[ZDan]

Originally Posted by Maxim

Tuning to understeer less without major changes like camber adjustment, tire size/compound changes, or weight distribution = tuning the rear to let go easier = reducing rear grip in the name of "balance".

If you've got steady-state understeer in a slightly front-heavy car, you will increase total grip by tuning in relatively more rear roll stiffness. You were overloading the outside front, retuning for less understeer alleviates outside front load and increases grip/load up front by allowing the inside front and outside rear to do more cornering work.

If you had been overloading the front, you will gain more front grip than you lose at the rear. To a point, but as long as we're talking about reducing a terminal understeer situation and not inreasing a terminal oversteer situation, this will hold true.

I've experienced this plenty. WRX guys are always trying to "fix" their understeer-prone front heavy cars. They tighten up the rear sway bar, they get more oversteer....but they haven't changed the amount of front grip, which means the only thing it accomplished is the rear of the car is letting go sooner.

You absolutely do increase the front grip by adding rear roll stiffness. You are taking load off of the overloaded outside front, giving it more grip per unit load, and adding load to the inside front, which at its low load level is still in the linear range of the grip/load curve. With more rear roll stiffness, under steady state cornering, you are adding total grip to the front and taking it away in the back.

Toyota went with softer springs and higher damping rates, which is going to result in more weight transfer,

In steady state cornering at the same speed, left to right weight transfer is a function of c.g. height and track width. So whatever the spring/damping rates are, at a given constant cornering speed, weight transfer will be identical.

Subaru went with tighter suspenders and less damping, and that will result in more even distribution of weight under load.

No it won't. Steady state cornering, weight transfer = cornering gs * c.g. height divided by track width, period. The stiffer-sprung car will NOT have more even left/right distribution.

At that point, the natural balance of the car will come out more....and with a slight front bias, this car will naturally have a little steady-state understeer.

(sigh) Again, this doesn't have to be the case. Tune roll stiffnesses to give more even load between the outside front and outside rear and you will reduce understeer and add to overall grip.

I would bet that the scion/toyota version will post ever-so-slightly lower skidpad numbers, reflecting a small reduction in rear grip.

Realistically, the differences are likely to be too small to measure, but again the advantage in overall grip will be to the car that doesn't overload the outside front as much, which (sounds like) should be the Toyota.

But yeah as you said the differences are likely to be extremely minor and almost a non-issue for most driving. .01 or .02 on the skidpad is not going to be noticable to most people.

Any difference is most likely to be smaller than car to car variations.


Consider a 2900 lb. 53/47 car, 18" high c.g., 65" track, steady-state cornering in a right-hander at 1g. With roll stiffness also at 53/47, you'll have:
1194 lb. LF, 343 lb. RF
1059 lb. LR, 304 lb. RR

You could add rear roll stiffness/take away front roll stiffness to get:
1126.5 lb. LF, 410.5 lb. RF
1126.5 lb. LR, 236.5 lb. RR

This would add MORE grip up front than you take away in the rear, due to the nonlinear nature of tire grip vs. load. In the 1st case, the outside front is making less grip per unit load than it or the outside rear are in case #2. The inside tires are so lightly loaded that they're in the linear region of grip/load.
Case 2 = more total grip and less understeer.

In reality, I bet that the roll stiffnesses on both cars actually bias more load onto the outside front, as this is normal for most cars. If the reports are true, the Toyota would do this to a lesser degree. Again, more total grip vs. the BRZ's greater overloading of the outside front.

But again these differences are going to be very minor and very subtle.