Shock absorbers - Rebound stiffer than Bump?

[April Dillon]

After looking through the car files in Race 07 and other rFactor derived games, I've spotted a trend I don't understand in the shocks. The rebound resistance rates are often much higher than the bump resistance rates. For example, on the DBR9 in GTR Evolution, the front slow bump goes up to 9500 n/m/s. While the slow rebound at this end only goes down as far as 12400 n/m/s.

Something similar occurs in FSONE2008 and the GT500 mod for GTR2.

Can anyone explain to me why this is? Specifically why you would want rebound to be stiffer than bump, and why the difference between them is what it is?

Thanks.

 

[Kennett Ylitalo]

Basically, the stiffer rebound limits the movement to reside more at the lower range. If it were the other way around, the car would be higher after every bumb and it's not desirable. This works at least in all sims this way.

 

[April Dillon]

I can see that, but when the arrangement is the way they are now, the car seems to end up lower after every bump, and so there's more risk of bottom-ing out. Isn't having them more equal desirable, so as to maintain the balance in ride height? Rather than risk going into extremes on either side of the scale?

 

[Kennett Ylitalo]

I'm no expert but i think so too, the ideal setup would be symmetrical. But if you have to allow the movement to be biased to one way, you most likely will choose the car to bottom out rather than the alternative.

 

[April Dillon]

Personally, I'm not sure. If the car bottoms out, that can unload the wheels, which I think is worse than a little more weight transfer than you'd ideally want. Oh well, I guess I'd rather just have the ability to tune the car either way, rather than be restricted like this by the setup options. Just to help me learn what kind of effect these things have.

 

[Rupe Wilson]

Have you had a look at "Race to play setup developer," it does not explain but tells you how to resolve a setup issues, EG.. left corner you have entry oversteer, a possible solution is to increase the bump on the front right, and decrease the rebound on the left rear... have a look maybe youl see a pattern....

 

[April Dillon]

I have, but that covers how the car feels, while I'm looking for a more comprehensive understanding of the physics.

 

[Marian Zelenka]

I always wondered too. I'm searching for real setup guides right now and found this document:

http://www.theoryinpracticeengineering.com/drift_mag/basic_damper.pdf

Maybe there is some answer to this question?

"Generally the rebound forces are greater than the compressive forces. This can be attributed to the idea
that the compression damping controls the unsprung mass while the rebound damping
controls the movement of the sprung mass. Because of the lower mass of the unsprung
mass, less force is required to control this motion. Another contributing factor is the fact
that during compression the forces of the spring and damper are in the same direction
while during rebound the spring and damper forces act in opposite directions."

"A shock very stiffly valved in rebound can cause an initial loss of
traction as well as contribute to a sort of jacking down effect where the unsprung mass is
pulled closer to the sprung mass after a series of bumps. This only occurs when there is
too much rebound versus compression and spring rate. This will also contribute to a loss
in traction once the shock bottoms out."

Interesting, still trying to figure out what does it mean.

 

[Rupe Wilson]

A shock absorber is basically an oil pump placed between the frame of the car and the wheels. The upper mount of the shock connects to the frame (i.e., the sprung weight), while the lower mount connects to the axle, near the wheel (i.e., the unsprung weight). In a twin-tube design, one of the most common types of shock absorbers, the upper mount is connected to a piston rod, which in turn is connected to a piston, which in turn sits in a tube filled with hydraulic fluid. The inner tube is known as the pressure tube, and the outer tube is known as the reserve tube. The reserve tube stores excess hydraulic fluid.When the car wheel encounters a bump in the road and causes the spring to coil and uncoil, the energy of the spring is transferred to the shock absorber through the upper mount, down through the piston rod and into the piston. Orifices perforate the piston and allow fluid to leak through as the piston moves up and down in the pressure tube. Because the orifices are relatively tiny, only a small amount of fluid, under great pressure, passes through. This slows down the piston, which in turn slows down the spring.

Shock absorbers work in two cycles -- the compression cycle and the extension cycle. The compression cycle occurs as the piston moves downward, compressing the hydraulic fluid in the chamber below the piston. The extension cycle occurs as the piston moves toward the top of the pressure tube, compressing the fluid in the chamber above the piston. A typical car or light truck will have more resistance during its extension cycle than its compression cycle. With that in mind, the compression cycle controls the motion of the vehicle's unsprung weight, while extension controls the heavier, sprung weight.
All modern shock absorbers are velocity-sensitive -- the faster the suspension moves, the more resistance the shock absorber provides. This enables shocks to adjust to road conditions and to control all of the unwanted motions that can occur in a moving vehicle, including bounce, sway, brake dive and acceleration squat.

 

[Dennis Phelan]

"With that in mind, the compression cycle controls the motion of the vehicle's unsprung weight, while extension controls the heavier, sprung weight."

Mind that well!

 

[Dennis Phelan]

Actually, in my mind I think fast for unsprung and slow for sprung.
Fast to keep tires on the road, slow to control body roll[weight transfer].

 

[Rupe Wilson]

http://www.race2play.com/forum/articles

 

[Günthar Rowe]

Well, there are two types on dampening too....

High speed and low speed.

High speed being the unsprung (wheel etc) mass movement up and down with little chassis movement like rumble strips and other impacts on the wheel.

Low speed being the sprung (chassis) mass movement up and down (things like quick body roll MoE in a chicane left to right)

 

[April Dillon]

Right, makes sense, so you can deal with bumps in the road and weight transfer seperately, rather than having to come up with a compromise for the two. But I always thought the speed at which the wheels moved up and down didn't matter as much as the force. After all, you get more grip by pressing the tire down harder into the ground. Not by hammering it down faster.

 

[Mark Guy]

Have you had a look at "Race to play setup developer," it does not explain but tells you how to resolve a setup issues, EG.. left corner you have entry oversteer, a possible solution is to increase the bump on the front right, and decrease the rebound on the left rear... have a look maybe youl see a pattern....

Sorry I just noticed the link now.

 

[Jim Cole]

Right, makes sense, so you can deal with bumps in the road and weight transfer seperately, rather than having to come up with a compromise for the two. But I always thought the speed at which the wheels moved up and down didn't matter as much as the force. After all, you get more grip by pressing the tire down harder into the ground. Not by hammering it down faster.

The speed does make a difference. The ideal is that your tires stay on the ground all the time. The reality is that on rumble strips, curbs etc your wheels may leave the ground for a short period of time. The faster your wheel can absorb the impact of a curb, the shorter the time that your wheel is not making full contact, and the faster your wheel can return to normal position.