19" vs. 18" Wheels/Tires

[Pat]

Originally Posted by ekam

18" vs 19" Comparison - The Pros & Cons of your track tire- Best Motoring International - YouTube

That video is a joke. While I like the intent, the "study" is executed poorly. The idea is to compare the performance of 18" wheels with that of 19" wheels. In order to do that, one must isolate all other variables. They did not do this. The only changes in the front wheels are diameter, which is perfect for the goal of this video. However, the 19" rear wheels are an inch wider than the 18" wheels. That alone can skew the results due to size and weight factors. Moreover, the front tires on the 19" wheels are 20mm wider than those on the 18" wheels. Even worse, the rear tires on the 19" wheels are 30mm wider than those on the 18" wheels. That's totally skews the results.
The cherry on top is when the test driver says, "I can feel the rolling resistance." Yeah...sure you can.

Ceteris paribus, if your top priority is performance, go smaller. If you care more about appearance, go bigger. Done.

[Johnny Danger]

Nobody runs 19's at the track . That's a given . We're talking about street applications here .

[Johnny Danger]

Originally Posted by jmatta

Johnny...I want some of those Salmon steaks!

As always, your car is a real looker...

I was thinking of adding some salmon steaks to the front spoiler . What do you think ?

[BYprodriver]

Originally Posted by Johnny Danger

I disagree with you assertion . Ultimately, it comes to reducing unsprung weight and rotational mass. If the overall circumference is consistent, regardless of diameter the the lighter wheel will create less inertia .

What would be the correct assertion if the overall circumference is inconsistent?

[ekam]

Originally Posted by Pat

That video is a joke. While I like the intent, the "study" is executed poorly. The idea is to compare the performance of 18" wheels with that of 19" wheels. In order to do that, one must isolate all other variables. They did not do this. The only changes in the front wheels are diameter, which is perfect for the goal of this video. However, the 19" rear wheels are an inch wider than the 18" wheels. That alone can skew the results due to size and weight factors. Moreover, the front tires on the 19" wheels are 20mm wider than those on the 18" wheels. Even worse, the rear tires on the 19" wheels are 30mm wider than those on the 18" wheels. That's totally skews the results.
The cherry on top is when the test driver says, "I can feel the rolling resistance." Yeah...sure you can.

Ceteris paribus, if your top priority is performance, go smaller. If you care more about appearance, go bigger. Done.

When you upsize, you're inherently going wider as well. For example, while you can get 7" wide, 17" wheels, you cannot find 7" wide, 19" wheels. Of course when you go wider wheel, you have to get wider tires. Hence the reason why if you have 17" you're running 255 rears, and for 18" you're running 265 rears and so on. In fact, that's the whole idea of upsizing... you should read up the tech article at TireRack.com.

The point of their exercise is that going wider & larger wheel which theoretically provide more traction & grip, does not provide better driver feedback and marginally quicker track time even in the hands of a professional driver.

[Mrmaddbrad]

Originally Posted by Johnny Danger

I was thinking of adding some salmon steaks to the front spoiler . What do you think ?

Just run them as spacers. Performance Achieved!

[DenverSteve]

Thanks Johnny. Post 18 is what I was hoping for. I can easily "chart" the difference in height and width on any tire converter. That doesn't give me the Boxster application I need. Both this and my first Boxster have always run factory wheels and tires. It's a little different than changing out the 32's on my Wrangler.

[Johnny Danger]

Originally Posted by BYprodriver

What would be the correct assertion if the overall circumference is inconsistent?

Here's an example:

The original factory set-up on my vehicle consisted of 18 inch Sport Design wheels

18 x 7.5 225/40/18
18 x 9 265/35/18

Current wheel set-up

19 x 8 235/35/19
19 x 10 275/30/19

Practically speaking, the overall circumference of the wheel and tire combination is similar (had I kept the fronts at 225 and the rears at 265 the circumference would have been virtually the same) . However, I have reduced approximately 60 lbs.+ of unsprung weight and rotational mass ! As far as acceleration, braking and overall handling is concerned, this has proven to be a night and day difference over the oem set-up . And, ride quality has not suffered one bit.

[cheetah]

Originally Posted by Johnny Danger

I disagree with you assertion . Ultimately, it comes to reducing unsprung weight and rotational mass. If the overall circumference is consistent, regardless of diameter the the lighter wheel will create less inertia .

Disagree or not, physics shows that with all else being equal (same tire circumference, same width, same rim style & density), the larger rim setup will have a larger rotational mass.

Most of the tires mass is on the tread (which diameter & weight doesn't change with rim size). Most of the mass of the rim is the on the circumference of the rim (which does change with rim size). Moving this large weight farther from the axis has an effect on the rotational mass. More of the weight differences comes from the metal rim vs the rubber tire changes.

I have enclosed a simple display of this from a 3D modeling program. The units are converted to simple rotational mass, but they show the comparison of an 16x8 rim with a 25" diameter tire to an 18x8 rim with a 25" diameter tire. This is a good test for our Boxsters.

The 18" rim setup shows a 23% increase in effective mass of the wheels, even though the overall circumference of the wheels stays the same.

[Johnny Danger]

The point that you're missing - is aside from comparing wheel/tire fitment of equal overall specifications, when there's a significant difference in weight between the two, this results
in a major reduction of unsprung weight and rotational mass .

[Pat]

Ekam, I disagree with your statement, "When you upsize, you're inherently going wider as well." There are plenty of 16" wheels out there that are wider than the stock Boxster 17" wheels. Additionally, there are plenty of wheels out there that allow custom barrel sizing and custom diameter. While I will agree most of the time taller wheels are also wider, saying the two factors are inherently coordinated is absolutely false.

[ekam]

Well disagree all you want but fact is almost all wheel manufacturers do not offer custom sizing (even so, it will not be at a price you and I can afford.) Add variables like wheel offsets and bolt patterns you're stuck with what they offer and that's that. Take a look at sites like Tirerack and see it yourself.

[stephen wilson]

Well, the factory 18's and 19's have the same section width, so no, it's not mandatory to go wider. He said the 19's are MORE sensitive, more like a race car, but that sensitivity may not result in quicker lap times for most drivers. The other problem is the Z wasn't designed for 19" tires, which are much more camber sensitive. Results will vary depending on the car in question.

[smshirk]

Originally Posted by DenverSteve

Johnny,

I am looking at a set of tires/wheels for winter wear that are 19" and the rear tires are currently 295/30's. The tire calculator shows them to be comparable to my stock 255/40/17's. The offset would go from 55 to 67. You seem to have much more tire/wheel knowledge than I. Do you think these will fit okay?

S.

The tire calculator showed that my 18 x 11 with 295 30s would fit on my 986S. It wasn't even close. If I used a wide enough spacer to get far enough away from the shock tower, the tires stuck way past the fender, plus the 295 30 diameter looked weird on the stock ride height. I've had this brand new set of Gemballa wheels and tires sitting in the basement for 3 years since selling my 996 and finally tried them yesterday.

I have a set of 18" CCW race wheels Complete Custom Wheel - Wheel Profiles in Boxster offsets that are very light but won't accept center caps. I've been trying to figure out a way to attach some caps without screwing the wheels up. They have holes drilled all around the center and the outside to reduce weight, so I'm thinking it wouldn't hurt to drill and tap a set of center caps in the middle. Does anyone have an opinion?

[Johnny Danger]

Originally Posted by smshirk

The tire calculator showed that my 18 x 11 with 295 30s would fit on my 986S. It wasn't even close. If I used a wide enough spacer to get far enough away from the shock tower, the tires stuck way past the fender, plus the 295 30 diameter looked weird on the stock ride height. I've had this brand new set of Gemballa wheels and tires sitting in the basement for 3 years since selling my 996 and finally tried them yesterday.

I have a set of 18" CCW race wheels Complete Custom Wheel - Wheel Profiles in Boxster offsets that are very light but won't accept center caps. I've been trying to figure out a way to attach some caps without screwing the wheels up. They have holes drilled all around the center and the outside to reduce weight, so I'm thinking it wouldn't hurt to drill and tap a set of center caps in the middle. Does anyone have an opinion?

What wheel/tire calculator are you using ?? There is no way that an 11 inch wheel will fit a 986 boxster . Let alone with a 295 tire .

[blue2000s]

Originally Posted by cheetah

Even if the 19" weigh less than the 17's, the effective mass (moment of inertia) is much higher. Simply put, the weight is farther from the axis of rotation.

I currently have 18's 225-F / 265-R on my 02 S, and am going back to 17's. The 18's were great until I did all my suspension mods (PSS9's, solid everything, etc.) but now I want a little more sidewall. When it breaks loose, it happens suddenly.

Also, I am getting some lightweight, Forged rims so the wheel weight will go down significantly. Weight reduction is my plan of attack for now, since I have a heavily modded 3.4L which puts out a lot of power already.

The big rims look great but have their downsides.

+1. I was going to post the same thing until I came across your post.