In this comprehensive article, we dive into a series of questions regarding vehicle dynamics as applied to race cars that you send to us! To answer you, we created a series presented by Bruno Finco. We will explore each of these questions with a focus on providing clear and balanced explanations. You can watch the complete video below:
Let’s begin by addressing the questions that have been raised and uncovering the rationale behind them.
Should I make a spring or an ARB (anti-roll bar) change to fix car balance? Why and when?
Typically, if you’re trying to only change mechanical balance, an ARB change will be better because it’s more isolated. You are only changing the roll properties of the car, trying to shift the load transfer either more forward if you need more understeer, or more rearward in case you need more oversteer. If you make a spring change, you change many other parameters in the car such as dynamic ride heights and aero balance.
So, for example, as you are braking into a corner, you will have a different amount of movement, meaning that you have a different aero balance at the corner entry. At different car speeds, your car will sit at different heights, again changing its aerodynamic properties. Also, if you make a spring change, you will also change how much bottoming your car has because if you run softer, it will change the pitch or the heaven and you can start touching the asphalt, and it could even influence your top speed since you have different ride heights for the front and rear axle.
So when would you do a spring change, if the anti-roll bars seem to be a much better option for changing mechanical balance only?
Well, sometimes your anti-roll bars will be out of range, so you already have your front anti-roll bar, for example, at the stiffest position, and your rear anti-roll bar either at the softest position or even disconnected, and you don’t have any margin for adjustment. It’s a very good practice to have your springs selected in a way that you still have range available for the front and rear anti-roll bars, up and down. Because if your anti-roll bars are out of range and you need to change springs during the session, you will lose a lot of time.
One last consideration is that when you stiffen your anti-roll bars, you could compromise your right behavior even more than the springs, this is another downside of the anti-roll bars. For example, when you go over curbs or over bumps on one side, since the anti-roll bar is connecting both sides, it will compromise and decrease, for example, your traction capabilities or even cornering capabilities further. So just be careful with running very stiff anti-roll bars for that reason.
How to define targets for mechanical trail?
Mechanical trail from the perspective of vehicle dynamics and driver feeling, will be influencing two main parameters:
- The total steering torque that the driver is feeling at the steering wheel.
It cannot be so high that the driver cannot hold the steering wheel, and it cannot be so low that the steering wheel is not coming back when the driver is releasing the force.
- Steering feedback
As the driver is steering, it feels a steering torque that will decrease. That happens because as you corner, the pneumatic trail of the tire will decrease. Once you are close to the peak grip of the tire, the pneumatic trail will be close to zero, so the driver feels the steering wheel angle a little lighter, and it’s an indication that the tire is close to peak grip.
While the steering torque is proportional to the sum of the mechanical and pneumatic trail, the steering feedback that we are discussing is the proportion between the mechanical trail versus the pneumatic trail.
In summary, you need to have a mechanical trail big enough so that the steering is coming back by itself, but on the other hand, you cannot have it so big that the pneumatic trail has no influence on the steering torque, otherwise, your driver will have no feeling from the steering wheel. So, each type of car and each type of tire will ask for a different range of mechanical trails, so you better learn about your car and do a few iterations so that you can find that.
You can do some simple calculations, especially if you have the tire model and the pneumatic trail. Otherwise, you should test it at the track and work with your driver to find the right number.
How do Ackermann and static toe change vehicle behavior differently?
Let’s discuss each of them to understand the differences.
For the Ackermann geometry, it is proportional to the steering angle. The more you steer, the more difference in steering angle you will have between the left and right sides. This means that it has a different influence on low-speed corners where you have a lot of steering angles and high-speed corners where you have small steering angles. So, in the end, for low-speed corners, you have a lot of influence from the Ackermann, while you have pretty much no influence in high-speed corners.
In the same way, corner entry does not change anything because you don’t have a steering wheel angle yet, while at the apex, it’s a lot more influential. When you are at the track, it’s hard to make an Ackermann change, very few race cars allow you to make a change, so you must work with what you have.
For the toe, it’s a constant value, meaning that you have the same influence for low-speed corners with a lot of steering angle or high speed. In the same way, you have the same influence at corner entry with very little steering versus at the apex. So, you have a constant influence on the tire performance or the axle performance. Another characteristic of toe, that could be an advantage or disadvantage, is that it is always there, even if you have no steering angle. So, on the straight lines, you have some sort of toe-in or toe-out that could generate tire temperature, but at the same time, it will generate some induced drag and reduce your top speed and also generate wear that you might not want.
One of the biggest advantages of toe is that it’s easily adjustable, so we are always making changes to the toe on the front or on the rear during a race weekend to achieve what we want. In summary, Ackermann will be proportional to the steering angle and will be more influential at the apex and for low-speed corners, while the toe is influencing everything.
What matters the most is for you to find a combination of Ackermann and toe that will give you ideal performance, meaning a lot of response from the front axle but enough stability on the rear. In the end, what matters the most is for you to find the ideal combination between Ackermann and toe, since the car is responding to both at the same time.
Since you typically cannot change the Ackermann geometry, it is important that you understand it and then find the toe that gives you the ideal front axle response.
What is the reason for using static camber, and why not use only the camber gain from the caster angle?
There are a few different reasons why we would still use static camber even though due to caster we can gain camber as we steer.
The first reason is that we only have the caster effect on camber gain on the front axle and not on the rear axle.
The second reason is that the camber gain coming from the caster is only influential when you have a lot of steering angle. This means that for low-speed corners, we do gain a lot of camber because of the caster, since we have a lot of steering wheel angle, however, for high-speed corners, we have minimal steering, meaning that we don’t recover as much camber on the outside tire as we need. The same is valid for corner entry versus the apex.
At corner entry, you have very little steering wheel angle; therefore, you don’t gain as much camber as you could want. You would need to wait until the apex until you have this camber, and many times you do want to have camber at the corner entry. Because if you add camber to the front axle, you might get more response, or if you add camber to the rear axle, you might get more stability. In summary, by running a higher caster angle on the front axle, you can decrease the static camber, but as we saw, it does not remove the need for static camber.
How can I adjust the damper bump and rebound to affect the car balance?
First, we need to understand in which phases of the corner the dampers will be most influential. Damping is proportional to speed; therefore, we need roll speed for the dampers to have any effect on the dynamics of the car. This means that when you are in a corner entry and you have some low speed, or at a corner exit, the dampers will be the most influential in terms of vehicle handling.
Now, at the apex where you have a more stable condition, it does not have any influence on the load transfer. Of course, the bumps on the road will influence the vertical load variation that you get, but not in terms of lateral load transfer. This means that the dampers will be effective at defining corner entry and exit behavior.
Now, how does it change the vehicle behavior?
The dampers can change the lateral load transfer distribution in transient. So when you’re cornering, you have load transfer on the front and on the rear. If you adjust your dampers, you could get more load transfer on the front or more on the rear, which will change the vehicle balance. Typically, the axle that transfers more load, you will decrease the grip on that axle, even though for corner entry, it could be a little different. If you have more camber or depending on the toe that you have, you could get the opposite behavior to what I just mentioned.
It is very important that you test at the track to understand what the damping is doing to your vehicle balance at entry.
Now, how do you work with bump and rebound?
Well, both will influence the vehicle behavior. What you must have in mind is that if you are changing the bump, it will affect the outside tire; if you change the rebound, it will affect the inside tire. But both tires define the final balance of the vehicle. So, if you’re only trying to change the transient load transfer and you want to simplify the issue, I’d think more about which axle am I changing, and then I change both bump and rebound, then if I should focus on the bump or rebound specifically.
If you want to go a little bit more advanced, then you can think, “Okay, I’m going to change only bump or only rebound.” But then in that case, you have to also keep in mind what other changes in terms of ride performance you will get as you make this asymmetrical change and also what differences in, for example, braking or traction you will get.
How does the steering rack position affect the roll center height?
If you’re using the very simple calculations of the roll center position, which is basically the intersection between the planes of the wishbones, it will have no influence since you’re not taking the steering rack or the toe link into account. However, if you do the proper calculation, for example, the one that we use in our software, Optimum Kinematics, changing the steering rack will indeed change the roll center height.
First, we should know that the roll center height is defined by the instant center, which is the point around which the wheel is rotating. If you change the steering rack, even though the vertical movement of the tire seems like it’s not changing, the tire will have a different bump steer, and the wheel will move in a slightly different way. If the wheel is moving differently and you have some sort of rotation, the instant center position will change. If the instant center position changes, the roll center height will also change.
In summary, by changing the steering rack position, you change the bump steer. The bump steer controls the movement of the wheel, which you will then change the instant center and therefore the roll center is also affected.
How does differential lock affect balance on power in corner exit?
In simple terms, more differential lock means more locking torque.
Basically, how much torque the differential can sustain between the left and right sides for corner exit. More locking torque makes the differential capable of transferring more of the torque to the outside tire if the system is asking for it. This means that as you are going back on the throttle and you are above the preload, you should be shifting more of the torque to the outside tire, generating more rotation. However, this is not an active differential, meaning that the torque is not automatically transferred to the outside tire. It only means that if this dynamic system is asking for more torque on the outside tire, the differential can provide that. Typically, it gives more oversteer behavior or more rotation.
Now, once the differential is locked, let’s say that when you are 60% or 70% power or on the throttle, the differential is already locked. Increasing the locking torque even further will not change anything. Once the differential is locked, it doesn’t matter what you change, the two wheels are rotating already at the same speed.
The question was about corner exit, but if you are interested in corner entry and the influence of differential preload, we have a whole video about that subject in our performance engineering series.
When is it better to increase the spring rate to increase grip?
Typically, increasing the spring rate will not directly help the grip of the tire. That happens because we’ll generally get more load variation, so, as you are driving over bumps, the load is varying more, which we know decreases tire grip. However, there are a few specific situations where increasing the spring rate will help you achieve more grip.
The first one is in terms of driver feeling or load transfer speed. Sometimes you need more support, you need less camber variation, you need faster load transfer, and a faster response of the car. In that case, increasing the spring stiffness will make the car easier to drive for your driver, especially if you are running a very soft car at a high-level racing series, in that case, a stiffer spring will not give you higher grip, but it will help your driver exploit that grip.
Another situation where running stiffer springs would increase grip would be if you need more aerodynamics, meaning that you want to run the car lower. Also keep in mind that you are also lowering the center of gravity for you to run the car lower; you will need stiffer springs so that you can have a higher level of platform control and avoid the car from bottoming and touching the asphalt.
Besides that, with stiffer springs, you get less camber balance variation, so as you are braking under traction or at different corner speeds where your car is sitting at different heights, stiffer springs will help you maintain your balance more constantly. This can also be a powerful tool to provide the balance your driver needs in different parts of the track. Increasing the spring rate could also have some contribution to tire temperature. It could have a small influence on increasing tire temperature, so you should also keep that in mind, even though you should test if that is an effective parameter on your specific car.
So these were some examples of situations where increasing the spring rate would help you achieve more grip.
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!