2013 Ford Focus ST Suspension Walkaround

[Pyrhel]

Една брутално яка статия! Но чеснто казано не ми се занимава да я превеждам на български и да я пиша, затова я слагам както е в оригинал.



What happens to a 2013 Ford Focus when you give it the ST treatment and plop a 252-horsepower turbocharged 2.0-liter four-banger under the hood? I mean, come on. That's a full 92 hp more than it started with. Things could go horribly wrong if it's not done properly.

As expected, the horsepower infusion has made the Focus ST significantly quicker and faster in a straight line. Our track numbers also tell us it's loads more adept at tearing up corners on tires that are no bigger than the ones you can get on a Focus Titanium. Along the way, the speedification process did not morph the Focus ST into an unruly handful.

Far from it. The Focus ST has manners. It responds the way you want it to when pushed, and you can push it quite hard indeed. This front-wheel-drive sidewinder is, dare I say, an honest-to-god driver's car that is a blast to drive.

It takes a solid foundation and good suspension tuning for all of the above to come true. So we hoisted our brand-new 2013 Ford Focus into the air on our Rotary Lift to see what we could see.



From here our 2013 Ford Focus ST appears to be nothing more than a bright yellow car with MacPherson strut front suspension.



Here we can see that the lower control arm (yellow) and the steering knuckle (red) are made of the same ferrous material as the base car. But the ST steering knuckle is unique because the steering geometry has been altered.



Typical of front-drive vehicles, the steering system (red) goes about its business aft of the axle centerline. The front stabilizer bar (yellow) loops up over the top of the steering rack on the way to joining its long, slender drop link, which in turn connects directly to the strut housing for the most efficient motion ratio possible. What all of this means is the stabilizer bar is not "geared down"; the Focus chassis can get the most out of whatever stabilizer bar the engineers choose to fit.



One of the biggest changes to the ST concerns the steering arm (yellow), which is cast as part of the steering knuckle. It's significantly shorter, which quickens the steering response dramatically.

The standard Focus has a linear 14.7:1 steering ratio. The Focus ST has a variable ratio that starts out at 13.7:1 around center and quickens to 10.1:1 as you add lock in corners. The variable part comes from ever-expanding rack tooth spacing as you move away from center, but the general increase in quickness comes from this shorter arm.

And so the Focus ST steers from lock-to-lock in something like 1.8 turns, whereas the standard Focus does the same in 2.6 turns. Most of this huge difference indeed comes from the quicker ratio, but a portion comes from the need to restrict the amount of lock somewhat so the shorter arm doesn't get too close to its over-center toggle point. The amount of lost lock can be seen in the ST's 39.4-foot turning radius, up from 36.0 in the base Focus.

Beyond all that, the steering arm's height has also been altered in order to change the ST's front bump-steer characteristics. The degree to which the steering tie rod (from the bellows out) is NOT parallel to the lower control arm as the two hinge up and down as the car rolls in corners is what creates small toe changes that we know as bump-steer or roll steer, and a change in the height of the steering arm alters this relationship.

I never found out how much and in which direction the height was altered, but I'm assuming the idea for something like an ST was to generate less roll understeer, less toe-out as the outside wheel compresses in corners.



As for the steering tie-rod end (red), that prominent kink is there to clear the inner flange of the 18-by-8-inch wheel when the steering is at full lock.

All Ford Foci employ the same L-shaped lower control arm, with the L pointing toward the back of the car. It hinges up and down on two bushings as the suspension compresses.

The forward bushing, the barely visible one at the crook of the L, handles the bulk of the lateral cornering loads because it is more or less in line with the ball joint and the tire contact patch. The stiffness of this bushing is the same as the base Focus.

The bigger and more visible one at the rear is primarily tasked with handling acceleration and brake torque reactions and the fore-aft component of pothole strikes. Such a load entering at the ball joint is turned through 90 degrees by the L-shape of the arm so it can be absorbed in the rear bushing as an inward force. Here in the ST this bushing is firmer than the base car because of the ST's much greater acceleration and braking potential and a somewhat lessened need to deliver a plush ride.



This in-out load path explains why the rear bushing (yellow) seems to hang off the side of the subframe, why the lower control arm slips into it from the front and isn't firmly bolted at the back. The way it's bolted to the subframe is fully consistent with the inward direction of the forces it is subjected to which, incidentally, will be more level with the ground than we see here with the car up on a lift and the suspension at full droop.



Just behind the point where this bushing meets the subframe is the all-important place where the subframe is bolted to the chassis, and here we can see the merest sliver of the subframe bushing.



It's easier to see the subframe bushing material when we look at the forward subframe mounting point. This white color gives this away as a polymer material that is stiffer than the sorts of rubber bushings that are commonly used in front subframe mounts, and this means something to us because the stiffness of these bushings has a direct impact on steering precision and feel.

Think about it. The steering rack that turns the wheels is mounted to the subframe that is in turn mounted to the car body through these bushings. Soft bushings are good for NVH and the isolation of road coarseness, but the "give" they introduce when the wheel is turned comes through as a sogginess in steering feel and a dulling of steering precision.

At first I thought these poly-something bushings were an ST upgrade, but a Ford engineer tells me the base 2012+ Focus has them, too. This makes sense of course, because the standard Focus has good steering feel, too, albeit with slower response.



As for the more obvious suspension pieces like springs and shocks, they've both been firmed up. The front spring rates are about 30 percent stiffer, but it's not all down to the needs of a sportier chassis because the 2.0-liter turbo engine and its plumbing and the six-speed transmission represent a bit of extra poundage to support.

The front struts use the same housing but the internal valving is significantly different to produce more damping in cornering situations, which are dominated by the low internal piston speeds associated with body roll, not the high-speed staccato hammering of bumps. "More low speed damping" is therefore a phrase that applies to the rate at which the suspension strokes, not the vehicle's speed on the road.



Perhaps the most obvious Focus ST front suspension difference is the size of the brakes, which makes sense when you think about 92 additional hp. These ventilated front rotors are 12.6 inches in diameter; a standard Ford Focus uses 10.9-inch ventilated front rotors.



Here we can see that the brakes themselves are still single-piston calipers, but the diameter of the piston within and the size of the pad have been increased to make use of the larger rotors.



A quick glance at the Ford Focus ST's rear suspension looks similar to that of other Ford Foci we've seen before.



Ford's so-called Control Blade brand of trailing arm multilink suspension is in use here. The thin trailing arm, a.k.a. the Control Blade (yellow), handles two degrees of freedom — fore-aft wheel location and brake-torque reaction — which means that only three other links are needed to fully locate the wheel, none of which we can see clearly in this view.



But we can see them here. The heavy lower link (red) carries the spring and the bulk of the lateral cornering loads, with a shorter lower toe-link (green) just ahead of it. Up top there's a single camber link that holds the top of the wheel at the desired camber angle. No word on whether or not it's a smidge shorter for a bit more negative camber, but I wouldn't be surprised.

Meanwhile, just to the right you can see the Control Blade coming in and the three mounting bolts (one is hidden behind the "AAA" tag) that make up the rigid joint that allows this arm to resist brake torque and do away with the need for any sort of triangular control arm or wishbone up top.



Here I've removed a panel so we can see just how long the Control Blade trailing arm (yellow) really is. We also get another glimpse of the toe link (green) and the main lower link (red).



But the rear suspension of the 2013 Ford Focus ST does have one significant difference in the form of a reconfigured rear stabilizer bar mounting point (yellow).

What you're looking at is an entirely new rear knuckle with a place for the stabilizer bar link to connect at a 1:1 motion ratio for maximum efficiency of the stabilizer bar. On a standard Focus the stabilizer link attaches midway along the lower link, just inboard of the spring at a motion ratio of just 0.5:1 or thereabouts.

If you looked at a spec sheet you'd see that the Ford Focus ST has a 22mm rear stabilizer bar, just like the standard car. But this new end link mounting point makes a world of difference because the 1:1 motion ratio seen here makes the 22mm bar behave like a 27mm bar would on a standard Focus. By going this route, the Focus ST gets about double the rear stabilizer bar roll stiffness without the need to carry around the weight or the bulk of that much larger bar.

If these numbers don't make sense in your head, you may have missed the engineering class where the professor explained that a stabilizer bar's torsional stiffness is proportional to its diameter raised to the fourth power.



The shock absorber (red) also enjoys the maximum efficiency associated with a 1:1 motion ratio because it, too, connects directly to the knuckle at its bottom end. And like we saw up front, its internal guts have been modified to provide more low-speed damping force to aid cornering.

That leaves the rear spring, which sits in a pocket in the lower link at a point that delivers its spring force at a 0.7:1 motion ratio. Here on the ST the rear spring is 20 percent stiffer, and most of that goes toward performance goals because our scales tell us the car's rear axle weight hasn't gone up much at all.

It's a pretty tidy package, too, and the upper end of the spring acts directly on the strongest part of the unibody. The hard rubber bump stop and its landing pad (white) sit just outside the coils.



This look from below shows the new stabilizer bar routing from a new angle. It's a long way from the pivot bushing to the new end link, which is a new source of inefficiency because the bar probably bends a little instead of twisting exclusively. But the ST still generates significantly more roll stiffness from its 22mm rear bar with this setup.



Larger rear disc brakes weren't necessary to the job, so the ST carries on with the same 10.7-inch solid rotors and single-piston sliding calipers found on other 2013 Ford Focus models.



All of the above rides on a set of 235/40R18 Goodyear Eagle F1 Asymmetric 2 summer performance tires. You'll want a spare set of something else if you live where it snows. The 18-by-8-inch rims they come mounted on are the same size as the optional ones offered on the Focus Titanium, but the ST wheels and tires are unique — and stickier.



Mounted together they weigh 50.8 pounds — close to the average for factory 16s or 17s but on the light side for factory 18-inch tires and wheels of this size and at this price range.

That's it. We haven't just seen acres of aluminum or trick adaptive suspension bits that would surely hoist the price way up. Instead we've seen a high-performing and entertaining hatchback that starts at less than $24,500.

The extra 92 hp that has been dumped under the hood of the ST is a wonderful thing, but it could have easily been a disaster. It's only fun if the chassis can keep up, and on that score the 2013 Ford Focus ST has no problem whatsoever. The behind-the-wheel experience proves just how much you can get from a few subtly redesigned chassis parts and a well-executed suspension recalibration.