Well this is a complicated questions.
For spring rates, it's going to depend on the weight of the car. A heavier car will need more spring, while a lighter car can get away with less. And it also depends on the weight balance, the wheel and tire size, what sway bars you have available, what dampers you want to run, etc.
So without knowing any of that, it's really hard to say exactly what springs you would want to run.
But for autocross or drifting, you are going to want a stiffer setup. I've run a 1000lb/in front and 600lb/in rear setup with stock sways, a 3000lb car and 18x9.5+12 with 235/40/18 all around for drifting. And the car has felt awesome. It could be a little stiffer, but most of the sloppiness is because of a lot of old bushings, so once those are replaced, I'm sure it will feel stiff enough.
I based these rates on the weight balance of my brother's car and a sprung mass natural frequency target. The natural frequency is the frequency at which the car will oscillate assuming no damping. There have been published guidelines for what ranges seem to work for certain types of cars. This is just a guide and isn't the final word in what springs you should use, but it will get you in the ballpark. This will get you the pitch and heave frequencies of the car, how the car will move under braking/acceleration.
After you get started with those numbers, you would look at what kind of roll rates those springs would give you. You use these numbers along with track data, roll center location, cg height and you can get your lateral load transfer numbers and total lateral load transfer distribution. This distribution will give you a good idea of which end of the car will be overloaded first or which end will slide first. Usually, biasing the spring balance a little towards the front and understeer is the best way to do it, even for a drift car.
With these preliminary roll rates, you would work in the sway bar values and the additional roll rate generated by them. This will give you the systems balance and the load transfer distribution that you can excpet to see.
And then you iterate. Over and over and over. You need to watch wheel travel values to make sure the wheel won't be moving too far up or down. This will factor in greatly depending on the camber and bumpsteer curves and will help you get a good idea of what kind of base alignment setting you want to run.
It's also possible to ignore the frequency calculations and look more at the wheel travel numbers, although both are directly related.
So you need to look at a lot of different things in order to get an idea of what spring rates, you're going to want. And that's not even getting into what damping you're going to need to control all of this.
As for weight distribution, you will hear a lot of people saying that a 50/50 balance is the best. It's definitely not a bad balance, but it may not be the best. Again, this depends on a lot of things. Look at different cars, some cars are close to 50/50 others are close to 40/60 or 60/40 and they can all be made to handle well. An equal car will probably be easier to setup when everything else on the car is equal, but that's usually not the case. On the supra, the installation ratios of the springs are very different from front to rear, which will make your spring selection different even if the car were 50/50.
But how the weight balance factors into the handling is with the lateral load transfer distribution. So you need to know the balance and the roll center locations and center of gravity heights and track widths. And depending on what all of these things are, an uneven car can be made to handle very neutral, you could even make a front heavy car understeer biased with the right spring selection.
The biggest downfall of having a front heavy car is under braking. Under lateral acceleration, the weight gets transferred from side to side. So depending on the spring setup, there isn't much change in the front/rear balance of the car overall. But under braking, depending on cg height, a lot of the load gets transferred from the rear to the front. So depending on your suspension geometry, sometimes with the more load getting transferred, even if the tires aren't getting saturated, the braking force capacity of the front tires is dropping just due to camber gain. So a car with more static weight in the rear would allow the car to be more balanced under braking.
A car with more static weight in the rear, would also help the car to plant itself better under acceleration. For both the braking and the acceleration examples, I am talking straight line, no cornering, only longitudinal force. So more load on the rear wheels under acceleration would allow the tires to produce more traction force as long as they aren't getting overloaded.
But it's going to take a lot of work to make a supra have a rear weight bias and even then it's probably not going to happen.
I would almost say to not worry about the static balance of your car, but to worry more about the placement of weight inside the car. The more weight that is between the wheels, the quicker the car will yaw. This is that polar moment of inertia that everyone always talks about. It's basically the moment around the vertical axis of the car. So more weight in the center of the car will take less to get it rotating. This yaw of the car will help it turn and you won't have to put as much into the car to get it change direction as quickly. All the heavy stuff hanging out in the bumper area has more negative effects on the handling of the car than if all that weight were right on the middle of the front axle, axle meaning centerline of the two wheels.
I know that most of that stuff has to be there, but I'm sure there are some things that can be moved to help balance out the car a little or to cut the weight down from the very front of the car. Moving the battery is one of the easiest and most beneficial things to do for this.
But after all that is said and done, when corner balancing the car, just make sure that you're diagonals are the same. This will help make sure that the car will act the same going left as it will going right. It is sometimes hard to get the front wheels to both weigh the same and the back wheels to both weigh the same, so sometimes some comrpomises have to be made. The closer to equal that everything can be the better, even if the front to rear ratio isn't 50/50.
Well I hope that at least gave you a better idea of how it works. Sorry I can't give you the exact answers, but I don't have anywhere near enough information to feel safe even making a suggestion. Although, the 1000/600 setup would probably work out just fine, but there is probably something better unless your car is exactly the same as my brother's.
Tim
