Lots of people with the head in the sand here, do you still think that earth is flat ;-) Great ring times 7,56 is good but there are better times and what do they use :-) you just shoot yourself in the foot with that comment.
Thanx Randy .......

 

OK Ace, answer me this: If the rear engine location has been abandoned by ALL other sports car manufacturers for years for basic vehicle dynamics flaws, why does Porsche continue with it for the 911? I guess they must be stupid, because "everyone knows" that it is an inferior design. Any properly designed sports car must be either front or mid-engined!!! If Porsche owners had any sense they would abandon those antiques and move to cars like the Boxster, Cayenne, 928, 944, 968, etc. 911s are obviously as out of date as an ash-frame Morgan Plus 4!! Some people just deny the obvious!

 

Alot of talk here but the question at the heart of this thread has not been answered. How do you fix the directional instability around bumpy (especially washboard) turns? Will a better set of shocks and/or springs help? Ive read that tires would help but there are virtually no choices for other tires. The only ones I've found so far are conti sport contacts. I've had these on a BMW M Coupe - they are not that good. I am ready to say good bye to this car and return to a much better "real world" handling BMW M car if I can't fix this major nuisance. Speeds around smooth race tracks are irrelevant when it comes to cornering on daily driven bumpy roads. This is my first Corvette, it has Z51 and a manual trans. If you have a suggestion, please let me know, I'ld like to stay with an american made car.

 

Let's see. 95% of the cars that have run the ring do worse than the C6 and a handful do better (at a much higher price point). So... what do the 95% who run it slower use? Methinks you just shoot yourself in the foot... ace!

Mike

 

GoodbyeC6, Toyo T1Rs are supposedly available in the proper sizes (check posts from alevy)...They are very sticky in both dry and rain and are lighter than most tires. They also have a softer sidewall (non-runflats) which allows them to smother small bumps better. The light weight allows the suspension to do its job better due to lower unsprung weight. I think going from runflats to a tire such as that could make a big difference for you.

 

Thanks for the info, have you done this to yours and gotten improvement in real world (bumpy turns) handling?

 

I have, but I do not presently own a Corvette. I went from Bridgestone SO3s to T1Ss (T1Rs are the latest model, recently released). I love the stick of the SO3s, but the ride and resistence to washboard surfaces has greatly improved.

 

I was a little unfair to say that the only reason why the C6 does well at the Ring is because it's running our coilovers. The C6 is a well priced, fast car right out of the box. It runs down some of the fastest street cars in the world. That doesn't mean the car can't be better. Better is cornering speed increased, fuild temps down and brakes that last more then a 15 minute session. Anyone that tracks their car hard knows all of the problems.
How many GT3's do you see with 300 degrees oil temp, water at 230, brakes that a baked, and diff that is burnt after 20 laps? This I think is where the 100,000 dollar car pays off.
But spend 100,000 on a corvette and you will have the fastest-most reliable car at the track for sure.

 

Question: Directional instability around turns?
Answer: Tires / Shocks.

I’m basing this answer on feedback from our own customers. As I am neither an engineer nor a racing tire expert, I would ask those who are for a good recommendation... What I do know about are transverse composite monosprings. We have made them since 1976 - adjustable height and rate systems for the C3 and now improved springs for the C4/5/6.

The coil-over / transverse monospring "debate" will go on forever (sigh). So here's my two cents...
Coil-overs are very good and so are transverse springs. They are just two completely different means to an end. For all out racing, the coil-over may be preferable for simple, affordable adjustability. They are very good for race cars - I personally have not been very pleased with a set I had on a test car. The handling was OK but the ride just about shook me to death on the street.

We may manufacture coil-overs in the future but only for C4/5/6 racing applications. The composite monosprings in the Corvette were not put there for cost savings, or even fit - but because they helped the car to ride and handle better. The design and production of composite springs for these cars is very expensive compared to coils or coil-overs.

"Cross Talk" debates can be very interesting (politically correct word):
While (of course) the sway-bar is MADE to transfer energy from one side to the other, the composite spring also does this (on our test fixtures) to a very limited degree. There were NO negative results from this energy transfer. In fact, on a dual mounted C3 spring, the transverse spring actually functioned like a sway bar. Picture this: When the right side of the spring went up, the center flexed down and the opposite (left) side flexed up also - kinda' cool huh. This technology is also used in adjustable front C3 systems and is being developed for C4/5/6 cars right now. Combining the adjustability of a coil-over with the fast response, weight savings and ride quality of a composite transverse spring should be a home run... Think about it. That’s all 4 now.

Alan G.
Vette Brakes & Products, Inc.
http://www.VBandP.com

 

Alan G.

OK I accept that the leaf springs, as executed in the C6, are not the problem. What tires/shocks do you recommend? Will they fix this problem? What is wrong with the tires? The F1 Supercar tires seem to be rated fairly well. Their adhesion on wet/dry smooth roads is comendable

 

The 300zx was/is notorious for having horrible wheel hop during hard launches.

 

GoodbyeC6,
You probably know more about tires than I do - I would really ask an expert - I would hate to spend big $$$ on the wrong ones...

Sincerely,
Alan G.
Vette Brakes & Products, Inc.
http://www.VBandP.com

 

The runflat sidewalls are very stiff, they have to be to make them runflats. Add to that stiff Z51 shocks, and there isn't enough compliance to let the suspension track the bumps. Think of it like a steel tired wagon with solidly mounted axles. No, or too little, compliance equates to a hard ride and skittering all over a bumpy road. Of course too much compliance gives a wallowing ride.

The F55 suspension was designed to deal with this problem by dynamically adjusting compliance to road surface conditions. If you don't have F55, then adjustable shocks, with separate bound and rebound adjustments, would let you manually tune the suspension for a given track surface.

More flexible sidewall tires would also provide more compliance. This is perhaps the easiest thing to change on a car with too little compliance. The downside to tires with more flexible sidewalls is that you have less control over the contact patch in hard cornering. OTOH they tend to give more warning of impending breakaway, and are more forgiving for the less experienced driver.

 

I completely agree with you to put the engine in the back qas Porsche 911 has is not a good desing i cant recall saying that it is good in fact i kinda hate the flatend beetle but it remain a fact that the say 911(from say 993 and forth) will handle better than any Vette outbreak them and so on. The best handling Porsch is the Boxster and almost all magazines agree to this it has the engine at amuch better place.
If you take the Corvette has front mount engine that isnt good either but both Porsche and Corvette has almost fixed the weight problems whit these designs. The ultimate is midengine car end of story with coilovers ;-). If we take Ferrari as an example they have the engine in the right place and have the right suspension but i just cant see myself riding around in a hotted Fiat:-).
I have had lots of cars abd no one of them have slided like the Corvette does in the corners you may called it what you want i call in crosstalk and iam curing my car from that.
To say that a coilovers are just for racecars what a joke i have had cars with coilovers if you dont buy the cheapest one they are adjustable you know :-), but i dont since your company dont sell coilovers it kinda pointless for you to say that the leafspring design isnt good, then you wont sell anything would you ?
The fact still remain the fastest cars around a track dont use leafsprings they use coilovers, when that isnt the case anymore i will gladly buy a new set of leafsprings but i will proably be six feet under before that ever happens ;-).

You have to forgive me for all bad spelling and so on in the above text since ebglish isnt my main lang. but then again you dont speak swedish so :-).

 

Shopdog - people on this forum respect your input, so I will to. But I take exception to somethings you state.

1) The Z51 Corvette is sprung rather soft in comparison to other cars I own or have owned, none of these have the skiddish response on bumpy turns. In my opinion the Z51 suspension has too much compliance and allows the Vette to wallow through certain road transitions.

2) Some of my previous cars have had racing tiires - very stiff sidewalls - still no instability on bumpy turns. The same cars had very stiff suspensions.

Some of the offline research and conversations I have had with tuning experts for Corvettes and other cars claim the following problems: The torsional rigidity of the Corvette C6 chassis is not up to the task of keeping the wheels planted ; The shocks are inadequate (not too stiff or too soft, just inadequate) to absorb the forces generated when hitting bumps. Both of these are inline with my experience so far.

The other cars I have owned (or still own) are BMW Z3 M Coupe, BMW M3, Lotus Esprit, Lotus Elise. The Elise is sprung much much stiffer than the Vette and it has NONE of the problems on bumpy raods. The Elise has an incredibly stiff chassis (no cowl shake at all - unlike my Corvette which shakes significantly) and Bilstein coil-overs. Putting softer sidewall tires or softer shocks on the Vette will just masquerade the real problem and exaserbate some of the car's short comings.
What the real problem is and whether this can be fixed I have not been able to definitively discern. I am still hoping to find this out before spending bucks to fix the problem or bailing on the car.

As a final note, I drove a non Z51 C6 before buying my Vette, the ride and handling in my opinion was all Boulevard Cruiser and not at all Sports Car.

Thanks for all your input to this problem.

 

I haven't heard the "cowl shake" complaint since the C4. The C5/C6 chassis is very stiff for a production car of its size and weight. About the only way to significantly improve it would be to go to a space frame or at least add a full roll cage structure tied in to the suspension pickup points.

I'm also leery of too high a spring rate for a road car. As long as the suspension doesn't bottom out, there's enough spring. And it is hard to bottom out a C5/C6 on any reasonable road surface.

Shocks, now that's a different story. I agree with you that the shock valving leaves something to be desired for an aggressive driver. The usual answer to that is to switch to adjustable high performance shocks such as Bilsteins. Reducing unsprung weight with lighter forged wheels and lighter non-runflat tires will also help, especially if you have added heavy performance brakes.

But the Corvette is not an Elise, and never will be. The Elise is lighter and has smaller moments. You can't make up for that with high rate springs and harder shocks.

 

I read in "All Corvettes are Red" that the big improvement in the C5 was the chassis stiffness over the C4. On the C4 the chassis was very flexible and to get the car to corner better stiffer springs and shocks were needed. With the C5's stiffer chassis the spring and shock rates could be lower and the car handle better and still have better real world driving charcterisitics. The C6 is a improvement over the C5, I know for I have owned 2 C5 and one C4. The C4 would shake you to death where as the C5/C6 is quite driveable everyday.

I would point out that the C6 on the skid pad displays some impressive numbers , I also know that the real world roads are not smooth like the skid pad. Maybe I don't drive my Corvette has hard as some of you but I have never been disappointed in the handling of my C5 or C6. I took a ride up in the hill country and on the curves that said 40mph I took them at 90mph and the G-meter only registered .52g's, I know I could have taken the curves much faster, and this was on a narrow two lane country road that was not smooth. I guess there will always be people that no matter how the cars are built will demand more, and that is okay, it keeps the after market companies in business. But I think for the majority of the Corvette owners the cars capabilities are far greater than most would care to explore.

GM has been using the leaf spring long time with changes from steel to composite along the way, if it was not a good design I would think they would have changed it by now. I would think it would be cheaper to put coil springs on shocks than the leaf spring, but I may be wrong. I know that some of the people I know that have the C4 changing over to the coil overs seemed to improve the handling, I have not met anyone that has coil overs on the C5/C6.

Anyway, that is my 2 cents.

 

I think I may be expecting too much from this car. It does have a perimeter frame, not a unibody or monocoque structure. I agree wholeheartedly that the Corvette will never and can never be an Elise, and good thing to. For many reasons, the Elise CANNOT be a daily driver like the Vette. As far as the cowl shake issue, the Vette chassis is stiff but not nearly as stiff as my Z3 M-Coupe or my M3 sedan where (both similar to the Vette in weight, size, mission and weight distribution). The handling of the Vette is remarkable, you can easily exceed posted speeds for turns by a factor of 2 or 3 (as is proper for any sports car) - unless the road is a washboard, in that case use caution! It is this unpredictable nature of the car I find troubling.

I am not willing to install a full roll cage (this has been recommended as a solution by several knowledgable folk). I feel this would ruin the daily driveability of the car.

Do you know if Bilsteins will improve the problem? Or will the chassis twist more due to the forces from the improved shocks?

There is a rear chassis stiffening structure available that does not appear to compromise the daily driver usefullness of the car - do you have any experience with the efficacy of these?

And (I really don't want to open the spring issue again but..) My knowledge as an engineer tells me that it is likely that a leaf spring will have a rate that increases more rapidly than a coil spring over the full range of suspension travel. So would a good set of coil overs alleviate some of these issues? Some have claimed that the ride and washboard-road-handling can be improved with a properly tuned set. Do you have any experience or knowledge about this claim?

Thanks again.

Ps. My C6 is bone stock Z51, no mods to brakes or tires. It has the polished wheels, don't know if they are lighter, heavier or the same as stock.

 

It'd definitely make entry and exit tougher.

I don't know if Bilstein has a shock out yet for the C6 with the correct valving, but better shocks are definitely needed. They don't necessarily have to be set harder, either. The trick is to get the correct bound/rebound ratios to control the wheels over a rough surface. You want the wheel to walk over the bumps, not just jolt hard against them and bound high off the road. As I said before, too high a spring rate or too hard valving in a shock will make the car skitter like a wagon on washboard roads.

If you're talking about the Elite Engineering stiffener plate, I have one of those on my car. I didn't buy it to stiffen up the chassis, I bought it to lower the exhaust heat that seeps into the car. But it does serve to stiffen the body, which indirectly helps prevent the chassis rails from twisting. I haven't noticed any real change in handling, but the body doesn't groan as much entering and leaving driveways.

With a monoleaf, the rate is constant throughout most of the travel. With stacked leaves (old style) the rate can be more progressive. Of course coils can be wound to have constant or progressive rates too. But the primary reason for the popularity of coilovers is that winding springs for any custom rate is *cheap* compared to re-engineering a polymer monoleaf.

Polished and painted are about the same weight, both heavy. You can save about 8 pounds per wheel with good aftermarket forged wheels. Non-runflats will save you a few pounds per wheel too.

 

Shopdog:

Thanks for all the info.

I was referring to the Chassis stiffener (looks just like a harness bar) made by Doug Rippie. It attaches only behind the seats so I am not sure how much it can do to stiffen the chassis as it does not tie into the front at all.

Doug Rippie and others offer Bilsteins as shocks and coil overs for the C5 and C6, I do not know if they are C5 shocks sold for C6 or specifically for the C6

I may try the Bilsteins and see if they improve the Washboard handling. I'll let you know if I do.

Happy Motoring! and thanks again