It takes more than a computer program to design an excellent suspension system, a very large number of variables need to be considered in any capable design.
Any competent performance automotive enthusiast understands that the most important aspect of any car’s ultimate performance is the ability to extract the maximum performance out of the tires. Everything on a performance car is there with the sole purpose of achieving this goal, including the driver. We are all truly fortunate in the hobby these days, as the tire technology has improved significantly over the last few decades, and even high performance “street” tires are better than many of the race tires of yesterday. But this also provides a challenge, most cars in the hobby are not set up to extract all the performance today’s tires can provide, with little ability to further tune.
At Lateral Dynamics, performance suspension systems are the core of our existence. But what is a “system,” and what are the issues with designing, building, and tuning? We have a somewhat different approach to looking at suspension than most folks, as with everything we do, a true, fundamentally based engineering assessment precedes any design work, such that the technical requirements of the design are completely understood up front. Sounds obvious but unfortunately this isn’t typically done with many suppliers in the hobby.
One way to look at a suspension system is to break it down into three fundamental groups: Motion, Rates, and Damping.
The ultra competitive GT Le Mans series has the baddest factory based hot rods on the planet. While there are many differences in the car configurations, the one thing they all do is to maximize the performance of the tires throughout the races. Race cars are nearly infinitely adjustable to help the teams achieve this goal.
Motion: All suspension systems utilize some combination of links and arms to control the motion of the wheels. It shouldn’t be a big surprise that virtually all performance vehicles rely on the “Short-Long-Arm” Double Wishbone configuration for the front suspension, as it offers the ability to compensate for contact patch changes as the suspension goes through travel. For the rear, there is a far larger variety of systems used, as both stick axles and independent configurations are popular depending on the era of car one has. Regardless of the configuration, “Kinematic” analysis is done to understand how the wheels behave when the car goes through the variety of roll, pitch, braking, etc, while ignoring the forces associated for simplicity. There are highly capable and user friendly software tools on the market to easily assess this, but the equations used to power them are all the same, well understood engineering fundamentals. While the equations are straightforward, the biggest issue is that each individual aspect of a suspension design virtually always interacts with a subsequent one, and the result is a complex set of balances and compromises – you just simply cannot design a “perfect, optimized” suspension system – you have to pick your battles. Couple this with variations in track and road conditions, driving style, tire characteristics, and a nearly infinite number of other variables, and you have one complex set of equations to solve simultaneously. Because of this, our philosophy in product design is to allow the ability to adjust the suspension components wherever practical. We could write a book on this topic alone, but the intent in this short paper to show that Motion is one discipline that is again, at the Core of our designs.
Rates: Just as Motion analysis is primarily derived from Kinematic analysis, Rates are also derived from classical mechanical engineering equations. When we look at rates, we are considering the forces associated with both the statics (sitting still) and dynamics (in motion). Most folks understand that the rate of wheel motion is largely derived from the spring rates, and the roll bar rates. But there is a lot more to it, and some very important aspects are simply not understood, or worse - ignored. The rate at which any wheel moves depends upon:
- Spring rates, roll bar rates, and their respective installation ratios. The installation ratio is defined as the distance that the spring/bar attaches to the control arm/axle, relative the center of the contact patch of the tire – and this value is squared when considering the resulting wheel rate. In other words, the further that you place the spring/bar away from the contact patch, the less effective it becomes at controlling the wheel rate. This is why cars with numerically low installation rates have sometimes HUGE spring rates. The further outward you can place these, the better, everything gets happier.
- Friction. Friction is the enemy, but it exists in every setup so the best thing you can do is to minimize it. This is dominated at the various suspension connections, we rely on very low friction pivots and balljoints to combat this.
Deflection: Deflection is also the enemy, and as crazy as it sounds, there are several suspension systems that are by design over-constrained, and wouldn’t allow the car to roll if there wasn’t deflection. Every successful race car is designed to be as rigid as practical, so the “Kinematics” of the suspension design is actually realized in real life. This is where a critical analysis of the specific forces of the entire system are accurately derived – again, we see all the time that many suppliers don’t do this, as the designs they come up with are in many cases, well, silly. Some elements of deflection:
- Bushings: Rubber, Polyurethane and to some extent Nylon/Delrin derivatives all squirm under pressure.
- Deflection/Bending: Don’t kid yourself, your car is probably a LOT more flexible than you realize. Subframe connectors and similar approaches can help, but the true way to minimize deflection is with triangulated 3D structures – in other words a well designed roll cage. For certain the primary consideration of the cage is for safety, but if you look at any contemporary race car, you will also see that the cage is designed to carry loads from the suspension, and that the links are triangulated.
- We go to great lengths to consider the amount of deflection a component sees in the real world prior to performing any design work. Further, almost everyone will first ask if a design is “strong” enough, but very few will also ask if it is “rigid” enough. Obtaining a rigid structure is the first order for our engineering approach, and in most cases, once it is rigid enough, it is also strong enough.
Damping. In a word: “Shocks.” While the shock system used will also affect the wheel rates, the main intent is to control the springing motion of the suspension from “ringing,” as well as to establish the rate that transitions occur (pitch, roll, etc). And even that is an oversimplification. Look at any professional race team, and you will note that they not only have a shock expert on the team, but that they also have the ability to completely tear down, re-valve, and physically measure the results – at the track. On top of this, enthusiasts are typically “brand loyal” to their particular shock supplier, and often bash the other offerings on the market. There are so many outstanding suppliers out there that we have taken the primary approach that we are shock agnostic – you can use whatever you want with our products.