2009 Volkswagen Jetta TDI: Suspension Walkaround

    You've read my references to a L-shaped lower control arm, and here's a good shot of one. The pivot axis is clearly defined by two bushings., the forwardmost of which is pretty much in line with the lateral loads that come from cornering. The rear one takes fore-aft shocks from potholes and big bumps radially as the impact pivots the arm on the forward bush.

The space ahead of the the arm is left open, allowing for better packaging of the front-engined, front-wheel drive (known as FF in engineering shorthand) radiator, accessories and forward crash structures.

Here the lower ball joint (yellow) is easily swapped-out with the removal of three bolts. Not much rust down here, either.

Most FF and AWD application use a rear-mounted steering rack (black) and forward-mounted brake calipers, especially if the engine is mounted sideways, as this one is. The steering rack nestles easily behind the engine and ahead of the firewall. It's all down to packaging.

 

Our Jetta has the usual ventilated front discs. Here (white) you can see where a chunk has been machined away in order to balance the cast-iron disc. Think of it as the reverse of wheel weights. The bleed fitting (yellow) of this single-piston sliding caliper is missing its protective rubber cap. Such fittings are always located at the highest point of the fluid chamber behind the piston so all the air can be easily bled out during brake bleeding.

Our Jetta has a form of multilink suspension that is centered around a blade-shaped trailing arm. You may have heard Ford's trademarked name for it: Control Blade. In a nutshell, this layout doesn't intrude on interior space very much, and it effectively divides the lateral-load cornering loads and fore-aft harshness duties between distinct links.

The blade-shaped trailing arm (black) bolts directly to the knuckle in two places (white), allowing the arm to control the fore-aft position of the rear wheel and its up and down arc all by itself.

Here we can see that the parking brake works by mechanically clamping the single-piston sliding caliper via the parking brake cable and an external lever (yellow).

This view shows how VW mounts the trailing arm/blade (green) to a massive bushing that soaks up harsh, fore-aft impacts. The blade is intentionally thin and somewhat bendy near the knuckle (yellow) so it won't cause binding or friction as the lateral links swing through their arcs. But, like a ruler on edge, the tall section is massively strong in a vertical sense.

And here are those lateral links, all three of them. The nearly-hidden curvy upper link (yellow) and straight lower link (green) define the camber curve of the rear suspension and carry most of the lateral load, a job they can do effectively because the big trailing arm bush takes on almost all of the harshness-prevention duties.

The rear toe link (orange) carries the rear spring and is much longer. The differing lengths of the front and rear lower links mean they'll swing in different arcs, thus causing the outside rear wheel to toe-in further when the suspension compresses during cornering. Called "roll understeer", this characteristic enhances stability in long sweeping turns.

Like the front, the rear stabilizer bar is direct-acting, too: its link bolts directly to the trailing arm/blade, which in turn bolts directly to the rear knuckle for a 1:1 motion ratio.

More color codes back here where the rear spring nestles into the rear toe-control arm/link thingie (black). As with the front, multiple applications of the suspension of this car call for multiple spring possibilities. The rear shock is direct-mounted to the knuckle (yellow) for 100% damping efficiency.

Compared to the short forward links, the long rear toe links nearly meet in the middle, where they they are adjustable for static toe-in via these eccentric cams (yellow). The Jetta's rear stabilizer bar bushings (green) are neatly packaged out of the way.

There isn't much intrusion into the trunk or passenger compartment with this multilink setup because it doesn't have a bulky coil over shock or strut. VW puts the Jetta's urethane bump stops atop the shock absorber's dust shield, rather than within it, because that's how they roll.

Other tidbits: the Jetta's average-sized wheel and tire assemblies (16 x 6.5 aluminum alloys with 205/55R16 tires) only weigh 43.5 pounds. They are mounted with five hateful lug bolts instead of lug nuts.

Our Jetta handles soundly, but it isn't exactly a sporty-feeling machine. But that's not the fault of the geometry of what we've just seen. Rather it's due to the family-oriented tuning of said suspension: the softness of the springs, the conservative valving inside the shock absorbers and struts, the hardness of the bushings (or relative lack thereof) the compliant nature of the tires and, something we are all familiar with, the diameter of the stabilizer bars.

These pieces (up to and including the internal construction of those tires) can all be tuned by the chassis development engineers at the proving grounds, a job I did for over 17 years. You don't have an infinite range of adjustment: the basic design of the suspension and it's geometry define how far you can go. That said, the biggest limit of all is the development targets laid down by the marketing department for a given model.

But here, on the Jetta's rear suspension at least, there looks to be a fair amount of sophistication and tunability. It's easy to see why a credible sporty-handling machine comes from the same chassis.

Dan Edmunds, Director of Vehicle Testing @ I forget how many miles because I took the photos three days ago, gimme a break!