Sorry in advance, this is a long one. Cliff notes My suspension causes my car to pitch, but adding weight to the trunk solves it. I hypothesized that the difference between my front and rear spring frequencies without the extra weight was either too great or too little - causing the pitching, but my modeling shows the exact opposite. My secondary hypothesis was that my rear spring rate is too high, but that too is shown opposite by the model. Lastly I play with using the suspension calculator to objectively pick and compare damper adjustment settings. Rambling description Last fall I modified my stock 2001 RS's suspension. Tokico D-Spec struts, Ground Control coilover sleeves, 320 lb/in 9" front and 280 lb/in 9" rear springs, 22-26mm rear sway (on 22mm) with 05+ mounts and poly bushings, solid endlinks front & rear, ride height is 14" hub center to fender line all around. Initially I had my subs out of the trunk. The car pitched badly no matter what the dampers were adjusted to. It always felt like the rear end pitched up sharply compared to the front. On certain freeways it would become a violent see-sawing. Then I put the subs back in the trunk and a hitch bar on (~200 lbs added to rear). The pitching is almost completely eliminated and the car feels much nearer to balanced than it was before. Links (I used the search! :biggthumpup So I started modeling up the parameters in the suspension calculator on nasioc. Based on this info: http://www.cars101.com/impreza_archive99_01.html http://forums.nasioc.com/forums/showthread.php?p=25611668 http://forums.nasioc.com/forums/showthread.php?t=178526 http://forums.nasioc.com/forums/showthread.php?t=954846 Numbers I figure my stock RS (manual swapped) with no spare dry weight (full fluids) is 2800 lbs with a 58%/42% distribution. That's: 1625 / 1175 (F/R) But I've made it heavier: Sound Proofing: + 20 / 30 Stereo Wiring: + 20 / 30 Misc. Parts: + 10 / 10 (bigger sways, etc.) Trailer hitch: + 0 / 30 Me (need a diet): + 125 / 75 Without subs Equals: 1800 / 1350 = 3150 lbs, 57.1% / 42.9% With subs Equals: 1800 / 1500 = 3300 lbs, 54.5% / 45.5% This is the best estimate I've got, since I haven't actually been weighed. Without subs With subs I am surprised to see that the difference in the model when adding the weight in the trunk is to make the rear spring frequency even further away from what the "flat ride" is. Without the subs in, a "flat ride" occurs at 111 mph. With the subs in, a "flat ride" occurs at 264 mph (I wish :eek3. And its basically saying that I should be sprung higher in the rear than in the front - that seems crazy? And why does it feel more balanced over bumps when the weight is in the trunk? Tuning Here's the plot for my D-Spec's from nasioc's official shock dyno thread: I've modified the spreadsheet from nasioc a bit to make it easier to pick a target damping percentage and relate that to an adjustment point. Its a simple algebraic rearrangement of the equation in another section. So if I want 100% critically damped rebound at 300 mm/second (iirc, this is what Back Road Runner said he liked as a starting point). When I plug that in, I get 285 kg. On the dyno plot, I look 285 up on the y axis and 300 mm/s on the x axis and that cross-references to 1.5 turns from full hard in front and about 4 turns from full hard in rear. What I can do then, is follow those lines on the shock dyno plot and fill in the forces for different piston speeds in another section of the spreadsheet (the one I pilfered the equation from), and it will draw a pretty graph. Don't ask me to interpret it. Although, looking at it - kinda makes sense that my ride is so bumpy. So let's see what all this hot 65-70% critically damped buzz is about. I'll try 70% at 100 mm/s. I don't really like how it peaks in the slow-speed range, and falls off below 100 mm/s. I have the car on this setting now, but don't have enough time driving it this way to really comment yet. I will probably try 70% damped at 50 mm/s next, to move that peak down in piston speed. New springs? So the ride with this setup has generally been, on average, too rough for my tastes daily driving it around the cities. Also, I notice that the spring frequencies I'm at are fairly high for a road car (so they say). I played with some numbers and found that something more like 225-250 lb/in front is in the "sports car" range, but it still says my rear spring should be higher - like 270 for 250 fronts. wtf is up with that? Feel free to discuss.
If this is a Final Exam, I'll take the "F" before the test starts. Although, i might learn a few things from it.
you should put this on a foam board display and use a laser pointer and have a class on this...good luck with your "discussion"
really, I don't know much of anything. I could be doing this all wrong. But then to those who say this is over they're heads - I dare to disagree. The hard work's all done. I'm just plugging in data and trying to relate what the numbers say to what I actually experience. Hopefully with the goal of using the numbers predictively to pick better system components.
For anyone looking to run aftermarket springs on adjustable struts, this is the "basic" 101 class you should go through in order to understand what you're doing. It's sort of the "you need to know at least this much in order to know what you're doing" type of thing. I commented over on Nasioc.
This is some good info... One of these days when i'm not lazy i'll have to read it all, i've always wondered how to choose the right setup...
Random thoughts: I think your weight assumptions are off, to start. If you are looking for this level of detail, you should really start with the right weight assumptions. Spring higher in the rear with subs (vs without)? Well, yes, you have more mass to support. If not, you could theoretically do away with springs if you get the mass high enough (trend line would go to zero). More mass = more travel from a given spring rate (that sets the freq). As for the freq... What is being calculated is the resonant (or natural) frequency of the system. You add mass to a system, its resonant frequency is lower (think tweeter vs woofer, or flick a thin sheet of steel and then a thick sheet). This, of course, is assuming other things don't change (motion ratio, rate). Is time delay accounted for in the calc (haven't gone back and read everything)? If the rear starts after the front, it will continue to oscillate for the time delta, assuming everything else is equal. If not, it could result in a 'pitch'. I see that it is mentioned in your pictures, but is it accounted for? if so how? I get the impression that it is a subjective thing to tweak your rates too, based on the way the comment is written. On your 70%, 100mm/s section, you mention peaks. Look at the scale, they are on the same scale as the 100%, 300mm/s plot. I am not sure what common velocities are used to represent street driving, but are dips under 100mm/s an issue? There are other things not accounted for, as well. Friction is potentially a big one (stiction, worn parts, overtightened parts, dirt, etc). This goes with the above statement, too, as stiction is a big factor in low speed bumps. I guess I am not sure where you are going with this... Having higher rates in the rear are counter-intuitive to you? ...or are you looking for something else?
Found a good link to read.... http://autospeed.com/cms/A_108167/printArticle.html It explains the relationship between spring rate and natural frequencies...
i'm going to come back and go through the calculations some other time, but to me it appears that when you don't have the subs in your car you are underdamped and when you have them in your car you have at least made critically damped, but most likely are overdamped you are very close as far as i can tell and this point it might just be easier to start using good ol' guess and check method however, you will not be able to tune your system in such a way that it works with both your subs in and subs out
