Inventor's corner: A new twist on a fully independent vehicle suspension
Edited by Leslie Gordon
One might think that after more than 100 years of advances in automotive technology, every kind of suspension system would have already been invented. But the “VXI” by inventor Winthrop Dada is an entirely new kind of vehicle-suspension system. Dada says it eliminates most if not all of the problems inherent with prior suspension designs. Dada calls his invention the “VXI” because the suspension links form a “V” or “X” shape, and the system is fully independent, thus the “I.” An off-road enthusiast who owns a modified Suzuki Sidekick with a strut front and a live-axle rear, Dada initially set out to build a high-travel independent suspension for rock crawling. He eventually realized the VXI also eliminates common problems such as wheel scrub and unwanted camber change.
Most cars at neutral resting compression — where the suspension is neither loaded or unloaded — are set up with the front tires at negative camber, says Dada. “The trade-off is less rolling-resistance efficiency for a little more traction midcorner.” The second prototype (V2) of the VXI allows control of the camber to optimize it for cornering. Users can set the camber link, which is attached to a “variable-angle knuckle” — it sits like any knuckle, but varies in angle — for as much or little camber change as desired. As the camber link is moved ever higher up the adjustment holes on the longitudinal arm, the suspension will supply progressively more camber change — both positive and negative.
“In V3, which I just started building, the camber link will instead be ‘active.’ Rather than adjustment holes on the arm, there will be a slot, which will let the link move up and down depending on the steering input. This makes the active link like an ‘on-off’ switch for either no camber change or as much camber change as you’d like, eliminating the effect of body roll on grip and traction,” says Dada. The active link will provide optimized camber at all times, eliminating the need for static negative camber and thereby boosting efficiency. With the active camber link, the VXI will offer no camber-change during bump/jounce and dive/ squat yet will provide full control of camber in bodyroll (while cornering). (A vehicle dives when its nose goes down while braking. It squats while accelerating and the rear lowers. Bump and jounce happen when the car hits a bump on one side of its suspension.) In these circumstances zero camber change is desirable because it maximizes grip. However while cornering, the VXI provides the camber change needed to maximize the tire contact-patch when encountering roll in midcorner situations.
Also important to the design are the “floating pivot points” which Dada says have never been used before. These pivot points attach either to the “wheel carriage” (also previously unknown) or the chassis at one end but are free to float up and down, allowing large amounts of wheel travel in a compact arrangement. Suspension geometry is maintained by the cross bracing of the suspension arms. “One reason the VXI can provide more travel than other independent designs is the floating pivot points let the suspension arms fold into each other compactly, sort of like a hinge,” says Dada.
In addition, because where the pivot points attach to the chassis are wide apart and high up, Dada says the VXI is more stable than other suspensions — kind of like the greater stability of a widebeam lifeboat as compared to a canoe. The roll center is higher than the vehicle’s center of gravity — higher than almost any other design — yielding enhanced stability. So the vehicle is less likely to roll over in case of an accident.
Importantly, the XVI also provides zero track change and the wheels do not move on any arc of motion, says Dada. “Zero track change means when two opposite wheels (e.g., front) go through compression, they never get further away from or closer to each other. This contrasts to other independent designs, including strut, double-wishbone, or multilink. A changing track width results in ‘wheel scrub,’ which makes the vehicle unstable, results in excessive tire wear, and is less efficient because the wheels are traveling in a crooked line. Although a live axle does provide a fixed track width, the wheels are still moving on an arc of motion — it’s just a longitudinal, not a lateral arc, meaning the wheel base is getting longer and shorter, rather than the track width getting wider and narrower.”
On suspensions such as double wishbones or struts, the tie rod is on the same arc of motion as the A-arms when the vehicle travels straight ahead. “But as soon as you turn into a corner, the tie rod is pushed outwards or inwards from the arc of motion of that suspension arm, which causes an unwanted steering effect known as ‘bump steer,’” says Dada. “In contrast, the VXI uses a tie rod that attaches to a sliding or telescoping ‘slip yoke’ steering input. Thus, the wheel carriage (to which everything on the suspension attaches) never goes out of phase with the tie rod — they always operate on the same plane, effectively eliminating bump steer.”
Another important consideration is “instant center,” which is the point on a double wishbone or a short-long arm suspension, say, at which the angle of the top arm intersects the angle of the bottom arm. “On conventional designs, the instant center moves around depending on where the suspension is and its deflection,” says Dada. “Live axles are pretty crude. But they do have a few good attributes, one of which is that the instant center is off to infinity. The VXI’s instant center is also off to infinity. The wider the instant center of a given suspension geometry, the more stable the vehicle, and the less weight-transfer during body roll. Independent-suspension engineers have previously struggled with the trade-off between designs with good instant center, roll center, and wheeltravel attributes, which all act counter to one another. The VXI optimizes all these aspects with no compromises or trade-offs.”
Dada says the VXI does have a few downsides. “Compared to, say, the double wishbone, the unsprung mass — the weight of the wheel, tire and suspension going up and down as the tire hits undulations in the road — is probably greater, he says. “Although not nearly as bad as that of a live axle, it’s probably similar to a strut design. Another weak area is the high number of parts. But, my hope is the tirewear performance and fuel-efficiency gains will more than offset the additional cost and complexity.”
This XVI can work on any two or four wheel vehicle, and even on airplanes. An Australian group is developing the VXI with Dada for a road-racing application — a time-trial event for super-modified production vehicles. Also, the U. S . military has expressed an interest for its next Humvee. Dada says he will grant free rights use to interested racing teams and individuals. He can be reached at [email protected].