Aerodynamics Fundamentals: Aero Balance
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Aero Balance
05.53
| 00:00 | If you've ever set up or driven a race car on track, you'll know that one of the most important aspects is to have good balance. |
| 00:06 | By balance we're talking about whether or not you'd expect the front or rear axle to let go first in a corner. |
| 00:12 | So for example, if we're in a corner and the rear steps out on us at the limit of our grip, it means that the car is oversteery or rear limited. |
| 00:20 | On the other hand, if the front pushes wide, we're understeery or front limited. |
| 00:25 | The balance of our car will massively determine the driver's confidence in the car, as well as how effectively we're using the grip from all four tyres of the vehicle. |
| 00:34 | A well-balanced car is typically a fast car. |
| 00:38 | Often car balance is tuned from the suspension, by stiffening springs or rollbars on the front or rear axle. |
| 00:45 | However, when you put aerodynamic devices on the car, you add another aspect, which is tuning the aero balance. |
| 00:53 | Let's imagine a simplified representation. |
| 00:55 | representation, known as a free body diagram, of a car. |
| 00:59 | This car will have a certain centre of gravity position, known as the CG. |
| 01:04 | Now, we typically can define the CG as either a location, or a percentage of mass balance on the front and rear axles. |
| 01:11 | For example, in a front wheel drive car, with the engine forwards, we may have 60% of the car's mass on the front wheels, and 40% on the rear wheels. |
| 01:20 | In motorsport applications, it's typical to express balance as a percentage of the load on the front axle. |
| 01:26 | In this case, we would call this a 60% mass balance. |
| 01:29 | You may also have heard of this as a 60-40 split. |
| 01:33 | If we have a rear-engined car, the mass balance may be further rearwards, say 40% front, 60% rear. |
| 01:39 | The mass balance changes the amount of lateral force on each axle required to hold a steady state corner. |
| 01:46 | So, let's say we have 90% of the car's mass on the front axle, then we need 90% of the car's lateral force applied to the front axle to hold a steady state corner. |
| 01:54 | 90% of the car's mass being over the front axle means that 90% of the vertical force of the car from gravity is applied to the front axle as well. |
| 02:04 | So, this all works out quite nicely. |
| 02:07 | This mass balance doesn't explicitly dictate the car's handling, as we can tune in any mass balance with the right suspension and tyre choices, but it does have a profound effect on the aerodynamics. |
| 02:19 | Let's consider that we just put a rear wing on our car, directly over the top of the rear axle. |
| 02:25 | Let's say this wing has minimal drag and just produces downforce. |
| 02:30 | We can see from our free body diagram that this force can only be reacted at the rear tyres. |
| 02:35 | So, any downforce we add on this device is exclusively on the rear axle, with no downforce being added on the front axle. |
| 02:42 | This will create a 0% forwards or 100% rearwards aero balance. |
| 02:48 | Now, grabbing our downforce equations from a few lessons ago, we can split them off into components for the front and rear axles. |
| 02:54 | If this car has a 50-50 mass balance and weighs 1000kg with a mu of 1, we can see that we would have 500 multiplied by 9.81 by 1, the mu, equals 4905N of lateral force available on the front axle before we exceed the maximum grip, and similarly 4905N on the rear axle assuming no downforce. |
| 03:20 | However, if we add the downforce of our rear wing, this picture changes a fair bit. |
| 03:24 | If that rear wing adds 100kg of downforce to the rear axle, but nothing to the front, what we get is the rear axle now having 500 plus 100, or multiplied by 9.81 by 1, equals 5886N of available grip force on the rear axle, while the front still only has the same force from before. |
| 03:48 | As the mass balance of this car is 50-50, both axles have to push the car into the corner to reach the maximum grip available, but the tires won't be able to sustain a higher cornering force, and in our case, this will apply when the front axle runs out of grip as it has a lower peak available grip force. |
| 04:08 | Essentially, with all this downforce we've added to the rear axle, we've achieved no improvement in cornering performance as the front axle runs out of grip, as if it would if there was no downforce on the car at all, because there's simply no downforce on the front end. |
| 04:23 | So essentially, if we want to improve the cornering performance of the car, we have to add downforce on both its axles. |
| 04:30 | This aero balance is typically targeted to be close to the car's mass balance, so that we align the aero force as well with the normal forces on the tires applied by the car's weight, and the lateral forces across the two axles as required by cornering like we discussed earlier. |
| 04:47 | So, if we had a 50% mass balance, we'd target a 50% aero balance. |
| 04:52 | If we had a 60% mass balance, We target a 60% aero balance, and so on. |
| 04:57 | In the real world though, we typically set this balance figure about 5-10% further rearwards for amateur drivers, and 2-3% further rearwards for pro drivers, as an understeery aero car is much easier and safer to drive than an oversteery aero car. |
| 05:13 | This brings us to the end of our aero balance module, so let's summarise. |
| 05:17 | Car balance is crucial for handling where it affects whether the front or rear axle loses grip first, influencing whether the car feels understeery or oversteery. |
| 05:26 | When aerodynamic devices are added, the aero balance, that's the downforce distribution between front and rear, must align with the car's mass balance to optimise cornering performance. |
| 05:37 | Ideally, the aero balance should match the car's mass balance, but in practice it's often set slightly rearwards to ensure safer and easier understeer dominant handling, especially for less experienced drivers. |
