Tuesday, May 12, 2015

Baby Has New Shoes, Version 1.1

"Meet the New Look... same as the Old Look..."  (apologies to The Who)

When I bent two rims on my Civic Si, it was an opportunity, not a problem.  The summer-only high performance radials on Hotshoe's Focus ST were a revelation to me.  Then Watchtower put a set of Michelin Pilot Super Sports on his Bullitt--and is very satisfied with them.

New shoes for the Si would be Dragon-oriented, because that's the only high-performance driving I do. But the Si is also a daily driver, so it needs to get down Interstates in the rain: super stickies were not an option.  Watchtower made tire selection easy by sending me a Tire Rack test against some comparables. Pilot Super Sports it would be.

Tire Rack was out-of-stock on 17 X 7 OZ wheels to replace my bent ones.  More precisely, my "Anthracite" color is discontinued, with only one in stock.  So I opted for 17 X 7.5.  They are "Matt Graphite Silver," which is less attractive.  But I wanted to keep the light weight of the Allegerita HLT line, so I'll live with it.  Surprisingly (and gratifyingly) the wider wheels weigh only about a half pound more.

This reopened the possibility of wider tires.  I stayed with a 45 aspect ratio (and thus 17-inch diameter) because 1) the ride is already firm enough for a street-driven car and 2) 40-series tires would make the wheels even more vulnerable to the potholes that created this opportunity to begin with.  A 235 tire is nominally about 0.8-inch wider than a 215, a bit less at the contact patch, so it is well-supported by the 0.5-inch wider wheel.  Clearance is very tight at the left rear because of a plastic shield on the inner fender well for the fuel filler pipe; otherwise it's ample.  If it is a problem, it should manifest itself in right-handers, with the left rear suspension in compression.  (The widest tire Tire Rack has tested on the Si, and recommends for it, is a 225.)  The heaviest balance weight needed was 2 oz.; the total weight across 4 tires was under 5 oz.  That's a tribute to Michelin's manufacturing tolerances.

As each tire will now support less weight per unit of surface area at the contact patch, I'll start with Honda's recommendation of 33 p.s.i. all around, and experiment from there.

The 0.5 inch increased wheel width and slightly more tire width is a
barely perceptible difference from stock, I believe because the
offset was changed by about 0.25 inch.  Clearance may be
an issue for the left rear only, because of the plastic
gas tank filler pipe shield.

Oddity: bouncing rubber balls (actually, orbiting rubber b-b's).  Keys shown for scale.  This one was the largest, at about
0.25 inches in diameter.  About a half-dozen of them came out of both old front tires (none from the rears).  My
mechanic believes they are flakes of rubber from the inner sidewalls, "chafed" loose by hard (cornering) use.
The flakes aggregate into spheres via centrifugal force from the rotation of the tires.  It's the reverse of the
physics of planet formation around a star: the flakes are contained in an orbit by the inner tread wall.

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