HONDA The Power of Dreams

SUPER GT is a racing series for cars based on production vehicles. There are two classes: GT500 and GT300. In the GT500 class, automakers Honda, Toyota, and Nissan each team up with a group company specializing in motorsports activities to develop vehicles for their team (in Honda’s case, Honda Racing Corporation (HRC) since 2022). For the automakers, it is part of their official corporate business.

GT300 is the class for privateers and comprises cars conforming to rules set out, for example, in GT3 regulations defined by the Fédération Internationale de l'Automobile (FIA) or GTA-GT300 regulations defined by GTA Co., Ltd., which runs SUPER GT. The numbers “500” and “300” represent the estimated horsepower of the vehicles, the difference being apparent in their on-track performance.

GT500 class car regulations underwent major changes in 2014 with a view to unifying them with technical regulations for the Deutsche Tourenwagen Masters (DTM). Previously, manufacturers could design their own chassis within the scope permitted in the regulations, but from 2014, they were compelled to use a common monocoque. Common parts were also specified for many of the components. The aims were to keep down development costs and prevent excessive competition.

In addition to the carbon monocoque, common parts included the gearbox, subframe, braking system, fuel tank, dampers, and drive shafts. The monocoque specified as a common part was intended for a front-engine, rear-wheel drive (FR) layout. Under the new regulations, cars were to be fitted with a 2.0-liter turbocharged inline four-cylinder engine at the front of the vehicle, replacing the 3.4-liter naturally aspirated V8 engine used up to the previous year.

The longitudinally mounted inline-four engine was secured to the front bulkhead of the common monocoque. Mounted at the rear, connected via the bellhousing, was a common gearbox. Secured in front of the engine and behind the gearbox were crushable structures also specified as common parts. The new regulations were configured around this basic layout.

Honda opted to use the second-generation NSX as its base car to develop a vehicle conforming to the new regulations. The second-generation NSX was unveiled for the first time worldwide as a concept car at the North American International Auto Show in January 2012. The car made its first public appearance in Japan in February 2012.

The second-generation NSX had a mid-engine layout with a specially designed 3.5-liter twin-turbo V6 engine mounted between the cabin and rear axle. Two motors were mounted at the front and one in the rear, and Sport Hybrid SH-AWD technology was deployed, optimizing torque distribution between front and rear wheels and between left and right front wheels.

While it was not possible to employ the exact same systems as the production model, Honda judged that for the car to compete as an NSX, retaining its mid-engine, rear-wheel drive (MR) layout and hybrid system was essential. Talks with the rules governing body and rival automakers ended with MR cars being allowed to compete on the condition that changes to vehicle regulations and common parts would be kept to a minimum and that performance adjustments would be made.

Development of Honda’s NSX CONCEPT-GT got underway.

NSX CONCEPT-GT

On paper, calculations concluded that mounting an inline-four engine, regulated to no more than 500 millimeters long, in the middle of the vehicle would require moving the monocoque forward 300 millimeters. If only Honda could design its own monocoque, it would be able to optimize the MR layout. However, the new GT500 regulations mandated the use of a common monocoque.

Honda discovered that moving the common monocoque forward 300 millimeters to realize an MR layout would bring the left and right corners of the front end of the monocoque too close to the front wheels. While this did not imply the layout was unachievable, it was expected to adversely affect aerodynamic performance.

GT cars use a splitter—a panel beneath the front bumper—to generate front downforce. Directing airflow well is crucial for effectively eliciting the best performance and the area behind each of the front wheelhouses plays a major part in that. It was predicted that the vehicle’s aerodynamics would be considerably diminished were the left and right corners of the front end of the monocoque to encroach on this area.

A decision was made to cut away the left and right corners of the front end of the monocoque while accepting it would diminish the monocoque’s rigidity (with adverse effects on the vehicle’s dynamic performance). Obtaining the governing body’s permission to change the specifications, Honda went ahead, opting to secure space behind the front wheelhouses and achieve the same aerodynamic conditions as a car with an FR layout.

The carbon monocoque designated as a GT500 common part

MR layout adaptation of the common monocoque intended for FR cars

The reason the monocoque was brought forward only 300 millimeters for the MR layout was that any further would obstruct the layout of the double wishbone front suspension. Bringing the monocoque forward affected the points where the suspension’s upper and lower arms attached to the chassis (subframe). The front legs of the wishbone arms could not be shifted forward because they would have interfered with the crushable structure. Only the rear legs, where they attached to the chassis, were moved forward, avoiding the monocoque.

To achieve this, the span between the suspension arms was shortened, though shortening it too much would affect the ability to maintain the necessary strength and rigidity. A 300-millimeter forward shift was the limit.

Converting from an FR to an MR layout was no easier at the rear. Using the specified common gearbox as it was would have seen the differential retreat toward the rear, though this, it was found, would have imposed an unrealistically large angle on the driveshaft.

The governing body was therefore asked to approve an MR gearbox as a common part for MR cars. While made by the same supplier as the common gearbox, it had dedicated MR specifications. The orientation of the gear cluster (containing ratio gears and a transmission mechanism) was changed from longitudinal to horizontal, thereby shortening the overall length. This was after determining to specify the same points for attaching the rear suspension as for FR cars.

Adjustments to suspension mounting points

In the original GT500 layout, a bellhousing for length adjustment was inserted between the monocoque and gearbox. This contained the clutch. For the mid-engined NSX CONCEPT-GT, the bellhousing was done away with to secure space for the engine. The clutch was housed at the front end of the MR gearbox. Even then, the center differential was further to the rear than in an FR car. As it stood, the driveshaft specified as a common part was not long enough. Approval had to be obtained to use a special long 61-millimeter driveshaft (from the same supplier).

Other modifications were required to secure the engine to the rear bulkhead of the common monocoque. There was nowhere to connect the engine to the base of the rear of the common monocoque as the diffuser—an aerodynamic device—was already beginning to rise up. Instead, the engine was secured using adapters fitted to mounts attached to the front cover. Mounts at the top of the engine were given additional bosses for securing it to the rear bulkhead of the monocoque.

Adjustments to engine mounting points

Changes were also made to the roll cage. This was because moving the monocoque forward 300 millimeters meant the roll cage interfered with the cabin. To overcome this problem, the tubing at the front was moved a little toward the rear. At the rear, a structure that could be partitioned was adopted in place of the original single integrated structure as the latter did not allow loading and unloading of the engine. The design was approved after it was proven the structural changes caused no strength issues.

Changes to the roll cage structure

Generally speaking, an MR layout has advantages over an FR layout in terms of the vehicle’s dynamic performance, but that is not why Honda made a point of adopting an MR layout. The motivation came from wanting to use the same technologies for both production NSX and GT500 NSX models. Because the regulations newly introduced in 2014 were intended for an FR layout, modification into an MR layout was not considered advantageous.

However, intentionally adopting an MR layout for the NSX CONCEPT-GT helped to promote better understanding of MR vehicle characteristics and was later useful for acquiring the know-how required to enhance performance.