About dampers in bump vs rebound

My experience tells me to screw them in the opposite direction to whatever I'd think to be useful

 

They're not quite that free - they do represent a constant change per step (like most other setup things). So you're going from 100N/m/s to 120N/m/s to 140N/m/s or whatever spacing it happens to be, not just completely random numbers.

One thing that means is even if you had realistic range of rates (based on min/max of an irl shock absorber) the in between 'clicks' don't necessarily match because the real one's subject to a lot more complicated interactions than the ingame one, so you can't just translate an irl setup sheet that says "10 clicks above minimum"

 

When braking the front bumpers (and rear rebounds) can be used to control car pitch.
The reason for making some adjustments with braking in mind is dependant of brake bias, therefore, for instance, if someone prefer a more front bias when braking may use a stiffer front bumper (and stiffer rear rebound) to try to limit the possibility of some understeer due to having the turn in to much front dependant (rear will have a lesser effect in turn in, due to a weaker rear brake pressure).
The physics aspect is that when braking the front axle normal force during deceleration will increase and rear axle normal force will decrease.
As such, during braking the front axle will need to dissipate more energy than the rear and that energy is accumulated in the front spring. A stiffer bumper will allow the spring to swallow that energy faster (a stiffer bumper will make the spring start working sooner) and so less energy is required to be absorbed by the tyre. As the tyre is less stressed it can easier stay within the optimum grip levels and thus is more manageable with less tendency to understeer due to overload.

 

Ok, so if they really mean that a soft bump/compression front damping causes a "too rapid" loading of the front wheels then I'd say that is wrong. The stuff on the rear rebound in post #115 is in line with my claims, though - too little rebound damping and the spring extends a lot ("rear pops up"), too much damping and the spring doesn't extend much but the tyre gets unloaded, quickly.

Are you sure the source you quote is trustworthy?!? I did electronics in a Formula Student team and I wouldn't trust half of the documents that were available. The terminology used in your quotes isn't consistent and not clear enough for a professional release.

 

It is like jumping on a bed, to hard and you will hurt yourself on the mattress, to soft and you will hurt yourself on the floor .

Ok, thanks for clarifying!




Damping......

 

They are clearly referring to spring compression. Otherwise it would not have sense.
When a car undergoes a deceleration, both front suspensions must extert a total force Ftot.
If we keep aside for simplicity tyre flexibility and damping, this total force is given by:
Ftot = Fs + Fd
Fs = k*u
Fd = c*v
where
k spring stiffness
u suspension travel
c damping coefficient
v suspension velocity
There is always a tradeoff between the two contributions, too little front spring compression is caused by too stiffer front bump dampers, and vice versa.
Consider also that this balance between Fs and Fd varies during the deceleration period, and also Ftot varies.
Fd is velocity dependent, so is bigger in the very first instants of braking, while Fs is still increasing because it requires a suspension displacement to occur.
As the braking phase goes on the Fd contribution reduces and the Fs contribution increases.

 

No, I'm not sure (the link was posted on page 5 of this thread, so you can look it up yourself), but there's another setup guide (for iRacing) that I saw which also says that stiffer damper compression on the front "slows down the car’s frontward weight transfer upon initial brake application". And when I think about it I find it quite logical (at least for not over-damped case). Load transfer is not complete until the front suspension is not fully (for the given deceleration rate) compressed, but stiffer dampers slow down that compression movement, so in theory it means the whole compression process will take longer. Now you might say, and I will agree, that stiffer damper also adds some rigidity to the suspension which in turn make the load transfer to happen faster, so the question is which of the two would prevail.
I think it might depend on the damping ratio. See, if it is zero, then the front spring will compress faster (but not instantly) than compared to a normal damper case because nothing counters that (and let's not discuss what happens next because we're not really interested in the case of broken dampers). But if the damper is infinitely stiff then it won't let the spring compress at all and the load transfer is going to be instant. I think that damping ratio going up from zero will initially increase the load transfer time up to a certain point and then it will fall back down slowly approaching 0. The question is where the heck that maximum transfer time value is and whether we actually need too achieve it.

 

The question is what that suspension velocity depends on and whether it is also affected by the damping coefficient. I think it does, as the speed in case of c=0 (no damper) will clearly be greater than in case of some damping present because the damper will slow down the movement.

 

I see some terminology that is shown at school. What I understand so far in my opinion.

What's mentioned in the tread

There's tons of shortcut & acronym. A couple of formula left & right.

CG= Center of Gravity
Damper reduce frequency(or vibration)
Specs or factors of a car dictate damper is due to; the force(mass, aero, acceleration, braking, gravity), the center of gravity of the car & the road condition
Sprung mass=The mass of the body of the car
Unsprung mass=The mass of the wheel, brake, tires & suspension components
Force=Mass X Acceleration(downforce, gravity, velocity)
Tire grip more with vertical load(or vertical acceleration or force)
Spring is measured by hertz(you hit a curb, it emit frequency, the slower you hit that curb, the lower the frequency is needed to maintain stability)


In general term if I understand well.
Higher frequency do less suspension travel.
A higher sprung mass resist more to frequency<center of gravity>A lower sprung mass resist less to frequency
Lower frequency do more suspension travel
Higher downforce promote less suspension travel

If I do the setup, I also have to consider the steering, gas, brake input that also add lateral longitudinal acceleration & I have to consider the time it travel between each suspension with the general above.

 

That sounds based on the sorta unhelpful idea that because the car tips forward, the CoG has also moved forward. When really most of the load transfer is due to the force of deceleration. Like, the CoG might move a quarter centimetre forward (changing it from 45-55 to 45.2-54.8 weight distribution if the car tipped that way without decelerating) but at 1g deceleration the effective front load is 80% of the weight of the car, and that part happens in the time it takes the front suspension to switch from carrying 45% to 80%, which happens sooner with stiffer dampers. (but is reached eventually in any case)

 

Could be, but everything I've read on the topic says that the effects of the movement of actual CoG because of suspension movements are so small in a typical car (even in an SUV) that it can be safely ignored, so why would anyone be writing a setup guide and trying to use that?

 

I don't understand how that can be.
As you said Ftot = Fs + Fd, therefore if Fd decreases Fs must increase and if bumpers are stiffer they compress less and store less energy (the stiffer bumper will also take longer to take energy then a softer bumper, and if it's slower to take energy the remaining energy must go somewhere else and that would be the spring), so it must be the spring to compress more to store the remaining energy.

 

I think we got to a point where we need professional help
@Aristotelis ?

 

"Stiffer" dampers, or better, dampers with a greater damping coefficient, are able to sustain more load at early stages of braking (maximum suspension velocity) because of a greater damping coefficient.
More load provided by dampers require less force exterted from springs --> less suspension travel.
Simple?

 

Somebody will tell him he's wrong

 

Let's consider a theorical suspension system like this:
m = 300 kg
k = 200 N/mm
under a triangular force that starts at 15000 N and linearly decrease until 0 in 1 secs.
After solving with a computer program (that solve the dynamic equations with Newmark method and sample the response at 0,01 sec) you'll have the results that follows.

Case A) low damping (5%)
Peak suspension velocity = 1,72 m/s @ 0.06 s (Peak damping force = 1335 N)
Peak suspension compression = 13,1 cm @ 0.12 sec

Case B) medium damping (50%)
Peak suspension velocity = 1,03 m/s @ 0.05 s (Peak damping force = 7973 N)
Peak suspension compression = 8,1 cm @ 0.14 sec

Case C) high damping (85%)
Peak suspension velocity = 0,77 m/s @ 0.04 s (Peak damping force = 10112 N)
Peak suspension compression = 7,1 cm @ 0.19 sec

As you can see, reducing the damping coefficient, you have more suspension peak velocity, but eventually a much lower damping force.
As expected, greater damping coefficients produces less compression of the spring with a peak of compression that occur later.

 

Wait a minute. Isn't the peak suspension compression the point in time when the load transfer has finished? In this case softer dampers reach this point faster than stiffer ones, so load transfer takes less time to complete

 

I'm also not sure how you came up with 15000N force (that's 5 times more than the initial load on the tire) and how accurate is the triangular force model. It definitely does not apply for the kind of braking you do in Monza T1 at least not for the first second of that braking where the deceleration (and so the force causing the load transfer) is probably more or less constant.

 

Not that simple, at least not for me.
Are you considering braking as a high speed compression situation?
I don't think it is, it's quite the opposite. Braking should be seen as low speed compression situation therefore the bumpers should take longer to reach maximum compression and as such a stiffer bumper will make the spring react sooner. With a stiffer dumper the time required to start using the spring is shorter and as the force required to compress the spring is greater than for the bumper there will be less suspension travel.
If I understand you correctly you're describing a fast bumper actuation that does apply to the situation when the tyre pass over a kerb, for instance, but not during braking.
On a kerb I'll want the fast bumper to take the most of the force (it will act as a very soft damper) to not require the spring to take any load and thus having a more compliance car in those situations (kerbs = high speed compression situation).
This is my empiric notion of how dampers work and the effect I expect does match the changes I make in setup (simracing, of course).

 

No. At high damping the force is mostly going through the damper initially and being slowly released into the spring, compressing it. During all that happens, the tyre was already fully loaded. The body taking a while to get to front fully compressed is absolutely not an indication that the load took a while to get to the front.

See it that way...the body, under braking, is trying to reach the new equilibirum which is front fully compressed. If it's going towards that state slowly, it's because there is a force preventing it from doing that, that force ultimately is coming from the ground and going through the tyre, no matter if it's going through damper or spring. If the body is moving quickly towards equilibrium, it because there is little force preventing it from doing so, so little force through the tyre.

The whole damping can be seen that way, I'm moving from grip state A to grip state B, the damping that is in that path changes the speed of the grip transfer, do I want to get to the new state quicker or slower.