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G-Force tour report
The Colorado section of SAE sponsored a tour of G Force
Precision Engineering in Golden, CO, on February 17th.
G Force is a race car component manufacturer and chassis
builder. Much of the component production takes place in
West Sussex, England, especially carbon fiber and other
composite parts. The Colorado location concentrates on
Indy car assembly and race support. G Force chassis cars
finished 1st, 2nd and 3rd in the 1997 Indy 500. They also
made major contributions to the Thrust SSC land speed
record effort.
The tour consisted of a guided ogling of G Force Indy
cars in various stages of construction. David Cooper
provided a highly informed commentary and fielded a
continuous barrage of questions with great aplomb. It
was an absolute nerd feeding frenzy. I absorbed as much
of the detail as possible. Here are a few observations.
1997 Indy car specs:
Weight less fuel and driver = 1620 lb. max.
Dimensions = 195"L x 37.5"H x 78.5"W
Wheelbase = 118"
Tires = 25.5 x 10 x 15 Front, 27 x 14 x 15 Rear
Gearbox = Emco IRL, 6 forward gears, H-pattern
Fuel capacity = 35 gal.
Engine = Aurora V8 NA approx. 700 BHP
Cost fully equipped and instrumented over US$450K
(I'm not certain the engine is included)
Basic construction:
The chassis core consists of a longitudinal stack of
four pieces bolted together: the carbon fiber tub is
bolted to the front of the engine which is bolted to
the cast alloy gearbox adapter ("bell"?) housing which
is bolted to the transaxle. In other words, the engine
is the central piece of the chassis structure. In
general, six small bolts hold each section to the
next. The front suspension is bolted to the tub. The
rear suspension is bolted to both the adapter housing
and the gearbox.
Additional construction detail:
The tub consists of upper and lower halves bonded
together over a system of machined aluminum bulkheads.
The control hardware is fastened to these bulkheads.
The fuel cell is in the rear of the tub between the
driver's seat and the engine. The heat exchangers are
cantilevered off the sides of the tub. Water is cooled
on the left side, oil on the right. Oil cooling is
given much more weight than we are accustomed to; the
oil-to-air heat exchanger is approximately 2/3 the volume
of the water cooler. No fans. Carbon fiber body panels
attach to the core chassis, the main piece being the
bottom pan and air tunnel (regulation Indy car tunnel
dimensions).
Much of the construction technology appeared to be right
out of Carrol Smiths "_________ to Win" series of chassis
engineering books although I'm sure there have been many
small advancements in the technology since Smith wrote
them. Still, I'd say the basic principles haven't evolved
much in the last ten years. For example, Carrol describes
a small, blade type anti-roll bar which is cockpit
adjustable by rotation of the blade cross-section relative
to the plane of deflection. Bingo and ditto for hundreds of
other such design features still in use today.
Suspension:
Classic double A-arm. Suspension members 4130 alloy heat
treated. Uprights lightweight welded box construction.
Stiff? You better believe it. Full droop vertical tire
displacement appeared to be about 1/4". Coil-over shocks
are mounted on top of the tub (gearbox adapter in the rear)
parallel to the longitudinal axis and operated via push rods
attached to the bases of the uprights coupled through pivot
arm assemblies to convert vertical suspension displacements
into longitudinal displacements. The spring coils are about
5" long by 2.5" I.D. by maybe 1/2" wire O.D. The pivot arms
- one per side - give the springs about a 3:1 mechanical
advantage over the upright push rods and also couple to the
anti-roll linkage. To paraphrase Dave Cooper, the suspension
geometry has not changed since Christ was a child. Perfect is
perfect.
Brakes:
AP Racing 6-pot calipers all around. Carbon brake rotors
and pads. Dave reported that the braking point into a corner
is dramatically delayed for carbon brake equipped cars relative
to steel rotor setups. I didn't get a full understanding of
why this was so but it seemed to involve better utilizing
tire grip during braking rather than ultimate braking
torque. Rotor size was identical between front and rear.
These cars don't dump all the weight onto the front tires
during braking like ours do. The car weight distribution
is biased toward the rear also.
Miscellaneous details:
Tiny little battery, no on-board starter. The car can only
be started in the pits requiring a start crew of four.
The 4 (?) disk clutch pack hangs out in the air inside the
engine to gearbox adapter.
The oil and water pumps are external to the engine block.
The small alternator is driven via a shaft extension aft
from the water pump.
The water heat exchanger inlet area is between 1/4 and 1/5
of the face area of the exchanger itself. (This is very
good news for those of us who visualize most of the frontal
area of our cars being devoted to the intercooler.) However,
very careful duct design is required to make this work.
Exhaust air from the heat exchangers is contained within
the body panels and routed aft along the engine block and
gear box, exiting at the tail underneath the rear wing.
Exhaust plumbing was not on display. Intake air is collected
in a giant air box bolted to the top of the engine. It's
intake duct for the box is immediately behind the driver's
head. (You think rock and roll concerts are hard on the ears.)
Individual, per cylinder intake horns extend vertically about
four inches into the interior volume of the air box.
An on-board data acquisition system (the deluxe package)
provides real time monitoring, via telemetry, of many vehicle
parameters during operation. For example, strain gauges sense
deflection in suspension members.
Well, I guess I better shut up. Sorry about the bandwidth;
the whole thing just made be giddy. If you have any questions
about Indy cars, ask Eric. ;-|
DeWitt Harrison de@aztek-eng.com
Boulder, CO
88 5kcstq