The requirements are quite detailed:
Quote:
Acceptable 'eyebrow heights' (the measurement from the centre of the wheel vertically upward to the edge of the mudguard) or 'bump rubber clearance heights' (the clearance between the bump rubber and corresponding contact point) for most passenger vehicles can be found on the Passenger Car Track List for 1970 and later vehicles .
Other requirements for lowering a vehicle include:
either front or rear, the suspension travel must not be reduced by more than one third of that specified by the vehicle manufacturer
the vehicle must also retain 100mm ground clearance which is measured between any part of the vehicle and the ground
all measurements must be carried out with the vehicle on level ground in its normal operating but unladen state
coil springs must not be shortened by cutting, and no suspension component is to be subjected to heating
lowering blocks may be used on leaf spring suspensions provided they are manufactured from steel, aluminium, or metal and to be positively located to the axle spigot hole and the spring centre bolt. Note: the axle mounting 'U' bolts are not permitted to be below the lower rim height.
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So I just looked up the list and apparently the "minimum eyebrow heights" for my model are 356 F / 347 R. So I got out a ruler, and that's almost exactly what the car is sitting at. Pretty sure it's all stock (previous owner wasn't aware of any modifications), and it sure as heck doesn't *look* lowered
but maybe it has been - or the springs are a bit old & tired
. Anyhow... I've double checked my measurements, so maybe I *won't* be lowering it (or lowering it any further, as the case may be)...
Minimum ground clearance of any part of the car seems to be about 140mm, and the bottom of the front bumper is about 200mm, so based on the above advice, I should look at extending the bumper downward about 60mm (about 2 3/8 in)