17" rims

[rocco16 at sbcglobal.net (LEF)]

  Oddly enough, newer rubber provides less lateral grip.  This is the exact reason racers take a brand new set of rubber and shave
  them to 3-6 /32.  Deep tread only exists for greater foul weather traction, and because people don't want to buy new tires every 6
  months.  Larger tread blocks deflect to a greater degree under cornering, causing greater slip angle, and greater deformation of the
  contact patch under heavy loads.  The extreme of this is brand new rubber, which still has mold release impregnated in the outer
  layer of tread.  In my prior example, the tires were as follows when tested:

  195/50/15 - (sidewall 3.84) - yoko avs, ~25% tread (perfect tread depth for autox)
  215/40/17 - (sidewall 3.39) - kuhmo ecsta 712, ~75% tread (mostly due to 'Anson influence')
  235/40/17 - (sidewall 3.70) - kuhmo ecsta 712, ~95% tread (about a month old)
  In this case the tread difference worked against the grain, yet the other variables still prevailed.  The yoko / kuhmo change also
  acted against the grain, as the yokos were a much softer, stickier compound.  An interesting point is that the final size sidewall
  is closer to that of the original yokos, yet there was another step increase in lateral grip, even despite the larger tread depth.


  You make an excellent point there, and have taken away one of my " gotcha's"  for your particular situation.  :)
  This brings up a question I've had for a long time: which is more critical in the handling qualities of a tire, assuming two identical tires (we are talking street stuff, not slicks) one used for 15,000 miles and down, evenly, to the wear bars...the other just mounted and taken for a 25 mile jaunt, just enough to scrub the treads clean of compound...which would be the preferred tire for Sunday's autocross??

  Would the responsiveness of the "shaved" tread be offset by the increased hardness of many-times-recycled-and-aged rubber?
  Would the grip of new, soft rubber be offset by the squirmy-ness of new, tall tread blocks?

  Would the grip of new rubber be more desireable than the crispness/controllability/accuracy of the stiff tread on the old tires??
  I don't know.... it might come down to personal preferrence.  What would be YOUR preferrence?

  (see, I don't think I know everything after all, David/Dan............. :) ...)



  Something else to consider - for a given series / sidewall height, I believe a larger wheel still results in an overall stiffness
  increase, as there is more sidewall acting about the contact patch area. 

  I'm not sure I agree with you ( surprise, surprise...:)..),  in theory anyway.    Here's why:
  given series:  (say, aspect ratio of 40)  a 205/40-14 will have the same sidewall height as a 205/40-18, and as you pointed out elsewhere, sidewall height has a lot more influence on overall lateral stiffness than wheel diameter....so for all intents and purposes, would they not be equal?

  I will give you this, however: in practical terms, you ARE right, since finding a 40-series tire in a 14" tire is as difficult as finding a 40-series 18" tire:  the smaller sizes seem to be available only in the larger aspect ratios, while the 18" tires are only available in smaller ratios.

  So... the larger the wheel size, the smaller (more desireable for performance) the aspect-ratio tires that are made to fit it.  So, theories aside, you are right... where it counts.....where the, ahem, rubber meets the road. :)

  $0.06
  :)
  larry
  sandiego16v

  Now up to $0.04 :) (good discussion though)
  Al