Tyres and effect on ride comfort, I'll give the very short obvious answer. As the difference in stiffness between the vertical stiffness of the tyre and the vertical bump stiffness of the suspension gets bigger then the influence of the tyre stiffness on ride comfort gets less - sorry but that was really obvious. One (of many) rule of thumb is that as the ratio of stiffness between two compliant structures (in this case tyre and suspension) gets around 7:1 or more then the influence of the stiffer item tends to get far less important, 5:1 is OK. So as the wheel rate (stiffness of spring plus ARB) comes down from the original very high std stiffness to what is suitable for a Aero8/Plus8 as a touring true GT car then that ratio gets larger and so the effect of tyre stiffness on ride comfort gets less.
Tyre stiffness is very progressive, gets increasingly stiffer as tyre is loaded as it is only a gas spring, the piston is the contact patch and the cylinder is the volume of air inside. The rate of progression (how quickly it gets stiffer) is related to the volume of air in the tyre (try compressing a bicycle pump to feel the progression in rate). A low profile tyre on a big dia wheel will tend to hold less air than a high profile tyre on a smaller dia wheel so stiffness of the low profile tyre will increase quickly when loaded; this is not really related to sidewall stiffness until the side wall is nipped by the rim, ouch! The sidewall has some effect but its stiffness is so small compared with the stiffness from the air pressure it can mostly be neglected.
Measure your contact patch area by coating a section of tyre with black boot polish then lower the car onto the ground so the contact patch sits on a A4 white paper, worth a look if bored. That is your piston size, measure the area. If you know the air pressure you can get close to the corner weight by the simple calc of course. Then if you know the tyre and rim cross section you can calc that cross sectional area and thus the air volume and thus the tyre stiffness rate progression. Hours of fun and finger bashing on a calculator, but not sure how far this gets you as the tyre choice is as much to do with grip and personal preference - stop waffling! Cheers Peter.
