Oversteer Under Hard Braking

[Ubersuber]

Originally Posted by 7thgear

weight transfer as an absolute value and it's visual representation are two different things


putting stiffer springs in the front will still have you transfer the same amount of weight to the front... only it will happen much quicker, but without the dive..


the lack of dive is what keeps your brain happy, but the reality is that nothing's really changed from a weight perspective, just a transient response.. which gives you confidence so you feel faster.

Weight transfer is actually a relative value, relative to braking force. More braking force produces more weight transfer. For wet roads the braking force is such that weight transfer may be only a quarter of the transfer on dry roads (obvious if you think about it for a second or two).

The moment changes even though the lever arm length remains the same (height of CG above the ground, ignoring anti dive and anti squat forces).

Therefore, anything that makes the braking more effective will increase weight transfer. Stiffer front springs often improve braking forces because all four tires can contribute more, resulting in more weight transfer and less dive than for a softer front springs. The poster you replied to had the effect correct but the explanation backwards. Stiffer front springs tend to shorten braking distances, all other things being equal, but by doing so they increase weight transfer (actually inertia forces tending to compress the front springs by torque forces acting through the CG).

Better tires also increase weight transfer, provided the road surface can deliver the grip the tires can exploit. Better tires on a snow covered road aren't going to work (unless they are snow tires of course).

[learnerspermit]

Originally Posted by Ubersuber

Weight transfer is actually a relative value, relative to braking force. More braking force produces more weight transfer. For wet roads the braking force is such that weight transfer may be only a quarter of the transfer on dry roads (obvious if you think about it for a second or two).

The moment changes even though the lever arm length remains the same (height of CG above the ground, ignoring anti dive and anti squat forces).

Therefore, anything that makes the braking more effective will increase weight transfer. Stiffer front springs often improve braking forces because all four tires can contribute more, resulting in more weight transfer and less dive than for a softer front springs. The poster you replied to had the effect correct but the explanation backwards. Stiffer front springs tend to shorten braking distances, all other things being equal, but by doing so they increase weight transfer (actually inertia forces tending to compress the front springs by torque forces acting through the CG).

Better tires also increase weight transfer, provided the road surface can deliver the grip the tires can exploit. Better tires on a snow covered road aren't going to work (unless they are snow tires of course).

Excellent information there thank you. Also rarely do I hear the term moment and CG in this reference outside of my work.

Thank You

[solidONE]

Alignment would be the first thing to check. I wouldn't be surprised if the rear toe was off due to the toe link adjusters (eccentric bolt assembly) slipping.

[Ubersuber]

Originally Posted by solidONE

Alignment would be the first thing to check. I wouldn't be surprised if the rear toe was off due to the toe link adjusters (eccentric bolt assembly) slipping.

Plus I'm pretty sure these assemblies are all put together on the subframe and then bolted up under the car. The chances that the factory alignment is within spec, much less ideal, are pretty small.

The front is McPherson strut so it's going to be pretty good slapped together at the factory. The rear is more sophisticated and, ironically, precise alignment at the rear has much more effect on handling than the front end does.

This car handles so well that any mis-alignment is going to be very noticeable.

[CSG Mike]

What kind of brake setup do you have on your S2000?

Perhaps you're just not used to having lower grip tires?

[NissanGuy]

The issue you're having is the inverse of the issue I'm having: I want the rear to slip under braking, but the front does instead.

Based on my experience with various setups on my Altima, spring stiffness profoundly impacts weight transfer. With the stock stuff, it oversteered all over the place under braking, and the stiffer suspension I installed, the less corner-entry oversteer I got, even though steady-state cornering balance didn't change much.

The main (only, as far as I know) mechanical difference between FRS and BRZ is the springs. FRS front springs are roughly 2/3 as stiff as BRZ front springs, and FRS rear springs are roughly 10% stiffer. That translates to the FRS being a much softer, much more oversteering car. Having driven both, the BRZ feels much more planted (less roll), much less oversteery, and way less comfortable than FRS.

I recommend you get BRZ front springs. And then give me your old ones haha. Along with, naturally, an alignment (might as well do front camber bolts and rear camber-adjust control arms too). If your car came off the same assembly line as mine, I can guarantee that your alignment is way off: One of my wheels had positive camber, another one had 2 degrees negative.

[totopo]

Originally Posted by Ubersuber

Weight transfer is actually a relative value, relative to braking force. More braking force produces more weight transfer. For wet roads the braking force is such that weight transfer may be only a quarter of the transfer on dry roads (obvious if you think about it for a second or two).

The moment changes even though the lever arm length remains the same (height of CG above the ground, ignoring anti dive and anti squat forces).

Therefore, anything that makes the braking more effective will increase weight transfer. Stiffer front springs often improve braking forces because all four tires can contribute more, resulting in more weight transfer and less dive than for a softer front springs. The poster you replied to had the effect correct but the explanation backwards. Stiffer front springs tend to shorten braking distances, all other things being equal, but by doing so they increase weight transfer (actually inertia forces tending to compress the front springs by torque forces acting through the CG).

Better tires also increase weight transfer, provided the road surface can deliver the grip the tires can exploit. Better tires on a snow covered road aren't going to work (unless they are snow tires of course).

given same tires, more weight transfer equals less maximum braking power. usually braking is tire limited.

[Roadcone]

tl;dr

if the car is oversteering under braking you're trail braking. I've had to train myself to no to do this since my last car was a staggered wheel setup and i HAD to.

[learnerspermit]

Originally Posted by CSG Mike

What kind of brake setup do you have on your S2000?

Perhaps you're just not used to having lower grip tires?

Hawk Hp plus, 4 piston Spoon Calipers, steel braided lines, and Motul fluid

They just checked my Alignment this morning

[Ubersuber]

Originally Posted by totopo

given same tires, more weight transfer equals less maximum braking power. usually braking is tire limited.

Braking should be tire limited, ideally, because brakes only stop the wheels, tires stop the car.

Pneumatic tires are unusual friction devices and empirically we know that, up to a point, higher tire load develops more grip. This is why heavy cars brake as effectively as light cars and corner as well also.

Aerodynamic downforce exploits this characteristic by loading tires vertically but not adding any real weight.

Weight transfer is really a momentum effect. Imagine a car without springs at all (infinite spring rate). Braking that car would not result in any dive at all but weight transfer would still occur. Weight transfer isn't really weight transfer, it is rotation around the CG in a vertical direction and perpendicular to vehicle motion, just as roll is rotation around the roll axis and in a longitudinal direction. The forces are independent of the springing but the body movement is dependent on the springing.

Weight transfer is an acceleration force (torque) tending to rotate the mass around its CG. The extent to which it actually does rotate depends on the freedom of movement but that does not affect the size of the torque force.

Braking forces create weight transfer, not the other way around. More braking force causes more weight transfer just as more cornering force creates more roll force. Brake force causes dive and roll forces cause roll. Without spring travel there is no movement but the force will be the same, so when the car goes over on the bump stops roll stops but weight transfer continues to rise as cornering speeds increase.

Spring rates cannot increase maximum possible grip but they move the grip curve/tire loading. Stiffer roll cannot increase maximum grip but it will deliver higher grip more quickly AND importantly, make use of the inside tire grip for longer so that the car will turn in more readily and will reach maximum cornering speed more quickly. Stiffer front springs do the same for braking forces, allowing the rear tires to grip better for longer as they unload more slowly.

Weight transfer will always be the same for any given brake or cornering g but the rate of weight transfer will be different with stiffer springs, reaching a maximum when all suspension travel is absorbed. Which is why hitting the bump stops can cause you a moment of a different kind.

[sluflyer06]

Originally Posted by learnerspermit

Hawk Hp plus, 4 piston Spoon Calipers, steel braided lines, and Motul fluid

They just checked my Alignment this morning

So it looks like you're toe'd out in the back? That could be why. (unless I'm interpreting that incorrectly.

[Racecomp Engineering]

Originally Posted by learnerspermit

Hawk Hp plus, 4 piston Spoon Calipers, steel braided lines, and Motul fluid

They just checked my Alignment this morning

That's one of the worst alignment sheets I've seen Not the numbers...just in terms of how vague it is.

Anyway, it's hard to tell but it looks like you have a little toe out front and rear. I can't tell how much because that sheet sucks.

- Andy

[MyEvilMonkeys]

So it looks like they passed the alignment. But what is unclear is the toe left to right. The numbers are within factory spec, but I would not like the toe out on one side. If you are going to be off a little, it's best to be off equally on both sides... Was this a dealer alignment or an alignment specialty shop?

[learnerspermit]

Originally Posted by MyEvilMonkeys

So it looks like they passed the alignment. But what is unclear is the toe left to right. The numbers are within factory spec, but I would not like the toe out on one side. If you are going to be off a little, it's best to be off equally on both sides... Was this a dealer alignment or an alignment specialty shop?

That was from the Toyota dealer this morning. When they tested it in the service lane. Also I know the old trick to tap the paddle to make the screen show it's in spec.