With a proliferation of alloy rims over the past 10 or 20 years, I'm wondering if technology has allowed them to become lighter and stronger? Is your average base wheel now much lighter than a base wheel on a Porsche, say, 20 years ago? Have the Alloys themselves changed a lot? How much lighter are carbon fiber wheels, and are they going to become more and more common?

 

An interesting point, but one problem with any comparison is that modern wheels are so much larger, both in diameter and width, than the older ones.

 

What he said... I promise you the modern wheel despite the thin spokes is way heavier than the old one ;-) There's probaly twice as much rubber around it too!



In all seriousness 20y ago, I'd say not really... The 996 had some thin spoke 18s and still less rubber... The trend towards bigger wheels and more tire - we're now using 20s - means more wheel material.. The carbon fiber wheels are a great idea to reduce the unsprung weight but still way to expensive... It is amazing to me the modern cars are as fast as they are when I carry one of those wheels by hand and realize how big and heavy they are...
I've started shopping on tire rack for tire weight specs now, there is a substantial difference from one make to another !!! sometimes 4-5 lbs ! People think light wheels but don't check tires...

 

It shouldn't be. Wheel weight doesn't affect acceleration more than weight on any other part of the car does.

Yes, unsprung weight affects vertical forces, shock absorbers and the like, so it certainly has its importance. But you said how fast, not how the suspension handled bumps.

Yes, heavier wheels and tires increase your moment of inertia, which means it requires more force to achieve the same angular acceleration. If your wheels were free spinning, that'd be your primary concern. When they're accelerating or braking the mass of a car, though, it's irrelevant, since the rotation of the wheels is only a method of transforming crankshaft torque into forward force, and the inertia of the wheels is no longer relevant to ye old F=MA for the car.

Before the peanut gallery starts in on the hand-waving and physics denial, you might find reading this thread helpful. It's an actual test of acceleration in a 1st generation NSX, measuring the effect of reducing total wheel weight by 27.3lbs. He saw a reduction in 1% on 20kph-100kph times, which since we're talking about velocity rather than distance, is a straight 1% increase in acceleration. Which is exactly what you'd expect from reducing the weight of the car by 1%, anywhere on the car, wheels or body.

That's just the first example I found. If you do a web search on the subject, you'll see lots of people arguing it's important without actually backing it up, and handful of people who have done actual tests and discovered that it's simpler than people seem to think.

 

That would really fill out the wheel well!!!

 

Gus, while the effect might be minimal, rotational mass and inertia do affect acceleration. If all else is equal, moving mass from the wheel and tire to the hub carrier, would not change the overall or unsprung mass. But spinning up a light wheel and tire will be easier, and thus accelerate the wheel (and car) faster.

Anecdotally, I can definitely feel the difference on my bike, where a lightweight wheel set and racing tires have a negligible reduction of total weight (bike plus me), but much bigger change in rotational inertia--and bike acceleration.

 

You did a good job of putting wheel weight into perspective, even though your examples do highlight that wheel weight does affect acceration... just not materially. A factor that needs to be taken into account is wheel diameter because stored energy goes up by the square of the radius change. If you increase the wheel size with lighter wheels, the forces offset each other by some margin.

I think it’s fair to say that we agree on the major points, which is that it is a lot more complicated than just putting lighter wheels on. There are many other rotating parts that have mass which affect acceleration, from the crankshaft to the drive shafts, and where the rubber meets the pavement. On that note, lighter tires will have a greater impact on acceleration than lighter wheels.

 

I mean, when the tire shop pulled off my HRE's (12" rears) they were shocked at how light it is. So it's being done, but it seems unless you pay a ton for CF rims or the like, most OEM wheels are still of the heavy fare.

 

That test you referenced was interesting, and the NSX owner did a reasonable job of trying to perform a fair test, but there are a number of reasons why his results do not settle this subject.

First, while he did his best to compare lighter and heavier wheels with everything else being the same, he did not account for all of the weather conditions during his two sets of runs. He mentioned that it was a bit cooler when he was testing the lighter wheels--which would help the engine develop a bit more power--but he didn't quote the bara metric pressure and relative humidity. Both effect engine output and would effect the results.

Furthermore, the rotational inertia of a tire and wheel is dominated by the tire, which has all of its mass further from the axis of rotation than the wheel does. So a slightly lighter wheel has a minimal effect on the rotational inertia of the entire assembly. This, of course, does suggest that wheel weight has less effect on acceleration than it does on suspension performance. But rotational inertia clearly effects acceleration and whether the tires and wheels are transferring thrust to the pavement is irrelevant.

And it's worth remembering that high performance engines tend to continue to have lightweight flywheels and smaller diameter clutches. The reduced rotational inertia of these components contributes materially to acceleration.

 

Interesting question. The new flow-form is basically forged, but unlike the old Fuchs alloys where the whole wheel was forged, with the new designs only the rim is flow-form. So a little bit lighter, and at least the weight is reduced at the rim where it has the most effect, but still not a big deal. The real improvement is CAD/CAM which is why we have, I believe the technical term is bazillions, of different looking wheels that are all the same.

Carbon fiber wheels are a lot lighter. But they have a lot more going for them than that. Carbon fiber wheels are also a lot stiffer. They flex less under load, retaining geometry better. Carbon fiber inherently dampens vibrations. So less road noise. Its even more repairable than aluminum.

Are they going to become more and more common? Already are. From Koenigsegg to Porsche. The only real downside is cost. Which in this market is probably more bragging rights than hardship. So expect more and more of them.

 

I was astounded by the difference in performance between the GT350 and 350R which they stated was primarily due to the reduced weight of the Carbon Fiber Wheels.

 

What performance difference impressed you? Acceleration? braking? Handling?

 

i was spoiled by BMW E36 wheels. Very light. My Panamera rear wheel/ tire tip the scales at 70lbs. Haven’t check the 991.1 GTS but I expect heavy. Seems that is the next frontier for weight reduction at a reasonable price.

 

It's also likely in reference to a Car and Driver article that was posted comparing the performance of the Aluminum spec wheels versus their lighter Carbon wheel option on the GT350R. Here's a screenshot that I'm sure everyone would be interested in looking at while referencing the benefits of a lighter wheel. The article and their data speak for themselves really.


Article - https://blog.caranddriver.com/tested...-fiber-wheels/

 

Forgot to mention, lighter wheels also provide better steering feedback or road feel.