I want to get into this whole high rake, low rake business but first I want to thank Skillshare
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1. Let’s chat a bit about rake then, because the word on the street is that cars with low
rake design concepts like the Mercedes and Aston Martin are suffering more than their
high rake competitors as a result of the 2021 floor regulation revisions.
2. So let’s look at: - what rake is and why teams might choose
to run one way or the other 3. – what the new rules are around the floor
of the car, and the impact they have on the aerodynamics
4. – what the suspected effects are on the teams and why it might have stymied lower
rake car designs 5. And while we’re at it we can take a quick
look at the efforts teams are doing to to recreate the aerodynamic work taken away from
the modified floors. 6. Rake then. If you draw a line along the
ground and a line along the floor of the car, the rake is simply how aggressive the angle
between them is. 7. A completely flat bottomed car has no rake
at all, whereas a car with its nose down and bum in the air has a high rake.
8. Mercedes has stuck pretty firmly to a low rake concept for many years, while Red Bull
have been among the most potently rakish cars in the field for the last decade.
9. To get to the bottom of these designs we need to consider underbody downforce and the
diffuser. 10. While most of the aerodynamic downforce
that pushes the car onto the ground to give it grip at high speeds comes from the top
of the car – generated by air passing over the wings and the chassis – over a third
of the downforce comes from underneath the car.
11. Downforce manifests as a result of differing air pressures separated by car bodywork.
12. If you have high pressure on the top side of, say, a wing element, and low pressure
on the underside, this generates a force acting from the high pressure area to the low pressure
area, effectively pushing the wing down. As wings are attached to the car, this downward
force pushes the whole car down at that point. 13. The floor of an F1 car is basically pretty
flat until just in front of the rear wheel centreline where it is allowed to rise up
a further 175 mm at the back in the area known as the diffuser.
14. This expanding volume underneath the car at the back essentially creates a fast moving
suction of air through the flat, low lying area of the floor which is much lower pressure
than the ambient air. 15. This happens because the airflow at the
end of the diffuser has to move with the ‘ambient air velocity’ – that is, just the normal
airflow speed around the car but with much less space to work with, the air under the
floor has to move much, much faster to keep up with the rate of air volume displaced.
16. This low pressure underneath therefore creates a downforce effect on the car, pulling
it down into the ground as the car moves at speed.
17. Now if we apply some rake to the car, lifting the rear end, essentially, we increase
space at the back of the car for escaping air and thereby increase the difference between
this gap at the rear and the squeeze at the front, amplifying the power of the diffuser
effect and increasing underbody downforce significantly.
18. A higher rake car has potentially bigger suction underneath, pulling the car into the
ground. 19. Ok so let’s talk downsides.
20. There are some issues with raising the centre of mass, and inducing roll in the car
and so on, but I’m going to strictly stick with aerodynamic effects for now.
21. If you have an area of high pressure air and an area of low pressure air, the high
pressure air is going to rush into the low pressure area until they equalise.
22. You can see this when you release the nozzle on an inflated balloon – the high
pressure air inside rushes out to the relatively low pressure air outside as soon as you allow
a gap for them to merge. 23. And so it is with a car’s underfloor.
If there’s an expanse of very low pressure air under the car, the relatively high pressure
are surrounding it is going to want to rush underneath the air, which ruins all the nice
clean suction running through the diffuser. 24. This is why, back in the hardcore ground
effect days, cars had skirts running down the sides – physical bits of bodywork that
blocked outside air from being sucked into the underbody airflow. These are now banned
as those cars had dangerously high cornering speeds.
25. But you can see how the more you raise your car with rake, the more you expose the
sides to the high pressure air around the car, and make your diffuser effect more vulnerable.
26. Now one of the ways teams have been tackling this is with vortex generation.
27. A vortex is just a tight, high energy whirlwind of air – you can generate with
cleverly exposed wingtips and aero devices. I have an aerodynamics video that goes into
more detail, but the essential thing you need to know is they can provide an invisible barrier
to separate your airflows. 28. So if you have a high pressure air volume
here and a low pressure air volume here, by generating a big and energetic vortex than
runs between them, you can stop the high pressure airflow bleeding into the other.
29. The main vortices are generated right from the front wing and shaped and amplified
by turning vanes and other aero devices along the car, but in recent years teams have been
adding slots to the long edges of the floors to generate vortices explicitly down the length
of the floor edge to create something of a “virtual skirt” protecting the low pressure
underfloor suction from the outside. 30. And while we’re on floor slots I should
mention tyre squirt, which is a hilarious name but here we are.
31. As the rear tyre rotates at high speed it pushes air down and around its footprint
with the track. This air naturally gets sucked into the low pressure area around the diffuser,
which decreases its efficiency. 32. By putting slots around where the tyre
meets the floor, it allows some of this high pressure air to escape out and away from the
underbody. The diffuser strakes – tall fences that run along the length of the diffuser
also help protect the underbody air from outside spill.
33. Mercedes seem to have preferred to keep a flatter rake concept to minimise aero inefficiency
that is inherent through these gaps in the side of the floor.
34. They also run a longer car than Red Bull to gain back from underbody downforce as the
downforce generated is proportional to pressure by area, so increasing the area of low pressure
under the car boosts their downforce. 35. So up to 2020 we had Red Bull with its
more powerful but potentially less efficient diffuser and underbody and Mercedes with its
less powerful but more efficient underfloor. 36. Now for 2021 we have a bunch of rules
around the floor that aimed to cut back on downforce across the field.
37. Firstly, the rear edge of the floor has been chopped. The actual rules define a forbidden
triangular area defined by a line 800 mm away from the car centre line, the rear wheel centre
line and a diagonal line between a point 800 mm along the car centre plane and 650 mm along
the rear wheel centreline. 38. So this essentially snips a corner edge
off your floor. 39. Immediately you can see this just creates
a bigger hole through which air can leak into the underfloor, reducing underbody downforce.
Wuh oh. 40. Furthermore, the regulations got rid of
all the slots and mandated the floor has to be one continuous surface underneath, so that’s
removed the ability to create those vortices and ‘virtual skirts’ along the edge of
the floor. Also wuh oh. 41. The tyre squirt takes a hit too as the
slots were able to help with that effect. Also, mini vanes under the rear brake ducts
were made illegal which would help guide squirted air away from the underbody.
42. That and the shortening of the diffuser strakes that hang wider than 250 mm from the
centre line all add up to a potentially huge increase in high pressure air bleeding into
the low pressure, diffuser-induced suction downforce under the car.
43. What a lot of people expected to see was teams like Red Bull taking the biggest hit
as their high rake floors were even more exposed with the reduced floor area and removal of
vortex generating slots. 44. But evidence to date, backed up by implicit
statements from Mercedes and Aston Martin, suggest it’s these low rake cars that are
facing worse consequences out of these changes. 45. And what it looks like, though definitive
answers are not yet forthcoming, is that -in balance- the gains from a ramped up rear end
are greater than the losses from the cut back, simplified floors.
46. By which I mean, all cars are losing aero efficiency in chopping back their floors,
inducing more leaks and disruption to their underfloor flow, but having a heaving big
rake on the car compensates for those losses far more than the cars left with their flatter
floors. 47. Plus, as Mercedes had to lengthen their
car to increase the underbody area, they now have all this length to try and seal up with
vortices generated further up the car, something shorter cars like Red Bull might find much
easier to do. 48. So while we’re talking about that, let’s
quickly look at some of the ways teams are trying to compensate for what they’ve lost.
49. If we look upstream, teams have gone much more hardcore in their mid-body aero devices.
Turning vanes and mid wings are stacked like venetian blinds in an effort to steer air
and generate and boost vortices along the edge of the car.
50. We’re seeing little turning vanes around the back of the car to help push some of the
air inwards and create flows that encourage the tyre squirt to tend away from the diffuser
as much as possible. 51. Now, while I say all this, there’s still
a lot of speculation on the 2021 regulations’ effect between cars of between rakes. It may
be far less about the rake and more about the areas of development, or any balance of
a combination of matters. We will have to keep watching and see what bears out as the
season unfolds to gain more clues and insight into Red Bull’s potential leap forward and
see if they can sustain this attack on Mercedes dominance.
52. And this is curiously a fairly moot talking point as in 2022 the revolution in regulations
means much, much more underbody aerodynamic work is allowed, which is incredibly exciting
but will have next to no connection to what’s happening with this year’s cars.