Formula 1 teams are reviving a design feature known as the mousehole in their 2026 cars, with an enlarged version dubbed the rathole. This slot in the rear floor aims to optimize airflow into the diffuser. Mercedes and other frontrunners have implemented variations, while McLaren has not yet.
The new 2026 Formula 1 regulations have brought back the mousehole, a slot in the rear of the floor that feeds into the diffuser exit. Previously seen under old ground effect rules, such as on McLaren cars, this feature has been enlarged, prompting some to call it the rathole due to its potential performance impact.
On the Mercedes car, the opening allows airflow to be drawn into the diffuser expansion area. Positioned after the diffuser ramp begins, it helps manage airflow spilling from the tyre's inner corner, where tread meets sidewall. The brake duct's inner surface, marked by a green line in analyses, contains this flow, with a small duct in its lower section directing more air inward.
A vertical splitter inside the brake duct redirects tyre-related airflow around the wheel's inner side, into the gap between tread and track. Louvres on the floor's top surface pull in air to counter low pressure under the car, connecting to the tyre squirt—the air squeezed by tread and track contact. This setup aims to maximize outwash and minimize inwash, enhancing underfloor sealing.
Mercedes has added a small wing above the floor to boost outward airflow. Most leading teams feature similar openings and secondary turning vanes on the brake duct, though McLaren currently lacks one, possibly debuting it in Bahrain.
Technical expert Gary Anderson notes the design's complexity: "The objective here is to maximise the outwash and minimise the inwash." He questions its legality, suggesting it occupies a grey area. As long as it remains a cutout rather than a hole, Anderson believes it complies with regulations, though the FIA may scrutinize it amid the 264-page technical rules.
