An engineer from Team Le Mans, a racing team that has competed for many years in top Japanese categories such as SUPER GT and Super Formula, once wrote the following in a post published on the team’s Facebook page:
Does changing the springs change the amount of load transfer under braking?
No. It doesn’t matter whether the springs are stiff or soft. The only thing that changes is the vehicle attitude.
It’s fine for a driver to say, “The suspension is so stiff that I’m putting too much load on the tires under braking,” but an engineer shouldn’t go around saying exactly the same thing.
Strictly speaking, the amount of load transfer does change by a tiny amount, so I’m a little reluctant to say that it doesn’t change at all. But the difference is so small that, for all practical purposes, it doesn’t change.
Drivers, however, will feel that the amount of load transfer has changed when the vehicle attitude changes, so they will casually say things that are completely different from the physical phenomenon actually taking place.
And that’s fine. A driver’s job is to drive fast. If they can go fast, that’s what matters.
Engineers are different, though.
To make the car better, they need to correctly understand what is actually happening.
Why does raising the front ride height shift the steering characteristics toward understeer?
Many drivers will say, “Because the load moves toward the rear.”
But an engineer shouldn’t say that.
No matter how much you change the ride height, the wheelbase barely changes, so almost no additional load transfer occurs.
This is a calculation by Tatsu that I introduced on this blog before.
The Relationship Between Ride Height Changes and Front and Rear Tire Loads
The calculation shows that changing the rear ride height by 10 mm results in only 0.8 kgf of load transfer.
Just like the example at the beginning, it changes a little, but not enough to really matter.
And yet, actually changing the rear ride height by 10 mm can change the handling so much that it feels like you’ve got a completely different car.
So the important thing is to ask why that happens and understand what is actually happening to the car.
As for the front ride height, raising it increases the roll center height and tilts the roll axis rearward. This increases the front’s share of the total load transfer, which moves the car toward understeer.
At the same time, though, initial response increases.
The main reason is probably the front toe-in.
And related to the roll center point, the increase in anti-roll effect also contributes to the initial response.
You often hear people say, “Put about 5 mm of preload on the front springs and they’ll hold the car up better and improve response!”
But if you change the preload without changing the ride height, the amount the spring is compressed, the position of the spring, and the force acting on it don’t change at all, except in the range below the preload force.
And completely independently of preload, simply raising the front ride height a little will increase initial response. It’s easy to do, so if you want more response, it’s a pretty handy adjustment to make at the track.
It’s a bit like the old shift from a geocentric to a heliocentric view of the universe. The physics at work in a car often involves mechanisms that go against our intuition and common sense, which is why having the right knowledge and understanding matters.
If you want to learn about suspension mechanisms online, I recommend the Tech Explained articles from Racecar Engineering and the technical articles from OptimumG. Both are available in English, so they’re good places to start if you’re interested.
Of course, ideally, I’d rather learn this stuff from a proper textbook than from information on the internet. Masato Abe’s Vehicle Motion and Control is one of the books I’d trust, and there’s also an English version, Vehicle Handling Dynamics. But it’s a pretty serious technical book, and you need a decent amount of knowledge to read it in the first place.
I bought it myself, but I haven’t made it all the way through…
Still, the information available online isn’t necessarily wrong. The two sources I mentioned above are technically sound, so I’d say it’s best to start with whichever one feels easier to tackle.
Then again, if you’re the kind of person who says, “I’m a driver, so I don’t care about mechanisms!” that’s perfectly fine too. Don’t worry about the difficult stuff. Just practice a lot and work on getting faster.
No matter how much correct knowledge you have or how well you can handle complicated calculations, it’s all meaningless if you don’t have the driving skills to actually go fast.
But if you’re an engineer who works on cars, it’s important to correctly understand what’s actually happening.
I’d like to be able to understand the physics of cars without being misled by all the misconceptions and crazy theories floating around out there.