First post over here (isn't that exciting). I was directed to this thread because I'm doing similar things with my speed6....
.... I almost have enough information to finish my spreadsheet for suspension frequency calculations. !
Sounds like another interesting build ! I've seen natural frequencies show up in the literature, and I can verify that it is an important concern, especially if you will street
the car. On my 2000 lb race prep'd Triumph, when I hit certain concrete highways where each "block" of concrete had a tiny pitch like a launching ramp (made by haulers), it was like riding a bucking bronko. But, In the 3 years I autox'd the car in the local non-SCCA competition, I won my class the 2nd year and bumped myself into a higher class and won that the 3rd year.
I saw your excel sheet on springs/suspension, and found this table:
Front Spring Selection
From above, we have everything needed to closely approximate suspension frequencies.
285: 1.30 Hz (stock)
400: 1.53 Hz
500: 1.71 Hz
600: 1.87 Hz
700: 2.03 Hz
800: 2.16 Hz
900: 2.29 Hz
, the frequency method started off with production cars, and then was applied to autox. But these target numbers change depending on the type of car ... rwd is different than awd (suggestions for AWD are in the link below), and they are often back calculated from what has worked in competition.
It is a simple up-down model, proportional to the sq-rt( K/M ). And when I back calculate, the weight used was some constant lbs, for each spring rate. For autox, I think the frequency used is for sprung-weight
, which is related to body motion, including brake dive, and corner roll. (edited 3-17-14)
I did find an excellent discussion in an engineer's forum about the subject, where they made a clear distinction between wheel modes and sprung body mods:
Greg and Brian are Suspension Engineers for a major auto co.
I read a bit more and you have target freq's of ~2.4 & 2.2 hz, consistent to what I have read for body motion in autox spring design. But I would assume the target freq's would be higher for a road course racer, like CR.
I made a M6 suspension spreadsheet about 8 years ago, based on Fred Puhn's book, but I did not include the nat freq calc.