Who says ‘tire guys’ don't understand suspensions?
DETROIT — Submitted as proof that not every invention at the 2001 Society of Automotive Engineers World Congress is some variation on a whiz-bang electronic device: Michelin SA's elegant and simple — no power required — rear suspension that allows the tires to work more efficiently.
Michelin's term for the new suspension isn't exactly simple, though: with no catchy moniker yet, the company calls it the “new axle system with optimized contact patch.”
The advance of the new independent rear suspension, say Michelin engineers, is that it passively uncouples vehicle roll movement from verticle wheel displacement. What that means to you and me is that this is a suspension design that gets the most out of your tire's embrace with the road.
If you “got it” in geometry class, you might understand what's going on here: Norman Frey, Michelin's group leader of vehicle suspension analysis, says that the new rear suspension design “optimizes the contact patch in a corner,” yet doesn't compromise that same contact patch when the wheel moves up and down to accommodate a bump. That's the traditional tradeoff in rear-suspension setup; if you want the wheel to stay in maximum contact with the road when the vehicle hits bumps or ripply pavement, that type of suspension geometry can't also deliver maximum cornering grip.
“There's always a compromise with a short/long arm (SLA) independent suspension,” says Mr. Frey. “If a system is optimized for roll control, it's compromised for vertical displacement.”
Again, a translation: A car can be set up with a lot of negative wheel camber, which in corners — and as the body rolls in a direction opposite the turn — helps the tires stay in better contact with the road. Just what you want at the racetrack. But vehicles set up with exaggerated negative camber are crummy at taking bumps, because as the wheel rises to accommodate the road irregularity, the tire contact patch is diminished even more than usual.
Michelin's innovation, then, with the new suspension system, is the design that its chief engineer, Michel Blondelet, recently patented — a design, he says, that “adds one degree of freedom to the suspension by decoupling roll camber from vertical displacement camber.”
The crafty new design isn't exactly magic, but Mr. Blondelet obviously “got it” in geometry class. His idea delivers on the road by adding a “mobile cradle” — along with its associated two link rods and various bushings — that allows the wheels to work independent of body roll in corners while retaining the optimum design for maintaining tire contact when the wheels move up and down for bumps.
Until Mr. Blondelet's unique design, existing independent suspension design couldn't do both. Humbly, he says all that he did was “change the way the subframe is mounted to the body.”
Mr. Frey says the design permits the vehicle to develop an extra 10% to 15% of cornering force, because the wheel can now “lean” into the turn in a fashion similar to the way a motorcycle wheel leans into a corner, maximizing the contact with the road.
Higher cornering forces and maximized contact patch may be a little esoteric for everyday customers. That's okay, though, because there are manifold benefits. Michelin adds that its new suspension improves safety by maximizing adhesion so that antilock brakes (ABS), traction control and stability control all can work more effectively.
And there's a final factor that's curiously at odds with the presumed goal of a tiremaker: The new suspension system actually will increase tire longevity. The Michelin engineers swear that the improvement of the tire/road interface has a quantifiable positive effect on tire wear.
Mr. Blondelet says the new suspension design could be employed for either unibody or full-frame, truck-type vehicles, but concedes that it likely isn't something that could be retrofitted to an existing chassis, given the new complexity of the subframe interrelationship with the rest of the chassis.
Michelin says its new suspension earned its first patent early this year and currently is being evaluated for production-vehicle use.