Finally got the rear ride-height to be where I want it:
Which creates a whole 'nother world of problems. When I did the coil-over conversion, the suspension was setup to work best at stock ride height and more, for rally. Setting it up this low means there is a disproportional amount of suspension droop available, and not enough travel for bumps. The spring rate, which was technically always a problem before, is also now a much bigger problem. I need something in the 325-400lb per inch rate at the beginning of the stroke, but 600-700 at the end of the stroke. Normally one would just buy a progressive spring, but they don't make them in the short spring height I have available to me. So I started thinking about stacking two springs, and what springs I would want.
Now, given that the red spring is 8" long, and they make 4" springs, it seems like a simple conclusion. Put a 350lb and a 700lb spring in the 8" space, and call it a day? I bounced the question off of a couple of racing buddies of mine, and then spent the last two evenings going to spring-physics classes with links they send me on the internet. Spring physics get absolutely brain bending.
If you stack two springs in series, you don't get a curve that starts at the lower spring rate and finishes at the higher rate. Instead, you actually get a single linear spring rate that is wildly different than you anticipated. Stack a 4" 350lb spring ontop of a 4" 700lb spring, and together the two will act as though they are a linear 233.3 lb spring. The top springs is multiplied by the bottom spring, and that is divided by the top spring plus the bottom spring, to arrive at your linear number: (TSxBS) / (TS+BS) = Spring Rate. In our case (350x700)/(350+700)=SR -> 245000/1050=SR -> SR=233.33
HOWEVER if one of the two springs reaches coil-bind, then you'll have the non-coil-binding spring acting at is 'normal' rate. So if we could make a 2" tall 350 spring, and a 6" tall 700 spring, at some measure of stroke the 350 spring would coil bind (ie: compress fully) and you'd go from a 233.3 linear spring rate to a 700lb spring rate. Hmmm...
In offroad racing, this is done by something called a crossover ring. This ring threads onto the shock body inside the springs. The spacer that is between the two springs cannot slide past the crossover ring. So as the springs compress, eventually the spacer will hit the crossover ring essentially stopping the top spring from moving any further -> now you have a bottom spring acting on it's own, thus it's now acting at it's labeled rate. So, put the 350 spring on top, with a crossover ring inside, and the 700lb spring on the bottom. The suspension will have a 233.3lb spring-rate until it compresses enough to have the spacer between the two springs touch the cross over ring. At this point it will transition to a 700lb spring rate. Moving the crossover ring up and down changes where the spring-rate change happens. We're getting somewhere now.
Now, I happen to know a 233.3lb spring rate is going to be much too soft to start with, as the car sinks to the bump stops on 300lb springs. So we need something in the 375-400 range to start. How the heck do I find a combination of springs that will do that? Wouldn't you know it...there are handy spring charts where someone has already done the excel math for you. Simply scour the chart to find a combined rates in the 375 range and...
aww crap. A 600lb and 1000lb spring combo will do it! I suppose I could put the 1000lb spring on-top (presuming I can get one in 4" length) and lock it out with the collar to end up with a 600lb rate? But I honestly don't know. This is the point where you call your friend Ray, of Trial Engineering. Before moving to Canada Ray worked as a suspension engineer for a very prominent World Rally Championship team. Everyone has a Ray, right?
Fortunately for me, Ray enjoys a good noodling session. He also explained to me the problem with using crossover rings, or planning on one spring coil-binding. The transition between combined rate and single rate isn't so much a 'transition' but more of a stair-step. So, in our original example you go from 233.3lb to 700lb rate almost instantaneously. Not a significant issue on a long-travel off road impact (think jumps in Baja) but potentially significant on a road car. You don't want to be setup and halfway through a tight mountain corner, suspension loaded almost to the crossover and then hit a compression. It could cause some handling issues.
Ray needed more info, so the suspension came apart for measurements once again:
The spring length as measured with the car at rest, along with wheel-to-fender distance. The suspension was reassembled without spring, and I cycled through full droop full compression taking measurements of the wheel-to-fender gap. All these numbers were sent to Ray, who started on some calculations. First off, he figured out the ideal starting spring rate for maximum comfort and compliance, while still holding the car off the ground at the desired ride height. He then used my ideal maximum rate (based on questions he asked about the cars handling in rally) to figure out what the ideal finishing rate would be.
Then he came back with some suggestions on modifying the car. I have way too much droop, and not enough travel. Raising the car 30mm would be ideal, which is hilarious because I just spent two weeks getting it DOWN 30mm! The bump stops are trimmed as much as they can be, and the shock is dangerously close to nearing bottom-out. So, my only other option is to move the lower shock mount. This will reduce the droop, but allow us to trim the bump stop further for more upper travel.
Commercially available springs are compared:
For now, Ray thinks he has found the best of the worst-case scenarios. One solution is to combine a progressive tender spring with a longer linear spring, planning for the tender spring to reach coil bind. The downside is how the car may handle at the transition. The second option is to simply get a custom spring wound in the desired progressive rate, not sure what that will cost me. The third option is to combine a shock extension with a custom lower spring perch to get me more room for mounting an off-the-shelf progressive spring which is (currently) too long to fit on my shock.
OR I take these rear arms off, mount up a spare set of factory arms with mounts, and look at what setups you guys are using on your lowered German Look cars. My current setup uses 2.5" inner diameter springs, but I believe some of you are using shocks with 1 7/8" springs, which allows you to use the factory lower shock mount (just moving the hole lower).
Definitely more research to be done here.