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We have been looking through several posts of this automotive course at the development of the vehicle in the so-called physical phase, studying the tests that are carried out on a vehicle before its production. The engineers are in the final stretch of car development: They already have the vehicle physically developed, now they have to adjust the car to get a correct behavior on the road. After this, there will be a production trial and Project Control will kick off the production (SOP: Start of Production).
CAR DYNAMICS TESTS
Vehicle dynamics test
We are going to briefly see some basic concepts of vehicle dynamics, to understand the tests that are carried out.
Basic notions of car dynamics
When driving a car, its stability, comfort and the path in the curve are important. Trajectory precision is really important to improve riding confiance. Among other parameters, they are very important: suspension design, weight distribution, car tires, crosswind and chassis stiffness. We will see some concepts later; avoiding the use of formulas, as always.
A car with poor vehicle design can cause the vehicle to body pitch or tilt, and not follow the line of the curve. A car is pitching by lowering or raising the front and rear modules, while tilting the sides of the car lean from one side to the other.
In understeer condition, the car's front wheels lose grip through a corner. This causes the front end to push towards the outside of the corner and for the steering to become useless. In oversteer condition the car steers too much into a corner, the car's weight is moving forward causing the rear wheels to lose grip. For neither of these two conditions to happen, design engineers perform a lot of calculations and tests.
CAR CHASSIS DESIGN
Car chassis design
We are going to see the main design criteria of a chassis from the point of view of vehicle dynamics, we already saw in previous posts of this automotive course some ergonomics and safety criteria. We are going to consider self-supporting chassis as they are the most common in vehicles, although there are also other types.
But first, we will make a special mention of the tubular chassis. These are widely used in some sports cars, small-volume car brands, exoskeletons (Ariel Atom type), Formula SAE, ect. These types of chassis are relatively easy to calculate and to manufacture. Tubular chassis don't require hiring sheet metal suppliers for the chassis or industrial robots for assembly. So the company will only need to have an industrial warehouse or a workshop for the assembly of their cars, instead of gigantic factories with highly automated assembly lines.
All you need is a welding table and a team of professional welders, as it's a much more manual process, it's ideal for low-production cars. If you have in mind to create your own car brand, this is a very logical option. The unit cost is higher, but the initial investment is much lower. Another advantage that interests us a lot, as automotive designers, is that with SolidWorks we can carry out the entire design of the tubular chassis, simulating the entire chassis from its behavior through finite element analysis (Fem) to the welds. On the other hand, for a vehicle with a self-supporting chassis it's more optimal to use a more powerful program such as Catia, as well as a much larger team of engineers. Therefore, you can design a vehicle with a tubular chassis yourself in a relatively simple way. Another option is to use monocoque or even hybrid chassis, which would be a mix between tubular and monocoque. Although they tend to be much more expensive, so they are usually reserved for high-performance sports or racing cars.
An important factor in car chassis design is torsional stiffness. We are interested in the highest possible torsional rigidity so that the chassis of the car doesn't deform and always maintains contact with the road on all four wheels, this means greater stability and a higher quality of driving. In vehicles with high torsional rigidity such as the Rolls Royce Phantom or VW Phaeton, the chassis practically doesn't twist, the suspensions will in charge of absorbing the irregularities of the road. In this way, the suspensions work much better than on a chassis with less torsional rigidity.
As designers, to increase the torsional rigidity of a chassis we can use more rigid steels (high-modulus steels 'HMS'), increase the number of welds and high-strength adhesives or use crossbars or even crosspieces. But all this must be done smartly so as not to consume more resources than necessary. Either due to the increase in weight that this implies or due to the increase in manufacturing costs, both for material and due to the increase in extra processes in the assembly line.
Engineers calculate this in two ways, initially with CAD (CAE to be specific), and then physically through the various torsion tests that we will see later. The calculation in a self-supporting chassis is somewhat more difficult due to the complexity of the chassis itself and the amount of elements, structural solutions and connections it has, but, when it comes to a tubular chassis we can design it in our own home.
That is to say, right now we are able to design any car with a tubular chassis from our home, with a series of basic notions and without an economic outlay. If we want to do a test with our CAD program, for a basic study of torsional stiffness, we must leave the rear part of the car fixed and exert the efforts on each of the support points of the vehicle's chassis, horizontally and upwards. We will alternate these efforts to see the behavior of the vehicle. Finite element analysis is a complex area, but this calculation is very simple to perform and you can do it even without specific knowledge of FEM, without going into complex topics such as vibrations, and always, in a static and with limitations. You can use Grabcad to download a chassis to simulate it.
As we saw, other factors also come into play, such as suspension design or weight distribution. Suspension design is really complex, and its tuning and layout is critical to vehicle dynamics. A correct adjustment of the suspension allows to completely change the car behavior. But the suspension design is too complex to fit in a single post.
Weight distribution: The ideal distribution tends to be 50/50. Although other distributions are also used, carrying from 55% to 65% of the weight of the vehicle distributed on the rear axle.
Be careful, a car with a theoretically ideal 50/50 distribution can have a bad weight distribution, this concept is much more complex. For example, even meeting 50/50 it would be a bad distribution if it had excessive weight distributed in the overhangs of the car, a too high center of gravity, the weight too concentrated in certain areas, or worse, a not symmetrical distribution.
The polar moment of inertia is also an important concept when designing a car, the closer the car's masses are to the center of gravity, the better. That is why many racing cars have a central engine, being much closer to the center of gravity the car is much more dynamic and with a much earlier response.
So a 50/50 distribution is a good start point, but it doesn't assure us that the car has a perfect weight distribution. Knowing this, we can start to design a chassis, starting from a block, then we define the basic dimensions including the wheelbase of the car. Then we can locate the elements of the car, trying that the heaviest elements are as close to the ground as possible, and we lower the center of gravity as much as possible. In high-performance vehicles, it is favorable that they are as close as possible to the center of gravity.
When a car brakes, the weight is shifted forward so the rear wheels can lose grip, causing the rear to slide out of the curve. The opposite occurs when accelerating, the front tires lose grip so the car will not turn properly since the vast majority of cars have the steering on the front axle only. In front-wheel drive cars, they would also lose the ability to accelerate when it goes out the bend, by losing contact with the asphalt.
When we talk about four-wheel steering we mean exactly that, cars like the Skyline R34 GT-R or the Mitsubishi 3000GT VR4 turned all four wheels, but that system ended up being abandoned. Luckily Porsche took it up again in the 2013 911 GT3 (991), but relying on actuators instead of the mechanical system.
Engineers can work on vehicle dynamics to improve car comfort and for better step curve and grip. The latter is an important design requirement in sports cars, but in utility vehicles without sports features it's a lesser important requirement, with comfort taking precedence.
In front-wheel drive vehicles manufacturers tend to approach 50/50, but this doesn't usually happen with sports performance and rear-wheel drive vehicles, where in addition, cornering is an important design criterion.
Let's try to make you deduce why manufacturers sometimes reject the 50/50 and tend to distribute a little more weight on the rear axle. Here some clues:
Therefore, we want to get away from formulas and go to logic, although we cannot explain everything since it is a really extensive field. Have you deduced it? We will give one more hint, which we discussed earlier.
So far, in these four previous points we haven't said anything that no one, with or without automotive knowledge, doesn't know. It's clear that we brake when entering a curve and that the weight goes to the front axle. But… Why don't we always use 50/50 if we have said that it is better?
The 50/50 distribution of the masses of the car is done statically, that is, we design the car and we distribute the masses, but we don't consider the movement of the car. But, vehicle dynamics make sense when the car is in motion. If we had a car with a theoretically perfect 50/50 mass distribution, when braking to enter a curve, part of the mass would go to the front axle and we would lose that ideal distribution.
For better cornering, we put a little more mass on the rear axle, so when braking and shifting the weight forward at cornering, we have a weight distribution as close to 50/50 at the moment that interests us the most.
Therefore, is the 50/50 distribution optimal? Of course it is, but the vehicle dynamics, as its name suggests, is somewhat dynamic, so we want the 50/50 when the car is in movement. When a car is parked, its weight distribution doesn't matter. Therefore, in a sports car with rear wheel drive, it's better to shift a little more mass towards the rear axle. This improves corner entry braking, and corner exit acceleration, by gaining more traction, allowing you to go faster.
So what weight distribution is ideal for a car? There is a tendency to think that the 50/50, although as we mentioned earlier, when the car brakes, the weight of the car goes towards the front axle, so here we have the car design key.
VEHICLE DYNAMICS TESTS
Vehicle dynamics tests
At this stage we already have the car almost finished, the suspensions calibrated and a well optimized chassis. Now we need the most fun: try it.
Tires are key in vehicle dynamics, although these have already been tested by their manufacturer, it's necessary to check how the model behaves with these tires. Tests are carried out both in the laboratory and on the track, with all the possible configurations of the tires with which the model will be sold.
Drivers and engineers check wet grip, fuel efficiency and noise. They also carry out the famous moose test, a evasive maneuver to avoid a collision in the hypothetical case that something gets in the way of the vehicle.
The vehicle will be taken to a test circuit with different elements where a multitude of tests are carried out. The test circuit for the car development is very different from the one used for racing, it has to recreate everyday situations such as turning in a roundabout.
It's not about going fast, but about evaluating different aspects of the car. The test track is normally surrounded by a speed oval in which to put the car to the limit, or it has speed sections, but the rest of the tests usually take place at low speeds. There is also an area called the dynamic zone, which consists of a large, smooth circumference without any type of obstacle where the pilot can execute any type of maneuver at high speeds without running the risk of colliding.
There is a great variety of tests, you can actually test almost anything, so the options are practically endless.
For example, braking tests are carried out on special surfaces, sometimes inflatable cars are added to have braking references. There are also sections of track with different roughnesses and characteristics. For example, polished cement to significantly reduce the grip of the vehicle, they also add a watering system to drive on wet if necessary.
In addition to playing with the different roughnesses of the terrain, obstacles and irregularities such as stones or small grooves can be added to check the comfort of the model and evaluate its noise. There are also areas with speed bumps, ramps, hydroplaning areas and water pool areas to test the off-road capabilities of the vehicle.
The amount of tests is such that sometimes these areas are also used to teach driving courses, especially if they are private circuits external to a car brand. Many of these tracks belong to external companies that offer their services to brands, so each track and the tests that can be carried out are totally different.
The car with which this type of test is carried out is, in theory, the same as the production one, but it is fully instrumented to collect all the data. The result is similar to what we see in the images.
Another of the most striking tests is the comfort bench, the vehicle rises on four posts, we saw it in the previous installment when talking about the MAST. Those posts are huge pistons that work independently.
In this way it is possible to simulate the strong vibrations suffered by the suspensions, as well as the rest of the chassis, when the vehicle goes at high speed on rough terrain. Years of continuous use can be simulated in a few days. It's a really tough test for the vehicle, since when in suspension, the entire vehicle vibrates completely as it receives the violent shocks of each of the four pistons.
The chassis dynamometer is also used. The vehicle is placed on rollers that simulate the different loads, through vibrations and movements. The engine department also uses these dynamometers to perform their tests.
This is the last test to be carried out on a car, with this we have the car fully prepared to go to the factory. We carry so many lessons that we even feel like there has been a long and tedious design process to reach that point where the SOP will be signed. At this stage, the car is ready to hit the road, and, in theory, completely finished. Now it remains to try it and prove it!
The vehicle is put to the test in extreme conditions, it's the only test that is carried out on public roads, although it is also done in a closed track. In total, more than 1 million kilometers are made to the vehicle between all the tests carried out. For example, the car is driven in countries like Finland to check the real resistance of the car at low temperatures, and then, in summer, it's taken to very hot environments such as the Jerez Circuit (Spain), or even in desert areas.
Different aspects are checked such as engine overheating, vehicle starting, the effectiveness of the different filters and other components against different dust particles, or the corrosion of the different elements in different environments. That is, if before it was tested in a laboratory that simulated different conditions, now those conditions are tested in a real environment. For this, the brands transport these vehicles to countries and areas where the conditions they seek exist.
Here we must clarify one aspect, depending on the area where a version of the vehicle is marketed, one type of test will be more or less important compared to another. It's known that a model can vary physically from one area of the world to another, in part, due to the tastes of consumers in these countries. But on other occasions, even if the vehicle is visually the same, it will have little noticeable changes depending on the country or region where it is sold. For example, particulate filters, some suspension settings, and even the thermal properties of the glass may vary. It makes no sense to have crystals prepared for really low temperatures in countries where it never snows.
This type of test is also a great opportunity to check the usability of the cabin, since the pilot spends many hours driving.
Once the car has been tested for a long time in the laboratory, it has been put to the test outside. So, are you ready to start production of the model? If so, see you in the following week when we will dedicate two posts to see how our car is manufactured. Later, only two posts will be missing in which a key point of the car will be seen, its commercialization, although we will see it from the point of view of the automotive design engineer or the car designer.
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