Ride natural frequency opinions?

[BluEyes]

Hey guys, I'm thinking about upping the springrate on my Camaro. Right now, I have 420#/in springs, which I've calculated gives me a natural frequency of roughly 1Hz. Everything I've read says that this is roughly like the ride in a modern sedan, not a sportscar, and I have no complaints about the ride being rough or harsh at all. It's actually smoother at some times than my LeBaron, probably because the LeBaron is bottoming.
So, I've been thinking of going stiffer to get the natural frequency up around 1.15-1.25Hz (500-600 lb/in) but have no idea what the ride would be like. Check out the sig, I don't have anything to compare this to!
I've heard that cars like the Miata and Audis have a natural frequency in this range, is this right? Is there anyone else who has calculated their ride rate and could tell me what they think of a ride up in this range?

[Ripper]

I'm currently using 1,6 Hz in front and 1,2 Hz in the rear (taken from memory... I need to look in my papers to see what spring rates they are and if it's exactly 1,6 or if it was 1,5...). The ride is Ok for the street (we have pretty bad roads in Sweden, but I don't think the ride is too harsch) but too soft for the track. The car is too soft and chassis reactions takes too long time and that's why I'm stepping up to about 2,4Hz instead up front. After the three link in the rear is made, I'll go with about 1,9-2,1 in the rear.
I'll also go with stiffer sway bars (1,25 and 1" today) which I don't remember if it affects the natural frequency.


I got a ride in my friends Pantera with 3,1 Hz and that wasn't bouncy at all. I could probably live with that on the street to, but I don't think the firebird chassis is stiff enough to take advantages of that stiff suspension. The chassis will probably flex too much if you don't have weld in SFCs and a good cage to stiffen the body.

[Norm Peterson]

For much of any street driving, it's preferable that the rear frequency be somewhat higher than the front, somewhere in the 10% - 20% higher range for most normal speeds. My '79 has undamped frequencies of about 1.25f/1.43r, which act more like 1.15f/1.31r if my shocks are up to snuff.

That's with ~640 lb/in springs up front and ~165's out back in a 3450 lb car. In my opinion, that's driveable anywhere that I have enough ground clearance for, and it may get stiffer yet for reasons involving competition. The disclaimer is that I generally prefer a stiffer ride than many . . .

Edit: added frequency/pitch plot (with about 40% critical damping IIRC).

Edit #2: added a frequency/pitch plot for the case where the front frequency is higher than the rear frequency. It's probably for a different car, but the results for front frequency higher tend to follow the same character. Much "busier" even in the top plot with the same 40% critical damping, but especially so in the bottom plot where the damping was cut in half to simulate dying shocks.

Norm

[BluEyes]

Well, definately seems like the springrates I've been looking at shouldn't be that bad after all! I've got a level ride spreadsheet set up with the equations from "How to make your car handle" so once I've settled on a front rate, it will give me the rear springs to match for my desired speed.

Ripper, you've sure got some stiff springs in there to get up to 1.6Hz. With my car, I'd be nearly at 1000#/in, but I've got roughly 1850# sprung weight on the front end. I definately see your point about stiffening the suspension past the chassis. 2.4Hz seems to be over 2000#/in? I wonder just what the roll stiffness is then?

I love those graphs Norm, really show how worn shocks would degrade the ride. What program did you do that on? I should be able to make something like that on Excell or TK solver, but can't remember that part of my system dynamics class...
I guess even though you might like a stiffer ride than most, ripper has you beat for the kidney-buster though, haha.

All this has me wanting to upgrade the springs on my Caddy as well. It's somewhere around 0.85 Hz - '70's soft. Just going quickly aound corners around town there is a perceptible lag in vehicle response!

[Norm Peterson]

Linky. (https://www.pro-touring.com/forum/showthread.php?t=13251)

[Norm Peterson]

I'm wondering if the front springs' motion ratio, squared, was included in the 1.6 Hz and 2.4 Hz math . . . ~950 lb/in springs being judged too soft for street use but combined with approximately 100 lb/in rear springs makes me wonder . . . 2400 lb/in wheel rates for ~2.4 Hz suggests that the car settles less than half an inch when you release the jack from first touch to the pavement . . . springs with 850 & 250 rates sound more reasonable as the intended upgrade (for about 1.5 Hz / 1.9 Hz - about where I want the STS car to be if I can find the right springs).

Norm

[BluEyes]

Yup, read that other thread some time ago. I guess I just need to get over the preconcieved notion of stiffer springs automatically sending the ride down the crapper (at least in the range of springrates I'm looking at)
I think my last post might have been worded poorly, I get over 2000#/in springrate for a 2.4Hz ride, about 550# wheel rate.

Norm, since I don't have a spreadsheet that includes the damping effect of the shocks, and don't know what % damping my shocks provide anyways (Monroe Sensatracs for now - budget) Is there a general calculation for the effect of shock damping on the natural frequency? I'm trying to figure out the no-pitch speed, but if the natural frequency is changed by the dampers, it seems that might throw my numbers off.
I notice that with your numbers the no-pitch speed does not change from undamped to damped natural frequency though. Is that typical assuming the shocks are good and all of the same brand/line?

[Norm Peterson]

Natural frequency shifts downward slightly as damping is introduced or increased. It's a pretty slow effect at small damping values, but is progressive in nature. I'm not at the computer with those files on it, but from memory it's a "SQRT(1 - [percent critical damping]^2)" sort of function.

Assuming that the shocks at both ends are of similar damping as a percent of critical, the frequencies will move downstairs by similar percentages. In such cases, the no-pitch speed shifts downward a little, since the time to travel one wheelbase needs to be the difference between the two periods.

I just use some generic values for damping in this analysis based on RCVD and other sources. Most shocks do not have constant damping over wide ranges of piston speed, and are asymmetric (bump vs rebound) anyway. But assuming a "good average" should yield better results than ignoring it entirely.

There's a fair bit of human tolerance for speeds that differ from a theoretical flat ride speed, possibly more closely associated with the magnitude of pitch jerk than pitch rotation itself. And as long as you have decent damping and have a reasonable match, things are mostly quieted down by the time one full cycle or so of these vibrations has occurred. IOW, don't get too hung up on chasing down oddball rates in order to achieve an arbitrary and theoretical no-pitch speed with great precision. The nickel's worth of philosophy is that the time for being picky is when you're developing the analytical tools, not for when you use them.

Norm

[BluEyes]

There's a fair bit of human tolerance for speeds that differ from a theoretical flat ride speed, possibly more closely associated with the magnitude of pitch jerk than pitch rotation itself. And as long as you have decent damping and have a reasonable match, things are mostly quieted down by the time one full cycle or so of these vibrations has occurred.

I've figured as much. The level ride speeds I'm finding from the rough figures I've got on my rides tend towards community speeds rather than highway speeds but nothing feels very unsettled at higher speeds either.

I wonder if I'd notice any pitching difference changing the level ride speed from say 20 to 60mph. Maybe I'll have to play around on a spreadsheet and see how the pitch response changes when one is above vs below the no pitch speed.

[BluEyes]

Hey, what's a good way to model shock damping on a ride oscillations?
I'm using the equations from http://www.efunda.com/formulae/vibrations/sdof_free_damped.cfm but the damping with a 0.4 damping ratio seems waaaay too slow, with oscillations continuing past 2 seconds.

[BluEyes]

Nevermind, I figured it out.