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I've asked this question many times over the past years, and the answers usually come down to this:
1. 50/50 isn't optimum for very high hp cars (and, IMO, the C7 is in this range, even in stock form) for putting power down. So, rear mid-engine cars (Corvettes since the C5 have been front mid-engine) have a distribution biased towards the rear. How much varies depending on the make.
2. For handling you not only have weight distribution, but also what's called the "polar moment of inertia". This is basically how close the components in a car's weight are its center of mass. Think of a skater performing a spin. Bring the hands in, it's easier to rotate. Extend the hands, it takes more energy to spin the same rate. So, in theory, a rear mid-engine can have a lower polar moment, which makes it turn quicker and easier. Also makes it less stable to steering upsets (whether intentional through the steering or externally).
I've asked this question many times over the past years, and the answers usually come down to this:
1. 50/50 isn't optimum for very high hp cars (and, IMO, the C7 is in this range, even in stock form) for putting power down. So, rear mid-engine cars (Corvettes since the C5 have been front mid-engine) have a distribution biased towards the rear. How much varies depending on the make.
2. For handling you not only have weight distribution, but also what's called the "polar moment of inertia". This is basically how close the components in a car's weight are its center of mass. Think of a skater performing a spin. Bring the hands in, it's easier to rotate. Extend the hands, it takes more energy to spin the same rate. So, in theory, a rear mid-engine can have a lower polar moment, which makes it turn quicker and easier. Also makes it less stable to steering upsets (whether intentional through the steering or externally).
Our C7's are practically 50/50 in weight distribution. My question is this, how will the C8 compare to the current GS C7 in handling?
First, handling has more to do with suspension tuning and tires than configuration.
That said, in a racing situation, braking at turn entry, and getting power down early on turn exit are critical, and here is where rear weight bias is a huge advantage. I raced 911s for years. Rear engine layout has been out of favor for decades, but in full race trim, a 911 with its "obsolete" rear engine is just unbeatable in these two areas.
A mid/rear configuration combines this rear weight bias advantage with lower rotational momentum. Its no coincidence that virtually every successful race car built since 1964 has a mid/rear engine design.
If I compare my ME GT4 to my C7, the GT4 turns in more quickly and feels like the car rotates around the driver seat. When the car slides, the whole car slides in a very balanced way and is more controllable with the throttle... that is, the rear end only comes out if I’m deliberately trying to do so and I can modulate it very easily. The C7 and GT4 subjectively feel like they have similar ultimate grip (the GS may actually have more with the massive tires) but the GT4 feels WAY more nimble and has way more direct turn in... some of that is suspension set up and some of it is the ME design.
On the street, the difference is small. You notice sharper turn in and a more tossable front end on the GT4 but the rotational difference is only noticeable at the limit on a track.
The FE C7 handles very, very well. If the C8 is built properly, it will feel more nimble and handle better at the limit with the ME layout.
The concept that 50/50 weight distribution is the best, ironically, was pushed in a lot of Corvette commercials in the 1960’s — as they were bragging about their getting closer, than later even closer, to 50/50% weight distribution.
The above three posts are highly accurate as to why having the motor just ahead of the rear axle is a better handling car than a FE.
Moreover, even thought the C7 is just about 50/50 weight distribution, that too is far from cutting edge on a front engined car, for the Ferrari 812 front engined Superfast, is now 47 F/53 R rear weight distribution.
To sum it up mass centralization is more important than weight distribution. Though distribution does matter. The Grandsport has a slight rear bias at 49/F and 51/R. One of the reasons it handles so dam good at the track. A C8 should easily handle better than a GS with Z06 power levels, if done right.
To sum it up mass centralization is more important than weight distribution. Though distribution does matter. The Grandsport has a slight rear bias at 49/F and 51/R. One of the reasons it handles so dam good at the track. A C8 should easily handle better than a GS with Z06 power levels, if done right.
The other factor is tires. If you can put larger tires on the rear, this maximizes the benefit of a rear weight bias. Again, look at the Porsche race cars from the 80's... the rear tires were huge. This is common practice today unless sanctioning bodies limit it. Even in the 60's, "cheater" drag-racing specials from the big three (fire bolt, etc) moved the rear wheels forward to shift the weight to the huge rear slicks rear under acceleration.
To sum it up mass centralization is more important than weight distribution.
Up to a point, the Ferrari 348 and F355 probably arguably had to LOW a polar moment and their drivers need at all times tpo keep the front end pointed in the direction the car is going.
All Ferrari ME models afterwards moved the radiators up front to increase the polar moment and decrease the externally induced rotation moment of the cars.
Last edited by MitchAlsup; 01-19-2019 at 05:37 PM.
Up to a point, the Ferrari 348 and F355 probably arguably had to LOW a polar moment and their drivers need at all times tpo keep the front end pointed in the direction the car is going.
All Ferrari ME models afterwards moved the radiators up front to increase the polar moment and decrease the externally induced rotation moment of the cars.
Some scribe in a auto magazine long ago used this term incorrectly and now everybody uses it in that manor. I design and build chassis. Now make a note of this please. First: Polar moment of inertia would apply to the chassis properties contained within it's structure only as an indication of rigidity. The Corvette chassis in this case, has a low PMI. IE, it flexes like a bitch. Two: 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 f.....g car. Third:
As you can plainly see, since the C5, Corvettes are hardly front ME cars by design. I analyzed this design somewhere in this forum. Check it out. The C7, including the ZR1 have 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). The C7 chassis' lack of torsional and bending rigidity and a very compliant suspension, optimises the friction circle at each tire and is much more efficient than other sports cars in this regard. It's beautiful by design. Oh yeah, you can't corner weight it either. I cover that also. If you loose it in a ME car, chances are you will crash. Radiators are not efficient at the rear especially now that HP numbers are out of sight. This why Ferrari and everyone else moves them up front. Quit making stuff up.
So, If I take a 5 foot 2x4 and hang a 10 lb weight on each end, hold it at its center of gravity and rotate it, it will tend to continue to rotate until I apply force to stop the rotation... based on my lowly medieval history degree, I'd ball park it at oh, 2.5 feet x 10 lbs of torque or thereabouts. But it will want to continue to spin.
Now, if I place those two 10 lb weights right in the middle, and rotate it, it will tend to accelerate and decelerate more easily, as there is less torque on the CG. Is this correct, or am i missing something?
Yep, that's essentially it. BUT, you could also have something external push on the end of the board, and it's going to move away from where you want a lot easier with a lower polar moment than a higher one. Essentially the tradeoff of maneuverability and stability.
So, If I take a 5 foot 2x4 and hang a 10 lb weight on each end, hold it at its center of gravity and rotate it, it will tend to continue to rotate until I apply force to stop the rotation... based on my lowly medieval history degree, I'd ball park it at oh, 2.5 feet x 10 lbs of torque or thereabouts. But it will want to continue to spin.
Now, if I place those two 10 lb weights right in the middle, and rotate it, it will tend to accelerate and decelerate more easily, as there is less torque on the CG. Is this correct, or am i missing something?
No, it will spin longer on its own when the weights are in the middle. A skater will spin longer with arms close to the body vs. extended.
Some scribe in a auto magazine long ago used this term incorrectly and now everybody uses it in that manor. I design and build chassis. Now make a note of this please. First: Polar moment of inertia would apply to the chassis properties contained within it's structure only as an indication of rigidity. The Corvette chassis in this case, has a low PMI. IE, it flexes like a bitch. Two: 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 f.....g car. Third:
As you can plainly see, since the C5, Corvettes are hardly front ME cars by design. I analyzed this design somewhere in this forum. Check it out. The C7, including the ZR1 have 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). The C7 chassis' lack of torsional and bending rigidity and a very compliant suspension, optimises the friction circle at each tire and is much more efficient than other sports cars in this regard. It's beautiful by design. Oh yeah, you can't corner weight it either. I cover that also. If you loose it in a ME car, chances are you will crash. Radiators are not efficient at the rear especially now that HP numbers are out of sight. This why Ferrari and everyone else moves them up front. Quit making stuff up.
the c4 corvette had a lackif torsional rigidity. The c5 was light years ahead of c4 torsional rigidity. C6 and c7 even more so.
the only vehicle that had a lessened torsional rigidity than the standard corvette c6 and that was the c6 z06.
the aluminum chassis of the c6 z06 was flexible.
c5 , c6 and then c7 have greater torsional rigity,
allows softer suspension and greater accuracy of suspension to work as designed to offer improved handling
Rear mid engine handling offer superior initial turn in.
the c4 corvette had a lackif torsional rigidity. The c5 was light years ahead of c4 torsional rigidity. C6 and c7 even more so.
the only vehicle that had a lessened torsional rigidity than the standard corvette c6 and that was the c6 z06.
the aluminum chassis of the c6 z06 was flexible.
c5 , c6 and then c7 have greater torsional rigity,
allows softer suspension and greater accuracy of suspension to work as designed to offer improved handling
Rear mid engine handling offer superior initial turn in.
It is a very complex subject. Notice how my efforts to explain PMI in layman's terms falls upon deaf ears. It's very sad. I've raced a bunch of C4s, C5s and my beloved C6. It took 'till the 2004 C5 Z06 to be faster than the C4 GS. The C4 had a perimeter frame like a 50s Chev Biscayne. The engine was far back compared to the C5, 6 and 7 which loaded the frame adversely. The long wheel base Vettes that followed have a unique structure while they transitioned from steel to aluminum. It's a perimeter, space frame, backbone structure. I love it. The focus of chassis design is the friction circle at each tire. The most important component is the load at each tire. Late model Corvettes are a little like go carts in that the chassis is part of the suspension. The bending and torsional displacement in the C7 chassis does not effect tire grip at either end. The back and the front of the car are doing their own thing. It would be impossible to stiffen this chassis without making it very heavy or using expensive materials. The long wheel base provides for a lot of leverage, the front tires are already loaded so the turn in is brisc. A ME car coming out of a turn under power will understeer but you will be able to get on to the power sooner. Always trade offs. The C7 Vette will out brake any car just because of tire loading. You have to have lightning quick reflexes with a ME car to recover from a problem you encounter at the limit.
I have a steel roll cage in my C6Z which changes the entire equation. It becomes a single purpose car.
C7Z after your jack it up...
....and after you let it down.
Before you can design a chassis, you have to know the materials you work with. A Strength of Materials course or post grad work is an essential part of mech. engineering. The C8 chassis will be a study in the usage of materials.
I don't know why I bother, it won't change a thing, but here goes.
TORSIONAL RIGIDITY IS THE TORQUE REQUIRED FOR PRODUCING A TWIST OF ONE RADIAN PER UNIT LENGTH OF THE TEST SHAFT. MATHEMATICALLY, IT IS THE PRODUCT OF THE MODULUS OF RIGIDITY AND POLAR MOMENT OF INERTIA .
Steel, G = 75865.126 N/mm GJ = 9.65*106 N-mm
Aluminium, G = 34131.536 N/mm GJ = 4.34*106 N-mm
POLAR MOMENT OF INERTIA would apply to the chassis properties contained within it's structure only as an indication of rigidity.
MODULUS OF RIGIDITY IS DEFINED AS THE RATIO OF SHEAR STRESS TO THE SHEAR STRAIN. EXPERIMENTALLY IT CAN BE DETERMINED BY THE STRESS-STRAIN CURVE.
The actual shape of the member counts the most in application. IE:.....
SECTION MODULUS: IS A GEOMETRIC PROPERTY OF THE CROSS SECTION USED FOR DESIGNING BEAMS AND FLEXURAL MEMBERS. IT DOES NOT REPRESENT ANYTHING PHYSICALLY.
TO DEFINE SECTION MODULUS, IT MAY BE DEFINED AS THE RATIO OF TOTAL MOMENT RESISTED BY THE SECTION TO THE STRESS IN THE EXTREME FIBRE WHICH IS EQUAL TO YIELD STRESS.
Your posts are extremely interesting. It is clear that you are expert in your field. But, to simplify, there are two takeaways...
1. Us layman are misusing the term "polar moment of inertia", so we should rethink this.
2. When we are discussing a high-performance/racing environment a rigid, well engineered chassis will always handle better than a cheaply engineered, flexible chassis regardless of the location of the mass.
That said, given chassis of equally good design, does a mid/rear configuration (where the mass is closer to the CG) have an advantage over a design where the mass is located at each end.
In the former case, would you agree that the car is more likely to spin, but also more likely to change direction with less cornering force from the tires?
Also, given a very experienced race driver, does this not create an advantage on a road circuit where the car needs to change direction quickly from turn to turn?
I think this has already kind of been said, but the engine although mounted to the frame still moves under acceleration and turning. Having the engine in the middle doesn’t disrupt the weight transfer as much in these scenarios. I notice in my Corvette the nose dives a good bit more under braking and turning than does my Boxster.
I haven't driven any of the competitive rear mid engine cars but I have instructed in a 458, a Lambo and in a couple of R8s. Riding in the cars on track you can definitely feel the difference in the way the car rotates in a corner. They are much more nimble than my C7Z. The interesting thing that I found was with the 458 driver who wasn't much more than a novice. At VIR he entered the second turn of the uphill Esses incorrectly so had the car pointed in the wrong direction as we went over the hump in the right hand turn. I was pretty sure we were going to run off the left side of the track before we got to the left hand turn. However, as the weight settled back on the wheels he turned the wheel to the right to steer toward the left hand turn Vs the grass and lo and behold the car did what he asked it to do. It pivoted to the right to keep the car pointed on track and then he turned the wheel to the left to go through the left hand turn. Snap, snap, snap. I am pretty sure if I asked my C7Z to do that it couldn't. No way it could pivot around its vertical axis that quick.
I've asked this question many times over the past years, and the answers usually come down to this:
1. 50/50 isn't optimum for very high hp cars (and, IMO, the C7 is in this range, even in stock form) for putting power down. So, rear mid-engine cars (Corvettes since the C5 have been front mid-engine) have a distribution biased towards the rear. How much varies depending on the make.
Mike
Mike, most Corvettes built since 1963 have had a 50/50 weight distribution or even a 49/51 weight distribution since from the 63 on they have all been front mid engine cars (engine behind front axle). The ones that went to 51/49 ratio were the cars with the big block engine which weighed 150 pounds more than the small block.
01-19-2019, 10:03 AM
