drag.

BUT, the way that a dorsal fin works will even out air pressure differentials between the two sides of the car at the back end, and where the air coming around over the clamshell top would otherwise be turbulent, so it will smooth its passage to the back of the car.  This will improve directional stability (good for the long fast straights) because by directing smoother flowing air over the back of the car's body and the wing, it'll help it meet up with the smooth flowing air from the underside diffuser.  The improved performance of the wing would allow for the right downforce level but with less angle of incidence, which all means less back end lift and less drag and better stability because movement forward of the Centre of Pressure is minimised, which allows for a less compromised suspension set up with less understeer dialled in, which means better handling and less tyre wear.  Pretty much a win all round at the expense of looking a bit sh*t, but the effect will be worth it to give more choices and control over set up. 

And it's not new - think Jaguar D-Type - they put the fin there for the same reason  .

MG Mark

[jimclark]

Audi R18 testing at Sebring this looks a lot lower to ground than the r15plus, plus i wonder how quick this will be in the future especially.

I don't think the ride height minimum has changed for this year but haven't checked.
From the ALMS boards "R18 fast lap was around 1:44. most laps were in between that and 1:48"

[jimclark]

Looking at a back end view, the fin is very thin over the whole cross-section,..............Pretty much a win all round at the expense of looking a bit sh*t, but the effect will be worth it to give more choices and control over set up.  

And it's not new - think Jaguar D-Type - they put the fin there for the same reason  .

MG Mark

Nice dissertation but you don't think that it's being mandated has anything to do with it's existence???    
(Hint: It does, to keep the cars from getting airborne whilst going sideways at high speed)

[mgmark]

Looking at a back end view, the fin is very thin over the whole cross-section,..............Pretty much a win all round at the expense of looking a bit sh*t, but the effect will be worth it to give more choices and control over set up.  

And it's not new - think Jaguar D-Type - they put the fin there for the same reason  .

MG Mark

Nice dissertation but you don't think that it's being mandated has anything to do with it's existence???    
(Hint: It does, to keep the cars from getting airborne whilst going sideways at high speed)

Just explaining reasons that it might be there, beyond the one that it might be mandated, and the other benefits of it............

MG Mark

[Kpy]

As jimclark says, the fin is mandatory. See this years regs 3.6.4.

3.6.4. Fin
a/ General :
A vertical rigid fin:
• must be added to the cars built as from 1st July 2010
(new chassis),
• is recommended for the other cars,
• will be mandatory for all cars as from 01/01/2013
This fin should be:
• Longitudinal and parallel to the car centreline.
• Perfectly located on the longitudinal axis of the car with
equal thickness either side of the centre line.
The fin should have a constant thickness (between 10mm
minimum and 20mm maximum).
With the car complete on its wheels, the visible area (lateral
view) of the fin should be >3000cm² from both sides.
The fin should be continuous without any holes or openings.
No other device can be attached to this fin.
The fin can be integrally fixed to the engine cover and/or fixed
rigidly to the chassis, rear wing and rear structure (on a
“bridge”).
Tools may be required to remove the engine cover and/or the
fin.
b/ Position :
The top edge must be straight and situated between 920mm
and 1030mm above the reference plane.
The leading edge must be straight and situated at the rear of
the roof (closed car) or at a maximum of 300mm behind the
cockpit opening (open car).
The trailing edge must be straight and situated between 350mm
and 450mm behind the rear axle centre line.
The bottom edge may be no more than 25mm above bodywork
surface.
c/ Geometry :
The leading edge, top edge and bottom edge may be made
round with a constant radius (the radius will be equal to half of
the fin thickness).
The trailing edge may be bevelled or have an elliptical form on
no more than 20mm.
50mm radius maximum is permitted between top/leading edge,
top/trailing edge, bottom/leading edge & bottom/trailing edge.
If the fin is attached to the engine cover, a maximum of 50mm
radius will be permitted between both parts.
d/ Deflection :
A static load test using 400mm long channel tool, 60 mm high,
positioned over the top edge of the fin, will be applied.
The middle of the channel can be positioned anywhere along
the top edge of the fin (with the most rearward position being
the rear axle of the car) so that it will not overhang either end
(side view blend radius will be ignored).
Load will be applied at the centre of the 400mm channel.
This test will be carried out 2 times on fin in situ so that
mountings to the chassis / bodywork are also tested.
For each test the deflection of the fin can be no more than
100mm (at any points) for a load of 100daN and any permanent
deformation must be less than 3 mm after the load has been
released for 1 minute.

[Canada Phil]

Hi Mark,
             Thank you for you explanation. If aside from looking a bit sh*t a fin has so many advantages why have we not seen them in recent years?
 Second question. what or how much in "English Money" is 100daN ? in the side load test.
 Phil

[lofty]

the fin cant be helpfull in a cross wind.in and out the trees walls and buildings down mulsanne it could be a bit scary.

[Doris]

Second question. what or how much in "English Money" is 100daN ? in the side load test.
 Phil

daN = decanewton = 1000 times the force of gravity.

Dx

[Canada Phil]

Second question. what or how much in "English Money" is 100daN ? in the side load test.
 Phil

daN = decanewton = 1000 times the force of gravity.

Dx

Hey Doris  Thanks
 Phil

[mgmark]

Hi Mark,
             Thank you for you explanation. If aside from looking a bit sh*t a fin has so many advantages why have we not seen them in recent years?
Phil

Phil,

No problem, glad you enjoyed the rambling.  Ledwinka started it all for the reasons I mentioned with the classic Tatra T87 teardrop design – actually, before that with Zeppelins.... Streamlining reduces drag, so gives a trade-off choice between top speed increase or a lower powered engine, but induces lift and instability which is countered by the dorsal fin. 

Jaguar needed more speed than they could extract from the C-Type and needed to resolve directional instability, particularly in cross-winds.  The need for good "aero" in relation to top speed has been long known.  Before it overheated, the "droop-snoot" C-Type pulled about 152 mph on the Mulsanne, around 8 mph better than the 1951 car, which had near identical power.  Wind tunnel tests on the D-Type by Jaguar (Bob Berry in Jaguar: Motor Racing and the Manufacturer) show a nose-heavy weight distribution of 53.5/46.5, reducing as speed increased to 51/49. So, while the whole car may have been lifting, the tail wasn't now lifting more than the nose and, with the fin added to the headrest, the Centre of Pressure wasn't moving across the car as well as forwards, so D-Type drivers could reportedly “scream along at 170-plus with their hands off the steering wheel”.  Jaguar measure drag as the power needed to maintain 100 mph, this was reduced by more than 28% against the C-Type and, even against the the "droop snoot", the D-Type was better by 10%. So, the new car was not only faster, it was more stable and gained effective weight on the rear at high speed. 

So if it was known about then, why did it fall out of use?  I don't know, but perhaps it just became less of an issue when rear-engined cars came on the scene but, even as they developed, you can detect a similar application of the principles in the 80s Group C cars at Le Mans, with the extended tails, narrower chord wings and long fins each side of the wing, compared to the WSC and IMSA equivalents - speed, drag, stability and handling trade offs again.  As design has continued to progress and teams are looking to shave fractions off, something that probably gives that advantage and makes it more stable in terms of suspension set up for mid-high speed corners would be a good thing, irrespective of aesthetics.         

And that’s before one looks at whether there is a positive effect of it on any actually intended directional change when taking a corner.  The fin might also promote yaw stability because the force generated by the fin when the car is turned would tend to re-align the car (i.e. when the car turns, the fin will generate an opposite pressure) which means that you might be able to set the suspension up to tend towards oversteer rather than understeer, which would the car more nimble but without the swift tail into the barrier moment that oversteer normally brings.  But that would be just surmising.......  

MG Mark

[Stu]

That explains it then