Rake: What is it and what does it do?
What is it?
Rake is a vital setup option in real life and simracing alike. The term is commonly used in motorsports to describe the difference between the front and rear ride heights of a race car. It refers to the intentional sloping or angling of the car's body, where the rear end (most often) sits higher than the front end. Imagine the front end having a ride height of 50mm and the rear end of 80mm. We then speak of having 30mm of rake. Also important for rake is the length of the car, as the driving factor behind it's effects is not the ride height persé, but the angle. A very long car (think a truck) will have less rake angle at the same height difference than a very short car (think a slot car ;) ). But since most cars in GT3 have roughly the same length, we can ignore the angle for now and only talk about the rake in millimeter.
The concept of rake is employed to optimize the car's aerodynamic performance and balance during racing. Having a higher rear ride height compared to the front has several effects. Keep in mind that car designs vary and that rake will have different effects, but also a different magnitude of effect on different cars. There will be cars responding drastically to rake, some will respond less.
Generally rake will increase downforce. Most modern cars will create both more front and rear downforce as the rake increases. Some cars will create more on the rear than the front, some will create more on the front than the rear. Some cars will have zones where there is more downforce created towards the front, but by adding even more rake you'll create more at the rear and vice versa. Some cars might outright lose downforce on the rear with more rake (old M6 for example) Assetto Corsa Competizione models all these variations in car design and hence each car has different rake preferences. Also be reminded that the ride height itself on each axle has its own impact on downforce. While increasing rake might add downforce, the ride height change itself might reduce or increase it further. Again this largely varies per car. And we'll provide some examples below. Keep in mind that all aero effects scale with speed and ride heights interact heavily with the springs. Soft springs will allow the downforce to change the ride height while driving, whereas stiff springs resist the additional aero load better and maintain more ride height. Thus changing spring stiffness from softest to hardest will lead to drastically different aero balance (and generally less changes in car balance) during the various driving situations compared to soft springs.
Weight Distribution, Springs, Stints.
Rake will also influence the distribution of weight across the car. With a higher rear end, there is a greater proportion of weight being shifted towards the front tires. Depending on where the engine sits, this allows you to utilize one end of the car more or less - whatever your desire is. Lower rake, so more weight resting on the rear tires, will give better traction and acceleration, stability during corner exists. However the car might not rotate as good anymore and shift the car more towards understeer in that situation. In turn more rake allows you to rotate the car better through corners, especially slow ones where the aero effect is muted. It also allows for more agile change of direction when the rear sits a little higher and "exposed" if you will.
An important factor for rake is also the position of the tank in the car. Of course the tank changes the weight distribution massively and therefor plays a role in the choice of rake. Even more important is the effect of the tank emptying over a race distance. The more fuel is burned, the less weight will compress the springs. That means that over a race distance the ride height of the car will change and with that all effects related to rake will come into play. These effects are more pronounced the softer the springs are. A soft spring will extend much more with 100kg removed as a stiff spring would extend removing the same mass. Assetto Corsa Competizione has a nice function to predict the aero balance as you go through a stint.
In single player practice, go to the Fuel & Strategy tab and fill the tank completely. Now move to the Aero tab. The full tank in the previous tab now is the base for whats next. The "Fuel Load Test" allows you to see how the rake changes from full to empty tank. I'm using soft springs in this example for a big effect.
This is how the car will leave the pits, with the ride heights as you set them up initially:
This is how the ride heights will develop until the end of the stint with soft springs:
As you can see the rear height changes by a whopping 5mm, while the front only increases by 1mm. This indicates that the fuel tank sits much further in the back of the car. Luckily on the BMW M4, the aero variation (the % at the bottom of the page) does not change despite the rake increasing and is 3.0% at the start and end of the stint, regardless of soft or stiff springs. This indicates that the car adds similar amounts of downforce to front and rear as the rake increases in this zone. However there is a lot of weight missing on the rear and this will likely hurt traction a lot. Other cars will have drastically different outcomes doing the same test. Do it, its totally worth it.
Here's the same example, but with stiffest springs, showing that the ride height change is much less:
Another vital resulting factor is pitch sensitivity. Pitch describes the change of rake under acceleration and deceleration of the car - so its movement on the longitudinal axis (or its movement around the lateral axis) of the car. The effects vary a lot per car and have to be considered for the car setup. Some cars aerodynamically become more stable when they pitch, as they increase the rear downforce more compared to the front end of the car with increasing rake. Some cars absolutely hate pitch as they react very sensitive to rake changes. E.g. the front downforce might increase drastically with more rake (pitching forward under braking) and make the car hard to handle. Most modern GT3s, especially the 2021 and later generation all became less pitch sensitive. With more novice drivers stepping into real race cars, they need to be easy to drive and hence engineers aimed to remove pitch sensitivity. However, it will still be a factor. The resulting rake effect of pitching can have two sources: The front diving and the rear rising. On cars with very good anti dive suspension it might only be the rear rising (e.g. AMG) under braking creating the effect. On older cars like the Lexus or Nissan it will be the front diving a lot. The main factors to control for the magnitude of the pitch impact are the springs and bump stops. The less travel you allow the car to have, the less pitch sensitive it will be. However, aero maps are not linear and you might get a more sensitive car with a little pitching forward and suddenly a less sensitive car with pitching it even more. This can be seen on the Porsche. With the front going too low it drops "out of the aero map" and the front suddenly produces less downforce, while the rising rear produces more. This somehow keeps the car stable, even though it should - under normal circumstances - become very tricky to drive.
The last consideration surrounds drag - the aerodynamic resistance of the car to go faster in a straight line. Some cars will have more drastic drag increases when rake increases. Some cars will barely have drag increases with more rake. Some cars will have drag increases when the rake becomes negative (that means the front is higher than the rear; this can easily happen with a soft suspension on fast straights, as the rear compresses a lot under aerodynamic load). Some cars in turn have reduced drag with negative rake.
Just to emphasis this point once more: How you set the car up in the garage, e.g. 50mm front and 80mm rear, resulting in a rake of 30mm is not the car you are having while actually driving. Ride heights are extremely dynamic. Under braking you might have as much as 50mm of rake (front goint down to 40mm, while the rear goes up to 90mm), while under acceleration rake might become very low or negative even (think the front lifting to 60mm and the rear squatting to 65mm = only 5mm of rake!). This has drastic consequences for balance and rotational capaticies of the car. Using springs and bump stops is important to manage this variation throughout the lap. By keeping the bumpstop range low and the bumpstops and springs stiff you can reduce the variation under braking and acceleration, maintaining a more similar rake in all situations.
Here's an attempt to give you a rough overview of how the cars compare. It's best you go into the setup screen and have a look at the %-change in the aero variation with each click of ride height on front and rear, but also in response to rake.
Pitch sensitive cars (in no particular order) that like low rake (~10mm)
These cars are also front ride height sensitive.
- Audi (2015 and 2019)
- Lamborghini (2015 and 2019
- Ferrari 488 (both)
- general old gen cars like M6, Jaguar
Pitch sensitive cars that like/tolerate/need medium rake (~20mm)
These cars are also front ride height sensitive.
- Honda (all)
- Porsche 991 I & II
- McLaren 720s
Pitch tolerant cars (doesn't mean the effect won't be there, but less pronounced) that like/tolerate medium and/or high rake (~20-30mm]
These cars are less front ride height sensitive and also do not shift balance too much in response to rake
- BMW (can be balanced with almost 0 to almost 30mm of rake)
- Aston (the most stable of them all in that regard, would probably take 50mm of rake without issues)
- Bentley (also likes a lot of rake)
- Lexus (needs plenty of rake to even turn)
- Mercedes (The mercedes still responds to rake, but it needs a lot of rake to turn in the first place, also the rear rises more than the front dives)
Pitch tolerant cars that can work both ways, low and medium rake
These cars are less front ride height sensitive, but still respond sensitive to rake
- New Gen cars like...
- Ferrari 296
- Lambo Evo 2
- Audi Evo 2
- McLaren 720s EVO (by the looks of it)
- Porsche 992 (given its layout its very pitch tolerant)
This is an attempt to sort the cars along their aero sensitivity. Don't take it as an exact representation and surely not all cars in one group really behave the same.
You can see, there are many considerations to make when adjusting your car setup, and especially your aerodynamic setup. It matters a lot which car you are driving and it also matters a lot how good the driver is in the car. A more sensitive car always requires a good driver and you might also be able to drive more difficult setups with better hardware (e.g. load cell brakes or direct drive wheel bases). Keeping control of the variation of rake is equally important as the rake in the setup menu itself. Even with a lot of rake, you can produce an understeery car, and even with very low rake, you can produce an oversteery car - it all depends on other settings and how they interact and affect the dynamic rake throughout the lap.