Damper rate graphs

[Sassman]

Im trying to calculate damper rates using Force vs Velocity graphs. Ive been able to calculate the slope of the graph for low speed damping and high speed damping, however im unable to figure out where the slope changes from low speed damping to high speed damping. Ive read some textbooks and technical papers and ive gathered that the point where this happens correspond to the point on a transmissability vs frequency graph converges for all damping ratios but i can figure out how to calculate where it is on the F V graph. (Im stuck at the same point in pitch, roll and ride)

[Claude Rouelle]

Good question. In a very short way the ratio of bump and rebound damping as well as the speed at which the low speed - high speed slope changes (it doesn't need to be the same speed for bump and rebound) is mainly depending on your track profile. Try to build a simple 1/4 car model simulation in the frequency domain and then in the time domain with a simple road profile step for example input step, sine wave, sine sweep, ramp up, ramp up and down, road spectrum (there are good info on the internet about graph of road excitation ^2/ frequency (m^2. Hz) Vs road Frequency). Try to display the road profile, spring and damper force, sprung and unsprung mass movement, velocity, acceleration in m/sec^2 and in RMS, damper force, velocity and acceleration, tire dynamic force. You will have to decide if your criteria is tire grip (tire contact patch load consistency) or suspended mass ride/ comfort or any compromise between the 2. There will still be need of fine tuning on the track with a good, sensitive driver but your damper ideal profile found from your simulation will not be far way from you needs. Next step would be a complete car model with 4 wheels, ARB, inertia etc... but you will know 80% of what you need is a 1/4 car model.

Sassman.... Who are you?

[dynatune]

Sassman,

you should go step by step on your learning curve but beware, the matter is far from simple because the damper is quite a non-linear piece of equipment (as you can see on any force velocity graph) which makes also post-processing not so simple (or even correct). If I may suggest have a look at our dynatune tool that comes already in the base version with elementary damping graphs and partially linearized transient responses in the time and frequency domain. Beyond that proven F1 and WRC post-processing algorithms calculate various metrics that would help you judge the quality of your spring/damper setup. Using the tool will allow you to enlarge your understanding and may (or may not) make you decide to go to more complex software.

Cheers,
dynatune, www.dynatune-xl.com

[BillCobb]

Are you saying you have data from a shock dyno and now want to reduce it to a few parameters (low speed, high speed, break point and for jounce and rebound?

That's just a constrained optimization problem easily solved with Matlab or Excel tools. Presume linear states at first and decide whether to go more complicated.

Or, are you trying to determine what the rates and break points ought to be??

[Flight909]

Sassman,
If I may suggest have a look at our dynatune tool that comes already in the base version with elementary damping graphs and partially linearized transient responses in the time and frequency domain. Beyond that proven F1 and WRC post-processing algorithms calculate various metrics that would help you judge the quality of your spring/damper setup. Using the tool will allow you to enlarge your understanding and may (or may not) make you decide to go to more complex software.


Can you give some examples for this metrics you mentioned? I don't understand how the metric being proven for a F1 (very important to chassis stable because of aero) and WRC (40 meter jump on gravel roads) will work on a Formula Student car?

[ChassisSim]

Sassman,

My friend I think you should find the following very helpful,

http://www.chassissim.com/blog/chassissim-news/the-damper-workbook

It will walk you through your ABC of calculating and interpreting damper rates.

Enjoy

Danny Nowlan
Director
ChassisSim Technologies

[dynatune]

There are some "elementary" metrics that define the ride quality of any car. The standard percentage critical damping is probably the most famous one and works excellent from F1 car to 40t trucks. Beyond that one can investigate dynamically "single" events with a so called linear ride step transfer function that suits itself for investigating those type events and the frequency ride transfer function that is typically applied on 6 / 7 poster rigs. These transient functions come with metrics that have proven themselves over the last 20 years and have for each type of car typical values. I have attached a picture of the interesting part of dynatune and you can see the various graphs with metrics.
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I hope the picture comes in a visible format, if not there is a downloadable pdf file on the website if you are interested.

Cheers,

dynatune, www.dynatune-xl.com