Question about Wheels

[Rolla]

I know that Porsche beefed up the Boxster S and 2000 or later to handle 18" wheel properly. What can be done to a 99 Boxster to enable it to handle 18" wheels? Thanks in advance.

[Tool Pants]

18" wheels are approved on 1998 and newer. It was the first year, 1997, that they were not approved.

And, for my editorial comment. I have never seen a Porsche approval for 19" wheels but there are Boxster owners who have them.

[antiriced]

Why are 18" wheels not recommended for use on the 1997 Boxsters? A sticky (r-compound) 17" tire is capable of loading the rear suspension as much if not more than 18" street tires so I wouldn't expect it is an issue w/ strength. Likewise, the rear sizes are similar (265 vs 255) - same O.D., width is within 10mm so the physical dimensions should be OK.

Hoping someone can elaborate - I understand there were numerous rear suspension changes made for the 1998 model year (including strengthening of the crossmember) but the popular M030 suspension upgrades are listed as covering all models 1997 to 2004 - this implies the geometry couldn't have changed that much...

[ar38070]

I believe it is the shortened sidewalls of the 18's. 18's are stiffer and heavier. Average road shock to the frame is greater. I believe the problem was cracking at the suspension mounting points

[antiriced]

Again, an R-compund DOT 17" tire is far stiffer than an 18" street tire.. no warnings about that.

I believe it is the shortened sidewalls of the 18's.  18's are stiffer and heavier.  Average road shock to the frame is greater.  I believe the problem was cracking at the suspension mounting points

<{POST_SNAPBACK}>

[ar38070]

Again, an R-compund DOT 17" tire is far stiffer than an 18" street tire.. no warnings about that.

Not saying that you are wrong but show us hard numbers that confirms this statement. Yes 17 DOT R's are stiff. Stiffer than street 18's with shorter sidewalls? Even if they are, 17 DOT R's are usually driven on better surfaces. Also the wheel is infinitely stiff compared to a tire sidewall. An 18 wheel/tire is then more likely (where the wheel is 18 inches and the tire is 7 inches) to transmit more shock to the body versus a 17 wheel/tire (where the wheel is 17 inches and the tire is 8 inches) regardless of the relative stiffness of the 18 versus the 17 tire.

You can argue this either way. Unless you can measure it you do not really know which effect is greater.