Continuing our series of questions and answers with Bruno Finco, today we will talk about Vehicle Setup and Vehicle Design, with questions sent by you! Let’s not waste any time and dive into the questions!
Why does toe-in on the rear increase stability?
When you put more toe-in on the rear, you are basically creating what we could call a slip-angle preload. You’re preloading the outside tire of a corner in the direction of the corner. This means that when you enter the corner, you start transferring load and generating lateral force, and this tire is already seeing a slip angle in the direction of the corner, which will help keep the rear stable since the tire with the higher amount of lateral load is already generating lateral force to keep the rear stable.
Even in a straight line, it’s going to give more stability in the braking zones. This happens because even though you cannot always see it, when you are braking, you have small movements of yaw and small amounts of load transfer. So, all of this happens when you have a little more load on the tire that is holding the car in a straight line and keeping it more stable. Not only do you increase stability, but you actually decrease car response, so, you don’t get as much car rotation because of the same reasons we discussed. This means that if you want to increase car rotation and stability is not an issue, one thing that you could consider doing is decreasing a little bit of rear toe-in. Just be careful because this parameter is extremely sensitive, so every millimeter that you change will influence a lot of the car’s behavior.
What factors go into selecting a suitable roll gradient range (deg/g)?
So basically, the roll gradient is how much roll you get for each G of lateral acceleration. The first thing you should do to define what roll gradient you are using is to do benchmarking. So typically, for a given car segment, be it a passenger car, a race car, a GT, a prototype, or a Formula car, you have a specific range of roll gradients that people typically use because they’ve learned that for that type of car, it’s what works best.
But what considerations should you have if you have to come up with a rough gradient range?
Well, first of all, you need to understand how much support you need. If it’s a Formula car, you need a lot of support, you have to have a very minor roll or very minor pitch variation. If it’s a GT car, you can have a little bit more variation, you are not running as low, and your aerodynamics is not as sensitive. Besides that, you need to think of load transfer. If your car is too soft, the roll movement, the load transfer, for example, will be too slow, and the driver will feel that he is not confident enough in the car because he needs to apply the steering and wait for a few seconds or tenths of a second to feel the car find a new stable position. In that case, it means that you need a stiffer platform that reduces the roll or pitch gradients.
On the other hand, if you have a really stiff car, for some types of cars and drivers, it is not ideal because the load transfers too fast. The driver does not have the time to find the ideal steering angle that he needs at that corner entry, for example. So, we also see that it’s dependent not only on the car type that you are working with but also on the type of driver. Typically, a professional driver will be able to afford a lower roll gradient, meaning a stiffer car, while we would see the opposite for an amateur driver.
One last consideration is that you should have some level of adjustment range. So, whenever you design a car, you should design your springs and anti-roll bars in a way that you can go from a stiffer platform, meaning a lower roll gradient, to a softer one, meaning a higher roll gradient.
I’ve seen people saying that toe-in on the front axle improves the turn-in response, while other people say that toe-out improves the turn-in response. Which one is true?
This is a very good question, and the answer is not that simple. There are many different reasons why toe-in or toe-out on the front axle would improve response. The first one is the slip angle on the front axle as you start to steer. In this case, more toe-in would help car response since you are preloading the slip angle in the direction of the corner as you start your corner. So, let’s say that you have toe-in when you enter the corner, and you transfer the load to the outside tire; it’s already pointing in the direction of the corner, helping the car to rotate more. So, in this case, toe-in is beneficial.
Next, we have the induced drag consideration. If you run toe-out as you steer, the inside tire will have a higher steering angle, and it will generate more induced drag because of this slip angle, which will help rotate the car since it’s a pro-yaw moment factor. So, from the induced drag perspective, toe-out would give more car response, which is the opposite of what we just saw for the slip angle preload.
Lastly, it all depends on the Ackerman geometry that this specific tire asks for. Some tires will need more toe-in on the outside tire, so toe-in would help. Other tires need less slip angle on the outside tire when it’s loaded, requiring more toe-out. Not only that, but naturally, as the car corners, it will have different slip angles between the inside and the outside tires, so from this specific perspective, do you need more or less slip angle on the inside and outside tires? It depends. Sometimes toe-in will help, sometimes toe-out will.
Now, as you can see, there are many different factors influencing the car’s response based on the alignment of the front axle, and sometimes they are contradictory. But this is why you see some engineers finding that with their car, more toe-out will provide better response and a stronger front axle, while other engineers will see that toe-in will provide this higher response. The best way to figure it out? Test it with your car. Make a big enough change to be able to quantify it. So, for example, for a professional race car, you could try going from three millimeters or two millimeters of toe-in all the way to two or three millimeters of toe-out back-to-back. Repeat this multiple times until you get a concrete answer.
How do I determine the ideal spring stiffness for a Baja car?
In the case of off-road cars, there are two main parameters that we will analyze when defining the spring and stiffness package. The first one is ride quality. In this case, it’s very important since the terrain is very rough, so we are trying to minimize load variation, minimize the impacts, and also work a little bit on driver comfort. This parameter is asking for soft springs. With soft springs, you are able to minimize the load variation on the tires.
Now, the second parameter is that you need some sort of platform control, not as much as with an aerodynamic car, but you still need to control the movements of the car, roll, pitch, but also jumps, and landings. So, you need to have some sort of platform control. What you should do here is first define roll and pitch gradients that are acceptable, as well as extreme maneuvers such as landing, to make sure that you are not bottoming out on your dampers too much. You try to use higher values, for example, of roll gradient and pitch gradient, so that you can run soft springs. In this way, you can find a balance between parameter #1, which is ride quality, and #2, which is platform control.
The trick here, though, is once you have defined this spring that you are trying to run, you should also focus on the damping characteristics that you need for that. If you don’t have the right damping, your suspension will not behave well, even though you select a relatively soft spring. It will not minimize the load variation you can get out of the suspension. So, by selecting the proper damping ratio for your system, you make sure that for that given defined spring stiffness, you are minimizing load variation while still having good platform control. Typically, a damping ratio of 0.7 could be a good starting point for your design, and from there, you can work with simulation or track testing to refine it.
How can I set my optimum damping ratios differently for bump and rebound?
First of all, let’s think about the overall damping ratio or the average damping between bump and rebound, this is a very good metric for us to follow, as this will be one of the main factors dictating car behavior. When we speak about the bias between bump and rebound, it will be heavily dependent on driver feedback, platform control, and dynamic ride. Let’s tackle each of them.
So, for example, if you want more support on the front when you are braking or on the rear while you are under traction, you would want to run more bump either on the front or rear axle. Now, if you want less movement on the rear when you are braking or on the front when you are under traction, you would play with the rebound side of the damper adjustment. If you would like less aggressive impacts over curbs, for example, then again you would go back to the bump adjustments. So, you are seeing a few examples where we would adjust only the bump or only the rebound.
Once you make this adjustment, let’s say that you decrease the bump to have a less aggressive impact over curbs. If you want to keep the same average damping ratio that we discussed, it’s very important, you could actually increase the rebound. So, by decreasing the bump and increasing the rebound, you create this bias, but the average damping, so the average potential of the car to dissipate energy over bumps, remains the same.
Now, one thing that you have to keep in mind when you do asymmetric adjustments of bump and rebound is that the car will have different dynamic ride heights. For example, if you are running a bump-biased damper setup, meaning that you have a lot more damping for the bump side than for the rebound side, in that case, when you are driving over a section of the track that is very bumpy, your car will sit higher. You have higher dynamic ride heights. That happens because since you have a lot of bump damping, the car cannot compress on the damper, so it cannot go lower, but you don’t have a lot of rebound damping, so the car can go up. So, it keeps going higher and higher, and then you end up with higher dynamic ride heights. On the opposite adjustment where you are running a rebound-biased damping, your car will sit lower around the track. Again, when you are going over these oscillations, the damper can compress, meaning that the car can run lower, but it cannot extend as much. So as a consequence of that, your car will have a lower dynamic ride height. Sometimes this could be a useful tool if you want to, for example, run lower in a specific part of the track, but also it can be harmful because since you are running higher or lower in specific parts of the track, you cannot keep as low ride heights as you would want. It all depends on the goals that you have in order to define the damping bias between bump and rebound.
What are the best setup changes we can make in a low-speed cornering issue?
If it is a low-speed corner, we know that aerodynamics will not have a big influence, so let’s discard those changes. We’ll focus on the mechanical grip, so grip coming directly from the tires or from load transfer. The first change that we should consider when we have issues in low-speed cornering is the lateral load transfer distribution. So, whatever axle we transfer more load to will decrease the grip on that axle while improving the other one. So, let’s say that we have understeer. We need to decrease the load transfer on the front axle. For that, we should soften that axle. We could use softer anti-roll bars or softer springs. But the opposite is also true. We could stiffen the rear axle by using stiffer anti-roll bars or stiffer springs. The result is very similar, at least in terms of load transfer. Even if you have an issue with the front axle, if you change the stiffness of the rear axle, you will help the front.
Besides that, we could think of setups directly affecting the tire. So, for example, we could change the camber. Typically, more camber will give you more grip, but it’s not always true, and also, we have to be careful with temperature distribution, tire failures, and regulation limitations.
The second parameter we should think about directly from the tire perspective is the pressure. Again, by changing the pressure, if you are at the ideal pressure, it will change or, if you put the wrong pressure outside the ideal window, you will influence the low-speed corner balance.
Now, thinking of all the set of changes you could make, since it’s a low-speed corner, we could use, for example, caster adjustment. If you increase the caster, you will gain more camber from steering that helps you recover the camber you lost on the outside tire, so that could be very effective for low-speed corners while not affecting high-speed corners. You could also think about changing the differential preload. In low-speed corners, you have a lot of wheel speed difference, meaning that the differential setup will also be very influential. Typically, if you increase the preload, you will get more understeer. We have a complete video about this topic.
Lastly, something that we have to keep in mind that is heavily influential on the car’s balance, not only at low speed but particularly at low speed, is driver input. So, if you are braking, if you are on the throttle, depending on how you apply the brakes, and how you apply the throttle, you will influence car behavior. Since in low-speed corners, you have a lot of deceleration with the braking and then traction out of the corner, that could amplify even more the influence of driving style on car balance. So, don’t think only about car setup, try to understand if it could also be coming from your driver. We have two videos analyzing driver inputs and how these inputs influence car balance, check them also!
These were the answers to your vehicle dynamics questions applied to race cars. If you would like to submit more questions like these, you can send them to our Instagram profile. I’ll be waiting for your next questions!