Hi Mark,
I think you are pretty close to the right number... 5/8" lowering at the coil spring should lower the car about 1". (At the rear, 1" spacer will give 1" lowering, due to the leaf spring setup.)
Besides trimming the spacers, there are a number of ways to lower the front of our cars.
For example, swapping to the competition or heavy duty coil springs will lower the front of the car about 1" too. The ground clearance at the front on my TR4 is presently 5" (as compared to 6" with stock springs, per factory manual specs), with the only change so far being the springs. The HD springs are fitted along with the original alu spacers, too, and are actually easier to install because just a little compression is needed to fit them in place. I've got a set of spacers that have been shortened to about .75" thick, but I think that might be too much for use on the street, would lower the car to around 3.5" to 3.75" ground clearance!
TRF is now selling several different thicknesses of aluminum coil spring spacers that might be useful. (See
https://www.zeni.net/trf/TR6-250GC/70.php?s_wt=1024&s_ht=768 ) They list them to raise the ride height, but of course they would lower the car if used as a substitute for tall stock alu spacers on our cars. Stock spacers are 1.25" thick. TRF's are 1", .75" and .5" thick. I'm thinking I'll try 1" thick spacers, either from TRF or by machining down another spare set I've got. (Note: TRF also offers similar spacers for the coils at the rear of IRS cars.)
In his book "Improving TR2-4A", Roger Williams mentions several times that the lower a-arms of the front suspension can be pretty easily inverted to lower the front of the car, too. The difference is about .66", so should also translate into about 1" ride height reduction. *However*, one thing Williams doesn't mention is that inverting the a-arms will also lower the bump stop .66", and that will effectively translate into a lot of increased upward travel for the front suspension. Unless something else is done - installing a taller bump stop or moving the bump bracket on the shock tower - I'd be concerned about over-extension of the front suspension/steering joints, bump steer or possible tire clearance issues. I'd also worry about using the shorter HD springs with this setup, since they would have little or no compression with inverted a-arms and just might jump off their seats over a hard bump.
With the current setup, I don't appear to have tire clearance issues. That's with 195/60x15" Goodyear Eagle HPs on KN Minator wheels (5.5J). The overall tire diameter is about 24", or roughly .5" to 1" smaller than original equipment tires. These tires are considerably wider than original and the tire/wheel combo puts them just barely inside the lip of the fender at the top of the wheel arch (no wheel spacers are installed).
The rear suspension of my car will be spaced also, eventually. Some custom made shims will be needed: 1.25" thick is a likely starting point. This is with heavy duty/competition leaf springs fitted and will require special U-bolts for mounting, as well. I'm pretty sure rear spacers would be different with stock rate leaf springs. When I bought the car way back in 1977, it had 1.5" spacers. But IMHO that lowered the rear way too much with stock leaf springs, so I switched to .25" spacers.
I should note that the HD/competition coil and leaf springs are pretty rigid! I'm sure they'll make for a fairly coarse ride once my car is back on the road. At the front, pushing down on the fender hard gives almost no noticible bounce (there is also a 3/4" sway bar installed on the front). At the rear, the HD leaf springs give a little, but not much!
Hope this info is helpful.
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