Vehicle Development - There’s More To It Than You Realize
There’s a reason that it takes automakers years to develop new sports carsby Alvaro Pendas, on
Perhaps the most important point is for the customer to be sure what it is they´re expecting from a sportscar. Speed? Comfort? Agile track handling? It is easy to see that it is not easy to reconcile all these aspects, and this is the reason why in many instances firms will offer several variants of the “same” sportscar. Let´s take the Porsche 911 range as an example; it includes models for everyday use such as the Carrera S all the way to the more extreme, flamboyant – and less comfortable – GT3 RS or GT2 RS.
It is up to the development, dynamics and product management teams to give the vehicle its appropriate driving qualities. Performance targets such as top speed, acceleration, maximum weight or cornering speed need to be pre-defined. In the automotive industry, such parameters are referred to as “vehicle attributes”. Driving an S-Class is a very different experience to driving an AMG GT-R; this is all down to their different attributes.
Test Tracks Play A Vital Role
In order to achieve the set targets, OEMs and suppliers send their engineers to testing tracks – or proving grounds. Within such facilities there are high speed test tracks, low speed handling courses, wet & dry handling tracks, bumpy roads and some of them even have testing rigs and design services within the complex.
One of the most important proving grounds is IDIADA, which is in Spain and is very convenient due to its all-year-round good weather.
Others include Nardo, located in Lecce (Southern Italy) which Porsche acquired in the year 2012. It is well-known for its high-speed 12 km long oval track and its dry handling course.
Engineers need to be able to measure both qualitatively and quantitively the progress made throughout testing to ensure performance targets are met. Data analysis softwares are widely used to enable the engineer to calibrate the different vehicle systems. Wintax, Vector CANape or Bosch Windarab are some of the most common (they are also commonly used in motorsport for data acquisition). Their purpose if to offer all information concerning performance, whether its vehicle speed, steering angle, total percentage of brake and/or accelerator pedal applied. Other parameters may include engine air intake temperature, velocity of each wheel or aerodynamic downforce (dynamic and static pressure sensors as well as strain gauge dampers will be used for this parameter).
The data acquisition tools will collect the values of the different parameters and express them graphically (normally with respect to time, but it is possible to plot any desired graph). The software is very versatile, and it allows for the programming of mathematical expressions if we want to infer certain vehicle behaviors such as understeer or oversteer. It will use the equations we input to create the graphs, so it´s essential that they are correct!
Analysis and Tuning Are Key
Back to the topic of supercars. These vehicles have features that make them easy to distinguish. They are wide and low to keep a low center of gravity and tend to deploy complex aerodynamic shapes. One of the key differentiating factors – among others – between this type of vehicle with respect to others lies in their ability to carry much more speed through the corners; such vehicle must also remain composed throughout cornering; it must not allow itself to be “disturbed” by the lateral accelerations experienced.
In other words, we will try to reduce the vehicle inclination angle in order to reduce body roll.
In physical terms we are talking about the roll gradient which is measured in degrees of inclination per “G” of lateral acceleration (where G = 9,81 m/s2 ). For a supercar, the roll gradient value can be of around 1,5 degrees/G whereas for an average sedan car it can be around 8 degrees/G. In order to achieve such low inclination value, supercars will use stiffer damper springs whilst sacrificing comfort, it´s wide low bodywork also help in this respect. We can quantify stiffness in terms of how much force we need to compress de damper by a certain displacement – this will yield the spring ratio, measured in Newtons per millimeter (N/mm).
There are other reasons for using stiffer dampers in this type of vehicle. If we consider that the bodywork lies relatively close to the ground to keep a low center of gravity, a set of softer dampers would allow for greater changes in bodywork ride height with respect to the ground particularly during heavy braking (the front could even risk bottoming out!). Now, one must also consider that the aerodynamic downforce is also affected by the bodywork´s height with respect to the ground (among other factors). The downforce increases with speed and this will add extra weight which acts on the vehicle´s bodywork (weight is a force, weight and mass should not be used interchangeably). A set of soft dampers would compress too much due to the extra downforce acting on them which would in turn allow the bodywork to come too close to the ground – once more increasing risk of bottoming out. Furthermore, as the aerodynamics of supercars is so reliant on the height of the bodywork with respect to the ground, and such bodywork vertical displacements allowed by softer dampers would mean you are moving away from calculated front/rear axle heights at which the aerodynamic performance gains are optimal.
The topic of suspension also brings us to the concept of vehicle balance, which is typically characterized by under and oversteer. Roughly speaking, in understeer the front wheels are “pushed away” towards the exterior during a turn and in oversteer it is the rear that gets out of shape which can frequently result in a spin – more on this shortly. Trained drivers will give their opinion during the development phase; however, we still require the software tools I mentioned previously to quantify these handling behaviors. One way of doing this is by measuring the lateral acceleration of the front and rear axles separately. This way, it is reasonable to assume that the axle with the greatest acceleration is the one with more grip. Bearing this in mind, we can define the following condition:
-* If the acceleration of the front axle is greater than that of the rear axle the result will be oversteer.
-* If the acceleration of the front axle is smaller than that of the rear axle the result will be understeer.
Using basic dynamics knowledge, we can come up with a simple mathematical expression whereby you subtract the acceleration of the front axle from that of the rear axle. If the result yields a positive value, we will get understeer and vice versa. If we decide to go down this route, then the acceleration sensors (gyroscopes) must be installed where the vehicle center line crosses the center of the front and rear axles.
Once the test has been carried out, we will analyze the results and take corresponding actions. The sway bars are components which can be tuned accordingly depending on the desired over or understeer balance. These are rigid bars that reduce body roll; this way we reduce weight transfer to the external wheels while turning and achieve a more equitable weight distribution amongst all four wheels. They can also be used to change oversteer and understeer:
• If we want to reduce understeer, we can reduce the rigidity of the front sway bar or increase the rigidity of the rear one.
• If we want to reduce oversteer, then we could increase the rigidity of the front sway bar or reduce the rigidity of the rear one.
In my previous article “Aerodynamics – it´s more than a fancy word” I discussed downforce at length. The distribution of aerodynamic downforce also plays a key role in vehicle balance. Too much front downforce will make the car prone to oversteer whereas too much aero load at the rear will cause understeer. Thus, it´s not just a question of “the more downforce the better”. A well distributed front/rear aerodynamic load will help reduce tire spin (it increases the normal force acting on the wheel); this is useful as it provides greater traction capacity and thus the vehicle will be able to have greater longitudinal acceleration at the exit of corners. Tire wear will also be reduced thanks to reduced tire slip.
It’s Important to Find Balance
The disadvantage of a greater aerodynamic load will always be greater aerodynamic resistance – drag – which will both increase consumption and reduce top speed.
Hence, as I mentioned earlier deciding the vehicle´s attributes is key! What are we looking for? As little drag as possible for higher top speed? More downforce for faster lap times but lower top speed? How about fine tuning for track times whilst hindering top speed as little as possible? The possibilities are infinite! One only needs to look at the Bugatti Chiron SS 300+ which is focused on achieving a low drag coefficient for high top speeds – and it managed no less that 490.5 km/h in Ehra-Lessein! An interesting fact is that it would have achieved 515 km/h had the test taken place in the Nevada State Road 160 thanks to the lower air density in this area according to simulations carried out by Volkswagen.
Data compiled during testing can also help in calibrating systems such as the ABS or traction control. Let´s first look at the latter. The idea of traction control is to fully or partially prevent wheel slip when the vehicle is accelerating. For it to function, the system requires information inputs from certain parameters to know what´s happening in each driving situation – whilst accelerating hard from a low-speed corner for example, moment at which rear tires can be especially prone to wheel slip.
In this instance we would need wheel speed sensors. The values will be sent to the corresponding ECU (Electronic Control Unit) which will transform the angular velocity reading of each wheel into values expressed in km/h. The total vehicle speed – “Vvehicle” – can then be calculated by adding the total speeds of the wheels and dividing by 4.
Thus: Vvehicle = (Vfl + Vfr + Vrl + Vrr) / 4
- Vfl = speed of front left wheel
- Vfr = speed of front right wheel
- Vrl = speed of rear left wheel
- Vrr = speed of rear right wheel
This way it is possible to know at what speed each wheel must turn to “agree” with the total vehicle speed (“Vehicle” in our expression). If a wheel is turning at a greater speed than the vehicle total speed, then the traction control will be actuated to slow down that wheel and prevent slip.
Lastly, a very similar approach can be used for the calibration of the Anti-lock Braking System (ABS) as we are also dealing with the speeds of each wheel. It is possible to know when a wheel is “locking” when, say, it´s translated linear velocity is 25 m/s and the total vehicle speed is 40 m/s. At this point, the ABS will be actuated and temporarily the brake caliper will temporarily “free” the brake disc of that wheel until its speed is in accordance with “Vvehicle”.
In conclusion, the development of a supercar comprises many technical aspects. A few have been briefly discussed here, concerning mostly attributes within dynamics and aerodynamics whilst also referring to common softwares used in automotive testing. Before making investments and mobilizing teams to proving grounds around the world, OEMs must know exactly what it is they want from the new vehicle, its position in the market, how they are going to go on achieving all vehicle attributes and… who the competitors of the new supercar might be!