LSJU

A better question might be what does GM know about it thanks to the likely millions of dollars they have pumped into the research over the years.

I think a great question is which part of the spring is the issue? Is it the cantilever part or is it the fiberglass part?

So let's ask why wouldn't other road cars use it? The short answer is, they have. Mercedes and Volvo have all made production cars using similar fiberglass transverse leaf springs. Fiat actually is the grandfather of the setup used on the Corvette. The 128 was the first car I know of to use a dual mount transverse leaf spring that worked as both anti-roll bar and ride spring. It was very clever but they didn't have the other part of the puzzle, the fiberglass technology. Since they were using steel they needed a stack of leaves just like a pickup. The result was they had to deal with two of the traditional leaf springs issues (neither apply to the Corvette). They had a spring that was heaver than a coil spring and they had the hysteresis (stiction) in the spring movement caused by the friction between the leaves of the spring.

GM actually has used the technology on the rear suspension of a number of cars including the W-body cars. However, much of this was in the 1980s when GM's engineering was very disjointed. GM in the 80s had a mix of brilliant, forward thinking ideas that are now common place but also many poorly integrated ideas thus the sum of the parts was more than the whole. For example GM was a leader in things like coil on plug ignition systems and ECUs. They had steering wheel button controls and touch screen displays in the cars in the late 80s. They also had the very innovative V8-6-4 V8 motors. Almost every bit of tech I just mentioned also suffered from GM of the 80s (the V8 was from the 70s). It was a great idea but wasn't ready for prime time when released. Now decades later all of that technology is rather common and available from competitors.

Anyway, the leaf spring was used on the W-body and other GM FWD cars. The advantage was better packaging. A coil spring takes up a lot of space. The leaf is very low profile and can lower the installed height of the total suspension package (This is why Volvo used it in the 960). For the FWD W-body cars it meant smaller shock towers (smaller body diameter around the shock as it didn't have to clear a coil). This allowed both better damper placement and more trunk space.

The down side with the leaf spring, and the reason why it's rarely used, is the leaf costs quite a bit more than a pair of coils. We aren't talking truck suspension here. Leaf springs on a Chevy truck cost a LOT more than the coils on the Tahoe. However, when you add the cost of all the suspension links needed to do the duty of two leaf springs on a truck you get the total cost savings. Well on the Corvette (and Volvo, and W-body cars etc) the leaf wasn't acting as a control arm so you don't get that double duty cost savings. Instead the leaf is only replacing two coil springs. Net result it costs more. So you will only use it when you have to.

When do you have to use it? Well Volvo used it because the 960 body was based on the live axle 940 and they didn't want the cargo floor sacrifices that would have been required if they used coil springs. A wide, flat rear suspension was ideal. Mercedes used it on the first Smart cars because the packaging in the front suspension was very tight. A flat leaf that spanned the front of the car and saved the need for a sway bar seemed worth the extra cost.
What about the Corvette? Well in the 1984s I'm not sure what GM was thinking. Again they were half the time showing off advanced thinking and at other times trying to figure out if the left hand knew the right hand was even attached to the same body much less what it was doing. I suspect that was why the W-body cars got this expensive spring. When GM looked at the C5 they were fully expecting to dump the leaf springs and move back to coils (at that time the C4 was the only Corvette to not have coils). However, the engineers found that they couldn't fit the needed coils where they wanted them. Yes, the aftermarket guys put coils in the cars but those coils aren't tested to OEM specs for corrosion resistance and cycle life. The engineers even considered pushrods and rocker arms to get things to fit. Clearly they were not wed to leaf springs if they were considering those options! In the end the went back to the idea of the leaf spring though the C5 design is much different than the C4 setup.

So why wouldn't Porsche or Ferrari use it? Because while it has basically no technical disadvantages, it has one big non-technical disadvantage. It costs more. If it was cheaper GM would certainly use it on the cheaper cars.

Let me start by saying Ferrari isn't afraid of Corvette technology given they are using the mag dampers that started with Cadillac (Haha your Ferrari has Caddy shocks ) and were later developed on the C5 and C6. No, Ferrari simply hasn't had the packaging issues. The Corvette has a very low front cowl and an engine between the front shock towers. Porsche doesn't have a low front cowl by comparison and since there is no engine in front why use more expensive springs. Ferrari and Porsche do have to worry about springs around the motors in the back but most mid engine (and the odd ball rear engine car) have high cowls in the back so you can simply put the springs up high (longer springs also means you can decrease the coil diameter).

Note that the whole Corvette rear suspension is hardly taller that the rear upright

The Porsche, with a higher rear cowl can extend the spring well above the suspension and uprights.


Since the leaf provides only marginal benefits in terms of reduced weight and provides no real advantage in terms of suspension control (for the most part a spring is a spring) then the only real advantage is packaging which is offset by higher costs. If you don't face the particular packaging problems GM faces with the Corvette you dump the leaf and go with cheaper coils and put the savings into other weight saving or performance enhancing technologies. This is no different than say Audi working really hard to make their engines VERY short. They do that because they have committed to a design that puts the whole engine in front of the front axle. No BMW I6 for Auid! Instead they do things like putting the cam drive at the back of the motor and other things that no other make does because they don't put V8's in front of the front axle! Porsche also tries to make very short motors since the entire motor is behind the rear axle.

OK, if you didn't sleep through or just skip all the rambling I will leave you with one last bit... Audi is looking at using fiberglass springs. Not leaves but a coil. They are chasing some of the same advantages GM went after, the spring is lighter than a steel spring.
(Scroll about half way down)
http://www.digitaltrends.com/cars/lu...ts-working-on/

(No proof readers where harmed or even consulted in the making of this post)

LSJU

I would say they are every bit if not more adjustable than most coil setups. How can you adjust the coils on a base Cayman. With the turn of a wrench you can perform the equivalent of adjusting the spring perch on a coil over damper on even a base Corvette.

The side to side shudder has nothing to do with the leaf. Remember the leaf is nothing but a spring. The same left side affecting the right side happens with a 911 thanks to the anti-roll bar.

If I get time I can draw the free body diagrams to illustrate this

GSIRM3

LSJU- Thanks for the information and actual facts.

Noah Fect

And we were listening to A Flock of Seagulls while checking out our girlfriends' leg warmers.

Detroit Shooter

Thanks for the information. You can tell by the photo that it is 70's tech.

Noah Fect

Great post. I'm not sure I agree with the idea that Ferrari refrains from using leaf springs because of cost, although there's certainly no shortage of places where Ferrari has compromised their quality with no other apparent motivation. (Was it really cheaper to use unstable interior plastics for 20 years...?) And I definitely can't get on board with citing Volvo sedans as if they disprove my point. Nobody buys a Volvo for its sublime road manners.

Not being a suspension engineer, my issue with a leaf spring probably does come down to its cantilevered nature. Up until the C5/C6 generation at least, there seemed to be no way to keep forces on one side of the car from showing up on the other side in perceptible ways. Don't ask me why anti-roll bars by themselves don't have that problem, but they don't. The effects I'm talking about will never show up in lap times or skidpad numbers. A lot of people seem to assume the case is closed at that point, and the engineers can go home. I disagree.

Perhaps the C7 has finally put this question to rest. I look forward to a chance to take one for a spin to satisfy my own curiosity.

LSJU

Thanks!

You are right that no one thinks Volvo and sports sedan. However, Volvo used the spring for the same reason as GM, it fit better than coils. That doesn't prove that it has no handling issues (it doesn't) but it does show that companies other than GM see the merit of the design.

Actually the C3 and C4 (rear) springs didn't transmit forces from side to side. The spring was rigidly mounted in the center and thus acted as two springs. The issues with the C4 were long and complicated but it wasn't a problem with the leaf. It's just that the leaf was the obvious difference. When was the last time you heard someone complain that the C4 used the halfshafts as suspension links (a concept not unique to the Corvette). One of the issues with the use of the half shafts is now the diff housing needs to be rigidly mounted to the frame since it's a suspension pickup point. The C4 also was not a very stiff chassis. The design was started as a T-Top and the engineers were counting on the roof to add rigidity to the chassis. The brass said make it a targa. Great, now the chassis was much weaker.

So we always hear people talk about chassis stiffness and how it helps handling. How does that work? Well you need a relative roll stiffness between the front and rear axles. That roll stiffness includes the front and rear suspension (easy to think of if we think of a pair of solid axles). So when we twist the front relative to the rear the suspension articulates and the chassis twists. We have in effect 3 springs in parallel. If the center spring (the chassis) is a flexi-flyer (C4) then you have to have really stiff anti-roll bars to get the total roll stiffness between the axles where you want it. That means you get GM suspension tuning of the 1980s, rock hard springs. Rock hard springs means a bump on one side disrupts the whole chassis. That is likely what you were feeling.

Now remember I said three springs in parallel (the front roll rate as a "spring", the chassis "spring" and the rear roll rate "spring"). Only the front and rear springs have dampers. The middle spring does not. This is part of why you want the stiffest center spring possible.

One more thing, earlier I said the C4 rear spring didn't transmit forces side to side. That was generally true but the spring loads were loaded into the diff housing. That diff housing wasn't perfectly rigid with respect to the rest of the chassis so you do have some level of left side goes up, right side goes down but it's very small and generally would be felt as poor NVH, not in chassis handling. Basically there were flaws in the C4 design but they weren't related to the thing that was different, the spring. They were related to things that weren't obvious but really mater, the chassis rigidity.

The C5 was worlds stiffer and hence much of the issue went away. Improvements in dampers and chassis rigidity have really helped the issue you are talking about. It was never a spring problem and the ultimate solutions are spring related. Incidentally, moving from a traditional LSD to an computer controlled diff will also help handling quite a bit. I posted about that in another thread.

kosmo

here's the aricle to the Edmunds video:

The Purveyor of Powerslides vs. the Prince of Precision
Published: 09/09/2013 - by Josh Jacquot, Senior Editor
Comparison Test 2013 Porsche 911 Specs and Performance 2014 Chevrolet Corvette Specs and Performance The slide, when it finally comes, is a long, predictable and quietly controlled thing.

And in the 911 it's handled with a smidge of countersteer and an equal measure of patience. Certainly Porsche has polished this car's dynamics well beyond what its design merits. But at the end, when the chips are down, stability control is off and you're up against the icy, unforgiving hand of physics, the 911 is still a 911. And it's going to behave like one.

Respecting physics is one of the necessary truths of driving fast cars fast. You'll learn that respect in this Porsche. Yet today's 911 lets its driver delicately dance with physics like few cars made. That the 911 remains composed — stoic, even — during a 150-foot slide at more than 90 mph isn't surprising. What is surprising is that we still love it. Because even when physics plan the way, it's the driver who directs the 911's path.

The fundamental question of this comparison, then, is can the 2014 Chevy Corvette measure up to the Porsche's greatness?

The Primary Target
We've already hailed the thorough competence of the 2014 Corvette Stingray and in the last two weeks it has razed two far more costly and powerful pieces of hardware — the Nissan GT-R and the SRT Viper — in our comparison tests. Chevy makes no secret that Porsche's 911 is the Corvette's primary target, in dynamics, refinement and comfort. So hard-fixed is that goal that a 911 Carrera S equipped with a PDK transmission was the only benchmark car purchased by the C7 team during development.

And that just happens to be the exact configuration of the car you see here. Though this example is thoroughly marinated in Porsche's options bin, it is the car Corvette engineers put in the GM crosshairs. Equipped with $47,000 in add-ons (including the $8,520 carbon-ceramic brakes and $4,050 Porsche Dynamic Chassis Control and Porsche Active Suspension Management), the 911 is, ahem, considerably costlier than the 'Vette. The $4,080 seven-speed PDK auto-manual transmission pushes the price to $144,350.


The seven-speed Stingray, though not inexpensive by any standard, is within reach of upper-class mortals. Our test car — outfitted with the $2,800 Z51 package (electronic limited-slip differential, dry-sump lubrication, 19- and 20-inch wheels, lower gear ratios), $8,005 3LT package and $1,195 dual-mode exhaust — totaled $69,375.

Let the games begin.

The Defining Differences
If there's one character trait that most profoundly distinguishes the Stingray from the 911 it's this: torque. And it is here that the big, deep lungs of American displacement score the first hit.

The 'Vette's 140 pound-feet advantage, which arrives 1,000 rpm earlier than the Porsche's torque peak, leaves the 911 driver seeing stars and stripes at every corner exit. The numbers tell the story nicely. Corvette: 465 lb-ft at 4,600 rpm; 911: 325 lb-ft at 5,600 rpm. So it goes without saying that if you're a purveyor of powerslides, the Stingray is your car.


What's more, the Corvette's 144 additional cubic inches continue the haymaking well into the rev range where it enjoys a 60 peak horsepower advantage. Its 6.2-liter V8 cranks out 460 ponies to the 3.8-liter 911 flat-6's 400.

But don't even think of writing off the 911. Its recipe for greatness might be more subtle, but it's not lost on anyone who gives it time. There's a coherence to driving the 911 that's only present in a car sharpened by decades of commitment to purpose. And that's what you get here: the promise that no matter how hard you drive it, someone has driven a 911 harder.

In fact, the 911 defined itself in this test as much by what it didn't do as by what it did. Among those feats were the ability to tolerate triple-digit heat without wavering, endure repeated launches without faltering and remain utterly composed throughout it all. The Corvette, partly because it was a preproduction car and partly because it cost half as much, simply lacked the same quiet confidence.

That's Not All
Despite its torque deficit, the 911 still managed to beat the 'Vette in acceleration testing. Its PDK transmission is merciless when it comes to doing important things, like shifting and launching — quickly. It helped produce a 0.3-second advantage in the quarter-mile (12.1 seconds at 114.9 mph vs. 12.4 seconds at 113.7 mph). Sixty miles per hour also arrived 0.3 second sooner in the Porsche, which hit the milestone in 4.0 seconds (3.8 with a 1-foot rollout as on a drag strip). The 'Vette needed 4.3 and 4.1 seconds, respectively.


The 911's launch control is unbeatable, and it's the reason Porsche prefers we test cars equipped with PDK transmissions. Even in the real world it's easy to access and use, so there are few downsides. Our test-driver beat the Stingray's launch control fairly easily but still couldn't match the 138-pound-lighter 911.

Full disclosure: The first C7 we tested in Michigan in June was quicker than this 911 in the quarter-mile, but we're not in Michigan anymore. Truth is, the realities of lower-octane fuel (91 vs. 93) and less grip played a role. But those factors were the same for both cars, which were tested on the same day at the same location.

Handling tests, however, favored the Stingray. Its 73.5-mph slalom pass is 2.8 mph better than the 911 could muster. It also eked out a victory over the Porsche on the skid pad, producing 1.05g to the 911's 1.04g. Finally, without the help of carbon-ceramic rotors, the Stingray stopped shorter from 60 mph (99 feet vs. 101 feet).

Track
But it's here, on the track, that the Stingray shows its real merit. Motivated by what is fundamentally a truck engine, it finds speed in places the 911 doesn't. Largely, it's the Stingray's ability to explode away from an apex that earns it the advantage. Performance Traction Management is deadly effective in making the laps both fast and easy. Turning it off, though it will more deeply engage the 'Vette's driver, ups stress without a proportional increase in driving reward.


If you want to hoon the 'Vette then, by all means, turn it off, but if you want to go quickly set it to Race Mode, warm the tires and stand on it. So potent are the Stingray's technologies here that they embarrass the Porsche in identical proportion to what happened at the drag strip. Think about the irony in that.

Steering feel and response, next to torque, are the Stingray's biggest allies. There's a confidence in the C7's steering beyond that of most every other sports car made today. Its front end sticks with intractable persistence. For evidence of this you need to look no farther than its front tires, which wear faster than its rears during hard driving.

The 911, for its part, remains an amazing car to drive hard. Its light but direct steering makes no concessions when driven at speed. Its brakes are so utterly capable that we began to think they're actually worth the cost of a nice sport bike. Its balance, communication and honesty at the limit are remarkable. And when it slides — and it will slide — there's both comfort and reward in bringing it back. It's not going to bite you, but there's no denying its fundamentals at the limit.


In the end, it's 1 second slower around the 1.6-mile Streets of Willows Springs road course.

Porsche 911 Carrera S: 1:25.6
Corvette Stingray Coupe: 1:24.6

Details Matter
But we don't spend every day at the track or the mountains. The 911, in daily use, comes to life in both observation and expression. It's here that Porsche's resources of refinement exceed Chevy's by an order of magnitude. It's here that the little things take over. Look at the 911. Really look at it. Appreciate its subtle purposefulness. Appreciate a shape honed on the mill of necessity. Now look at the Stingray's details, its ducts, vents and its facets.

They might be functional, but when measured by the yardstick of the equally effective 911, they're also gratuitous.


Now get inside the 911. Touch it. Operate its controls. Drive it down the street and notice what you don't hear. Talk to your passenger. Listen to him. Do the same in the 'Vette. Appreciate the 911 with both your senses and your heart. Measure the difference not in numbers but in nuance.

The Daily Grind
Chevy moved mountains in improving the Corvette's interior, but there's still a vast gap between these cars in quality. Everything you touch in the 911 is laser-micrometer precise and right-now responsive. The differences matter. In the 911 you move and it moves with you. In the Stingray you punch the touchscreen twice and wait. You step cleanly into the 911. You descend into, over and around the Corvette. And on a hot day, the 911 smells like leather. The Corvette smells like chemicals.

Ride comfort is a wash. Chevy's magnetorheological dampers are magic and they make the Stingray's ride every bit as refined as the 911's. But you'll compete for your passenger's attention with the Corvette's tire and road noise.


If the 911 utterly dominates the Corvette anywhere, it's here, in the ever-important words between the numbers.

What Our Hearts Say
So we're stuck. Stuck with a decision no enthusiast can fairly make. Picking the 'Vette is the obvious choice since it's supported by ample empirical data. At the end of the day, we can't deny that the home team nailed at least one of its primary targets. Making the Corvette as quick and rewarding as a 911 is a big task, and it's been fully accomplished. The Corvette, when driven hard, is as good as the 911, maybe better. There, we said it.


That it costs, in this case, less than half as much is pure gravy.

But then there's the undeniable reality that the 911 is the better car. Whether we're taking our kids to school or adding subtle countersteer to correct that big slide, we'd rather have the Porsche. It's the car that wins our hearts so it's the car that wins this test.

kosmo

Edmunds (same guys as above):
Best Coupe: 911
Best sports car: Vette

hmmm


http://www.edmunds.com/car-reviews/f...u-can-buy.html