Spitfire and GT6 Rear Suspension Lowering Block
Here's a calculation for estimating the thickness of lowering block needed to go from an initial camber setting to a more negative one:
target block thickness "T" = effective axle length * sin(initial camber - target camber)
because for small angles, sin(theta) = theta, this can be approximated to:
T = effective axle length * [180 * (initial camber - target camber) / 3.14]
where initial and target camber are in degrees. Note that block thickness means net thickness--i.e., the thickness of the middle portion that goes between the bottom of the spring and the top of the differential. This simple formula is valid for the small angles of only a few degrees being dealt with here. Effective axle length is the distance from the swing axle u-joint to the bolt that attaches the swing axle trunnion to the vertical link. In the case of the "long" later model swing axles (used on later mkIV Spitfires--FH50001 and up starting in February 1973--and all 1500 Spitfires), this measurement is 16 inches, and for the shorter swing axles used on earlier cars it's 15 inches. This small difference in axle length makes a small difference in the calculation and the actual results. Roughly speaking, each 1/4 inch of block thickness generates nearly one degree of negative camber change, as shown in the plot below:
For example, if you are starting with -1 degrees and you want to achieve -4 degrees, that's a -3 degree change and so a 3/4 inch block will do. Knowing that 1/4 inch of block thickness generates nearly one degree of negative camber change is handy because commercially-available lowering blocks come in 1/2, 3/4 and 1 inch thicknesses, corresponding to almost 2, 3 and 4 degrees of negative camber change, respectively. You can zero in on other change values if you machine your own block. Be aware that the use of any lowering block thicker than 0.5 inches net will necessitate using longer studs to attach the transverse leaf spring to the differential, which in turn may require modification of the spring/differential access cover to provide clearance for the longer studs.