Quote Originally Posted by JohnL View Post
I doubt it's actually the volume of air in the higher profile tyre that causes it to be "slightly dull" and "bouyant/floaty" compared to a lower profile tyre. I think its more likely that it's the sidewall compliance itself that causes this. The problem with seeing the difference is that the two tend to go hand in hand, i.e. low profile and less air volume, easy to mistake the affect of one for the affect of the other.

The sidewall stiffness of a low profile tyre tends by necessity to be stiffer than sidewall of a higher profile tyre. But, if the sidewall stiffness were the same with both kinds of tyre, then both kinds of tyre would 'compress' to the same degree for the same loading or impact on the tread and behave more or less the same up to a certain point, but after that point the tread on the lower profile tyre would 'bottom out' against the rim before the tyre with the higher sidewall would. This would cause an instantaneous and very harsh impact on the rim itself, causing an effective abrupt increase in the tyre 'spring rate' (to infinity) that could cause handling issues as well as rim damage. The issue is the distance from tread to rim and sidewall stiffness, not the air volume.

In reality the sidewall of the lower profile tyre is very likely to be significantly stiffer than that on the higher profile, at least partly to avoid this potentially catastrophic problem (though some higher profile tyres will have quite stiff sidewalls, rally tyres for instance). As a consequence of this an increased loading or an impact to the tread will be absorbed with greater resistance by the stiffer sidewall of the lower profile tyre than with the softer sidewall of the higher profile tyre. The occupants of the car will feel this as the tyre being less 'dull or 'bouyant/floaty'.

I hope I'm being clear, it's harder to explain than you'd think!
yeah, your explanations are understandable. im just not too good with the mechanical explanations you post in the other threads. my mechanical knowledge is quite limited compared to my physics knowledge.

but back to the start, i believe the bridgestone adrenalin tyres (forgot to mention earlier) use a softer sidewall than most high performance tyres from what im hearing. so it is rather similar to using higher sidewall profile tyres made out of an equally soft compound. but swapping to a bigger rim size can bring out more of the steering quirks outta the suspension setup. since the swap to low profile tyres and bigger wheels, i can notice the tyres going over every bump and the places where i would be able to 'float' over with the high profiles now trigger bump steer. but thats probably more due to dead shocks and increases in responsiveness of the tyres thats causing it. should be sorted out shortly once i can sort out the problems with my coilies.