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I stand by the idea that the car should be designed for amazing handling first. There are a million ways to make it a plow for the mass consumer, then give them the ability to change it on their own later
I agree - if you go by Tadge's earlier comments, he grew up with early rear-engine Porsches in his family and he admits that with the C8, he was deathly afraid of poor handling and or snap oversteer. He indicates that the team approached handling from the contact patch to the frame, and tried to optimize every component in between for a safe chassis that was neutral with the track settings. Tire construction, cg and roll centers, intermediate connection point bracket stiffnesses, frame stiffness, etc, they treated it as a system design.
First, you have to get the terms right. You can't transfer the weight unless you move one component of the car to another location, like the battery for instance. When you accelerate the load, the mass is transferred thru the Cof G.- Load transfer.
Another thing, Polar moment of inertia is not the correct term to use for car handling, it applies to solid shafts or complex bodies like the vehicle chassis which can be an assembly of point masses.
PMI should not be confused with (second or planner moment of inertia ) used for bending and torsional calculations of above examples. For handling description purposes, the term "planner moment" must be used. it can assume positive or negative magnitudes according to its axis position.
C7, the driver is behind the cg, C8 the driver is closer to and right in front of the cg. It'll be a different feeling inside both cars in a turn. C7, engine and rear are further apart, higher second mass moment I would have thought,...……. C8, engine tranny and driver all close together. I would have thought a lower second mass moment here for the C8. Not so says Tadge….. talking about the second mass moment.
Terminology - I've used Planar moment in the past, primarily for beam bending. Static deflection of an I-beam under a cantilever load, PL^3/3EI, where I is the planar moment (second area moment) of the I-beam cross section about the bending axis for example. Planar accounts for the
For dynamic accelerations, such as accelerating a rotating body or a body's resistance to rotational acceleration, I'm used to using a mass moment (second moment, similar to a planar moment except accounts for mass moments and not specifically area moments), which relates the torque required to produce angular acceleration of a mass. Both are second moments. Planar has units of ft^4 and mass moment has slug-ft^2.
I think we both understand them, maybe we can agree on a terminology as long as we know what we're talking about, specifically. If you have a different use of Planar moment I'd like to hear it. I'm not a chassis expert.
I stand by the idea that the car should be designed for amazing handling first. There are a million ways to make it a plow for the mass consumer, then give them the ability to change it on their own later
It seems like GM has done exactly that. This is the first car I've read about (gimme a few years before I can own one) that apparently has such a significant change in handling with only an alignment change to stock hardware.
^^^
Yep, unlike modern Porsche's (etc) that some posters are referring to as "it's all driver skill in managing Oversteer," it was NOT the same with early Porsche's (or Corvair's of that era.) Yep they were fun to race BUT the average driver was not skilled dealing with high speed skids if going to fast for turn or being forced to take other than an optimum line. Here are two references that may be of interest and explain why the C8 design was more critical and changes like camber can have significant handling differences:
Super Chevy Interview: Jul 25, 2019 TJ: “We knew all of the bugaboos that other brands had to discover and correct over a period of many generations. … We were always sensitive to the car's rear mass with the mid-engine placement. We had to do it right the first time."(obviously talking about Porsche)
Vette:"How is the turn-in into a corner on this car compared to the C7?"
TJ: "It is lightening quick, we were really paranoid about it, too much mass in the rear causing a classic handling problem. We knew all of the bugaboos that other brands had to discover and correct over a period of many generations. This included the rear wheels breaking loose and causing oversteer or closed throttle snap that caused the real wheels to react and also create oversteer. We were always sensitive to the car's rear mass with the mid-engine placement. We had to do it right the first time. There were many little details we had to design into this chassis to correct that, but the driving experience is amazing. The car is very linear and it was a surprise to us how the frame architecture allowed us to reduce higher tire pressures and sticky compounds. We were able to back away from some of the extreme tire compounding we used in the previous generation."
November 15, 2017 Road and Track re the Corvair, "To avoid spinning the car, I have to counter-steer almost immediately after initiating the turn. But I could see how the lightly trained driver might get into trouble. ... The average driver wasn’t equipped to handle an over-steering car."
Steady state and predictable neutral, slight oversteer or slight understeer characteristics can all be used to your advantage on a race track. I could care less if the C8 had slight oversteer or slight understeer characteristics.
But moderate to severe understeer (not saying the C8 has it) is a different ball game. With more understeer, for example, comes the need for more aggressive and more precisely timed trail-braking on entry and very smooth and properly timed transition from braking to throttle.
Very few drivers can do this that precisely and that consistently corner after corner, lap after lap, specially as you are driving multiple different lines to overtake or navigate through traffic or - the BIG one - when track is going from dry to wet or wet to dry and grip is changing on different lines.
That is what Speed Phenom and others here don't have the experience or have it but do it naturally and don't consider the implications of severe understeer - did he ever track the car in the wet and/or with nannies off?. That, in fact, was the issue with the early 911s. The real issue was the amount of trail-braking required into a corner for turn-in - get the timing or smoothness off and you'd get a snap. I heard some of the reviewers from MotorTrend et al complain about both the understeer AND the unpredictable snap on the C8. So there may be something there.
If you are driving (err ... co-driving with the computers) in Sport 1 at Spring Mountain at way below the limit you may think the car is fantastic. Try to push it with the nannies off and it is a whole different ball game. In the meantime, yeah, dial -3.0 of front camber on your street car to be able to do a simple track day
Have you seen all the youtube videos of people that have ZERO talent, that buy expensive cars , turn off all the controls and then proceed to wreck the car leaving cars and coffees? lol... GM would get the pants sued off of themselves selling a car that had inherent oversteer...
Hell, BMW took a load of grief selling the E36 M3 that was neutral. They had to redesign the front suspension geometry, and put staggered tires on it to make it understeer more, out of fear of being sued.
(yeah, I still own the 95 M3 I bought 25 years ago before they "fixed" it...)
Of course I am serious. Watch the Throttle House video of the C8 and you can see their car has excessive understeer. They don't just talk about it, they show it on camera. This is a Z51. Inexperienced idiots are going to crash C8's regardless of how much understeer the car has. They will think they are heroes and turn of the nannies and then make mistakes because they don't know how to drive. If GM is so paranoid about safety and driver experience why was the C7 and Camaro more neutral?
I think this is something GM can easily address and likely will as the car matures in stock form. I just don't buy that you have to induce excessive understeer just to protect people.
Terminology - I've used Planar moment in the past, primarily for beam bending. Static deflection of an I-beam under a cantilever load, PL^3/3EI, where I is the planar moment (second area moment) of the I-beam cross section about the bending axis for example. Planar accounts for the
For dynamic accelerations, such as accelerating a rotating body or a body's resistance to rotational acceleration, I'm used to using a mass moment (second moment, similar to a planar moment except accounts for mass moments and not specifically area moments), which relates the torque required to produce angular acceleration of a mass. Both are second moments. Planar has units of ft^4 and mass moment has slug-ft^2.
I think we both understand them, maybe we can agree on a terminology as long as we know what we're talking about, specifically. If you have a different use of Planar moment I'd like to hear it. I'm not a chassis expert.
The "Polar Moment of Inertia" is defined with respect to an axis perpendicular to the area considered. It is analogous to the "Area Moment of Inertia" - which characterizes a beam's ability to resist bending - required to predict deflection and stress in a beam. These idiots keep on using the wrong term to describe a car's responses. Makes me sick. The pretenders.
Just a few things. First. Polar moment of inertia would apply to the chassis properties contained within it's structure only as an indication of rigidity and not. The C7 Corvette chassis in this case, has a low PMI. IE, it flexes like a bitch.
2.. The correct term for what you are describing is Polar or Planor second moment of inertia which is the mass moment of inertia or the rotational motion resistance of an object, in this case the whole car.
3. The C7 ZR1 has two large mass centroids placed at each end of the chassis where the bending moment is the least.(Allows a lighter structure than ME cars). Note the position of the engines in the above images. A chassis is a bridge and it's loading determines the design construction. The C8 has to be heavier than the C7. The C7 chassis' lack of torsional rigidity and a very compliant suspension, optimises the friction circle at each tire and is much more efficient than the C8 and the Porsche in this regard. It will perform better on a constant radius, constant speed turn and out perform the other two cars on braking.
The tires will last longer because of the more even loading. In constant speed events the C7 Vette will beat the other cars.
4. Check the weight of each car. 3300, 3600, 3000, and 2650 lbs for the McLaren. makes a big difference in the numbers. Accelerating the mass on these particular cars the C7Vette will be at a disadvantage on most road courses that have long straights and hard braking areas.
5. The C7 Vette will have a slower initial or incipient rotation rate about it's vertical axis at entry or turn in, but that means, an indication of a spin is well within the average human reaction time. You loose it in the other cars, you're done. These cars are only usable because of electronic and technological wizardry. https://www.oreilly.com/library/view.../ch6sec4.xhtml
[QUOTE=JerryU;1601762380]^^^
Yep, unlike modern Porsche's (etc) that some posters are referring to as "it's all driver skill in managing Oversteer," it was NOT the same with early Porsche's (or Corvair's of that era.) Yep they were fun to race BUT the average driver was not skilled dealing with high speed skids.
Ahh, you compare a Corvair and a Porsche.... I won't comment.
Yes, it's all about driver skill, and how come Mario Andretti, and Sterling Moss won in every car they ever drove, and Eddie (under) Cheever drove 180 F1 Races with the best equipment, and never got a podium?
Here's the deal....
Great Drivers in Good Cars beat Good Drivers in Great Cars...BUT...
Great drivers in Great cars beat everybody.
Mediocre Drivers in Great Cars buy the beer, complain, and keep Race Tuners in business.
And Great Drivers in Mediocre Cars become DE instructors, and bitch and moan on Web Forums.
C7, the driver is behind the cg, C8 the driver is closer to and right in front of the cg. It'll be a different feeling inside both cars in a turn. C7, engine and rear are further apart, higher second mass moment I would have thought,...……. C8, engine tranny and driver all close together. I would have thought a lower second mass moment here for the C8. Not so says Tadge….. talking about the second mass moment..
Aviation is my first love, 6 year air force ret. and 15 000 hrs total time. You name it and I've flown it. Seating position is critical for sensory perception. Each person's is calibrated differently. If you have perfect vision, you may not be fighter pilot material and you may not even be tracked to transport.
There are other parts to the brain that determine whether you track to fighters. My son and wife have perfect vision but lack spacial orientation or situation awareness. My son can only drag race and not road race. In a aerobatic glider, the pilot is forward of the Cg which is at the center of lift. In an aerobatic plane, the pilot sits aft of the Cg which is also at the center of lift. A glider pilot and a F1 driver spine is almost horizontal and a fighter or aerobatic pilot's or a NASCAR driver spine is vertical. Most aviators are not capable of aerobatics.
In a 911, you sit further forward of the Cg than a C8 and a C7. Most drivers are not capable of driving a car at the limit. Even then, it is rare for a race car driver of another discipline to drive NASCAR. Schumacher could not have driven NASCAR. If you don't have these faculties, you can't grasp what has to be said here as far as having to drive a car of questionable chassis and suspension design. Like the kid in the video hasn't a clue but few even know why. It's like color blindness. So many people have to go through life oblivious to their surroundings.
My sensitivity in chassis design comes from flying and down hill skiing. I learned to tune shocks and determine spring rates on moguls. Closing your eyes over moguls tells you of the futility of active suspensions and electronic grannies to assist the driver. Like an aircraft seating position, even with all the grannies in play in a car, if you have well calibrated natural sensory perception, you can sense the intervention of the grannies, tire technology, etc. and you are also vary aware of what the result would be without these interventions. The first thing I do after setting up a car is to drive it on a wet or dirt road where it is much more difficult to oppose the laws of physics artificially. My son can only be a reactive driver. I sensed that his Porsche GT3's rear end of the car left up to it's own devises, would overtake the front end in short order. I had an Austrian friend who could drive old 930 turbo in any conditions under or over the limit. It is possible to take advantage of the idiosyncrasies of a poorly designed chassis if you understand the dynamics.involved. I personally don't like such cars.
So, in closing, engine tranny and driver in close proximity may not be the ideal. How to set up and drive a tight or loose car. That is the question here.
To be continued. My old brain is burning.
Ahh, you compare a Corvair and a Porsche.... I won't comment.
Yes, it's all about driver skill, and how come Mario Andretti, and Sterling Moss won in every car they ever drove, and Eddie (under) Cheever drove 180 F1 Races with the best equipment, and never got a podium?
.
Yep, apparently you're too young to know Porsche of the mid to late 1960's. In SCCA Corvair's were very completive! That was the era Tadge references his Dad driving his Porsche at the limit and caused his "Fear of Oversteer in the C8 design!"
Our Chief Metallurgist at the R&D Lab where I worked raced an early Corvette in SCCA then switched to a '67 Turbo Corvair Spider that had 180 hp, about the max power of Porsche's in-the-day! He was very completive! We were always discussing ways to increase power "within the rules!"
Then you're also to young to do anything but read about Andretti and Moss, at 77 I saw them race.
SIDEBAR
Recall Moss's comments about the Maserati Birdcage he drove when he was 30 and said it was the best handling car he had been in (referenced in my CarTech Pro Series book on automotive welding referring to the gas welded small tubes in the 66 lb Birdcage chassis.) Reminded of my friend Tom who had ~40 Vintage race cars (also ~40 old Great Lake race boats, all restored.) That's a pic I took of him in the "Corkscrew" at Laguna Seca Vintage races. He was a "purest" insisting on the original repro tires, brakes etc. Recall telling him he needed better brakes for his 1951 C-2R Chrysler hemi powered Cunningham he also raced. He'd come down the straight, leading the pack into the Andretti Hairpin and by the 2nd turn be last! I said he needed better brakes BUT he wanted to feel exactly what the driver's in-the-day felt!
Looking at the book about his cars he had published to donate proceeds to his college, he also had a car Paul Newman drove, the Honker II, an AC Bristol, Cobra, GT40 Mk III, Jag XKSS, Porsche 550 Spider. Tom, was born the same year as I but unfortunately died unexpectedly 10 years ago. After attending his Celebration of Life and seeing his many Lake Racing boats I bought some of what Tom owned. Albeit mine are a bit smaller! Have over 50 cars in my Man Cave. My Baby Bootlegger Lake Racing boat he had fully restored is my biggest model at 36 inches.
This is pic I took of Tom in his Birdcage that I have on my Man Cave wall. Great guy, very unpretentious!
