At the April 26, 1992 round at Road Atlanta the Nissan team suffered multiple tire related failures that resulted in the destruction of two race cars.  Both failures occurred during the race, and most unusually, within about 100 yards of one another on the track, although 41 laps apart.  Nissan GTP driver and multiple IMSA GTP Champion Geoff Brabham relates, “After 10 laps I came onto the straight to see pieces of tire all over the road and Chip’s (Robinson) Nissan upside down in the middle of the road.  I knew what had happened and in our wisdom, or lack of it, the team and myself decided to continue but stop more often for tires as I was running 3rd at the time.”  But on lap 55 it happened again and this time Brabham’s Nissan ended up on its roof, “The marshals did not know what to do and after a very long while it was my crew who picked the car up enough for me to get out, it could have been a very nasty situation.” (IMSA's Bob Laubach relays that this incident was the culmination of a number around this time period that ultimately led to the creation of IMSA's traveling safety team.  No longer would track safety be the responsibility of the local safety crews)

In addition to the Nissans, Juan Fangio in the All American Racing Toyota Eagle MkIII also fell afoul of similar tire issues.  From the cockpit Fangio had witnessed both Nissans’ accidents and the team decided to tell him to slow down fearing a similar failure, “They said to me, ‘Juan, there is a tire problem, and we are on the same choice as the Nissans,” says Fangio.  “The big problem was that my mind didn’t allow me to slow down.  You can do this when you feel something wrong in the car, but this was not the case, because the tires were good until they were not any more.”  Fangio’s tire delaminated at the most inopportune place on the track, the dip between turns 9 and 10 and ultimately not too far from where both Nissans went off, “The only problem was to slow down the car from over 200 miles per hour, going through the big dip on the twisty straight, miss the bridge and make the turn."

Goodyear’s concerns about downforce apparently predated the Road Atlanta incident; while Goodyear were developing the 1992 tire they inquired with the teams regarding the downforce levels they were running.  It turned out that Goodyear had underestimated how much downforce the cars were generating by some 20%.  Naturally the tires were designed to the information the teams provided and not the Goodyear estimate, but the point was that by 1992 the aerodynamic pace of development had quickened to the point where even the tire companies were caught by surprise.

Ultimately Goodyear struggled to come up with a conclusive reason for Nissan's tire failures, being unable to recreate the mode of failure in testing, and could only offer that the tire batch in question was at the limits of its life cycle.  And while that most certainly was the case, it isn’t much of a stretch to say that the downforce loads the cars were carrying exacerbated the situation.  But another finger might point at the static weight of the cars coupled with high downforce loads.

Of all the IMSA GTP competitors, the Nissan was carrying the most overall weight.  This was primarily a factor of the IMSA regulations at the time and ultimately Nissan was in mid strides to correct this by changing their philosophy to that of their competitors.  On IMSA’s sliding scale of weight vs. engine capacity, the Nissan weighed in at 2100 lbs as they were running the maximum turbo capacity of 3.0 liters.  But all of Nissan’s competitors, Toyota, Intrepid, and Jaguar, were taking advantage of the weight reduction possibilities granted by IMSA of utilizing the Group C specification underfloor.  Under IMSA rules, competitors running to Group C underfloor regulations were allowed to take 100 lbs off the overall weight of their car.  And while the Group C spec underfloor regulations put (slight) limiting factors on the dimension of the tunnels (reduced tunnel exit size and mandated flat bottom area ahead of the tunnel leading edge), and in theory made them less effective at generating downforce, ultimately it didn’t really work out that way as downforce levels continued to increase even as teams adopted Group C spec tunnels in order to take advantage of the IMSA allowed weight break.

Contrary to the Nissans, Toyota came in at 1830 lbs running a 2.1 liter turbo in combination with a Group C spec underfloor.  Toyota coupled a small capacity 4-cylinder engine (which therefore weighed less on IMSA's sliding scale to begin with) with the weight reduction allotted to the Group C underfloor.  Toyota aerodynamicist Hiro Fujimori explains, “We knew that the Group C style tunnel lost downforce by making the tunnel exit smaller, but it would make the same downforce or more as a big tunnel simply by adjusting the contour of the tunnel.”  So nothing lost from an aerodynamics perspective and certainly something gained (free weight reduction).  And while it took Toyota a few years to iron out the reliability of their highly stressed 4-cylinder, in the end it proved to be the equal of the front runners in terms of power, even while being a counter intuitive choice. 

At the extreme end of the weight scale, the Jaguar XJR-14 was the lightest GTP in the field.  From 1990 onwards (upon the announcement of the pending changes in Europe for 1991), Group C derived 3.5 liter formula prototypes were allowed to race in IMSA at 1750 lbs.  In combination with the Group C spec underfloor, the all up regulation weight was a controversial 1650 lbs.  Ultimately the IMSA-ized XJR-14 gained weight as components were beefed up and TWR never did reach the 1650 minimum with the car.  But even in its debut race, the Miami GP, the Jaguar was tilting the scales at a lithe1680 lbs.

Obviously Nissan was cognizant of the advantages of all of this and was working hard to put the NPT-90 on a diet for 1992.  A reduction in turbo engine capacity to 2.5 liters would move the Nissan 100 lbs down the weight scale.  And fitting a Group C spec underfloor to the car would make their new target weight 1900 lbs; a full 200 lbs under where they started the season at.  This would be achieved by analyzing every aspect of the car’s design and finding weight wherever it could be pared.  This would mean ounces off some items, pounds off others.  But destroying two cars so early in the season certainly complicated Nissan’s issues; in order to prove these new, lighter items, the team needed two cars to be circulating the test track, not to mention the race track.  But at Road Atlanta the Nissan was still at 2100 lbs; the weight reduction program was in-process and bits were only just trickling onto the car.

While the root cause of the tire failures was never determined, the Nissan’s more than 2100 lbs of static weight in combination with around 7000 lbs of dynamic “weight” (Kas Kastner indicates the failures occurred while the cars were still accelerating and at an estimated 185 mph) probably put them in the best position to exacerbate any underlying tire issues.  And even Nissan’s Trevor Harris hinted to as much in On-Track magazine at the time (June 12, Full Chat section) referring to the Atlanta weekend, “Ours is the most highly-loaded car vertically,” and "Under certain circumstances, we can reach 5,000 to 6,000 lbs of load."  Harris also points out that the failures seemed to occur from the heavier cars to the lighter cars (with the lighter cars actually sustaining no failures).  Compare the Nissan's 2100 lbs to the Toyota’s 1830 lbs. or Jaguar’s 1680+ lbs. with about the same amount of downforce and it begins to make further sense.  And even if the Nissan's downforce advantage was only along the lines of a couple hundred pounds, by adding that total to the static weight difference you're very quickly approaching 500 lbs of additional tire loading, if not more.  

Though we'd be remiss in not mentioning an additional suggestion to the source of the failures.  We've been told it could have had something to do with running lower than the recommended tire pressures in order to gain a traction advantage.  This suggestion came to us courtesy of another competitor, one that was also running massive levels of downforce but did not have any tire issues at the event.  Though naturally this is just further speculation.

Regardless of the cause, the result was detrimental and shook Team Nissan to the core.  Trevor Harris simply stated, “We never really recovered from that weekend.”  Indeed, the team was unable to consistently field two cars following Road Atlanta and their 1992 title bid fell flat (though their second place in the Manufacturer's Championship flattered to deceive; they only won one race that year).