How Much Height and Which Springs to Use
Height is a big issue with spring overs. The most common question is, “I forked down $1,600 for an Old Man Emu suspension kit (or some other lift kit), can I still use it?”
Yes, you can, but for every inch of lift you have, add another 5-6” for the conversion to it. So if you have 2.5” lift springs, you are going to have about an 8.5” lift, which is tall, more than tall enough to run 37’s on any Land Cruiser. General folk lore is that a SOA on somewhat flat, stock springs will easily allow you to run 35" tires. In some cases, you can even run 37” tires with a SOA with stock springs.
Generally, because you are lifting your rig so much, you are also increasing your vehicle's center of gravity. Ideally, you want to lift your vehicle as little as possible when doing a SOA conversion. If you are planning on running 35 or 37" tires, your rig will be more capable with the smallest amount of lift you can give it room to adaquately clear the tires, but keep your center of gravity down. This is *especially* true with wagons that are off-roaded.
The minimum height of a SOA (5-6" lift minimum) naturally allows all Land Cruisers to be able to get over 90% of obstacles you should be taking the vehicle over in the first place.
De-arching your springs
The best springs for a SOA in my opinion are stock with a mild add-a-leaf put in. I’ve run a good variety of springs in both stock formation and with different add-a-leafs in them (mostly because Cruiser springs go flat so easily), and I’m now a firm believer in use of add-a-leafs. This gives you a somewhat minimum lift over 6,” which will allow you to run 35" or 37" tires, but has the strength to hold up to the SOA setup.
It is commonly said that the best springs for a spring over axle conversion are soft, stock height, broken-in leaf springs. This is simply NOT true unless you want a very soft, unstable susension.
If you think about the physics of the SOA vs. SUA setup, in a SUA the u-bolts naturally keep an arch in the spring and make them last a long time. But if you take a good set of perfectly healthy springs from a SUA and put them SOA, without making “strength additions” (aka extra leafs or an add-a-leaf), then you will notice within days or weeks that they will become very flexy and may start to negatively invert. The direction the u-bolts naturally pull in, in a SOA setup, naturally wants to flatten them out, versus the stock spring-under-axle setup which pulls your springs into the arched position.
On a wagon, it is almost mandatory to use an add-a-leaf at minimum. On a 40 you might get away without one, but I wouldn’t recommend it. The best add-a-leafs are the long, somewhat soft and flexible ones. You can always get a leaf that is much longer than your pack and simply trim them down as well and most spring shops will carry them.
Also, when doing a SOA there
are usually a number of smaller leaves towards the bottom of the pack.
When you think about it, each of those extra leaves will make for another
1/4” of lift for each spring, so I have typically removed the smallest
two leaves, and run about 5 leaves total including the add-a-leaf, and
have had great results.