What return from going with lighter wheels?
 

What return would I expect from going with a lighter wheel to replace the factory FR-S wheels?

I am considering Enkei RPF1's in a 17"x7.5" +38 offset. These wheels weigh 15.2 lbs. according to TireRack. That's approximately 5 lbs less than the factory wheels.

What will this do for my car? How big of a change does reducing unsprung weight actually have on a car?

Rule of thumb I've always heard is 4x the weight for unsprung. There's also maths for how far it's from the axle but meh.

So 5lbs a corner = 20lbs = same effect as removing 80lbs of body weight.

A pound saved in the wheel/tire is worth more then a pound saved elsewhere. Its rotating mass, you should notice a difference in acceleration, braking and steering.

The general rule of thumb is that reducing rotating weight at the tires by 1 lb is about like reducing 2lbs of static weight. However there is a lot more to it than that... for instance 5 lbs removed from the tires themselves (combined) will have a greater effect than 5lbs removed from the driveshaft.

If we use this calculator http://robrobinette.com/et.htm a 2750 lb car with 200 flywheel hp has a 1/4 mile et of 14.70 seconds
If you reduce the weight of the car by say 40 lbs (5 lbs per corner = 20 lbs * 2 for our rule of thumb) to 2710 lbs then the 1/4 mile et is 14.63 seconds. It's about like having 3 extra horsepower, or approximatly 1.5% more on our fictious car. So for the lighter wheel you will see a bit quicker acceleration times, and a very slight increase in fuel mileage.

There are also handling characteristics that will become more apparent as well. Firstly lets make a few assumtions lets say we have a 2750 lb car with equal corner weights so 687.5 lbs per corner and our unsprung weight is 50 lbs per corner. Then our sprung to unsprung weight ratio is 13.75:1 What this number ultimately means is how much the chassis is upset by bumps. The higher the number is the less bumps will affect the chassis. If we then reduce our unsprung weight to 45 lbs per corner our ratio becomes 15.278:1. This means that not only is a bump less likely to throw off the car from its line but that there is also effectively more force available to push the tire back to the pavement (it's not actually that way but its a simple way to explain it). Also because there is less weight the damping becomes more effective. That can be a plus or a minus depending on how effective the damping was for the given conditions prior to the weight change, though more than likely it will be a plus as I don't know of any car that is perfectly damped.

Basically, the engine has to work less to get the wheels up to speed. Therefore, you have better responsiveness in acceleration. The brakes also work less to bring the wheels to a stop. The OEM 86 wheels are very light; so I don't see the reason to shell out unless you go wider, which will help your lateral traction tremendously.

21 pounds is not very light in my book. Enkei or OZ 15 pound wheels is what I call very light!

Yeah 21 pounds for a 17x7 wheel is heavy. Not light at all. The lightest wheels in that size will be around 12-13 pounds which would be a substantial weight savings. I'm trying to see if I can find a set of Velox PG-5s around here...no luck so far. 12.5lbs for 17x7 though!

fixed

Sorry about that, I was misinformed. The 16s weigh in at 17 lbs a piece though.

Motegi traklites are forged and the 17s weight 14 lbs. $250 a wheel.

Thank you to everyone for the info.

Thank you DEnd for the very informative answer.

And thank you Captain Snooze for correcting my typo. That was way off.

one thing that is being ignored here is how unsprung weight will effect the suspension. Think of how much more your arm has to work when you lift a dumbbell that is 25% heavier. Same goes for the springs and dampers on your car. heavier wheels keep the suspension from reacting as fast as it would with less weight simply because of the inertial forces of the increased weight.

on a similar chassis, (miata) 5 lbs in wheel weight IS butt dyno noticeable in both centrifical force related ways (accel, decel) as well as compression and rebound related ones (comfort, lateral grip, etc)

So from what I read are only upsides, no downsides to going lighter? :)

Lighter wheels, depending on construction technique, may not be as durable as heavier wheels.

Lighter wheels make your wallet lighter.