I've pretty much decided on the Koni shocks and DSS coilover kit. Now for the Spring Rates:


NDgsx% DSM Wiseman% Car: 90 Eclipse GSX%%% Most of these kits are over sprung and tuned all ass backwards. These numbers are off the top of my head from some things I calculated a couple months ago, if it's not dead on accurate don't kill me The front motion ratio of a 1G is .92, the rear is .844. What does this mean? The rear suspension has a higher mechanical advantage and so the rear wheel rate is much lower than the actual spring rate. Say you have a 300lb spring the rear wheels will see a rate in the low 200s (take the spring rate and divide it by the mechanical advantage squared). Don't be afraid of higher rates in the rear, it's not as bad as it might seem. Given a 3000lb curb weight (my car with driver and a half tank, I'll have lower rates due to my lower weight) and a 60/40 weight distibution and a unsprung weight of 80lbs per wheel (total random number, figure 40lbs for wheels and tires, wheels bearings, hubs, brakes, it adds up quick), with a target rear natural frequency of 2.1 cycles per second (this on the stiff end of the scale) you end up with a target spring rate of 300-325lbs. Now you take the wheelbase of your car, figure the time it takes to travel that distance. In my case I went for 80mph, about my average lap speed. You subtract that time from the rear natural frequency to find the front natural frequency. When you do this the front suspension will travel up and back down before the rear wheels hit a bump. This helps keep the chasis settled over bumps by reducing porposing (sp?). In this case you end up with a front spring rate of around 300lbs. Unless you have added a significant amount of weight to your car I wouldn't be looking at rates that are higher than this by a large amount. Stiff springs do NOT make your car “handle” better. They may increase the roll stiffness of the car, in turn keeping a flatter contact patch with the same static camber setting, which increases cornerning power in a steady state. However when you hit a bump those extremely high spring rates will not allow the suspension to do it's job and the chasis will be upset. If you're on a smooth road course or parking lot this may not be a large factor, however on the street it is. If you need increased roll stiffness after setting the springs with a high natural frequency increase the stiffness of your roll bar, they are far more effective at reducing body roll. Use the relationship between front and rear roll bar stiffness to determnine the handling characteristics of the car. A larger sway bar does NOT INCREASE TOTAL WEIGHT TRANSFER, it increase the weight transfer at that paticular end of the car. Total weight transfer is determined by centrifugal force, the height of the center of gravity, and the track width of the car. THAT IS ALL. If you increase the stiffness of both sway bars by the same amount you will reduce body roll and that's it. You WILL NOT INCREASE WEIGHT TRANSFER. You may reach a point where you begin picking up one inside wheel, at this point reduce the roll stiffness at that end of the car and set the static negative camber so that you get even tire temps during cornering. Blah this is a long post and probably a total waste of time, but I've been wanting to say this for a while. Save yourself tons of money and testing time and read before you buy. Suspension tuning is not a black art, you just have to know a moderate amount of math and engineering to figure it out.

Dennis Grant (2G): (http://www.wincom.net/trog/autocross/2G_ESP.html) Spring rates are an item of even greater debate than tire size. Time and a lot of work is starting to reveal the ideal rates. The things to keep in mind are that DSMs have very forward weight distributions, and that the motion ratio of the rear shocks is slightly higher than the motion ratios of the front shocks. This leads to using a lot more front spring than rear spring. My current setup is 900/400 front/rear. The temptation is always going to be to crank up the rear spring rate (or the rear bar rate) in order to get the car to turn. Resist temptation! What you are doing is sacrificing rear grip, and overall grip, in order to balance the car. Don't unstick the rear, stick the front! Note to run spring forces this high, you need very good shocks. Note too that properly matched shocks will use a lot less shock force than you might expect - stiff springs, soft shocks is the way to go. The suspension must be able to move. Done properly, the ride is actually pretty confortable. If you are pissing blood after a drive, your shocks are probably too stiff.

Random Thoughts: Me guessing at how suspension works…

suspension.txt · Last modified: 2010/06/16 13:42 (external edit)
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