When it comes to tuning the car there are numerous variables we need to consider. Some of these include but are not limited to: driver experience, track conditions, circuit type, dampers, springs, tires, alignment, aerodynamics, engine setup, etc. With all this information one can easily get overwhelmed. While one tune might feel ideal for one driver, it could seriously lack in other departments for another driver. This is where the G-G-V diagram comes into play. The G-G-V Diagram allows us to graph the cars performance in a simple 3D graph, because of this it is often refereed to as the Cars Performance Profile. An image of a G-G-V diagram exported from PC2Tuner is given below,
Looking at the graphs above, on the left we have a 3 dimensional graph, the bottom two axis are longitudinal (accel/braking) and lateral (cornering) acceleration values in terms of G, and the vertical axis (Z axis) is the vehicle speed in km/h. The 3 graphs on the right hand side, are the same graph but in reference with only one axis (2d). The top right is the longitudinal axis, this graph deals with the cars acceleration (-Gs) and cars braking (+ Gs). The middle right graph is the cars lateral axis. This axis deals with the cars cornering force (+G is right turn, -G is left turn). The bottom right graph is the view of the 3d graph from above. It gives us our traction circle. If you are unfamiliar with the traction circle I highly recommend reading my first blog post on tires.
The Job of the Race Engineer
In terms of the G-G-V diagram the whole job of the Race Engineer is to figure out a way to widen the entire profile in all directions (longitudinal, lateral, and speed). If we can accelerate harder and brake harder, than the profile of the car on the longitudinal axis will be wider, this distance is denoted by the black arrow on the longitudinal axis graph in the picture below.
If the race engineer is able to widen this distance, it means we will be able to accelerate up to speed faster, and brake later, resulting in decreased lap times. The same is true for lateral axis and traction circle, the larger the profile the faster the car will be. Knowing this, we can use the G-G-V diagram to rate a tune, which tells us what the limit of the car is. For this car we can see from the data above that the traction limit is right around 2Gs, if we were to try a tune and it was only giving us 1.8Gs we can safely assume this tune would result in a slower lap time, than the tune that can pull 2Gs.
Now that we know what the G-G-V diagram is, what the overall goal of the race engineer is, lets talk about each axis separately and what can be done to widen the profile.
As I mentioned above, the Longitudinal axis is the axis of the car that goes from the front to the back, it deals with our braking and forward acceleration (throttle) values. Our braking values are denoted by positive Gs (yellow line), and our acceleration due to throttle is denoted by negative Gs (green line).
Longitudinal acceleration essentially deals with two things, aerodynamics, engine performance (including gearing), and braking. Lets look at each component below,
Aerodynamic down force is arguably one of the strongest tools to achieve faster lap times. We can determine the aerodynamic effects of a car by looking at the slope of the green and yellow lines in the picture above. When looking at braking, we can see that as our velocity increase, the amount of braking Gs we can pull also increases. However with aerodynamic down force comes drag, we can see this with the green line, at the top of the green line we can see our car is pulling nearly 0Gs this means that the available torque generated by the engine is equal to the aerodynamic drag and rolling friction, essentially saying our car has reached top speed as we are not accelerating any longer. When determining how much downforce to use for a partciular track, its important to consider where in the profile most of your time is spent. We only want higher levels of downforce if we are braking often, and above the speed threshold where the effect of aero kick in (140 km/h) in this example.
Forward acceleration (Engine)
When it comes to engine tuning, we are limited to final drive, or gear ratio changes to help widen the performance profile. This section of the profile is engine limited and nothing other than swapping a larger engine into the car will make much of a difference.
When braking, there are essentially two things we can do to widen the profile, we can change our brake bias, or brake pressure. You should adjust the brake bias so that both front and back brakes lock up at the same time, this creates a neutral steer condition (all 4 tires slipping the same amount), then adjust your brake pressure so that you minimize the amount of brake locking occurring on the track.
The lateral axis is the axis of the car that goes from one side of the car to the other, it deals with cornering (-Gs is left cornering force, and +Gs is a right cornering force). Lateral acceleration deals with our suspension setup, alignment, and down-force. To get the most out of our ability to corner we want to widen the distance between the purple and blue lines below,
Like the aerodynamic section for the longitudinal axis, the more down force we generate the more cornering force we will be able to pull. When tuning this parameter we want to consider where the majority of our time is spent in the performance profile for the circuit we are racing at. If are mostly cornering at lower speeds, then we will most likely want to reduce our total down-force, and vice versa.
When it comes to suspension setup there several variables we must consider, I talked in great detail about suspension tuning on my last blog post found here.
The traction circle is a great way to view our overall traction force without looking at the effect of aerodynamics. To read more about the traction circle please read this blog post.
Now that we know what the G-G-V diagram is, why its important, some of the parameters that we can adjust to change the profile. Lets use it to compare two tunes.
To analyse these two tunes we will refer to the tune on the left as the "green tune" and the tune on the right as the "blue tune".
Looking at longitudinal axis, the green tune has low speed acceleration near -1.3G, while our blue tune is at -1G. The green tune has more aerodynamic downforce because the slopes of our lines are steeper. The braking force on the green tune is around 2G while the blue tune is at 1.8G. Overall, the green tune has a larger profile than the blue tune, and ultimately is faster in the longitudinal axis.
Next, the lateral axis. The green tune is the clear winner here, at all speeds the car is pulling approximately 2.5G while the blue is only pulling just over 1.8G. The green tune is cornering faster, which means reduced lap times.
Looking at the traction circle, we can determine that the radius of the green tune is slightly larger than the blue tune.
This analysis was straight forward as the green tune was the clear winner for all 3 axis. Sometimes you will have a car that is faster in one axis but slower in another. It is your job as the race engineer to figure out where the majority of the time the car spends in the profile for the circuit. If the car is mostly doing straight line acceleration and braking, than you want to optimize the car for the longitudinal axis, if the car is mostly cornering than you want to favor the lateral axis.
The G-G-V diagram is a great tool to calculate the limits of a car setup and compare a car setup to another setup. We can identify where a car is lacking by analyzing each axis.
PC2Tuner developed by racingsimtools.com is available for $10, in addition to being able to tune suspensions, alignment, gearing, etc. The tool also provides a way to analyse tunes such as the G-G-V diagram, and neutral steer channel. Additionally all members gain access to the PC2Tuner Tune Database where members can submit their tunes for various cars. The program can be viewed in the racingsimtools store located here