Hello everyone! Quick question: have you lowered your car?
As a responsible MX-5 Miata owner, I still have enough self-control not to slam it completely.
That said, if it were up to my emotions, I’d slam it straight into the ground.
When I was younger, I remember someone I knew who did time attack saying, “I lowered my car and knocked a full second off my lap time!”
I remember thinking, “Really? A whole second? Isn’t that basically within the margin of error?”
Back then, anyway.
Now I know better. Once you start chasing lap times, a whole second is enormous.
Lowering the ride height lowers the center of gravity, which improves the car’s performance. Although looking back, I suspect part of that gain also came from finally getting a bit more negative camber.
Lowering the center of gravity is incredibly important for a sports car.
Why does it improve performance?
Because tire grip has a characteristic where it becomes increasingly difficult to gain more grip as vertical load increases.
That means if the tire loads aren’t evenly distributed, the grip lost on the lightly loaded tires is greater than the grip gained on the heavily loaded tires. The result is less total grip across all four tires.
In other words, the more evenly the tire loads are distributed, the better.
Lowering the center of gravity reduces load transfer for a given longitudinal or lateral G, so the load distribution stays more even and less grip is lost.
So if you’re chasing even a tenth of a second… yeah, you’re going to want to lower the car as much as possible.
But there’s something you have to watch out for.
There was a time when people commonly lowered their cars simply by cutting the springs on the factory suspension.
That lowers the ride height—but it also reduces bump travel.
So what happens when bump travel gets shorter?
During braking or cornering, the suspension bottoms out much more easily against the bump stop. When that happens, load tends to concentrate on that tire, which means even more load is taken away from the tire on the opposite side, making the load distribution even worse.
At very high loads, tires become even less efficient at generating grip.
You end up with more understeer on corner entry, more oversteer on corner exit, and lower overall grip. In severe cases, excessive tire deformation from overload can dramatically increase rolling resistance, making the car actually slow down.
The same thing can happen with lowering springs if the ride height is reduced too much.
The tricky part is that on a smooth racetrack, you can sometimes get away with it. The car still feels… okay. It’s just somehow slower.
There’s another reason for this. Lowering the ride height also reduces the effects of anti-dive, anti-lift, anti-squat, and anti-roll built into the suspension geometry, making the car even more likely to bottom out.
That’s why running an extremely low car is basically a constant battle for bump travel.
By the way, “bottoming out” generally falls into two categories: (1) the suspension is already bottomed out at static ride height, and (2) it isn’t bottomed out at static ride height, but it bottoms out while driving.
Soft urethane bump stops also change stiffness depending on how much they’re compressed. A lightly compressed bump stop feels completely different from one that’s crushed flat.
For the purposes of this article, though, once the suspension becomes significantly stiffer because it’s reached that limit and can no longer travel properly, I’ll simply call that bottoming out.
When people start talking about bump travel and rebound travel, another question often comes up: “What’s the ideal bump-to-rebound travel ratio?” Some people say 50:50. Others insist it’s 40:60.
But those numbers are generally based on normal road driving at legal speeds, so if you’re building a track car, you don’t need to take them too seriously.
For example, the front-to-rear wheel rate balance directly affects the car’s handling characteristics, so spring rates matter. Front-to-rear ride height also matters because it changes the inclination of the roll axis.
But the bump-to-rebound travel ratio itself doesn’t directly affect how the car handles. As long as you have enough travel left, you’re fine. If the suspension never bottoms out or tops out, the ratio itself is just a number.
Of course, you still want a reasonable safety margin, but that’s why the priorities for a track car are different from those for a street car.
On my own MX-5 Miata, for example, the suspension reaches its travel limit before the damper bottoms out. At the front, the limit is tire-to-body contact. At the rear, it’s arm-to-body contact.
That means the travel ratio measured at the damper rod has nothing to do with the suspension’s actual travel limits.
The front still has plenty of room before tire-to-body contact occurs, but the rear is much tighter. That’s why I actually built a DIY clearance gauge out of body filler on the suspension arm, so I can monitor exactly how much room is left before arm-to-body contact.
So the important question isn’t “What’s the bump-to-rebound ratio?” The important questions are: does it bottom out? Does it top out? If it does, how often and how severely? Those are the things that determine whether your tires can actually deliver the grip they’re capable of.
Instead of worrying about some travel ratio, it’s much more useful to know how many millimeters remain before bottoming out—or before topping out.
For an MX-5 Miata, topping out usually isn’t much of a concern. But on cars like the front suspension of an AE86 or the rear suspension of many FF cars, wheels can lift during cornering, so depending on the car, rebound travel deserves attention too.
Back when I was still running the factory suspension, I happened to meet professional racing driver Takayuki Oi at Takasu Circuit. He told me, “You’re braking too hard.”
With the stock suspension, trying to use the tire’s full potential under braking causes the front suspension to bottom out. As a result, braking a little more gently—and avoiding bottoming out—actually makes you faster.
At the time, I couldn’t quite accept it. All I could think was, “The tires feel like they still have more grip.” It wasn’t until much later that I understood what he meant.
Listening to what the tires are telling you is important. But if the suspension isn’t working properly, the tires can fool you.
You can spend endless hours perfecting your alignment and damping setup, but your grip may still be capped by something as simple as the damper bottoming out or arm-to-body contact.
It’s worth checking whether your suspension is running out of travel somewhere and preventing the tires from delivering the grip they’re capable of.