I've been plodding along with stock 14" bottle caps on my Spec E30 so far. Conventional wisdom out there is telling me moving to the lower profile 15's would reduce my lap times by a good second. I was pretty close to buying a set of new ones when some nearly free ones turned up. They each weigh 18 lbs compared to the 13 or 14 of the new wheel choices I'm looking at. Is 20 lbs extra unsprung weight really worth worrying about on a road race car? I'll probably snag them anyway, but I'd like to hear arguments either way.
My vote would be to go for the lighter wheels. I though the rule of thumb was every pound of unsprung weight was worth about 5 lbs of sprung weight, and that every pound rotating mass was worth about 10 lbs of static. Being both unsprung and rotating I don't know if those should add or multiply, but either way it's not "just" 20 lbs, it's 20 of the worst pounds you can have (other than flywheel mass).
I changed from curb friendly stock Neon Alloys to Rota slipstreams, and from Toyo 888's to Goodyear RS tires this year. The wheel/tire package is over 5 pounds lighter. My home track has a chicane, where the fast line is to bounce the curbs on both sides. I am now passing cars in that chicane that passed me in the same place last year. The composure of the car in the rough areas is like night and day.
Get the lighter wheels.
yeah, it's worth the coin to get rid of rotational weight. GRM published some excellent articles by David Vizard several years ago which dealt with exactly this subject.
Streetwiseguy wrote:
I changed from curb friendly stock Neon Alloys to Rota slipstreams..
Same here. Changing from 14" stock Sport alloys (16.5# ) to 15" Rotas Slipstreams (12#) bought me 1.5 seconds on average.
Of course, changing from 215s to A032Rs likely accounted for a good bit of the improvements as well.. 
Thanks boys. I will pick them up anyway because they are case-of-beer cheap but they will be spares.
Changing tires along with the wheels makes it difficult to say which made the improvement.
Also, heavier wheels do have a use (other than holding down a canopy :) - Rain Tires. Thats the one point that having heavier wheels / more inertia is better.
Kendall
iceracer wrote:
Changing tires along with the wheels makes it difficult to say which made the improvement.
The grip of my new tires is better, but the feel of the car over rough surfaces is completly different. Its as if i had the shocks revalved.
