Piston Slap: Si or No to 'Minus 1' Tire Sizing?

TTAC Commentator 70Cougar writes:

Sajeev,

I am probably the first owner of a Civic Si to ever want to downsize his rims, so I’m looking to the expert on -1 matters: you. (Woot! –SM)

I have a 2007 Honda Civic Si sedan with the factory 17-inch wheels. I’ve always preferred the same generation EX’s 16-inch five spoke rims, and I wouldn’t mind a smoother ride and cheaper replacement tires if I still get decent handling performance out of the 16s.

Would the 16-inch rims fit around the brakes on the Si and otherwise safely function on my car? Would there be a significant decline in handling performance?

Sajeev answers:

I should be the reason why bangin’ big-ass wheels are dumped for smaller ones!

The first determination is front rotor size. Rears are almost always smaller, therefore irrelevant. Snooping around RockAuto, the Si has a much larger front rotor compared to its more plebian counterparts. Oh dear:

• Civic Si front rotor diameter: 11.81″
• Civic Other front rotor diameter: 10.30″

The caliper part numbers are also different and, if this thread is correct, increased surface area implies the Si calipers are larger. But are they taller or wider … or both?

It’s possible an 11.8-inch rotor and caliper clears a 16-inch rim. Googling up snow tires, this thread says 16-inch steelies fit, so test fit your preferred 16-inch cast-aluminum wheel. If Corvette ZR1 peeps can -2 down to an aftermarket 18-inch wheel wrapped around their fancy ceramic stoppers, isn’t anything possible?

Your last question is indirectly referenced in the ZR1 hyperlink. If the aluminum 16-inch wheel fits (less unsprung weight than a steelie) and if you buy a high quality, high performance 16-inch tire (plenty of grippy summer tires on TireRack) you won’t lose a drop in performance.

I betcha new 16-inch wheels with those aforementioned summer tires will spank the stock 17-inch Michelin Pilot All-Seasons around a road course. Perhaps you can run 16-inch summer tires and “ruin” your factory Si wheels with winter tires? That’d be something!

[Image: Shutterstock user PPstock]

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.