MaxPF
AGNTSA
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Negative arch is largely misunderstood. Whether it is detrimental depends on the construction of the spring and how much unloaded arch it has. Every spring has a maximum amount of compression it can tolerate without yielding (permanent deformation). Some springs, such as the stock height springs on my K5, are negative arch when sitting at normal ride height and can arch further negative during normal suspension travel. The springs I currently have are 2-1/2" lift, which as you noticed sit pretty much flat at static ride height. These springs can negative arch 5-6 inches without any damage to the spring. OTOH, my stock height rear springs have much more static positive arch, and it is unlikely they can tolerate much negative arch. It is a matter of overall travel, really. This is why I left the overloads in place on the rears, to limit upward travel.
So, my simple answer to your question is: I don't know. I don't have enough knowledge of your springs to determine how much compression travel they can safely take. The safest bet is to limit up-travel to stock. If you know someone with a stock H3, measure the amount of distance between theor bump stops and the mating pads on the axle, and use that as your guide.
Most high speed off-road rigs use coils all around. Leaf springs have the advantage of being inexpensive: they eliminate the need for locating links (although Panhard rods and traction control bars are often added), they are generally rugged, and simple to set up. They have the disadvantage of having a lot of internal friction and more effective un-sprung weight, both of which are negatives for ride and handling. For an H3 SAS I would run coils, and use a 3-link+panhard or a 4-link+Panhard. You could use twin trailing arms like many SA pickups, but they don't work as well for large amounts of suspension travel.
Excessive flex inevitably compromises vehicle stability due to the excessive compliance needed. The only time this can be overcome is to use a sway bar, but the bar then limits flex. I think 10-12" of travel is perfectly adequate for the part-time wheeler.
I take it when you say "you can clearly see how solid axles and high-speeds don't work too good together" that you are being sarcastic? That rig looked to be soaking up the bumps quite well. I should point out that a 10b or D44 is better than a D60 when it comes to high speed, simply due to the fact that it is less massive. Less unsprung weight is ALWAYS better for high speed operation.
I do think IFS has an advantage in high-speed off-road situations, but it needs to have a lot of travel (read: expensive). It you are hitting the bumps on every hole or whoop, you'd be much better off with a SAS with decent travel. At low speed where articulation is needed, SA's clearly keep more of the tire's contact patch where it needs to be. This is not so much due to suspension travel but more to the way in which solid axles articulate. In traversing terrain the SA "walks" the terrain, and as the axle articulates it does a better overall job of keeping the tire's tread parallel to the surface beneath. As with so many things, it's a tradeoff.
I take it you got the iShip notification on those?
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