Ivan Viera

I have read that europeans C7 competitors have chassis rigidity of over 30,000 Nm/degree so how does the C7 frame rigidity compares with Mclaren, Porsche, BMW etc?

RC000E

C7's frame rigidity is on par, and exceeds European counterparts when speaking in terms of "open air" platforms. This is the debate and issue some have had with the design philosophy of the C7, designed solely as an opened air platform. Initial design targets/CAD indications were in the 14,500 NM/Degree range for the C7, which exceeds opened top Porsches, BMW's, etc...or is at least on par.

Look at dedicated coups though...Aston, Mclaren, Porsche...and torsional rigidity numbers are damn near double for dedicated hard roof coupes. Even a 996 era Porsche Turbo was in excess of 27k nm/deg.

With the C7, it's rigidity has been steadily improved upon, with each generation of Corvette. It should be pointed out though, that even in the C5 era, where the car with the newly introduced hydroformed steel frame exhibited an approx rigidity of 9,800 nm/deg, the Z06 was profoundly stiffer with a fixed roof. It's a real shame that the fixed roof has been abandoned in the C7.

Shaka

The importance of chassis properties depends how you are going to load it and what your design objectives are. Length and width and modulus and modulus of elasticity of the construction material will have a bearing on how the chassis performs. Viper, F458, and other mid engined sports cars load their chassis at the highest bending moment, therefore they have very short wheel bases in order to keep the construction weight down. Aluminum has higher torsional resistance than steel but poor bending. Stiffness numbers don't mean a thing. Optimizing the tire friction circle at each corner is the objective. 911s are stiff as hell but they have high tire wear rates on any road course simply because of where the engine is located. The tires don't share the load properly. The beauty of a Corvette, C5 and up, is that the high friction circle numbers are achieved with a very compliant suspension and a very flexible chassis. Try jacking your car up at the jacking points and measure your door gaps at the top, I dare you. You can't perform corner weights on it as you would do a 458. The engine and gearbox are placed at the extremes where bending moments are the lowest. Here in lies the beauty of the design. Quit comparing it to other designs. This allows a long wheel base which in itself reduces longitudinal transfers but requires less mass between the wheels. You can brake later and accelerate earlier in and out of turns as a result. The engine and gearbox form two major mass centroids over the front and rear tires instead of just one in the centre like it's competitors do. There are two near independent systems with a low couple. Marvellous.

Important. If you begin a load transfer on your scales to achieve your semetric read outs , the chassis will absorb the energy without registering a load increase or decrease at the diagonal wheel. That energy will release at the least opportune moment and upset the car with a resultant shake and the chassis regains its preload when stable again. Lowering your car by eye can cause handling problems.

Forget your chassis stiffness and high comliance and thank Dave Hill for his genius and center mount plastic leaf springs. The leaf springs load in the center where the backbone or center box section of the chassis can take the most torsional loads. amazing.

The proper way to do corner weights with or without a roll cage, especially with coil/overs is as follows. Jack the center of the rear subframe and lower the front subframe onto two scales. record numbers. Do the same for the back. Then place all 4 corners on the 4 scales. No wheels should touch the ground during this excersize.
If there is a discrepancy correct by adjusting the back only with spacers on the bottom of one of the rear scales to achieve the same numbers when weighed individually. Measure the height of each scale from the ground plus any support block you may have used.
These are important reference numbers and you don't want to do this again when doing corner weights.
Mark the floor where you took the measurements for future reference. Those numbers won't change unless you hit something or add or subtract or move stuff around on the car like the battery or something. Put bags on the seat that weigh the same as your ***. Full fuel is good. Make sure you have equally worn tires with the same air pressure. Once again, weigh back and front separately. Keep the car level by removing the rears scale when weighing the front and visa versa. When front is on the scales, check your ride height, preferably on chassis hard points but fenders will be OK. Record numbers. Weigh all four corners. If the ratios are the same as the original chassis xmember readings, leave well alone otherwise adjust at the back only.

RC000E

Shaka, good answer, but the "magic" you speak of is all part of the compromise. I agree overall, and I think your first sentence really serves well as a preface to your point: The design objective and how you load it. Since C5, the car has been designed as an open top platform, therefore the objectives and targets have surrounded that ideology. Even in the C5's case though, it was understood that torsional stiffness has it's benefits, hence how the C5Z was created. Looking even then, the simple addition of the roof added substantially to the cars rigidity...that was public knowledge.

I was a bit confused by the credit to Dave Hill on the suspension and center attached leafs, as that existed prior to C5. Granted the design of the suspension was evolved but, you lost me a little there. Can you clarify?

I take your points and agree, due to the weight distribution and suspension design, it works WITH the fact that the cars torsional stiffness isn't say, that of a dedicated coupe. Using the "transaxle"/torque tube/front mid mounted design is fantastic, and the Corvette is proven...without question. As with Porsche over the years, they've made the best of their "design quirks" with engineering advances and/or workarounds.

Where you lost me, and maybe I'm misunderstanding (as is the nature of text and quick typing), is where you seem to imply that torsional compliance is desired and/or beneficial. As if to say, that Corvette engineers, given a blank sheet of paper, are going to design a ton of compliance into the design...I have to say not. This is a series of compromises being made to maintain opened air. Take the opened air requirement out of the equation, and you have a different Corvette on paper, from day one....higher rigidity is an absolute granted bonus, which likely leads to mass reduction in other areas of the car, etc. It's a domino effect.

Certainly, it's been shown in the past, via various formula sae programs, that torsional stiffness increases are beneficial, only to a certain degree. Cars at various levels of racing are often designed with torsional stiffness and roll stiffness in parallel. So, I'd agree, that as a street/sports car, there is a point where targeting an unnecessary degree of rigidity only leads to unnecessary mass.

The fact remains though, that Corvette engineers, over the past 15 years, have justified the Z06 as hard roof for a reason though; stiffness allows the suspension to be more effective. The more rigid the chassis, the more predictable the behavior of suspension and steering. Additionally, a number of enthusiasts buy these cars, with the hopes of modifying them. Remove the tranverse leafs in exchange for slightly higher rates, coilovers placing loads further outboard on the shock mountings, and you start to exploit the lack of rigidity. For this reason, many enthusiasts may very well want a better platform to modify, don't you agree?

In the end, I believe that you and I are of the same overall conclusion though, that saying "I'm not buying this Corvette because it's torsional stiffness is 'x' and Porsche's is 'y' is certainly the wrong approach." Sae judges talk much about installed stiffness, beyond that of core torsional stiffness calculations, as the installed stiffness is really what makes the car perform end of day. Torsional rigidity is only one piece of the design pie, so to speak.

In summary, I agree with much of what you're saying. You've pointed out how GM has made opened air work, but my point is that end of day, Z06 with a fixed roof is the recipe of a better car, but removed for the sake of sales targets (so be it). Z06 seems to be saddled more and more by the desires of non-enthusiasts. Open air, heavy/luxurious interiors, etc. If I'd have my wish I'd see this car (C7Z06) more as a ZR1...a true grand tourer, with the Z06 reserved for the enthusiast base, much like the GT3/GT3RS, and make the car more focused.

Shaka

Well, answering to the stock holders needs also provides us with the best value in sports cars even if it means robbing the truck division's expensive tooling to pull it off. Vette sales today compare with late 70s. That means old farts like me have to get in and out of them. Hill's solutions are amazing and Tadge only built on it with the legacy remaining in the CAD system of Hill's design which came from McLelland.
You would never build a chassis like the C7's from scratch, it defies definition but not logic. There are still features that a 50s Chevy Biscayne shares in the design. You can take an old C5 and trash anybody at an autocross meet. You can also go racing on a lower budget than a Miata or a Nissan 370.
The roof added 12% gains in structural performance. Hills double mount of the spring increased roll resistance without changing spring rates in roll as do ARB would do.
Watch how a C7 pitches and rolls yet look at the numbers and comfort levels.
Guys think coil overs are cool on their Vettes, but they could encounter very strange handling behaviour that can't be analysed if it is not done right. Drive an X6 BMW, it still has 70s cowl shake.
The C7 CG is much lower than its competitors and the roll couple of the roll centers benefit enormously. A big plus for leaf springs and pushrods I say.
It is a great chassis but nobody dare copy it. Gaining stiffer chassis by raising the Cg, IE, stronger green house is not a good trade off. Cg first. The design of the backbone in the C7 prevents high door sills.(good for old farts)
A properly designed roll cage will give you a completely different animal to tune. There is just so much compromising that can be done for the street but nobody can do it like Vette engineers can. Why, it can go over railroad tracks like Loeb's Citroen now. Amazing.
If you have go cart experience, you will learn chassis dynamics via your ***. Like you will really understand flying if you begin with gliders. Go carts also have no damping in a structure that has stiffness adjustment. If you ski moguls, you will also understand damping and compliance in your chassis. Your ***.
In the case of C5 C6 and C7 chassis design, you can't apply handling solutions that work on other cars. If a Viper's chassis flexed like a Vette, you would not be able to tune the shocks but it is easy with a Vette because isolation. The entire drive train acts like a damper. Marketing has to deal with people who don't know sh one t. If chassis numbers were published, Corvettes would be a laughing stock despite the brilliant performance numbers. Same with fast shifting gearboxes. They come with severe penalties.
However, to feel the stiffness of a 458 or a Porsche is something to behold. Only recently have American cars had European chassis. A Vette is still old school but Yankee ingenuity triumphs to keep it competive. Germans just can't build a chassis like Cadillac can especially the boys at BMW. Be proud, buy American.

stevebz06

So do I understand you correctly as saying that GM chassis engineers have accomplished miracles by getting great results out of an inferior design? You're not saying that chassis flex is desireable are you?

Shaka

Clairvoyantwolf

How many times has the pushrod V8 been declared dead

How many times have leaf springs been stated as a negative

Performing miracles with "inferior" designs seems to be a matter of course for the corvette team.

Ivan Viera

Here is some data of different cars:


Alfa 159 - 31.400Nm/degree
Aston Martin DB9 Coupe 27,000 Nm/deg
Aston Martin DB9 Convertible 15,500 Nm/deg
Aston Martin Vanquish 28,500 Nm/deg
Audi TT Coupe 19,000 Nm/deg
Bugatti EB110 - 19,000 Nm/degree
BMW E36 Touring 10,900 Nm/deg
BMW E36 Z3 5,600 Nm/deg
BMW E46 Sedan (w/o folding seats) 18,000 Nm/deg
BMW E46 Sedan (w/folding seats) 13,000 Nm/deg
BMW E46 Wagon (w/folding seats) 14,000 Nm/deg
BMW E46 Coupe (w/folding seats) 12,500 Nm/deg
BMW E46 Convertible 10,500 Nm/deg
BMW X5 (2004) - 23,100 Nm/degree
BMW E90: 22,500 Nm/deg
BMW Z4 Coupe, 32,000Nm/degree
BMW Z4 Roadster: 14,500 Nm/deg
Bugatti Veyron - 60,000 Nm/degree
Chrysler Crossfire 20,140 Nm/deg
Chrysler Durango 6,800 Nm/deg
Chevrolet Corvette C5 9,100 Nm/deg
Dodge Viper Coupe 7,600 Nm/deg
Ferrari 360 Spider 8,500 Nm/deg
Ford GT: 27,100 Nm/deg
Ford GT40 MkI 17,000 Nm/deg
Ford Mustang 2003 16,000 Nm/deg
Ford Mustang 2005 21,000 Nm/deg
Ford Mustang Convertible (2003) 4,800 Nm/deg
Ford Mustang Convertible (2005) 9,500 Nm/deg
Jaguar X-Type Sedan 22,000 Nm/deg
Jaguar X-Type Estate 16,319 Nm/deg
Koenigsegg - 28.100 Nm/degree
Lambo Murcielago 20,000 Nm/deg
Lotus Elan 7,900 Nm/deg
Lotus Elan GRP body 8,900 Nm/deg
Lotus Elise 10,000 Nm/deg
Lotus Elise 111s 11,000 Nm/deg
Lotus Esprit SE Turbo 5,850 Nm/deg
Maserati QP - 18.000 nm/degree
McLaren F1 13,500 Nm/deg
Mercedes SL - With top down 17,000 Nm/deg, with top up 21,000 Nm/deg
Mini (2003) 24,500 Nm/deg
Pagani Zonda C12 S 26,300 Nm/deg
Pagani Zonda F - 27,000 Nm/degree
Porsche 911 Turbo (2000) 13,500 Nm/deg
Porsche 959 12,900 Nm/deg
Porsche Carrera GT - 26,000Nm/degree
Rolls-Royce Phantom - 40,500 Nm/degree
Volvo S60 20,000 Nm/deg
Audi A2: 11,900 Nm/deg
Audi A8: 25,000 Nm/deg
Audi TT: 10,000 Nm/deg (22Hz)
Golf V GTI: 25,000 Nm/deg
Chevrolet Cobalt: 28 Hz
Ferrari 360: 1,474 kgm/degree (bending: 1,032 kg/mm)
Ferrari 355: 1,024 kgm/degree (bending: 727 kg/mm)
Ferrari 430: supposedly 20% higher than 360
Renault Sport Spider: 10,000 Nm/degree
Volvo S80: 18,600 Nm/deg
Koenigsegg CC-8: 28,100 Nm/deg
Porsche 911 Turbo 996: 27,000 Nm/deg
Porsche 911 Turbo 996 Convertible: 11,600 Nm/deg
Porsche 911 Carrera Type 997: 33,000 Nm/deg
Lotus Elise S2 Exige (2004): 10,500 Nm/deg
Volkswagen Fox: 17,941 Nm/deg
VW Phaeton - 37,000 Nm/degree
VW Passat (2006) - 32,400 Nm/degree
Ferrari F50: 34,600 Nm/deg
Lambo Gallardo: 23000 Nm/deg
Mazda Rx-8: 30,000 Nm/deg
Mazda Rx-7: ~15,000 Nm/deg
Mazda RX8 - 30,000 Nm/degree
Saab 9-3 Sportcombi - 21,000 Nm/degree
Opel Astra - 12,000 Nm/degree
Land rover Freelander 2 - 28,000 Nm/degree
Lamborghini Countach 2,600 Nm/deg
Ford Focus 3d 19.600 Nm/deg
Ford Focus 5d 17.900 Nm/deg

RC000E

This is an interesting moshpit to decipher...



Here's how I see it...to see what effect there is within Corvette, you have to look at the entire history of the car...where it began, who was involved, etc. Had the car continued on the path it started on, and that Zora had it on, I think it'd be a very different car. Fact was though, the American market didn't support the idea. In it's early inception, soldiers understood the beauty of a drivers car, but the "musclecar" era quickly redirected the cars purpose.

I see the 70's as the era when Corvette really deviated from what it began as. It wallowed in this era of indecision through the 80's/early 90's between the poor health of GM, no real direction for the car, etc. Then Dave Hill and GM got behind the idea of Corvette and re-energized it, though it had the expectation of success. It had to earn its keep to survive. In this sense, the car was not really a HALO as much as it is, just a piece of the pie that HAS to make money the same as a Cobalt, Camaro, etc. I think with C5, the idea of becoming European and the AMERICAN nature of the Corvette buyer began to "clash" so to speak. Dave Hill made the very best of those compromises though, and made a hell of a car out of it, that sold.

My fear though is, that Tadge/Post GM bankruptcy tight wires/Cadillac dependency will lead to a Corvette that is becoming maybe...too muddled. It's trying to do too many things. Trying to spread it's costs, trying to be luxury/speed/HALO all in one, etc. In my opinion Corvette has done some great things overall, and my ONLY hope is that continued success allows the team to start to divide some of those jobs a bit instead of packing them all in one car. Porsche is a great model of that in some ways...they have a fantastic lineup that really doesn't make compromises. You can have the automatic/hair in the wind canyon carver, to the all out track monster. GM and Porsche two very different brands though, one built upon the basis of performance, the other build upon the basis of the working man.

SBC_and_a_stick

Great read. I do have a question. Is the isolation employed in the 'Vette on purpose the reason why the chassis doesn't communicate as well as cars that use springs and a stiff chassis? The car has grip, no doubt, but is some of the driver feedback lost?

Shaka

Everybody would like long wheel bases but the penalties are very high. Even with exotic composites, the multi million dollar super cars and open wheel race cars have short wheel bases. Keep the loads at the extremitees and you can keep the structure light. A light long chassis flexes, period. Tadge got it as stiff as he dare. Aluminum is a bitch to work with. Stiff in aluminum needs more real estate. The car must meet crash test requirements which compromises the structure for other requirements such as the ones needed for car enthusiasts. It is a massive jugling act and the Corvette boys met enviable objectives. It is truly wonderful. To imply that a Vette is an inferior product is a display of extreme ignorance.
Short wheel base cars are unforgiving and poor grip will require Senna type reflexes to tame.
I can't see Corvette going to mid engine ever. Corvette is a relatively high volume high profit product. Mid engine will screw that up big time.

Check this Lambo chassis. They have steel structures bolted to the main carbon structure which perform different duties.





My roadster frame (DOT and EPA certified) was designed for pure joy and feed back. No roof, doors, PS, air or golf clubs here. 1850LBS with 400 Chevy HP. Mild steel is magic and predictable. Aluminum or composites would take up much more needed real estate to achieve what I wanted a car to be. Also a front engine car uses the engine for energy absorption. Weight is the enemy.

Ivan Viera

Another great chassis Acura saids stiffest chassis in its class.


http://autoguide.com.vsassets.com/bl...pace-frame.jpg

Clairvoyantwolf

The car also weighs 3,800 lbs

RC000E

I don't think anyone here is implying the Corvette is an "inferior" product. It's inferior (and superior) to other in certain senses, but the comparisons often become apples to oranges...even if they fill the same market space.

Chris Harris tends to be a VERY perceptive driver from the seat and his perception going from the 991 Carrera to the C7 Z51 was that the Carrera's chassis was much more communicative and stable over undulations in the track surface. The chassis stability is very perceptible, allowing the suspension to be more effective.

End of day, both cars achieve their targets, but Corvettes are doing it with a degree of brutality, while cars with more refined chassis are doing it with a degree of precision and finesse. The challenges Corvette faces in terms of its buyer pool, versus where it wants to be in the market...it's a conflict they are trying to balance quite clearly.

Simple as going on this forum. You make a post about a new wax, you'll be at 10 pages in a day. You post about a drag strip you'll be at 5. You post about a road course or a race car...you'll be at 6 posts for a week. The buyers of this car, don't match the direction the car needs to go long term...so it's trying to stretch this rubber band more and more to meet everyones needs...and the car is getting stretched thin...bottom line. Have they done a hell of a job stretching...definitely.

RC000E

By all reports though, the car doesn't feel like it at all. That's a hybrid also...at this stage all these hybrids are tanks. They can put down power, get big exit speed, but a light chassis will own them. Jack Olsen has a mildly modded 72 Porsche and takes turns at Willow faster than the Porsche 918. Redirecting all that weight, making a tire carry it all...the game is changing a lot in how to accomplish being fast.

This is an American NSX...I think Americans hands in it will be its demise. It's going to be an OK car for sure, but....it's like a sequel with a different director...chances of it being the magic of the first one is slim.

Ivan Viera

Hybrid is the future sooner or later is going to make to the Vette. The could have make the NSX lighter with a Carbon tub and body panels but the price would have sky rocket.

Shaka

Quote:"So do I understand you correctly as saying that GM chassis engineers have accomplished miracles by getting great results out of an inferior design? You're not saying that chassis flex is desireable are you?"

The 991 is a great car and so is a F458. Neither car's chassis is refined more than a Vette.Take those 3 cars straight off the showroom floor and let them race the 12 hours at Sebring. Drive them in a city all day and then drive them to California from Florida, summer or winter. My money is on the Vette doing everything better even at a 100 000 miles. Front engine, mid engine and rear engine!
The only one that can be driven on the limit without grannies safely is the Vette. The Porsche has a light steel stiff structure only possible because of it's dimensions, but they hang the engine off the back which presents other problems like excessive tire wear because the 4 corners can't be loaded properly. The car has a brake bias adjustment to the front wheels. McPhersons in the front is hardly a refinement. Granted, stiffness has it's virtues but you can do that with small structures only but then you have to leave out the golf clubs as a result.
Both the Ferrari and Porsche use ordinary modern production methods unlike their big brothers and for that matter, a Viper. Google the space age construction of the C7 chassis. The 458 and 991 are really nothing special. Fiat and Volkswagen helped them bring their cars to the 21st century.
Take all that plastic off under the 458 and you will be horrified.
The Italians surely know how to refine a car for fun, but nobody can refine a car for comfort like we do, it's a cultural thing.
German cars are refined for the autobahns, they are horrible every where else. Drive a CTS V and then a E63.
The Porsche and the Ferrari have to have excellent rubber, and electronic differentials to function. Porsche even has electronic engine mounts that lock and a dead feeling electric steering system. Only American engineers could fix that. Is a F1 car chassis refined? Sebastian Loeb probably wouldn't think so. Refined is a tricky word but I know what you mean. I did mention that a C7 can go over rail road tracks better than any Vette before, but you still know it;s American.
OK, so it was a bit of a stretch to use a Corvette chassis for a Cadillac, but I don't think anybody buying a Vette is under the illusion that it is a luxury grand tourer but many people use it as such.
Corvettes are the most handicapped race cars ever. It's competitors chose small displacement high revving engines with lots of valves and cams and things and Corvette, using less materials, chose cubic inches and push rods and low revs. For this they are penalized. Make all the GT2 and E class cars run stock except for safety items and let's see the results then.
One thing that would 'refine' a Vette chassis to my mind, is a multi link rear suspension like Porsche and Ferrari have. Why, even F1 would have it if it wasn't banned.
Here is the Michelin OPT concept but let's keep the leaf spring please.

Clairvoyantwolf

Shaka I must say that you posts are quite fascinating. So to ask, what advantage would a multi link rear suspension have over the double A-Arm the car is already using?

Shaka