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i don't anticipate that i'll get iron clad answers to this question, but i'd like to open it up for discussion, anyway.

what's the deal with rebound stiffness, meaning, what does the NUMBER represent? if i set my front and rear rebound at 10.5, what exactly does this mean?

lets say i have my front springs set to 200, and my rear springs set to 400, hypothetically speaking.

i'm going with the premise that a high rebound number (ie:20) means a slow rate of spring expansion, while a low rebound (ie:1) represents a rapid expansion of springs. (i'm just hoping that someone doesn't post in reply to this thread, and turn my world upside down, telling me i have it all backwards, like it did with the -/+ toe deal). but anyway..... 

if my FRONT SPRINGS are at 200, and my REAR SPRINGS are at 400, and i set my front rebound at 8 and my rear rebound at 8, are my rebound rates the same? or is the rebound rate relative to spring stiffness, meaning, would i have to set my REAR REBOUND TO 16, to achieve the same expansion rate on both ends of the car?

my guesstimate is 'YES' (but, hey, i ask because i've been wrong before)

 

Rebound damping controls the rate of extension as the
suspension rebounds away from the wheel wells. Adjusting
front rebound damping stiffness fi ne-tunes your car’s balance
going into and out of corners. Increasing front rebound damping
stiffness increases transitional understeer. Decreasing front
rebound damping stiffness increases transitional oversteer. Try
different damping stiffnesses in the front and rear to fi ne-tune
your transitional understeer/oversteer balance.

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Remember- Drive it, like you stole it.

An heres another answer I found

Damping — There are two different settings under damping—bump damping and rebound damping—and while both settings refer to different things, the end result of the adjustments are nearly identical for both.

In general, too much damping of the suspension will make the vehicle less stable on rough roads. Too much damping effectively keeps the suspension from working properly as it doesn't allow the springs to respond to the road. However, as with many other settings, damping can be used to counteract the effects of understeer and oversteer.

The results of altering the bump damping and the rebound damping are virtually identical. Increase the damping in the front and you'll increase a vehicle's tendency to understeer, while drecreasing the damping in the front increases the chance of oversteer. Conversely, increasing the damping in the rear springs increases the car's chance of oversteer, while decreasing the damping in the rear makes the car more likely to understeer.

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Remember- Drive it, like you stole it.

How does this stuff work in the real world? Don't you want a spring that slowly deviates from its starting position ( bump ) but quickly returns ( rebound ) ? I bought some gas-charged shocks a while back that would seem to push the springs back into position ( rather than just offer resistance to movement by varying degrees ) This would imply a negative rebound. I had always thought that high rebound settings allowed the springs to quickly return to where they were. Hopefully there is some kind of in-game telemetry that could settle this. This and diff settings have to be the most confusing parts of tuning a car, you don't really know what you are adjusting. The manual doesn't say and it is very hard using the telemetry to find out what effect your tuning is having.

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GT = P00hhead (two zero's )

Yes, the shock will "pull apart" at the same rate if both are set to 8. Rebound and compression rates are not relative to the stiffness of the springs, the only control how fast those springs react to the road surface and weight transfer.

BrodeurSux:

...if my FRONT SPRINGS are at 200, and my REAR SPRINGS are at 400, and i set my front rebound at 8 and my rear rebound at 8, are my rebound rates the same? or is the rebound rate relative to spring stiffness, meaning, would i have to set my REAR REBOUND TO 16, to achieve the same expansion rate on both ends of the car?

My tenative hypothesis is that setting the rebound rates the same will result in the same rebound speeds regardless of spring stiffness. Let us pretend that the rebound values represent a suspension extension rate of fraction/inches per second (Setting of 1.0 means 1 inch per second, Setting of 20.0 means 1/20 of an inch per second).

Set both front and back rebound to 8.0 in your scenario, and both ends of the suspension would extend (after initial compression) at a rate of 1/8 and inch per second.

Any effect the spring stiffness would have would be determing how much distance the damper must move in the first place.

Definitely not a cast-iron theory. More like a tin-foil hat.

It took me ages (for which read about a year!) to get my head around damping, and no doubt someone will correct me on this. I tend to leave damping adjustment until late on in the tuning cycle as I'm still wary of making big changes, but here's how I understand it:

Bump damping - controls the rate at which spring compression is counteracted. Generally I leave the front setting at half the rebound rate. I usually start at half the rebound rate on the rear too. In a RWD, if the car is a bit squirrelly out of corners, and I'm happy with the balance through corners and don't want to play around with the diff settings any more, I'll look to lower the bump setting by 0.1 (maybe 0.2). That will create less resistance to the spring compressing so allowing the rear wheels to bite a bit more. However, if you have a FWD car which is well balanced through the corner, but understeers heavily when you apply the throttle, you can moderate this understeer by raising the rear bump setting by 0.1 or 0.2 - the extra resistance slows down the compression of the rear springs reducing rearwards weight transfer.

Rebound damping - controls the rate at which spring extension is counteracted. Once I have this set I rarely play around with it.

The part which played havoc with my brain was working out how dampers worked. Conside a spring which compresses fully when you place a 200kg weight on it. If that spring is fitted with a bump damper, it will still compress fully under that 200kg weight, but will take longer to reach the fully compressed state.

Likewise, if we now remove the weight, the spring will return to it's original length. Fitting a rebound damper will not prevent the srping returning to it's original length, but would be expected to slow that process. In both cases, a soft (low) setting would allow the spring length to change faster than a stiff (high) setting.

As to what the in game values mean, I have no idea. The only insight I can give is that as far as I can tell they are independant of spring values.

Hopefully this makes some sense. Even more hopefully, it's not wrong

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"My mistakes are not errors, just predictions immediately proved wrong." - Murray Walker

http://www.forza-tuning.net

GT: Spiny Anteater

The way to think about dampner settings is, the higher the number, the more RESISTANCE it provides against spring energy.  Everytime you hit a bump, the spring takes on energy.  Without resistance, it would take a while for that energy to disipate, assuming you never hit another bump.

Most people call them shock absorbers, which is the wrong term.  The springs are what absorb shocks, be it bumps, potholes, or FIA curbing.  The dampers are there to turn spring motion into a controlled release of energy.  The better you do that, the more balanced a car will be.  They have an added value in racing, in that you can use the dampers to control weight transfer.  Properly controlling the weight coming OFF the inside tires in a turn, or the rear tires in braking, or the front tires in acceleration (rebound dampning), and controlling the weight being transferred TO the outside tires in cornering, the front tires in braking, or the rear tires in acceleration (bump dampning) goes a long way to making a car fast.

Using the 200lbs/400lbs springs with 8/8 rebound dampning scenario you posted, without knowing any other variables, I'd almost have to say the back end of that car will be very tough to control on corner entry, especially if you like to trail brake, as it will let weight come off the rear wheels too quickly.

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SCCA Spec Miata
Spurs Astros Cowboys Aggies
Racecar spelled backwards is Racecar
Assist free sim racer

That's funny, I thought FM2 was about the racing...

Disclaimer: this post is just related to Forza, and may not be of any value in real world tuning - it's more of a practical application guide. It is also based on MY observations, and seem to work for me. It also contains much of the same information as earlier, but hopefully easier to understand.

To answer your original question, I don't see any relation between spring weight and damper values. The spring rates change automatically when you add weight to the car but the damping stays the same. I think dampers are more related to the amount of mass they need to handle, or weight distribution.

How the maths behind rebound works in this game is still a bit of a mystery to me. It doesn't seem to be fully compatible with RL - possibly due to the lack of consequences for bottoming out and the complete absence of wheel hop/lift.

- Low rebound can give a bouncy and uncontrolled ride, probably because the suspension doesn't "come to rest" when the spring is extended.
- High rebound seems to force the springs to extend faster, which can mean sudden loss of grip when initiating weight transfer.

- High rebound gives a tighter, more stable feel.
- Low rebound (to a point) can give more overall grip in transition phases.

The trick - I think - is to find the amount of rebound damping that makes the car begin to bounce and wobble when releasing throttle/brakes, and then increase the value slightly until you feel it's stable.

I've personally found that a 3:1 or 4:1 ratio of rebound:bump gives the most "positive" feel.

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GT: PzR Ske

i'm trying to figure out how Forza math relates to performance on the track. the stock settings of rebound are always mathmatically proportionate to the spring stiffness, and i don't really understand the connection. if you look at stock springs on any car, then look at the rebound, the rebound stiffness is always in proportion (within 1%) to the spring stiffness.

so last few days, i've been messing with rebound ratios (front to back).

to digress, i'm solely interested in lap times, so when i tune, i tune for specific performance on a turn by turn basis. bottom line is, if i can go through a three turn sequence without lifting off the throttle, i'll be faster than if i have to lift.

my first example is Sunset. coming out of turn 3, my goal is to go full throttle into turn 4, lift very slightly before turn 4, then nail the throttle all the way through turns 4 and 5, hugging the wall on the right side.

if i hit it right, i'll be at about 154 MPH before i have to break for the hairpin. this is where rebound comes into play for me.

bottom line is, as far as rebound relates to lap times, stiffer rebound allows you to hold your line, and maintain traction through high speed curves. in that instance at Sunset, if you can't go through that sequence full throttle without hitting the wall, raise your rebound. trust me on this.

my best example of how rebound affects speed is the hard left hander at Suzuka, after the long straight. i've watch replays from the LB, and the fastest drivers (S-Class) take that turn at around 148mph. i've tried taking that turn with a variety of different tunes, and different cars. with lower rebound, i've been at around 136mph, hanging onto my line by a thread. with higher rebound, i have a Zonda where i've taken that turn as high as 154mph, like the car is on rails.

a 10 mph difference in speed on a single turn can translate into a loss or gain of .500 seconds, depending on how long the straight is after the turn.

i'm still experimenting with rebound rates, but my feeling at this point is that there really isn't any reason to stick with the Forza 'balance' set between spring and rebound stiffness.

right now, i'm setting my rebound the same, front and rear, and raising it until i can hold my line at high speeds on the turns where it's called for. so far, it's paying dividends.

i was running my Lambo LP640 at Sunset. orginally i had my front rebound lower than my rear, to match the variance in my spring stiffness. its AWD (understeers like an S.O.B.) and back heavy (mid-engine) so my rear springs are significantly stiffer than the front.

with the stiffer rear rebound, the back end was a little loose on turn exit, which i compensated for by reducing the rear acceleration on the differential. a good lap for me was mid 1:22's.

if rebound rates are unrelated to spring stiffness, then i'm at a loss as to what the benefit is of setting my rebound at different rates, or why Forza's stock set-up has set a relationship between the two.

so, i set both front and rear rebound the same, and set the bump at 25% of rebound, both the same.

i immediately feel the car is more stable, i'm holding turns easier at high speeds, and the back end feels tight. this enables me to increase my rear accel. on the diff. significantly.

after half a dozen laps, i set my personal best, and i find i'm now flirting with 1:21's on my lap times.

i'm going to keep exploring this rebound phenomenon, but as it stands, if my lap times continue to improve, i'm sticking with it. ; )

Once again, thanks again for the knowledgeable folks coming out with, well... the knowledge. It is appreciated and practiced by other lurkers here.

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Tango Tuners Catalog

TT Seller

Remember though, that too much rebound will not allow the springs to return to their 'normal' position quickly enough, and can create other problems, like really slow transitions through a sequence of turns like the ones you described at Sunset Infield.

Once again, it's all about compromise, a word that seems to crop up a lot when talking about set-ups.

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SCCA Spec Miata
Spurs Astros Cowboys Aggies
Racecar spelled backwards is Racecar
Assist free sim racer

That's funny, I thought FM2 was about the racing...

Eh, that's only really a concern with cars with significant body roll. With low CoG stuff with high roll stiffness, Forza 2 doesn't really provide significant enough penalties for bottoming out, nor model sufficient bumpiness in the tracks to keep you from running a serious tie down setup-take a look at some of the 333SP scoreboard cars. They practically run in the ground. You can just about get away with running a go-kart as long as you avoid kerbs, and there's probably some cars even in S that you could drop the CoG low enough it might work to just try and minimize weight transfer all together in fast corners. Looking at one of those 333s though, it actually seems to let the aero hold the car down against really soft suspension instead of rebound, and let the weight transfer happen in low-speed corners where the loads are inherently lower. There's also a stupid-stiff rear bar and high rear rebound in there to make it skid/steer like a go-kart and let the downforce sort out the rest, mind you.

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carbsmith wrote the following post at 08-05-2008 3:26 PM:

Eh, that's only really a concern with cars with significant body roll. With low CoG stuff with high roll stiffness, Forza 2 doesn't really provide significant enough penalties for bottoming out,

that's my biggest beef with the physics in this game. there's no negative consequences, (Ie: loss of traction from bottoming out, and/or suspension wear). i routinely set my front ride height as low as it will go, regardless of how soft i run my front springs. this also allows me, especially with my AWD cars, to run a higher ride height in the back, (without having to go too high), and get a little bit of rake to reduce the inherent understeer.

the lack of negative consequences also allow you to run your bump stiffness really, really low. the high rebound, low bump set-up is a mainstay in this game. the in-game suggestion is to run bump at 50% of rebound. personally, i don't know any fast racers who sets their bump that high. i run my bump anywhere from 20% to 33% of my rebound, and have found that higher rebound allows me to hold tighter lines in turns, and increases my high speed curve speeds. the low bump seems to give the benefit of a tighter turning radius on corner entry, and in steady-state, lift-coast, low speed turns (IE: the last turn at Laguna Seca, the hairpin (i think it's turn #6) at Sebring, and the hairpin (turn #6) at Sunset.

I've actually printed this thread and added it to my tuning bible. Now, if only they had a "printer friendly" view. I really like Brodeurs posts, well written and all, but I don't need six copies of him pre-smoke.

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Tango Tuners Catalog

TT Seller

Tabbed as well.  I'll have to try to read all the explanations when I'm not already drowsy. 

+1 on the Printer Friendly view.  For these types of threads, I just copy and paste into Word. 

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GT=Aenyn1
CST - U.S.A.
UMKC Med School Class of 2012

Thirig wrote the following post at 08-06-2008 10:48 AM:

I really like Brodeurs posts, well written and all, but I don't need six copies of him pre-smoke.

LOLOL !! - bro, you almost made me spit out the mouthful of coffee i was quietly sipping on.

 

Good, I was a bit worried that you would take it the wrong way. Sometimes it's hard for a normal guy to crack a joke on the boards without p......g in someone's coffee.

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Tango Tuners Catalog

TT Seller

Right, honestly I haven't read all of the posts in the thread, so if this has been covered before, I apologise! Also, if I am wrong about any of what follows feel free to correct me, but to my knowledge this is correct.

I just wanted to add my $0.02 to the discussion by briefly (and probably very vaguely) explaining what damping actually means. I don't know how much help this will actually be or indeed if it's already been posted, but I find it quite interesting and useful all the same.

First off, a key thing to understand is harmonic motion. A good example of this is a mass suspended on a spring. When the mass is moved vertically from it's resting position, it will continue to oscillate up and down. I'm sure you've all seen this before, so hopefully you know what I'm banging on about. In a perfect system, this up and down 'bouncing' motion would continue indefinitely. However in practice, you will notice that the amount that the mass moves away from it's resting position decreases over time. This change in amplitude is caused by air resistance on the object. Eventually, the motion stops completely. This is an example of damping in a physical system.

A cars suspension is similar to this system, albeit bigger, faster and a hell of a lot louder. When a car is driven and the suspension is compressed (either over an uneven surface or due to cornering), the body of the vehicle is set into an oscillation, much like the mass that is suspended on the string. The suspension in a car contains dampers, which are used to apply resistance to the harmonic motion of the car, much like the air resistance in the spring-mass system described earlier. The aim of damping in a car is to limit the amount of up and down oscillations to it's optimal level.

There are three distinct types of damping; under-damping, over-damping, or critical damping. In an under-damped system, the system is allowed to oscillate multiple times before coming to rest. Conversely, an over-damped system will take a long time to return to the resting position of the spring due to high resistances. The ideal form of damping is known as critical damping. This is where the system returns to equilibrium position perfectly and in the least time possible without any further oscillation occuring.

Applying this to actually tuning in Forza is where it gets shaky for me as I'm quite inexperienced on that front. In theory I would think that an under-damped car would perform poorly, as the suspension is allowed to bounce around after it is deflected. This will result in the body of the car becoming unstable for a long period of time after the suspension is compressed. An over-damped car will have a suspension that takes longer than necessary to move back to it's equilibrium. While I don't think it would have quite the same negative impact on performance as an under-damped car, a car that has too much damping will be less responsive than it could ideally be. When tuning, you are aiming to produce a close to critically damped system, so that the suspension returns to it's equilibrium position as fast as possible, while not continuing to move up and down. This should mean that the car is both optimally responsive and stable. Of course by altering the balance of damping between the front and rear of the car, its overall behaviour can be altered at will.

To relate somewhat back to the original question, a high damping number (such as 20) would presumably represent a spring that has a lot of resistance applied by the damper, meaning the spring would tend towards being over-damped and taking a long time to go back to its original position. On the other hand, a very low damper setting (such as 1) means a small resistance from the damper, and hence the spring tending towards under-damping and resulting in a car that will feel more skittish after cornering or over a rough surface.

I hope this wasn't all too irrelevant or poorly explained, but personally I find that looking at the way the physics of a real car works helps in understanding how to optimise tuning settings. Plus I'm a nerd so I find that all this is rather interesting!

[Edit: Just read more posts, what I've said has been covered by others. I've just added some geek stuff in there for good measure]