Almost There

This is a thread with the purpose of discussing what the optimal weight distribution would be for the corvette assuming real-life engineering constraints for a FMR car were lifted and say the distribution could vary between 60:40 and 40:60.

LS1LT1

I'd like to see it back to the perfect 50/50 (which according to some probably isn't actually all that perfect anyway LOL) that it once was but that's not an easy task so I would be happy with it just maintaining anything within the 52/48 to 48/52 range and dialing in the suspension to work well with it.

Jinx

So, rear-mid-engine then?

What's the 458 Italia's? What did Colin Chapman believe?

Does the ideal change for an AWD car?

What's more important, optimizing distribution or mass centralization? Weight distribution is a fun stat like Cd but it's really just a clue and not an answer, isn't it?

hig4s

Depending on the model, the current distribution is either 51f/49r or 50/50!!!!

But if you look at RME supercars, many have a rear weight bias, around 43f/57r, which Lamborghini calls "perfect".
F1 race cars are around 46f/54r,, not sure if that is optimal, but it is by F1 rules, mandatory!!

jackhall99

It's right there now.

Racer X

It is not a simple one size fits all balance. It depends on the height of the CG and pitch axis and braking and acceleration capabilites of the car, as well as the anti-dive and anti-squat capabilites of the suspension.

Under braking the CG will shift forward, you don't want it to shift so far forward that the front brakes are overwhelmed and the rear brakes do little. You don't want the car to have too little weight on the front end during cornering and acceleration that the you lose the abiltiy to plant the front end for directional control or cause understeer.

Also the weight distribution front to back is not enough, it is where the mass is along the length and width and height of the car and in relation to the rotaional axes of the car. All of this will affect the handling of the car, and of course it's acceleration and braking capablities.

LS1LT1

C8forT

I agree with Racer X.

A while back a Car and Driver article compared the braking and handling of the 911 (something like 39/61) to the Cayman (about 45/55). http://www.caranddriver.com/features...11-gt3-feature This article is worth a read.

There are some advantages to having some rear weight bias. One of the drivers for Corvette Racing noted in an interview last year that the 911s in the ALMS series can get into the power sooner than the C6R coming off of corners due to having more weight/traction on the rear wheels. Also, when breaking, weight bias tends to shift forward, helping to balance out the weight distribution under breaking. That said, I believe the Z06 and ZR1 stop shorter than the 911, but this might be due to their lighter weight overall.

Nissan claims a 53/47 ratio is optimal to provide a 50/50 balance when the car is accelerating out of corners since balance shifts rearward during accelleration. But the GTR is AWD, and thus has the front wheels helping to accelerate the car out of the corners - so it is not as relient on rear wheel traction to accelerate as a RWD car. Ferrari, on the other hand, uses something like 47/53 on its RWD 599.

For straight line accelleration, a rear weight bias will help get traction to the rear wheels, and hence should help get the car out of the hole quicker.

MitchAlsup

Most cars are tuned for oversteer/understeer balance by increasing the cornering loads at one end of the car or the other. This increasing of load on a particular tire actually decreases the total amount of cornering power the car can apply to the road. Thus, one gets rid of oversteer in an old <hopped up> Mustang by increasing the front roll stiffness, causing the front end to wash out at the same time the rear end washes out.

Formula One cars are so advanced that they cannot be competitive and be tuned the same way road cars are tuned. Remember a couple years ago when the Mercedes car had a new set of front suspension arms made to move the front wheels farther forward? They did this (at great cost) in order to balance the car without sacrificing cornering capabilities.

In this aspect, RacerX is completely correct. The perfect weight balance is dependent on the width of the tires, the CoG of the car, and certain aspects of the car's acceleration capabilities.

And the calculation is far from direct! The perfect weight distribution for a car on a skid pan where there will be no longitudinal weight transfer will have each tire carry its proportinate width in weight. So, here, if you had a 50%:50% weight distribution, you would want the front tires to be as big as the rear tires.

When longitudinal acceleration is part of the equation, you need enough tire at the front to deal with the forward weight transfer under braking, and enough tire at the rear to deal with throttle induced acceleration. Under braking you want the front tires to give up just a shade before the rear tires give up. Under acceleration, you want the front tires to give up just before the rear tires give up (to avoid snap spins). Thus, suspension geometry also comes into play when yo thought you were only considering weight distribution!

Shaka

You are correct except for CG migration. CG can't move unless you remove your golf clubs from the trunk or have a movable ballast or something. You can have load transfers which is not the same as weight transfer or mass transfer. The ideal location of the CG on a 4 wheel vehicle will depend on its application.You can't scale it up either like from a Miata to a Vette because of moments of tire contact to CG. Corvette C6 Z06, F458, Camaro GT and a Porsche GT3 all have different front to rear loading due to location of the CG. The height of the CG compounds load transfer because of a greater moment. All good things that pushrod engines, transverse leaf springs, hydro formed perimeter frame and wooden floor boards provide. The Vette and the Porsche have lower CGs because of one has a OHV engine and ideal chassis structure and the other has a flat engine offset by a heavy steel body. The aft CG of the Porsche increases the moment under lateral loads and therefore requires much more grip on the rear tires in proportion to the other cars. See all those Porsche spinning out on Saturday night last? Under braking, the Porsche front tires receive less load transfer than the 458 and the Vette. The 458 will be able to distribute the braking forces more efficiently between back and front than the Vette but rhe Vette will stop quicker. These four cars perform about the same in the GT class of Grand Am where aero grip is policed severely. The addition of aero in the GT e class in the Lemans series alters things drastically and the position of the rear wing of each car in its regulation box differs greatly to counter their respective chassis properties.
The Z06 Carbon is 3 seconds slower than the ZR1 around the Ring despite having 135hp less because the less effective tire loading on the ZR1 due to the extra weight of the supercharger. Consensus amongst chassis engineers is that a front engine rear drive sports car should have a rear bias in the region of 48/52. 50/50 or less gives the car a slight push which is good for most driver preferences but that Z06 Carbon would equal the ZR1s Ring time if it had 48/52.
Aero packages alter the dynamics of mechanical grip drastically.
The 4 above mentioned cars each have differing handling characteristics. The Corvette Z06 and the Camera have equal loading front to back in lateral acceleration. You are accelerating the mass and the concentration of this mass has an effect on handling dynamics. The Corvette has essentially two mass centroids because of the location of the engine and the gearbox. The other 3 cars have more of a concentration of mass. The Corvette is able to have a lighter chassis construction than the Ferrari because of this. The concentration of mass on a beam generates higher bending moments. Aluminum is not strong in bending but is stronger than steel in torsion. These differing configurations have varying yaw and pitch angular inertia, incorrectly referred to as moment of inertia. A Corvette in a spin with far less load transfer per wheel than the other cars, makes it far more controllable and predictable. Now For those who wish to have a completely grounds up change in a the C7, do not appreciate or understand chassis dynamics. I bet the C7 is just a rebodied C6 and so it should be. Its one hell of a car and it is cheap and doesn't have a gas guzzler tax.

Flies Lo

You guys are right about weight distribution and it's relationship to handling...don't forget polar moment of inertia, though...

Two different cars can have close to 50/50 weight distribution, but with different polar moments of inertia...a high pmi car would have its mass concentrated close to both axles (like a Corvette, or a Porsche 944), and a low pmi car would have it's mass concentrated close to the center of gravity, like a (rear) mid-engined car (Boxster/Cayman)...

The lower polar moment of inertia car will tend to respond to turn-in faster, giving the perception (?) of "better" handling...

911's have most mass in the rear and as a result they act like pendulums...

I guess there are so many variables affecting handling that tuning a suspension can be like playing with a Rubik's cube.

Racer X

I would agree that the center of Mass does not move. I submit the effective center of gravity does move. If the center of gravity is the singularity where the weight is balanced 50/50 fore/aft, right/left top/bottom. As you brake the weight transfers forward, so therefore the fore/aft balance point has shifted, by definition the center of gravity has shifted dynamically. However, the mass is in the same place on the car, so the center of mass has not changed.

Maybe I am using the wrong definition of center of gravity.

The problem is that we often use weight and mass to mean the same thing and we shouldn't.

I do believe in terms of automotive dynamics it works as I described, perhaps I have misused terms. If so, I stand corrected.

MitchAlsup

Lower polar moment cars get out of control faster as well. The car responds faster to any imputs, from the driver or from the environment. This is one of the reasons Ferrari put the radiators back at the front in the 360/430 and away from the motor in the TestaRossa/438/F355. The later cars were 'faster' than most drivers could "use". Faster being in response steering inputs, and from external inputs (patch of wet road, patch of oil, big bump, high winds,...)

Shaka

We could get very **** about this and the winners do. It is generally accepted that amonst auto engineers, Cg is used rather than Cm. Other science disciplines may use both such as inter planetary rocket scientists. It is not meaningful to speak of center of gravity as being different to center of mass here on terra firma. The movement of the center of gravity is determined by Newton's gravitational laws applied to planets and stars and not to Corvettes. In other words, if the gearbox was the size of the moon and the engine was the size of earth, the Cg would be different to the Cm and it would shift depending on its location in the solar system at any given time. When people use the term "center of gravity" they are referring to a point in a body which moves as a point like particle. This is OK for a body in space but in chassis dynamics, there are other forces to consider.

If you generated a map table of peaks of mass of a Corvette, you would get the largest spikes or peaks at the engine and gearbox location. If you accelerate this mass in a high G turn for instance, the opposing force is generated by the tires. Any variation in this opposing force will cause an equivalent yaw force. This is called yaw angular inertia for lateral acceleration and pitch angular inertia for longitudinal acceleration and not 'Polar moment of inertia' as used by scribes. Engine designers get very picky with this distinction.