Aerodynamics: The Tale of Longtails.
Longtail cars, with their extended overhang behind the rear axle have been around since the days of GT1 racing and were implemented to match ever increases emphasis on aerodynamics amongst the competition. The McLaren F1 GTR Longtail and the Porsche 917 LH are two well known examples.
The longtail is used to modify the aerodynamic performance of a racecar by manipulating the mechanical advantage of the aero package in two ways:
Locating the wing further back behind the rear axle increases its leverage. In a system where the front axle serves as the fulcrum (a class 3 lever system) this exerts a larger downforce on the axle resulting in increased wheel loads. Nothing is free though; whatever vertical load the rear axle gains, the front axle loses. Moments must always balance.
A secondary effect of this extension is that the longer overall length provides a larger underfloor area for negative pressure to act upon. This also allows more efficient diffusers to be designed. The result is a larger level of downforce generated.
They are usually combined with extended front splitters which act in exactly the same way for the front axle.
This ‘longtail’ rear extension is used to manipulate the CoP (Centre of Pressure) location rearwards. Generally the CoP should usually be very close to the CoM from a vehicle dynamics perspective to maintain chassis balance as speed increases.
With rear-mid engined cars like the Porsche and the McLaren, the CoM has a rearward bias, so the ability to create significant downforce and to generate it in the right place on the body is a very useful freedom.
It’s all about leverage. The front splitter, rear wing and underbody aero all generate a pitching moment on the sprung mass. Providing more leverage gives the rear aerodynamics more advantage over the axle.
So, remembering that in equilibrium, all moments must balance.
Moving further, this allows us to show.
As d2 increases relative to d1, the leverage moves towards unity (a value of 1). The smaller d2 gets, the larger the moment becomes and the rear wing's leverage of the rear wheels increases.
Another demonstration of forces and moments in motorsport, there are many and they’ll undoubtedly pop up again!
Regulations these days limit front and rear overhangs behind the axles, which put an end to real long tails, but it was great while it lasted!