Cees,
That is why the XWX was so dominating in the production classes (back in the olden days, we had to use the actual tire the car came with in the stock classes). The sidewall was so stiff, the tires almost felt like 16s when on the track. Also one of the reasons that led to the changing of the wheel drop-center design. The tires were and still are some of the most difficult to mount without damage.
H