The Turtle Wax Trans Am Series has introduced a rule change which serves to ban Garry Rogers Motorsport’s controversial rear windscreen set-up.

GRM’s three cars, unlike other Mustang-bodied vehicles, had come to exhibit a droop in the rear windscreen and trailing end of the roof panel at speed, which its series-leading driver Nathan Herne claimed was discovered by accident but subsequently achieved deliberately.

Competitors have now been issued a technical bulletin advising of a change to the series’ technical regulations which will take effect before the next round.

Article T6, entitled ‘Chassis & Bodywork’, will have the following added (exact wording subject to final approval by the Australian Motor Racing Commission):

It is prohibited to modify or manipulate any bodywork mounting structure. Each bodywork mounting structure (i.e. support strut, primary or secondary frame, or other structure) must be fitted as per the manufacturer’s specifications, including any mounting structure locking mechanism, in its entirety.

Accident damage repair must ensure compliance with this regulation or as otherwise authorised at an event by the TD, in which case the TD will be the sole arbiter during an event with regard to the interpretation and application of these regulations and any decision made by the TD in this regard will not be the subject of any protest.

That new rule text is included in a section of the technical bulletin concerning bodywork/aerodynamics, which draws attention also to particular rules within the T2.1 ‘Modification’ and T6 articles, namely that:

• T2.1 a) Each Automobile must remain unmodified, in compliance with all aspects of these Technical Regulations and identical in all respects to the production make/model as supplied by the original vehicle manufacturer.
• T2.1 b) Any aspect relating to the construction, modification and/or preparation of the Automobile including the location, fitment/mounting of any ancillary component that is not specifically authorised in the present regulations is prohibited.
• T6 d) It is prohibited to manipulate or modify the body panels or their fitment.

While Herne claimed the droop was motivated by gamesmanship, notwithstanding that it had seemingly gone under the radar for multiple rounds before being brought to prominence after the Queensland Raceway event, it is thought that it would have achieved extra rear downforce given the increased airflow onto the wing.

Also on matters aerodynamic, checks will be introduced after it became apparent that front splitter heights can be “manipulated from the factory position” and, from next year, new rules regarding the position of the front splitter/undertray.

In another rule change which takes effect in 2023, the tolerance for wheelbase variation has been dramatically reduced.

Instead of competitors being allowed an inch (officially, 25.4mm) either side of 2640mm, the tolerance will be reduced to just 5mm.

The stated reason is to address the “trend appearing for premature rear gearbox seal and rear bush wear.”

Round 6 of the National Trans Am Series takes place at Sandown as part of the Shannons Motorsport Australia Championships on September 16-18.