Do lightweight wheels make the car faster?
 

So I'm debating on taking off my Enkei M52 17x7.5 (20 lb) wrapped in 225/45 Michelin PSS (22lb) and pushed out with spacers (approx 1.5 lb)
and swapping them for Flowform720 GTF1 17x8 (15.5 lb) wrapped in 245/40 Michelin PSS (23 lb) and no spacers for a total savings of 5 lbs at each corner.
I planned on doing this to get better acceleration and performance out of it and not so much for the looks since I am happy with it presently.
Then I got to looking at what real world difference is and not just seat of the pants dyno data and came up with this article below.

Their 4 lb difference is pretty close to the 5 lbs at each corner I hoped to save but their results tell a different story that what I was led to believe on wheel/tire weight performance.
So what's the deal?




http://i.imgur.com/rJLu2HW.jpg
http://i.imgur.com/bwZqgkZ.jpg
http://i.imgur.com/4j5E7aC.jpg
http://i.imgur.com/78W6tUV.jpg

Narrower, shorter, sticker tires will make a bigger difference in performance than lightweight wheels. What lightweight wheels/unsprung mass will do is improve the ride quality. Suspension can work more efficiently and quickly with less force/mass for it to have to control, this becomes more apparent the worse the surface condition is - smooth surfaces minimize the differences. I'm using a 17lb wheel (17x7.5 +40mm offset Enkei Fujin) and 19lb tire (205/45-17 Continental ExtremeContact DW) setup and would never want to go back to close to stock mass again...car is much more nimble and responsive to input, and more stable with the associated CG drop from the tires. And less effected when the road surface becomes less than optimal...

That article seems to contradict all of my beliefs and butt dyne "sessions" so I can see why you would be confused. However all I can really do is give a real world experience that I felt when moving to lighter wheels. My first set up was a 18x9.5 Rays Gramlight 57dr with 255/35 tires and the car did feel a bit slower than the stock 17s. However when I finally decide to pull the trigger on some light weight wheels I ended up with 17x9 RPF1s with a 255/40 tire. For me it was a night and day difference in acceleration. I actually felt the car move while in a high gear at a low RPM. I feel that maybe the reason why I felt such a difference was because first of all I went from a 18in wheel to a 17in wheel, I upgraded from a street tire to an r compound tire and I went from a wheel weighing almost 22lbs to a wheel weighting 15.6 lbs. In your situation I don't feel the difference will be that noticeable and the resources may be best used elsewhere. Just my two cents

What about the spacers, wouldn't you have to tack that weight onto the Enkeis? Surely that's extra weight the hub has to rotate yes ?

I think the biggest issues with their test is the buttery smooth track. But also note in that little blurb in the end with a larger difference the lighter wheels were slightly better.

As their drivers noted, lighter feels better while driving. Sometimes that confidence is more important. But 4 or 5 lbs is not that big of a difference.

205?

A tenth of a second on a short track or cone course is not enough to account for human error between laps.


There's also rotational moment of inertia to consider, which is simply put, how much the wheel resists the torque applied to it. It is possible, although unlikely, for a heavier wheel that has more of its mass concentrated towards the inside to actually have a smaller rotational moment of inertia than a lighter one. That and/or the fact that the tires should affect it much more than the wheels could account for the machine not noticing a difference.

it also helps the steering feedback

I agree with most of what has already been stated... just want to add a couple small points and clarify:

1) Rotating mass has a noticeable affect on braking...
2) As mentioned, the radial distribution of rotating mass makes a large difference. Your spacers don't affect acceleration/braking as much as your tires do, since the weight is close to the axis of rotation.
3) Light wheels can flex more contributing to slower lap times - ideally you want very strong, light wheels.

Unsprung mass helps handling, as stated above; however the original article changed too many things (as I fear you are doing) to draw a conclusion about the wheel's contributions to speed. Tire width contributes to rolling resistance and grip, and as mentioned diameter effectively changes final drive ratio for all gears. Almost all of a tire's weight is at the farthest point from the axis of rotation.

All of these changes are small/subtle, and the effects can be additive or cancel each other out. If your car is NA and you are trying to maximize acceleration and performance, I personally would stick with 225s. A NA car really does not need a 245 for performance and the 225 will weigh less and have less rolling resistance. That said I doubt the difference is meaningful to anything other than your wallet.

Here's a question for you as I'm on the fence between 245 40 17 and 225 45 17. The 245's are a lb lighter so do you feel that with the added rolling resistance but lighter weight it would be a wash on straight line performance? I'd imagine overall the 245's would be better when you consider cornering in the equation of overall performance? Thoughts?

It depends on the brand and model of tire - I will take MPSS as an example.
245/40R17 is a pound heavier - and only 0.3 inches shorter, I seriously doubt anyone would feel a difference. The 245 will be more expensive, and with NA power levels very, very few people will be able to take advantage of the additional traction. As mentioned in the original quoted article the overall tire diameter can make a noticeable difference in speed.

Really this is a coin toss for you... unless it is part of a larger set of complementing changes I seriously doubt you will notice any difference at all

It depends on what wheels you are putting them on too. You aren't doing yourself any favors squeezing a 245 on a wheel that should have a 225 or less on it, and messing up the rigidity of the tire's sidewall. Literally all you're doing is causing the sidewall to bend inward in a way that it wasn't designed. This ultimately is doing nothing but tacking on extra width (weight) at best and reducing your tire's ability to do their job at worst. Alternately, getting wide wheels and stretching tire that is too narrow onto it is going to save you some weight but will also negatively affect your tire's ability to do their job.

right, thanks. Want to go 8" so I can use a 225 or 245.

if you're worried about that 5 lbs, just go on a diet. :/