Why Some Cars Lift Their Inside Wheels in High-Speed Cornering
When physics makes for entertaining photography.
On the race track, it’s always an event when tires lift off the pavement. It’s typically obvious what causes two- or four-wheel air, but there’s a third form that is a bit more mysterious. Small cars like Mini Coopers, Mazda 2s, Honda Fits, and Volkswagen GTIs occasionally go three-wheeling, and there’s a specific reason why it happens.
Three-wheeling is when a small car, most commonly a hatchback, lifts its inside rear wheel under hard cornering. This could be on track or on an autocross course, and it indicates that the driver is pushing the car to its limits. On front-drive cars, the inside rear wheel lifts, while the inside front wheel lifts on rear-wheel-drive cars. But what are the physics behind three-wheeling?
What Is Three-Wheeling?
Three-wheeling is best explained by those who understand the complex workings of racing suspensions and those who have experienced one-wheel lift on track. Lee Grimes, the Automotive and RV Product Manager for performance and racing suspension specialists Koni North America, ticks both boxes. Not only has he worked for Koni for 26 years, he’s also raced in SCCA club racing for over 30 years.
"Ultimately, it's going to be happening in corners, and it's going to depend upon the amount of loading and tire grip, as well as overall chassis and suspension stiffness—most specifically, spring and sway bars," Grimes said. "Of all of the suspension components, the shock absorber is probably going to have the least to do when a car goes up on three wheels while cornering."
Because wheel lift is not caused by one thing, it depends on the relationships and balance between a variety of components.
"The first thing you tend to look at is the stiffness of the sway bar on that front or rear end and then you look diagonally at the spring rate on the far corner,” Grimes said. “Because the sway bar ties the left side to the right side, if the load wheel is down, inherently it's going to be having some downward resistance on the other side. If it turns out that it doesn't have that stiff of a sway bar, maybe you look and see if the opposite side is really compressed. This would indicate that the car might be relatively softly sprung."
Grimes' personal experience racing Honda CRXs over the past three decades comes into play here. He shared that his current race car is more stiffly sprung, so it doesn't lift the inside rear wheel to the rate that some of his cars used to in the '90s. This is because he was running in a less-developed class with three-quarters, or even two-thirds, of the spring rate that he's running now.
Are There Any Adverse Effects?
Depending on who you ask, you get a wide range of opinions about the effects of three-wheeling. While perusing through performance-oriented forums online, it seems like just as many people say it's detrimental to the car's overall handling and grip as those who say it's not a big deal.
According to both Grimes and Eladio Rodriguez, a Chassis Engineer at BMW Motorsport North America, it's the latter.
“Picking up a wheel, inside front (typical BMW) or inside rear (most FWD cars), is a natural occurrence of dynamic weight transfer," Rodriguez said. "It is not necessarily a bad thing. You know it's going to happen and just drive through it."
It might not happen every time, but when it does, it shouldn't make a huge difference in what the driver feels.
"When you're going around a corner, weight is going to transfer from the inside to the outside of the car,” Grimes said. "If the car weighs 2,400 pounds, it still does when it's totally heaved over in the corner. That inside tire may only be carrying 10 percent or five percent of the general weight, and therefore has no need for load and gripping ability."
It might not make a major difference while it's happening, but consistency is always important when racing, both for driving adjustments and wear and tear on the vehicle.
"Ideally, engineers would like all four wheels on the ground, but unloaded corner contact patch is not a priority," Rodriguez said. "We can only hope that it happens uniformly from left to right for proper tire wear."
Although the viewers will always notice this happening, the driver might not even know.
"I've never been in a situation where I felt like I knew or felt the transition from four tires to three.” Grimes said. “The grip feel doesn't change, and balance feel doesn't change—it's pretty dynamic and wild to watch—but so long as it isn't a huge amount of lift, it's not an issue."
What If There's Too Much Lift?
Too much lift could be an indication that there's a bind in the suspension. Bind is when something mechanical is preventing the suspension from doing its job—meaning blocking the suspension's full, natural range of motion—which could be something as simple as an improperly sized sway bar end link.
Bind can make your suspension do things that were unintentional, and it can be a difficult thing to chase. For instance, when adding more shock rebound to the rear dampers, the car should exhibit more corner-entry oversteer that helps rotate the car. But if it instead exhibits understeer, there's a problem.
"Almost every time, when something isn't following the rules, it's due to a bind in the system that's causing the car to react differently," Grimes said.
In my own experience, I'm almost certain I dealt with bind when I started tracking my Mazda 2 in 2016 and still had a big aftermarket front sway bar on it. I was running OEM sway bar end links that weren't allowing the bar to work appropriately—the front end was ungodly stiff for the damper-spring combo I was running. Ultimately, I removed the front bar entirely when I threw more purpose-built coilovers on, and that probably eliminated any concerns in one fell swoop, as the car then handled quite well on track. It was wonderfully neutral, yet the right mix of inputs could inspire the car into some predictable oversteer.
For anybody who watches or engages in performance driving, It's good to know the physics of how three-wheeling works. There's nothing inherently dangerous or alarming about it, but instead is a product of the right mix of chassis, sway bar, and spring stiffness. Too much inside tire lift might be a concern, but it could also mean that the car has the chassis rigidity of a wet noodle or is in desperate need of some new springs. If you ever happen to catch coverage of the Sports Car Club of America's Runoffs, particularly its B-Spec race, impress your friends with why it looks like those little Mazda 2s, Mini Coopers, and Honda Fits are about to topple over.
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