First, the SC400 shares the same suspension design as the MkIV Supra. It has upper and lower control arms and shocks, not Macpherson Struts like most other modern cars. The front and rear springs are both 5" diameter, although the front springs taper to fit into the upper control arm. The rears appear to be straight.
I managed to find a pair of used front shocks and springs on eBay for $10 which I bought to experiment with, and then bought a bunch of used circle track springs also on eBay for $7 to $12 each. Turns out 5" diameter is a common size.
Stock front springs, 368 lbs per inch:
Stock rear springs, 154 lbs per inch:
Front shocks with circle track springs:
Close up of the springs:
On the left is the stock spring. It's about 12.5" uncompressed. About 11" of that is at 5" diameter. In the center is the stock shock with the circle track spring. Notice it's a bit shorter, 9.5" overall. On the right is the stock spring and shock together.
The taper on the stock spring need to be there to fit into the upper control arm, and since the spring is a little short, and adapter will be needed. I've got a few ideas on how to solve this such as: a solid machined piece or a threaded piece and huge nut to create adjustable coil-overs. I'm not sure what we'll do, but I can continue experimenting on these parts off of the car.
Based on what the stock front corner weights should be (975 lbs), the stock spring should be about 9.9" compressed. The circle track spring is shorter, but this one is 650 lbs per inch, and I have some up to 1100 lbs per inch. At 650 lbs per inch, the spring will be compressed to about 8". If you add a 1.75" adapter (to fit inside the control arm), it would sit at about the same height as stock. I've also got a pair of 800 lbs per inch springs that would sit higher, so they may need to be cut to fit.
To keep this cheap, the front springs might be usable on the back as well. That would significantly increase the rear spring rate without adding too many dollars to the car. This would go from 368/154 to 650/368 or 800/368. Seems reasonable.
In the end, adjustable stiff springs will keep the car from diving too much or leaning too hard in the corners, and it should handle better. And the greatest part is that instead of used Supra springs or a used lowering kit to keep in budget, once this is done, we can swap out spring rates to tune the car even better. Too bad we need to continue to use used shocks. That's crap can racing for you!
Just in case anybody is interested in seeing the front shock assembly disassembled, here it is. On top, from left to right, is the destroyed boot, bump stop that we'll probably need to cut, and top shock mount. In the center is the front shock, and at the bottom is the front spring:
What's the OD of the shock of the little bit of the bracket? I wonder if we could get like a 2" piece of steel or aluminum tubing, then tread the outside of it to create a coil-over.
ReplyDelete1.75" is the shock tube OD. I think that a threaded sleeve over the shock tube that sits on the stock spring base could be made to allow a new adjustable spring base. This would be a complicated machining project, however.
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