Springs and dampers

[Erunanethiel]

I understand that dampers are there to stop the springs from bouncing up and down contiously after a bump. But I also have read that they also make the rate of roll. I do not understand how this works. I mean I have never felt, for example while breaking, that my front tires have loaded, or my car dived earlier when it had stiffer dampers. It doesnt make sense to me either, how can it slow down the weight transfer? As soon as you turn left for example, just whe the car starts to turn, its weight instantly transfers to the right side tires, how could you possibly change how fast that happens. The only thing you could do i think is changing the steering angle more gradually and thats it

[Jersey Tom]

As soon as you turn left for example, just whe the car starts to turn, its weight instantly transfers to the right side tires

Not true.

It's a dynamic system - there's a time delay to change equilibrium states. No different than an undergrad engineering class demo'ing a spring/mass/damper system bobbing along. The inertial reaction happens instantly with body acceleration, but it can only be reacted at a rate proportional to spring rate and displacement, and damper rate and roll velocity. The unbalanced force then goes into the roll (etc.) acceleration of the sprung body.

To be fair though, in a case of textbook flat road handling... dampers at realistic rates will have a trivially small effect on your mechanical balance and handling. They will be dwarfed by the effect of main springs, anti-roll bars, track widths, and tire spring rates.

What can be easily lost though is that if you really crank the damping rate up (say well past critically damped in roll) you can speed up load transfer even though you're slowing the roll rate.

Where you will likely find more application of dampers is with vertical road content.. and trading off tire load variation and [aero] ride height variation. You may also find in some race series people running dampers outrageously stiff on one side of rebound/compression, as a crutch for dynamic ride heights.

[Erunanethiel]

As soon as you turn left for example, just whe the car starts to turn, its weight instantly transfers to the right side tires

Not true.

It's a dynamic system - there's a time delay to change equilibrium states. No different than an undergrad engineering class demo'ing a spring/mass/damper system bobbing along. The inertial reaction happens instantly with body acceleration, but it can only be reacted at a rate proportional to spring rate and displacement, and damper rate and roll velocity. The unbalanced force then goes into the roll (etc.) acceleration of the sprung body.

To be fair though, in a case of textbook flat road handling... dampers at realistic rates will have a trivially small effect on your mechanical balance and handling. They will be dwarfed by the effect of main springs, anti-roll bars, track widths, and tire spring rates.

What can be easily lost though is that if you really crank the damping rate up (say well past critically damped in roll) you can speed up load transfer even though you're slowing the roll rate.

Where you will likely find more application of dampers is with vertical road content.. and trading off tire load variation and [aero] ride height variation. You may also find in some race series people running dampers outrageously stiff on one side of rebound/compression, as a crutch for dynamic ride heights.

Can you please explain the first part a bit more? Like walking me throu what happens?

[Jersey Tom]

Well, where would you like to start? The car reacting to roll is really analogous to going over a bump. You have some inertia or "rotational weight." You have an equivalent spring (the roll stiffness). And you have some roll damping.

So if you abruptly pitch a car into a corner, the load transfer can't happen instantly. The only things pushing down on the tires (other than their own weight) are the forces coming from the springs and dampers. Takes time to create the displacement in the springs and the velocity in the dampers.

[Erunanethiel]

Well, where would you like to start? The car reacting to roll is really analogous to going over a bump. You have some inertia or "rotational weight." You have an equivalent spring (the roll stiffness). And you have some roll damping.

So if you abruptly pitch a car into a corner, the load transfer can't happen instantly. The only things pushing down on the tires (other than their own weight) are the forces coming from the springs and dampers. Takes time to create the displacement in the springs and the velocity in the dampers.

So what happens is, when you turn left, the car rolls to the right, but the weight is not transferred to the right side tires yet, the dampers first react to it and then when dampers reacted to the velocity then springs have absorbed the impact, then it goes to the tires?? So the weight transfer of the body only goes to the tires only after the springs and dampers dealed with them, right?

[Greg Locock]

Some of the weight transfer is via the roll centre height, but yes, that's the basic idea. In order of time the RCH happens the quickest, then the springs, then the shocks.

[Jersey Tom]

So what happens is, when you turn left, the car rolls to the right, but the weight is not transferred to the right side tires yet

The car doesn't roll instantaneously, is what I'm getting at. Doesn't take very long - even with a very abrupt steering input it may only take a fraction of a second to transfer the load - but it's not instant. And yes, for a very slow, gentle corner the "dynamics" are minimal.

And it's not to say that reducing roll (with a stiff setup) will reduce load transfer.. total amount of steady state load transfer stays the same. But the speed at which it happens is a function of the roll inertia of the car, the roll stiffness, and the roll damping. (And yes, any geometric LT effect).

[Erunanethiel]

Some of the weight transfer is via the roll centre height, but yes, that's the basic idea. In order of time the RCH happens the quickest, then the springs, then the shocks.

Doesnt the roll center height determine the aount of body roll rather than how fast the body roll happens?

[Greg Locock]

no it does both

[Erunanethiel]

So what happens is, when you turn left, the car rolls to the right, but the weight is not transferred to the right side tires yet

The car doesn't roll instantaneously, is what I'm getting at. Doesn't take very long - even with a very abrupt steering input it may only take a fraction of a second to transfer the load - but it's not instant. And yes, for a very slow, gentle corner the "dynamics" are minimal.

And it's not to say that reducing roll (with a stiff setup) will reduce load transfer.. total amount of steady state load transfer stays the same. But the speed at which it happens is a function of the roll inertia of the car, the roll stiffness, and the roll damping. (And yes, any geometric LT effect).

What I understood is, the load transfer to the outside tires due to the body roll cant be instant due to existence of the spring and the dampers. But I cant understand why body roll cant be instantenous with the car turning.

[Jersey Tom]

What I understood is, the load transfer to the outside tires due to the body roll cant be instant due to existence of the spring and the dampers. But I cant understand why body roll cant be instantenous with the car turning.

Well if the car rolled from 0 to 2 degrees instantly then your load transfer would have been instant. Why can't the car roll instantly? In a word - inertia.

Imagine your car runs out of gas and you have to get out and push it a few feet off the road. You get behind it, put your hands on the rear of the car, and push. What happens? Does the car instantly jump 5 feet forward? If not, why? What's the step-by-step of what happens?

[Erunanethiel]

What I understood is, the load transfer to the outside tires due to the body roll cant be instant due to existence of the spring and the dampers. But I cant understand why body roll cant be instantenous with the car turning.

Well if the car rolled from 0 to 2 degrees instantly then your load transfer would have been instant. Why can't the car roll instantly? In a word - inertia.

Imagine your car runs out of gas and you have to get out and push it a few feet off the road. You get behind it, put your hands on the rear of the car, and push. What happens? Does the car instantly jump 5 feet forward? If not, why? What's the step-by-step of what happens?

I undertand that you have to overcome the inertia to the change anything. But wouldnt the inertia be the main determent between you turning the steering wheel and and the car acutally turning, rather than car started turning and body starting rolling?

[olefud]

Things have to happen. When the steering wheel is turned the tire points in a new direction but first –and progressively- has to assume a slip angle by distorting the tire contact patch. Then the car’s polar moment of inertia pushes back as the car rotates about itself to point in a new direction. As the car moves through the curve path centrifugal force acts on its CoG to develop a force couple between the CoG and vehicle roll center that causes the car to roll against opposing forces from the dampers, springs and antiroll device. All this monkey motion (roll and yaw) internal to the system sops up energy and movement and delays the actual steady state turning dv/dt of the vehicle proper.

[Jersey Tom]

I undertand that you have to overcome the inertia to the change anything. But wouldnt the inertia be the main determent between you turning the steering wheel and and the car acutally turning, rather than car started turning and body starting rolling?

Well there's inertia in yaw and there's inertia and roll. So bottom line, roll can't be instantaneous, and neither can load transfer.

[PlatinumZealot]

Well, where would you like to start? The car reacting to roll is really analogous to going over a bump. You have some inertia or "rotational weight." You have an equivalent spring (the roll stiffness). And you have some roll damping.

So if you abruptly pitch a car into a corner, the load transfer can't happen instantly. The only things pushing down on the tires (other than their own weight) are the forces coming from the springs and dampers. Takes time to create the displacement in the springs and the velocity in the dampers.

So what happens is, when you turn left, the car rolls to the right, but the weight is not transferred to the right side tires yet, the dampers first react to it and then when dampers reacted to the velocity then springs have absorbed the impact, then it goes to the tires?? So the weight transfer of the body only goes to the tires only after the springs and dampers dealed with them, right?

It think he is saying the load has to come through the spring and dampers first before it is felt by the chassis. The resulting force that the chassis feels will be damped by the shocks and springs so it won't move instantly and directly with the source of the load.