wide vs. narrow wheels/tires

[Leonardo]

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[solidONE]

Originally Posted by Leonardo

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[NoHaveMSG]

Originally Posted by why?

Has nothing to do with who is smart. It has to do with having a conversation and exchanging ideas. It seems most people that track twins have decided 245 or 255 17's are the way to go.

Guys like CSG Mike actually do recommend the 225.

The track you are running makes a big difference though.

The tire demands at PIR (Look at that stupid thing )

https://portlandraceway.com/?/fan_guide/track_map

are totally different then a track like ORP

[venturaII]

Originally Posted by NoHaveMSG

Guys like CSG Mike actually do recommend the 225.

It depends on how hard you're working the tires...if you're at a track with fast sweepers, long straights, etc, then a 225 could certainly be faster than a 245. But at a track that works the tires harder and top speeds are lower, a 245/255 could be a better choice. And likewise, in applications like autox which work the tires even harder and speeds are even lower, the big tire combo is almost certainly the fastest way around. I doubt CSG is recommending 225s for that application.

[rvoll]

Originally Posted by venturaII

It depends on how hard you're working the tires...if you're at a track with fast sweepers, long straights, etc, then a 225 could certainly be faster than a 245. But at a track that works the tires harder and top speeds are lower, a 245/255 could be a better choice. And likewise, in applications like autox which work the tires even harder and speeds are even lower, the big tire combo is almost certainly the fastest way around. I doubt CSG is recommending 225s for that application.

Wider tires certainly have more lateral grip so I certainly agree with the thrust of your argument. But from what I've gleaned over the years, a lot depends on the tire choice. If you have a track tire that has a lot of stick, 225's may well be enough that "works the tires harder". We talk about tire width a lot, but in my experience, the right tire makes a much larger difference. For the weight of this car, 225 should be more than effective, even at the track. Autocross is a different animal and much harder to determine the right wheel/tire combo. However, my testing in the past has said that wider tires in Autocross generally have given me faster times. That said, my experience has been in other cars, not the twins....

[nikitopo]

Originally Posted by NoHaveMSG

Guys like CSG Mike actually do recommend the 225.

I am not sure if CSG Mike has ever tried a 205/55R16 tire on this car. Issue is that you never got the racing spec trim in US with the smaller tires and let's face the following. The tread width of a 205/55R16 tire on a 6.5" wheel is 7.4". The tread width of same tire on a 7" wheel, which is quite common in Gazoo 86/BRZ races, might be a bit more. The tread width of a 225/45R17 tire on a 7.5" wheel is 8.1". So the difference on tread width is pretty small and I expect to not make a big difference even on a tight track. Additionally, the smaller 16inch wheel has the advantage of better weight distribution. It is not always about total weight, but where the weight is distributed. You need to have a minimum rotation inertia on a tire/wheel combo.

Reference on specs: https://www.tirerack.com/tires/tires...del=ADVAN+A052

[NoHaveMSG]

Originally Posted by nikitopo

I am not sure if CSG Mike has ever tried a 205/55R16 tire on this car. Issue is that you never got the racing spec trim in US with the smaller tires and let's face the following. The tread width of a 205/55R16 tire on a 6.5" wheel is 7.4". The tread width of same tire on a 7" wheel, which is quite common in Gazoo 86/BRZ races, might be a bit more. The tread width of a 225/45R17 tire on a 7.5" wheel is 8.1". So the difference on tread width is pretty small and I expect to not make a big difference even on a tight track. Additionally, the smaller 16inch wheel has the advantage of better weight distribution. It is not always about total weight, but where the weight is distributed. You need to have a minimum rotation inertia on a tire/wheel combo.

Reference on specs: https://www.tirerack.com/tires/tires...del=ADVAN+A052

I think the biggest issue with the 16" comes with fitting big brakes so I am not sure how much experimenting has been done with them over here.

[nikitopo]

Originally Posted by NoHaveMSG

I think the biggest issue with the 16" comes with fitting big brakes so I am not sure how much experimenting has been done with them over here.

There were some users on the Gazoo races complaining about the brake rotor size and how much they can handle the extra wear when the pads and tires are becoming every year better and better. The Gazoo 86/BRZ races are really a 10 lap race every time, so don't expect that much with these brakes. If you are a professional racer and looking for something better, then a 17" wheel is a better option.

[venturaII]

Originally Posted by nikitopo

The tread width of a 225/45R17 tire on a 7.5" wheel is 8.1". So the difference on tread width is pretty small and I expect to not make a big difference even on a tight track. Additionally, the smaller 16inch wheel has the advantage of better weight distribution. It is not always about total weight, but where the weight is distributed. You need to have a minimum rotation inertia on a tire/wheel combo.

The tire width difference is something like 10%, so that's well within a reasonable range of expected improvement/increase...no one is looking to double the width of their tires.

And a 16" wheel actually makes the distribution of the weight of the wheel/tire unit worse, not better. The wheel is always going to be lighter than the tire. By adding to the sidewall, and reducing the diameter of the already lighter component (wheel), you're shifting more weight to the outer circumference of the unit (rubber and steel cord weighs more than the forged aluminum that would be in their place). Overall you might be a bit lighter than a comparable 17"/205 combo, but the difference would probably be offset by the increased inertial mass out at the circumference. Which makes any advantage purely one of unsprung weight, not rotational.

But the whole issue is academic and not likely to matter one bit either way.

[rvoll]

Originally Posted by venturaII

The tire width difference is something like 10%, so that's well within a reasonable range of expected improvement/increase...no one is looking to double the width of their tires.

And a 16" wheel actually makes the distribution of the weight of the wheel/tire unit worse, not better. The wheel is always going to be lighter than the tire. By adding to the sidewall, and reducing the diameter of the already lighter component (wheel), you're shifting more weight to the outer circumference of the unit (rubber and steel cord weighs more than the forged aluminum that would be in their place). Overall you might be a bit lighter than a comparable 17"/205 combo, but the difference would probably be offset by the increased inertial mass out at the circumference. Which makes any advantage purely one of unsprung weight, not rotational.

But the whole issue is academic and not likely to matter one bit either way.

We can go on theory all day, but this remains a complicated topic whereby the only way to truly know is to test alternatives at the SPECIFIC track you run. There are some truisms from those of us with track experience like knowing that putting wide tires on this car does not give you better performance at the track except in very special circumstances like autocross. Or that tires make much more of a difference than wheels. When it comes to 16" or 17" wheels I submit that it depends on a number of aspects and there is no simple answer. How the car is setup, what the track is like, how big the brakes are, changes in suspension, etc. So it may be true that you are both right...

[venturaII]

Well it's just math, not theory. But either way it's not going to make me change my 225/50-16 setup. I'm quite happy with how well it works.

[solidONE]

Originally Posted by NoHaveMSG

The tire demands at PIR (Look at that stupid thing )

https://portlandraceway.com/?/fan_guide/track_map

are totally different then a track like ORP

It's beautiful... look at the elevation change.

[nikitopo]

Originally Posted by venturaII

And a 16" wheel actually makes the distribution of the weight of the wheel/tire unit worse, not better. The wheel is always going to be lighter than the tire. By adding to the sidewall, and reducing the diameter of the already lighter component (wheel), you're shifting more weight to the outer circumference of the unit (rubber and steel cord weighs more than the forged aluminum that would be in their place). Overall you might be a bit lighter than a comparable 17"/205 combo, but the difference would probably be offset by the increased inertial mass out at the circumference. Which makes any advantage purely one of unsprung weight, not rotational.

This is really contradicting of what we know so many years about smaller wheels. I checked in tire rack the weight of A052 225/50R16 and the weight of A052 225/40R18. I chose these sizes, because they are easier comparable. Both have similar diameter and same weight 22 lbs and similar tread width 8.3" vs. 8.4" . The difference is only on the wheel and if you compare same models, then the 18" wheel will always be considerable heavier than the 16" wheel. So, the smaller wheel has better rotation inertia. The issue on wheel size is really to be able to clear the brake rotors. You never need a bigger wheel than that. Unless, you focus on the looks ...

[ZDan]

Originally Posted by venturaII

And a 16" wheel actually makes the distribution of the weight of the wheel/tire unit worse, not better. The wheel is always going to be lighter than the tire. By adding to the sidewall, and reducing the diameter of the already lighter component (wheel), you're shifting more weight to the outer circumference of the unit (rubber and steel cord weighs more than the forged aluminum that would be in their place).

The smaller-diameter wheel/tire will have less rotational inertia vs. same-width/same-diameter tires on a larger-diameter wheel.
1. Most of the weight of the tire is at the tread, which is in exactly the same radial location for same-diameter tires.
2. Despite slightly more sidewall, tires for smaller-diameter wheels weigh the same as same-width and same-diameter tires for a larger wheel diameter (I just checked nine different Extreme Perf tires in 225/45-17 and 225/40-18 on Tire Rack). And since the 1" smaller wheel size has the tire bead .5" radially inboard, rotational inertia is going be be (very slightly) *less* vs. same-weight 1" larger size, as some of the weight is further inboard.
3. As opposed to tires, which weigh the same, you *do* see increased weight with larger-diameter wheels, typically 1 lb. per inch diameter for lightish-weight wheels. And unlike with tires, that weight isn't radially in the same place, it's 0.5" further out. More mass, further from centerline => increase in polar moment of inertia. As you point out, though, the wheel weight isn't nearly as big a contributor to polar moment as tire weight, so not a HUGE effect on overall rotational inertia, but still, the larger-diameter wheel plus tire will have more.

But the whole issue is academic and not likely to matter one bit either way.

Here's a test Car and Driver did a while back testing different diameter tires and wheels. https://www.caranddriver.com/feature...d-tires-tested
225/45-17 was 0.1s quicker to 60 and in the 1/4-mile vs. 225/40-18, 1mph faster in the 1/4, so yeah, pretty small difference. But in 0-100, the larger-diameter setup was 0.6 seconds slower. At a track with longish straights, that could be a significant difference in lap times. That's with same make/model tires and make/model wheels, only difference being 17" vs. 18".