I couldn't help but notice my post on this went unnoticed. Here's a quote from someone referring to an experiment conducted by Dodge on this topic. Note that with a wider tire, the contact patch of course got wider as well, but also LONGER... Overall patch area increased by even more than the width change would dictate by itself.
Quote--- "So lets go back to "Mechanics of pneumatic tires" perhaps they know what they are on about. In this book Prof Clark (Dept of Engineering Mechanics, Uni Michigan) talks about how the different constructions of a tyre will change the contact patch and how you cannot compare data from aircraft tyres with that of cars, tractors, trucks, bicycles etc. I thought it interesting that while a tyre with little external tread or shoulder region (i.e. aircraft/bicycle tyres) has a contact patch of an ellipse "for an automotive tyre a somewhat different set of relationships exists due to the fact that the usual construction involves the use of a relatively heavy tread, particularly in the shoulder region. In this case any significant contact spreads over the entire width of the tire between shoulders so that the contact area tends to have essentially straight parallel sides, and the width of this contact area is nominally independent of the tire deflection."
Thus for an automotive tyre the width of the contact patch is dependent in tyre width. However the length of the contact patch is dependent on tyre deflection, width and rolling radius. This was confirmed by experiments conducted by Dodge. They compared two 14 inch (26.7 inch diameter) tyres, one 7.5 inch wide and the other 8.0. The wider tyre consistently had a longer contact patch (not a huge increase mind you but a significant one) over a range of deflection values (0.5-2.0 inch). They then got the wide tyre and measured the contact patch length at a given deflection at three different pressures (15,24 & 30 psi) and the length was basically unchanged. They repeated this from 0.2-2.0 inches deflection at 0.2 inch intervals and got the same results. They then took the 8 inch wide tyres and increased the diameter to 28 and 30 inches. Again a small but significant increase in contact patch length occurred. "
---------- How much of this is due to structural stiffness is unknown to me, but note that even with extremely low (15psi) tire pressure the contact patch remained essentially constant at a given vertical deflection (of course the load was less). I only wish this fellow had posted the load numbers too. It would be interesting to see...
I'm not arguing with you Dave, just throwing something into the mix here. I don't know all about this yet and am experimenting right now with prediction models for this very thing, so hearing some input is just fine with me. However, it's not quite as clear cut as most people believe (yourself excluded).
Todd Wasson Racing Software