About dampers in bump vs rebound

Thanks, Phil, this explanation makes sense. However, I decided to see how this works in AC and took BMW M235i Racing (as a car with no aero and adjustable dampers) to the Endless floor track.
My test was applying max brakes as quick as possible at 260 kmh. I tried 0 front bump and 0 rear rebound, then 12 front bump and 0 rear rebound and finally 12 front bump and 12 rear rebound.
If the Car Engineer app didn't lie the damping ratios were:
0.31 for 0 front bumps
0.63 for 12 front bumps
0.89 for 0 rear rebound
1.3 for 12 rear rebound

For each of all three test runs I recorded the video of the braking showing Telemetry app at 60 fps. Here're the videos: https://www.amazon.com/clouddrive/share/m1xaMJt7ynhKLK5qaIl6miANoZS5ehQqS1EtpMsbDlj

Then I loaded the files into the video editor to go frame by frame and see the max values for suspension travel, damper velocity and front tire load. To my big surprise the difference was minuscule for all three cases. In all tests it took 20 frames (so, 1/3 of a second) counting from the very beginning of the brake pedal travel to reach the max front load value on the telemetry screen and these max values of of the front left tire load were 679, 677, and 674:






Max damper velocity reported was 0.22, 0.21, 0.21. And you can see it on the screenshots that the shapes of all the lines are basically the same. Now this is something that I do not understand at all. How come changing damping ratio from 0.31 to 0.63 didn't change jack ****?

 

Quite possible I'm seeing this completely in the wrong way, and so I ask.
As a simplistic rule I soften the front springs to have more front grip and/or use an stiffer rear springs to make the car rotate more (the car is more willing to point to the corner).
After I find what it seems to be the sweat spot I look at the dampers. I see dampers as a way to tweak the spring setup, a fine tuning tool.

If overall I find that car's handling is good enough but there are some spots that still need improvement (a particularly corner or braking point) I use the dampers to try to make things better.

Again looking at it as a simplistic rule I soften the front dampers if that track point requires increased front grip and/or stiffen the rear dampers.

However there are some cases that the rule doesn't apply (must be the exceptions that confirm the rule, I guess).

For instance, if I notice that in a certain corner is better to brake late and inside the turn I would stiffen the front bumper and/or stiffen rear rebound to not having that corner spitting me out of track.

My reasoning is the following, and although the effect is what I expect, perhaps I'm seeing it wrongly.

If I'm on a soft front springs and a soft front bumper I consider that the front is being loaded to much due to the soft bumper, to much energy is being transferred to the front axle.
So, with a stiffer front bumper the energy put in the front axle will be more evenly distributed between bumpers and springs due to the following: a stiffer front bumper will slow the energy absorbed by the damper but as the energy in that particularly situation is the same as before the bumper adjustment (same speed before braking and identical brake force applied) the remaining energy has to be absorbed in the same time but now more by the spring.

If the bumper now is absorbing less energy because is taking longer to do it, it's the spring that is working sooner than before because it will have to react more in the same time frame.
Also, as the force required to compress the bumper is less than the force required to compress the spring, and the same amount of energy must be dealt with within the same time, that means the spring now will hold more energy but with a little less suspension travel than would happen with the softer bumper.

In a simplistic way I see it like this: if instead of a damper I'd use a stick with the spring, that stiff stick would absorb very little energy and that would mean the spring had to do all the required work, but if I'd use some sort of damper, like a tennis ball, together with the spring surely the ball will absorb more energy than the stick and, therefore, the spring would have to absorb less energy now. As the stick is much stiffer than the ball, the suspension travel will be solely dependant of the spring rate, but once both the ball and the spring will be taken in account and considering that the ball would be compressed sooner, faster and more than the spring, the suspension travel will be greater, although by small extent (just as much as the compression of the tennis ball) than when using the stick instead of the ball.

Does this reasoning correspond to what is truly happening regarding dampers and springs?

 

What's unclear about this?
MoTeC's licence does not permit the usage of this software for AC and therefore it's illegal to do so.
The only video games that I am aware of where you might use MoTeC are GTR2 and Gran Turismo 6.
(Maybe also GTR1, can't remember.)

 

From the installer:

LICENSE
MoTeC hereby gives you a non-exclusive license to use the software i2 Standard in conjunction with data generated by any MoTeC logging device, or that generated by MoTeC licensed 3rd party software. i2 Standard may not be used in conjunction with data generated either directly or indirectly by non MoTeC products unless that product has been licensed by MoTeC.

 

I think the issue is due because some user try too hard to use realistic value. I'm a scrub, so I'm more interested to get simulation value.

 

No, as PhilS and me have written before, the suspension load is given by 2 contributions Fd and Fs, each of the 2 reach their peak in different time instant. First peak is Fd, then follows Fs peak. It's the summation of the 2 forces Ftot = Fs+Fd that gives the total suspension force. The peak of Ftot is indeed in the middle of the two, more shifted to left (near Fd peak) or right (near Fs peak) depending on damping ratio: more damping makes the Ftot peak to be reached earlier (in practical terms, the suspension accomodates the weight transfer demand earlier), less damping makes the Ftot peak to be reached later (in practical terms, the suspension accomodates the weight transfer demand later).
For the same system of the example above under a triangular force of 5000 N that linearly reduces to 0 in 1 sec, this are the Fs, Fd, and Ftot graphs considering 45% and 85% damping ratio:

Note as the Ftot peak moves to the left as the damping ratio increases: i.e. with "stiffer" dampers less time is needed to achieve the maximum force response of the suspension.

It's a basic numeric example that I did in 5 mins, putting intentionally an higher force in order to make results cleaner and straightforward.
However, being a linear system, whatever is the input force, you'll obtain an absolute response scaled proportionally, but exactly the same in terms of relative values. You can check it by yourself looking at the graph above, for a triangular force of 5000 N the peak of Fd is 3370 N that is exactly
5000/150000 = 1/3
of the value Fd = 10112 N obtained before.
And last, also using an actual non linear variation of the suspension force during the deceleration period, I can demonstrate you that eventually there is a very similar response, though of course not identical, but the general concepts above are always valid.

 

No, I'm not describing only a fast bumper.
The damped system I've considered is a very simple one, with a linear viscous damper, i.e. the relation between force and velocity is linear:
Fd = c*v
whatever the velocity range, the force is given by multiplication of the damping coefficient.
Real life dampers are much more complicated, the force is related to the velocity with non linear relations that depend on the velocity range.
Here's an example of a Koni damper:

where you can see the low (A) and hi (B) velocity range and the different response in bump and rebound.
As you can see, the slope of the graphs that represents the coefficient of damping is not a constant value as in the linear damper of my example: for rebound you have a bilinear relation that means two values of the coefficient of damping, at low and hi suspension speed, like we have in Assetto Corsa.

 

What's with the attitude? I asked a simple question. It comes as a surprise to you that not all reads EULA for every single piece of software we install?

Thanks for the answer I guess.

 

Assuming there was a huge overshoot that you simply cutoff from the images then it looks normal to me. If there was no overshoot we have a problem.

Changing from 0.3 to 0.6 will make a difference hard to notice in terms of timing the inital compression trace on a video but it will be there in telemetry. Try something like the shelby cobra if you want to see more obvious behavior.

 

Yeah, I can see that initially (until 0.06s) Ftot is greater for the stiffer damper, but then the slower damper takes the lead, so suspension with the stiff damper reaches the max Ftot at 0.08s and the max is about 5600N, but with slow damper it would reach 6000N by the same time and then peak at ~6200N about 6200 0.02s later.

It would be nice to see how all this changes if the force remains constant because that's what happens for the first couple of seconds when you brake from high speed into a slow corner.

 

The videos have everything if you'd like to take a look. Not sure what you mean by overshoot, but right after the moment those screenshots were taken the suspension would go up and the load would go back down until about 510 (502 for rear rebound 12) and then it will go up again this time reaching 610 for 0 bump, 600 for 12 bump, and 607 for 12 rear rebound. So I'd say the initial suspension overshoot (by 1.6mm) is already on the screenshots because suspension travel value then (after all major undulations subdue) stabilizes at about 70.8mm.
Will do the same test with Cobra later.

 

No, your reasoning sounds like if the damper and the spring were connected sequentially (e.g. spring sits on top of the damper or the other way around), while this is not the case and they are connected and work in parallel.
Your example with the stick illustrates this incorrect thinking. If there's a stick instead of a damper then the spring won't absorb any energy at all and everything will go directly to the tire.

 

I suppose... It can be explained in another way.

In my opinion
The damper role is to reduce the frequency received by the spring by slowing down the compression as more force is added to it. Technically the spring is what contain the energy.

^^^
Correct me if I'm wrong

 

Here it is, a 5000 N force staying costant:

 

Very nice, thank you @Glaurung. This one shows the load being transferred faster in the beginning with a stiffer damper even better than the previous set.
I gotta say that I like the stiff graph much more, load overshoot is almost two times less (~850N vs ~1600N), nearly zero suspension overshoot and it settles down at 0.3s vs 0.5s.
Maybe that extra load with softer damper is what they actually meant. I think everyone can see how confusing this topic is and that's probably why so few people really understand it

 

The damper also contains some energy because the oil heats up while it is being pumped through its valves at high velocity. It then dissipates this energy into the air

 

I think I'll not try to go any further.

I though that a very stiff suspension (thus the analogy with a stick instead of a damper) will unload the tyres so much after a bump that very little energy will be put in the tyres and the contact patch with the road would be significantly reduced.
I imagine that such a suspension, lacking the damper, would make the axle in question to bounce tremendously, because the spring will absorb almost all the energy and would release it just after, sequentially, thus reducing grip and traction significantly. If in the front axle it would be quite difficult to turn the car.

I also imagine that the spring is in fact the first part of the suspension to absorb energy (and the most part of it) starting by keeping the weight of the car and then the rest of the effects of car movement (braking, cornering, acceleration) and the dampers will only absorb a small part of the energy as their main purpose is to control the spring movement.

In braking the front springs will load more than their natural state and the dampers will control how long they take to load and then unload. The braking process is relatively long, with a significantly amount of energy being supported by the front springs and therefore is a slow speed compression.

Passing over a kerb is a fast process, without that much energy being transferred to the spring, thus a high speed compression, which happens quickly and thus can be controlled by a a fast (softer) damper.

But as I said I'm probably seeing it wrong as this isn't not even close to my expertise. I'm just a mere curious with an empiric perception of things, nothing more.

 

There's an real life example on post 110#

Maybe he got me on his ignore list.

 

I think you're mixing up a fully rigid damper (stick in place of the damper) and a fully soft damper (blown struts). The completely rigid one will have no suspension travel and absorb any energy into the tires or chassis.

 

Damper role is to control absorption and release of energy by spring.
Frequency of those parts (unsprung mass) is whatewer it takes because it is dictated by surface.

On the other hand, frequency of sprung mass (chassis) is what you want to be as low as possible.