Think about this.
Suppose you take a car with its stock suspension, put high-grip tires on it, and take it to the track. The car rolls so much that its response feels slow and it’s very difficult to drive.
What would you do to make the car easier to drive?
Increasing the damping force will have some effect, but it won’t solve the problem at its root. So I think a lot of people would try stiffening the springs or anti-roll bars. After all, a car that rolls less and feels solid and stable will respond to steering inputs better than a car that feels soft and floaty.
We all know from experience that “stiffening the suspension improves response.” And yet, for a long time, I had never heard a logical explanation of the mechanism behind it that really made sense to me.
The key point is this: when a car rolls a lot, its response feels slow while you’re turning the steering wheel and the car is still rolling. Then, once the roll finally stops, the car starts responding properly.
For example, if you ask ChatGPT why this happens, depending on how you phrase the prompt, there’s a pretty good chance you’ll get an answer that sounds convincing at first, such as, “Because energy is being stored in the springs,” or “Because the tires can’t respond until load transfer is complete.”
But if you reply, “The load is already acting on all four tires from the beginning, and load transfer is simply a redistribution of that load. Since tires have load sensitivity, wouldn’t lateral load transfer actually reduce the total grip of the left and right tires?” ChatGPT will admit that its previous explanation was wrong, and after that, you often don’t get any particularly useful answer.
I looked through various papers and technical literature, but I couldn’t find anything that really explained this part of the mechanism. I was left thinking, “Hmm…”
Then, about a year ago, I finally came across an idea that could explain at least part of this mechanism quite neatly.
I’m sorry that the paper is in Japanese, but here it is:
Analysis of Effect of Roll Movement on Easiness to Change Direction Using Lateral Acceleration Input
The author, Saiichiro Oshita, worked at Fuji Heavy Industries (now Subaru) and Kanagawa Institute of Technology, and has collaborated with Masato Abe — yes, the Vehicle Handling Dynamics guy.
The title includes the phrase “easiness to change direction,” so some of you might be thinking, “Wait, what does this have to do with sports driving?”
Don’t worry. The paper is very much about the response you get when you turn the steering wheel.
Here’s an excerpt from the introduction, translated into English:
“In vehicle development focused on handling, it is common to increase the stiffness of the springs and anti-roll bars, as well as damping force and lateral stiffness. In this report, we added a mechanism to the model shown in Fig. 1 in which the tire contact patches move laterally toward the inside of the turn as the vehicle rolls, with the center of gravity as the origin, in order to quantitatively evaluate the effects of these factors on handling. With this mechanism, it became possible to evaluate these factors because the lateral force generated by the tires decreases as roll angular velocity increases.”
I’ll skip the specific equations here, but the paper shows that tire lateral force—the combined equivalent cornering stiffness of the inner and outer tires—decreases as roll angular velocity increases, and also as the distance between the center of gravity and the roll center increases.
Put simply, the idea is this:
“While the car is rolling, the tires keep moving laterally toward the inside of the turn relative to the center of gravity, and that reduces grip.”
The grip we’re talking about here—the cornering force generated by each tire—comes from the force produced when a tire that has been deformed by cornering tries to return to its original shape.
The tread surface of the tire is in contact with the road. When the body tries to pull the wheel sideways due to centrifugal force, the tire deforms laterally because the tread is gripping the road while the wheel is being pulled sideways.
But a tire has spring-like properties, so when it deforms, a force acts to return it to its original shape. The lateral component of this force, relative to the direction of travel, is what we call cornering force.
The thing is, cornering force is ultimately the force generated by a tire that has been pulled and is trying to return to its original shape. So unless you pull the tire hard enough, it won’t generate a large cornering force.
But when the distance between the tire and the vehicle’s center of gravity grows due to body roll, the “pulling” force ends up weaker by that same amount.
Strictly speaking, the tire is pulled directly by the wheel, and the wheel is attached to the subframe via the hub carrier and suspension arms—so rather than the center of gravity of the entire vehicle, we’re really talking about the center of the sprung mass. Either way, while the distance between the tire and the center of gravity is “stretching,” the pulling force is weaker.
It’s a bit like pulling on a chain—the chain itself doesn’t stretch, but while the distance between the two ends is still increasing, the pulling force doesn’t get through at full strength. Once that distance settles, the force comes through fully.
And as a result, the cornering force—the force trying to return the tire to its original shape—also decreases.
But once the tire has reached the end of its movement, the “pulling force” is properly applied again. That’s why, once the roll motion settles down, the car finally starts responding properly.
Oh! That makes sense!
The mechanism behind why stiffening the suspension improves response may not be explained entirely by this principle alone. But after spending so long feeling like I had no real clue what was going on, finally finding an idea that explains at least part of it this clearly was incredibly satisfying.
By the way, if you keep making the suspension stiffer and stiffer, the improvement in response eventually hits a ceiling.
That’s because you enter a range where other elements, such as the body and tires, become the limiting factors rather than the suspension stiffness itself.
The importance of body stiffness is discussed all over the place, but tire stiffness is also extremely important.
This article is getting pretty long, so I’ll save the rest for the next one…