Let’s talk about Zenvo’s multi-axis hydraulic rear wing fitted to its 1200hp TSR-S model by taking a quick walk back to the aughts, when I was an engineer for a Pirelli World Challenge team.
It was the 2004 PWC race at Mid-Ohio where our chief mechanic would take an important first step towards becoming a race engineer. Our driver, a wealthy gentleman who did his best while running near the back of the field in a Dodge Viper, complained of dire understeer.
Given that he was normally clueless regarding what the chassis was telling him, I was pleased to hear a well-formed opinion on what he needed to go faster. With the chief mechanic having expressed an interest in the engineering I was hired to do, I ran through some tuning options we could consider to alleviate the understeer. We started by looking at tire pressures, spring rates, and softening the front or stiffening the rear anti-sway bar, then he blurted out, “what about adding more rear wing?”
That opened a discussion about forces and levers, and how a change at the back of the car would affect the front. Namely, how his "more wing" suggestion would dial up the understeer by making more downforce with the rear wing. As one of my mentors did with me, I told him to think of the car like a skateboard: Step on the back of the board and the front wheels come off the ground. Going his route, with more weight pushing down on the back of the car, a reduction in weight on the front tires would only exacerbate the understeer. Problem worsened, not solved.
The proverbial light bulb went on over his head, and since then, he’s taken that basic kernel, learned a massive amount on his own, and gone on to become a race-winning sports car engineer. That "aha" moment, of realizing that nothing happens in isolation at the front or the back of a car when setup changes are made, struck me when footage of the Zenvo TSR-S and its wacky rear wing hit YouTube.
Using hydraulics to pivot the large rear wing up, down, and side to side, it's been hailed as a “brilliant” device that “maximizes grip.”
Here's the theory behind it: While cornering, the inside rear tire has fewer forces pushing it onto the track surface; Zenvo’s tilting wing attempts to compensate by pulling the wing down and out of the airstream on the most loaded side and using the portion standing tall in the wind to apply downforce to the least loaded rear tire.
The line of thinking goes "hey, the loaded tire is in good shape, but what about the inside tire that isn’t working as hard? Let’s take rear downforce away from the loaded side, apply it to the one that lacks a little bit, and boom, both rear tires are working hard and we’re cornering faster!"
In the magical world of isolation that doesn’t exist, Zenvo’s rear wing is a cure-all. In reality, where drivers need an acceptable chassis balance to go quickly around corners, it’s ignoring one of the true engineering basics: Diagonal forces.
Let’s take a right-hand turn as an example. Hop in the TSR-S, blast down a straightaway, and come to our right-hand corner where, as depicted in the track video, the hydraulics pull the wing’s left side down and tilt the right side up high.
While turning right, the car naturally rolls to the left, and as it navigates the corner, the left-front and left-rear tires share most of the cornering responsibility. On turn-in, the left-front bites and changes the car’s direction, and as that initial cornering phase is completed, the car’s left-rear tire takes on a greater share of the cornering load.
In our right-hand cornering scenario with the TSR-S wing, stability and balance at the back of the car is modified to help the right-rear tire. Got it. And what about the left-front-tire, which is still responsible for the initial change of direction? In understanding how forces act diagonally across a chassis, increasing right-rear downforce to help the inside right-rear tire will also act as a lever across the left-front tire.
In tipping the rear wing to add downforce to the inside right-rear tire, the Zenvo, using the skateboard analogy, takes weight off the left-front tire ... the one tasked most heavily with turning the car. It explains why, in the track day video, the driver almost parks this 1177-horsepower machine in the right-left corner sequence leading onto the straight. It also explains why neither supercar manufacturers with immense wealth and aerodynamic development resources nor racing series that use active rear wings have gone this route. In fact, of all the things the TSR-S achieves in the video, rapid cornering is nowhere to be found.
I asked a few high-level race engineers to weigh in and see if anything was missed.
"Tilting the wing will change the downforce it produces on each side and impart a roll moment reacted from the center of pressure of that side of the wing to the mount against the moment on the other side," said Jeff Braun, a championship-winning race engineer whose experience spans IndyCar, NASCAR, and IMSA. "This would try to roll the car and warp the car by causing a diagonal dynamic corner weight change."
Translated to English, Braun said: "Understeer."
Race-winning engineer Brad Goldberg, who looks after one of the Ford Chip Ganassi Racing GTs in IMSA and at Le Mans, was equally confused by the real-world problems created by the tipping wing.
"There is definitely some form of diagonal cross weight effect," he said. "The part I don’t fully understand is how they are figuring out where the wing needs to be. The greenhouse must affect the air around the wing, and it cannot be linear. The angle of attack must be different for any kind of wing position. Maybe they have figured that out, but that would be countless hours of work in wind tunnels. I’m curious how they balance the car as it would seem to me it would almost understeer off the road in every high-speed corner."
Indy 500-winning race engineer and Andretti Autosport technical director Eric Bretzman got straight to the point on why those who’ve had this idea before left it on the drawing board.
"My initial response would be to just get better tires and find somebody crazy enough that can actually use the 1200hp," he said. "There would have to be a lot of wind tunnel and tire data to say that the wing is working throughout that range and is complementary at the right places. Maybe just increasing angle of attack for brake zones and traction and decrease it on the straights would be the best use."
On paper, the Zenvo’s rear wing makes total sense. But insert a living human being into the cockpit, start tipping the wing in the corners, and you have performance problems that go beyond the 2D world.
Want to make it hard to go quickly through a right-hand turn? Apply weight to the right-rear. Want to slow a car through lefts? Pile weight onto the left-rear. It’s engineering 101.