Rear Leaf Springs
The easiest way to change the stiff of the leaf pack is to remove leaves (softer rate) or add leaves (harder rate). You can usually add a leaf without changing the wrapping brackets, any more and you will have to make new wrappers. You can get custom leaves made up for about $15-$25 each.
Rob Dixon's tarmac rally setup is a custom leaf which matches the length of the 2nd-from-bottom leaf, except his custom leaf is 8mm thick, this is then placed on top of the leaf pack. Karl Skewes Jap-drift-spec setup was combining two KE35 leaf packs to make a 220lb 7-leaf pack.
A common performance trick in the 70s when everything had leaves was to make thick metal brackets (like an = sign) and bolt the front of the pack together where each leaf ended.
Another method of reducing tramp is to add a thick leaf on top of the front section only of the leaf pack. I have read about people getting another top leaf (as in the one with the leaf eye), cutting the eye off, cutting the leaf off behind the U bolt, and attaching this above the original top leaf, to good effect.
If you're using old leaves, a cheap way of reducing tramp is simply to swap the leaves from side to side. After all the years of winding one way, reversing the winding will make the leaves stiffer temporarily. This may change back in a short amount of time, or it might take another 30-odd years :)
Beyond this, you can setup a trailing arm system, or 4-link/5-link , or a Kaltracks rear end, but these are all too complex to be within the scope of this article.
There are a few ways of lowering leaf springs.
Firstly, you can use lowering blocks, where a block of steel is placed between the diff and leaf pack. The ride height of the car is lowered by the thickness of the block (ie: 30mm block, your car sits 30mm lower). This is pathetic American lazy-performance bullshit. Lowering blocks increase the torsional leverage on your leaf springs, which means you get increased axle tramp AND your leaves have greater force acting on them, so they bend out of shape quicker.
Another far better method is to invert the leaf eyes. The top leaves are heated until they become pliable, they eyes are unrolled and rolled back the other way, then heat treated again so the metal hardens properly. Most professional lowering kits you can buy are just an inverted-eye top leaf. This will give you a good couple of inches of drop.
Lastly, you can have your leaves reset. This de-arches the springs, which is alright to a point, you don't want your springs deforming past flat though.
Wagon Slapper Bar
Wagon leaf packs have a large bar on the bottom of the pack, not attached to anything. This is to stop your Corolla bottoming out when you're carrying around big heavy loads in the back (as you're able to with a wagon, not so much with a sedan). Jonny Rochester tried this slapper bar in a performance capacity when he had his KE38 wagon, but found the progression from soft stock leaves to hard slapper bar was too hard too fast.
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