[Maxim]

If stiffer suspension didn't affect weight transfer, we'd all be riding on pillows.

A stiffer suspension, or thicker roll bars, absolutely will affect how much weight is transfered from side to side during cornering. The whole body of the car moves over the unsprung portion of the vehicle. The total weights being transferred are of course the same, but the rate at which they are transferred is going to be different....which is going to affect how much load each tire is bearing, because it affects how the suspension "sets up."

Squishier suspenders = more side-to-side shift in weight distribution during cornering = less even load on each tire = reduced grip.

This is the exact same reason that it is easier to launch more softly sprung (rwd) cars. Upon launch, the softer springs allow more weight transfer to the rear, which increases rear grip. Conversely, stiffly sprung FWD cars are easier to launch, since less weight transfers away from the front tires...

Just to check on my reasoning, I loaded up Forza 4 last night and did some experimentation. I used a car with 53/47 weight distribution (RWD converted last-gen celica). Set up on as close to stock settings as I could get with an adjustable suspension, keeping camber and toe the same, a small reduction in spring rate accompanied by a small increase in damping rate...lowered the lateral grip figure by .01 at 60mph (no change at 120) and resulted in a slightly looser tail, especially during trail braking or throttle lift, because the rear was moving around more. Meanwhile, increasing the suspension stiffness a bit while decreasing damping rate resulted in no change in lateral grip but a little bit of understeer.

Increasing rear roll stiffness increased oversteer (dramatically increased it over uneven surfaces) but did not change total grip, while increasing front roll stiffness increased understeer but resulted in much better steering response.

These are the predictions I made beforehand.....so....yeah. ?? I strongly suspect that I am using terminology differently than you two and it's resulting in some confusion. The stuff you just described as weight transfer, I would have called weight distribution....when I think weight transfer, I think of dynamic changes in suspension load and the weight of the car actually shifting around before the suspension takes a set. Is this the source of the confusion?

I'm trying to get at what happens DURING suspension movement (while lateral forces are changing), and from what I can tell you're addressing the balance of the car once the suspension has taken a set (once lateral forces are constant)

To put it another way, yes, I understand how increased rear stiffness will cause weight to be more evenly distributed at the front tires, theoretically increasing front grip, but the trade off for that is that at initial turn-in, one of the rear tires is going to be recieving a much higher "spike" of the lateral acceleration, since the total amount of energy being transfered is the same, but the time in which it is transfered is shorter due to increased suspension stiffness. With a higher spike in instant load to the outside rear tire, that tire is more likely to exceed the available amount of traction and thus begin to slip, and at that point the inside rear tire will not have enough traction to make up the difference, which is going to result in the entire rear beginning to slip, causing oversteer.

It sounds to me like this is exactly what is happening with the cars, per the reviews I've read. The Toyota has softer front settings than the Subaru (I've specifically read that), which means the rear is stiffer in comparison to the front than on the Subaru (which is going to increase oversteer, because the rear tires are subjected to higher instant-load during initial turn in), while the Subaru is a little stiffer overall, but harder in the front than the rear, which is going to lessen instant-load on the outside rear tire (and increase it on the outside front) which will cause understeer.

The car and driver review of the Subaru mentioned this, how the car displays a little bit of understeer at turn-in.





Again though, the differences are really really small....most drivers probably won't notice much difference between the cars, except maybe that the ride in the Subaru is just a little harsher due to the spring rates.