Originally Posted by footie
This assumption that acceleration or topspeed will lessen due to bigger rim size is something I can't work out, only if the width increase can the performance decease but I still think the width would need to be quite big to be really noticed.
I don't have this problem. I understand full well that if the bigger rim size is accompanied by additional weight, or the distribution of the weight of the rim is more unfavourable then there is a performance penalty, in much the same way as any weight increase in the car. The difference in unsprung weight is amplified due to the rotation factors other have alluded to above.
I seem to remember a discussion paper (of either Porsche or Honda) on the effect of PCCB's where they approximated that a reduction in unsprung weight was the equivalent of a 4x reduction in sprung weight. i.e. lets say ceramic rotors saved you 5lbs per corner this would be the equivalent of taking out 20lbs from the car (eg seats or exhaust). This is obviously weight reduction for dummies and doesnt take into account weight distribution etc
All I would argue is that the difference between the OEM 18's and 19's is insignificant in this regard and I bet if you put a vbox on the same car and tested 0-60's etc you wouldnt even get .1 of a difference in the outcome.