2012 Tesla Model S Signature Performance Suspension Walkaround
A Look Underneath This Groundbreaking Electric Vehicle
At first it was easy to see Tesla as another EV propulsion system company, an upstart startup that made waves by bolting its very impressive electric powertrain into a modified Lotus Elise. But there was always more to it than that. The Tesla Roadster was merely Step One in a master plan that would lead to a whole range of Tesla electric vehicles to be designed and built in-house.
The 2012 Tesla Model S is the first of these, and after a recent three-day fling we came away thoroughly impressed by its acceleration, handling, accommodations for five and general sophistication. Oh, and this rear-drive electric sedan can rack up 265 miles on a single charge or finish off the quarter-mile in 12.6 seconds.
Impressed as we were by the Model S's EV powertrain, we wondered what the more traditional mechanical bits were like, the stuff that they didn't necessarily design from the ground up in their first vehicle, the Tesla Roadster.
A closer look at the suspension would reveal a lot, we figured, so we hoisted the Model S up on our rotary lift, removed a couple of tires and had a peek.
At first glance the front suspension appears to be a double-wishbone arrangement with a high-mount upper arm (yellow). The steep angle of recline indicates there's a fair amount of anti-dive geometry at work here.
A quick look below reveals the lower wishbone isn't the one piece A-arm we were expecting. Instead it's a pair of links, each with its own ball joint. This dual-pivot design moves the steering pivot axis out to the virtual point where the links intersect.
By so doing, the point where the steering axis reaches the ground is moved closer to the center of the front tire's contact patch, a condition known as a "zero scrub radius." We can't tell if the scrub radius is precisely zero or not just by looking, but the general goal of this design is to make it much smaller than it would have otherwise been with an A-arm and a single ball joint.
The tie rod (yellow) from the electric-assist rack-and-pinion steering unit acts ahead of the front axle centerline. From the look of things, this front steer arrangement wasn't merely chosen for maximum steering precision; its use also opens up space that allows the massive 85 kWh battery pack (red) to extend forward as far as possible.
Liberal use of aluminum keeps unsprung weight (and overall weight, of course) at low levels. That massive front knuckle is hollow, too.
And the front stabilizer bar's connecting link (yellow) attaches directly to the knuckle for a nominal 1-to-1 motion ratio, a move that squeezes the most efficiency out of the stabilizer bar so it doesn't have to be any bigger or heavier than it needs to be.
This bellows (yellow) conceals an air spring that encircles a Bilstein monotube shock absorber that runs up the middle. They're optional on the Model S base model and come standard on the Performance, Signature and Signature Performance versions.
A computer-controlled compressor and accumulator maintain the correct vehicle height no matter the passenger load. Nominal ground clearance is 6 inches, but two high clearance settings can be manually selected if a steep driveway looms or if you're parked next to a high curb. At freeway speeds the Model S automatically lowers itself to something less than 6 inches to reduce aerodynamic drag and increase efficiency.
A tiny linkage (yellow) connects to a height sensor to continuously keep tabs on the suspension height so the ECU controlling the air suspension can work out what needs to be done.
Meanwhile, the steel upper wishbone employs a riveted-on ball joint. As small as the upper arm is, the cost of aluminum must not have justified the modest weight savings that would have been realized.
Our Signature Performance Model S was equipped with a Sport package that hasn't yet been released for production. How can we tell? The Schrader valves we saw on all four Bilstein monotube shocks are only found on so-called "take-apart" shocks that are used in development. These valves allow the gas to be removed easily so the shock can be taken apart quickly in the field, rebuilt with different valving, then re-gassed.
Engineers conducting a typical damper tuning session can cycle through four to six such changes per day, depending on how quickly the technicians can remove and replace the shocks and the length of their drive route.
Four-piston fixed-caliper Brembo brakes clamp onto 14.0-inch cast iron one-piece ventilated front rotors that are 1.3 inches thick. If that doesn't seem like enough for a 4,770-pound car, consider this: 1) only 47 percent of that weight rides on the front axle and; 2) most routine stopping power — the first 0.20 to 0.25g of it, at least — comes from the main AC induction motor operating in regeneration mode, and it's connected to the rear axle.
Besides, the proof is in the stopping, and this Model S stopped hauled itself down from 60 mph in just 108 feet with no observed fade or degradation in performance after six repeated attempts.
This small part line reveals that these fixed-caliper brakes are bolted together from two parts, but these bolts need never be removed because the brake pads slide easily out the back through the open window.
The four weights riveted on to the pad ears are there to quell vibration and nose.
The rear end of the Model S sits on a multilink suspension, but all we can see from here is the toe link (red) and a single upper link (yellow.)
This closer look shows a couple things. First, the toe link (red) and the upper link (yellow) are the only two links visible topside, which means the lone upper link is strictly a camber link. Second, both are made from aluminum extrusions that have been sliced to the desired thickness like so much Play-Doh or deli meat.
At first glance the lower arm looks like an H-arm, but it's really an A-arm because it only connects to the rear knuckle in one place. In other words the lump at the end of the red arrow is not part of the knuckle.
It turns out it's a short link whose sole job is to hold the wheel in the correct fore-aft position in the face of torque loads created by acceleration and braking. Yes, a lower H-arm with two connections to the knuckle could do the same, but then toe-in would be harder to optimize as the suspension strokes up and down. In fact, it would be more or less locked in.
The existence of this short link — termed an "integral" link — provides a degree of freedom that allows the toe link room to work and create beneficial changes in toe-in as the suspension moves. It allows the lone upper camber link to concern itself only with holding the upper end of the knuckle in the correct camber orientation.
In other words, the integral link allows the bushings on the ends of these links to be more easily optimized for their specific task. It eliminates some of the conflicts of interest that can make bushing tuning an exercise in managing trade-offs when links are asked to do two things at the same time.
This end view of the lower A-arm gives us a peek at the integral link (red). The rear stabilizer bar and its drop link (yellow) connects to the lower arm just outboard of the inner pivot in what looks like a 0.6-to-1 motion ratio.
Meanwhile, the lower end of the rear monotube shock absorber (yellow) connects directly to the aluminum rear knuckle. Its stands nearly straight up, so the motion ratio for the shock (and the air spring that surrounds it) is as close to 1-to-1 as makes no difference.
In the background we can see that the Model S's toe link (red) has an eccentric cam built into the inboard connection point for static toe-in adjustment.
Here's a close-up of that rear air spring — the bellows that protects it, at least.
Like we saw up front, a linkage connected to a position sensor (yellow) is here to continuously monitor the height of the rear suspension so the computer can make the necessary air pressure adjustments.
In the background a large opening (red) shows that the rear suspension subframe is not only aluminum, it's hollow.
Here's another peek at the aluminum rear subframe.
We've been ignoring the dual brake arrangement so far, even though you've seen glimpses along the way and have probably worked it out for yourself. The small single-piston sliding caliper (yellow) is an electrically actuated parking brake, nothing more. The actual working brake is a four-piston caliper similar to the one we saw up front. Both are made by Brembo.
This Model S is fitted with the proposed Sport package, which includes 21-inch Michelin Pilot Sport PS2 summer performance tires. The front tires are sized at 245/35ZR21 (96Y) and the rear are 265/35ZR21 (101Y).
But 21-inch wheels and tires are also available without the Sport package, and in that case all four tires are Continental Extreme Contact summer performance tires sized at 245/35ZR21 like this one. The standard tire is a Goodyear Eagle RS-A2 performance all-season 245/45R19 tire at all four corners.
The front wheels on the Sport package (and all four on the 21-inch summer tire package) are 21-by-8.5-inch aluminum alloy wheels with a 40mm offset. The Sport package wheels are 21-by-9.0 inches, but they were prototype parts with no offset marked on them.
The underbody is as smooth as it gets. This car shows a little evidence of past nose rub incidents, but it's not visible in the least when the car is on the ground. The adjustable-height air suspension offers two higher ride height settings for relief on steep driveway cutouts, but you do have to remember to deploy it.
This view of the front suspension shows one reason why front-steer was specified by the designers: That gray expanse is the battery box, and as you can see it protrudes nearly to the front axle centerline.
The length and breadth of the 85 kWh lithium-ion battery pack is the reason why it only needs to be 4 inches thick, why it doesn't impinge on passenger or cargo space. It stretches from wheel arch to wheel arch, side sill to side sill.
The floor of the car begins to taper up a couple inches aft of the rear axle centerline.
Air that flows under the car is kept in line by these four fences so it can't readily escape out the sides as the pressure drops during its journey through the expanding volume of this functional-looking diffuser.
No word on exactly how much downforce it generates, but the ingredients visible here seem substantial enough to make a rear deck lid spoiler unnecessary.
From what we've just seen it seems that Tesla has some very clever engineers on the payroll. The 2012 Tesla Model S appears to be much more than a sophisticated battery and electric motor encased in a beautiful wrapper. Dig down farther and the bones look very sound indeed, suggesting that Tesla has serious all-around potential to be around for the long haul.