Designing Installation Ratios for Dampers

[Chris Lane]

Hi Everyone!

I have a few questions regarding the installation ratio design process, if anyone is willing to help me out.

I am nearing the end of the design stage for the suspension system, and I am a little confused about how one might 'design the I.R.' for a particular damper? RCVD doesn't really explain the process, so is there any other books or papers I should be reading?

[Chris Lane]

Hi Everyone!

I have a few questions regarding the installation ratio design process, if anyone is willing to help me out.

I am nearing the end of the design stage for the suspension system, and I am a little confused about how one might 'design the I.R.' for a particular damper? RCVD doesn't really explain the process, so is there any other books or papers I should be reading?

[Frank]

It is preferable to set the "installation ratio" to a large value; which is the same as saying "motion ratio" to a small value.

This gives the minimum damping rate (2 N.s/mm is typical for the small shocks I've played with) that your shock can be "tuned to", decreasing bad hysterises etc, but check that you won't run out of shock / spring travel http://fsae.com/groupee_common/emoticons/icon_frown.gif


My advice:
Write a quick dirty spreadsheet with installation ration as an INDEPENDANT variable.... for example:

independent variable (units) [ballpark numbers, for a fairly stiff fsae car]
mass of car with driver (kg) [290]
weight split (%) [50:50]
NSM front (kg) [24]
NSM rear (kg) [22]
Front Undamped natural frequency (Hz) [2.641]
Rear Undamped natural frequency (Hz) [3.530]
Front damping ratio (non-dim.) [0.7]
Rear damping ratio (non-dim.) [0.7]
Front installation ratio (non-dim.) [0.690]
Rear installation ratio (non-dim.) [0.800]


and it should output the dependant variables:
Front Static Spring compression, assuming no preload. (mm)[24.6]
Rear Static Spring compression, assuming no preload. (mm) [15.9]
Front Spring Rate (N/mm) [35.0]
Rear Spring Rate (N/mm) [47.3]
Front Damping Rate (N/mm) [2.96]
Rear Damping Rate (N/mm [2.98]

Now, vary the installation ratio and notice what it does..


Hope this helps

[Chris Lane]

Thanks Frank, this helps a lot. I just have to ensure that the low extreme of the adjustment ranges of the damper does not exceed these minimum calculated values.

Common sense I guess...

[murpia]

Frank's method is a good way to learn about it, but in my experience you also have to take into account your required wheel travel, your required rising rate (zero is a requirement too!) and your actual available damper travel.

Don't forget the rules specify a minimum wheel travel.

Regards, Ian

[flavorPacket]

Yes, just as they specify that the car is built for a weekend autocrosser, and should be reliable...