Gary Anderson on the 2026 Return of an F1 Design Trend
The Mousehole is Back: A Controversial Return in F1 Design
One of the most talked-about aspects of the new 2026 Formula 1 cars is the resurgence of the 'mousehole'. This design element, seen below in the McLaren car under the old ground effect regulations, is back with a twist. This year, the slot in the rear of the floor leading into the diffuser exit has been enlarged, prompting some to call it the 'rathole'. This design change could potentially disrupt the performance dynamics of the cars, sparking intense debate among engineers and fans alike.
The lead illustration showcases the opening on the Mercedes car. It is believed that this opening allows airflow to be pulled through it into the diffuser expansion area, similar to the effect of mouseholes we've seen before. But why would you want to reduce the performance of the diffuser with such a big opening? It's not as simple as that.
The slot is positioned well after the start of the diffuser expansion area. The inner surface of the brake duct contains the airflow spilling off the inner corner of the tyre where the tread meets the sidewall. It even has a small duct in the lower section to allow more airflow lower down into another inlet route.
Internal to the inner surface of the brake duct, there is also another vertical splitter. This splitter contains the flow spilling off the tyre corner and helps redirect it around the inside of the tyre into the void where the tyre tread comes away from the track surface.
To complicate the airflow structure even more, the small louvres on the floor's top surface allow airflow to be pulled through them. These louvres help reduce the airflow being pulled under the floor by the low pressure underneath the car and connect it to the tyre squirt, which is the airflow created by the squeezing effect from the tyre tread meeting the track surface. If this can be achieved, it will improve the performance of the underfloor in this area.
I have highlighted the tyre squirt with purple arrows showing what will probably try to go inboard and red arrows showing what will probably try to go outboard. The red arrows are longer than the purple arrows because the objective here is to maximize the outwash and minimize the inwash. If that outwash can connect up to the airflow that these louvres set up, then more airflow will be extracted outwards, improving the sealing mechanism of the underfloor.
Mercedes has also added what looks like a small wing section above the floor, increasing the power of that outward airflow in that area. Where this inlet duct is positioned is behind where the diffuser ramp starts and it could be just about where on the inner surface the diffuser boat tail sweeps inwards fairly dramatically around the gearbox. If it sweeps inwards too dramatically, it is a prime area for airflow separation - so optimizing the airflow on the inner diffuser wall might just mean that this opening is required in this area to reduce the risk of airflow separation further outboard.
As we can see from these pictures, most of the other frontrunners have, to some degree, an opening in this area. They also have the secondary turning vane mounted to the inner surface of the brake duct. McLaren (for now) is the exception, although from this picture, it could be just around the corner. If it's not, I'm sure we will see its interpretation of this opening in Bahrain.
Is it Legal?
We often talk about the grey areas, but grey is simply a lighter shade of black and to get that you mix white with it. As with many areas of discovery across these new cars, I doubt that much is black and white and I'm sure the FIA will have a close look at it. However, I doubt that the height of the lower edge of the diffuser sidewall is defined even in the 264 pages that make up the technical regulations. As long as it's not a hole and it's a cutout, I don't see it being a problem.
