ACO 2004 Rear Diffuser vs. Toyota Eagle MkIII GTP Rear Diffuser

[Dauntless]

I was drooling over the photos of Toyota's GTP on MulsanneMike's website this morning when it struck me that the ACO 2004 rules would seem to produce a rear diffuser strikingly similar to the very effective diffuser employed on the all-conquering MkIII.

From the 2004 ACO rules on Mike's website:

3.5.2 - Rear diffuser
An inclined surface (rear diffuser), flat, continuous and rigid is mandatory underneath the car and at the rear.
a/ It must be inclined relative to the reference surface and it must comply with the maximum volume defined by
drawing n°1 ;
b/ No part of the diffuser must be more than 200 mm above the reference surface and its rear end must be plumb
with the perimeter of the bodywork ;
c/ The panels joining the rear diffuser to the reference surface must be vertical. In addition, from the rear axle
centreline to the rearmost edge of the diffuser, the outer panels joining the rear diffuser to the reference surface
must remain parallel to the longitudinal centreline of the car ;
d/ A maximum radius of 10 mm is authorised to connect the rear diffuser to the vertical panels ;
e/ A maximum of two vertical fins may be added to the rear diffuser, but their surfaces must :
e.1 – be at right angles to the diffuser ;
e.2 – be flat and parallel to one another and to the longitudinal centreline of the car ;
e.3 – be positioned symmetrically about the longitudinal centreline of the car.

Now, having read that, check out this photo of the rear diffuser on the Toyota...

http://www.mulsannescorner.com/toymkiii-5.html

Notice that there is a full width flat reference plane ahead of the diffuser, then the diffuser rises to what appears to be no more than 200 mm. And there is one rules-compliant fin per side.

The Toyota was an exceptionally grippy car. Perhaps I underestimated the down force potential of the new rules...

[pirenzo]

the Toyota still wasn't as grippy as modern cars i don't think however...

[gttouring]

the toyota may not have had modern cars rubber development...and wing developments, but the undertray is a fresh air bit isn't it.
From flat bottom to this ahh it feels good

[JAG]

The main thing is we won't have to worry about top speed/humps etc. anymore as the more air gets under the car the more downforce is created , whereas currently if any air gets under the car at high speed it is likely to flip.

With the extra drag a 2004 cars creates top speed may be reduced despite having more power.

Sounds like the ideal compromise to me.


Powerful cars without the top speed problems. Afterall whats the difference between 200mph and 215+mph as the cars were doing in 1999 despite only lapping at the speed of the current cars.

[Dauntless]

The Toyota wasn't as grippy as full-tunneled cars of the era, but I suspect that it is comparable in downforce to today's cars due to its length and width, plus the semi-tunnels. The best of today's cars have better suspensions and especially better dampers and tires than cars of 12-15 years ago. I think that goes a long ways towards explaining the fast lap times of today's cars, in spite of their lower horsepower.

[MulsanneMike]

I'll go out on a limb and say there is little difference between the Toyota's underfloor and the ACO spec. For starters the Toyota's wasn't simply an flat plane as mandated by the ACO for next season. It's a graceful arc. The MkIII's rear exit height is well above 200 mm. Also, the strakes are arcs, not parrallal to the centerline (as would need to be per ACO'04), that's a very large difference as they are generating a nice vortex because of their arc relative to the centerline. I haven't checked their forward leading edge compared to ACO'04, but I suspect they are well forward of that.

Now lets look at the MkIII's aero performance, these numbers come directly from Hiro Fujimori:

Downforce:
4061 lbs. @ 150 mph, with 929 lbs. of drag
5848 lbs. @ 180 mph, with 1338 lbs. of drag
7220 lbs. @ 200 mph, with 1652 lbs. of drag

Peak downforce: 9,000+ lbs. at 200 mph

Lift-to-drag ratio: 4.37:1 (the comparative L/D is decieving to below as GTP cars were pretty dirty at high downforce levels as they had prodigous hp to move them through the air and were less constrained by drag than say contemporary Group C cars)

So that puts the MkIII's downforce easily 4500 lbs. (at peak) more than any contemporary LMP/LMGTP. Consider that the ACO'04 rules are aimed at a 25% downforce reduction with a 10% drag increase. So if a top LMP generates 4500 lbs. for 1000 lbs. drag (4.5:1), you'll be looking at 3375 lbs. downforce for 1100 lbs. drag (3.06:1). I'm pretty confident we'll see similar reductions for '04 cars despite the designers best efforts. Give then a few years. Suffice to say, the ACO'04 rules will not be producing 9000+ downforce monsters, ever.

[cybersdorf]

Pity.

[Dauntless]

Indeed...

[Dauntless]

Originally posted by MulsanneMike
I'll go out on a limb and say there is little difference between the Toyota's underfloor and the ACO spec.

Mike, just to clarify, did you mean to write that "there is a little difference"...?

[C_g]

Also remember that the 2004 cars will not have sidepods that extend all the way down to the reference plane. Instead they will be bevelled, much like keel on a boat. Imagine a rowboat with wheels, and that will give you (an exaggerated) idea of what the car's underfloors will be like.

[MulsanneMike]

Oopps, should read "little similarity..."