How much does wheel weight affect performance?

[venturaII]

Originally Posted by nikitopo

I measured once a 205 tire on a 6.5" rim and another on a 7" rim. The rubber on the ground is more with the bigger rim. The 225 tire will give a bit more rubber, but less than 3/4+".

Of course the same size tire has slightly more contact with a wider rim. I'm keeping the same size rim width as stock, so the contact patch is 20mm (.79") wider with the 225s. And that's not even accounting for variation in contact patch size going from one make/model tire to another, which has considerably more variation than just the theoretical numbers. Again, look through TireRack's specs for the various tires and sizes and it'll show you measured section width on a specified width rim, as well as the contact patch size for that particular model tire. Using my RT615K+ tires as an example, my 225/50 has 8" of contact when mounted on a 7" rim. 205/55-16 doesn't even exist with that model, so going to a 215/45-17 to compare, it only has 7.5" of contact on the same width rim. Theoretical numbers are just that - theory. Reality differs, sometimes significantly.

[nikitopo]

Originally Posted by venturaII

Of course the same size tire has slightly more contact with a wider rim. I'm keeping the same size rim width as stock, so the contact patch is 20mm (.79") wider with the 225s. And that's not even accounting for variation in contact patch size going from one make/model tire to another, which has considerably more variation than just the theoretical numbers. Again, look through TireRack's specs for the various tires and sizes and it'll show you measured section width on a specified width rim, as well as the contact patch size for that particular model tire. Using my RT615K+ tires as an example, my 225/50 has 8" of contact when mounted on a 7" rim. 205/55-16 doesn't even exist with that model, so going to a 215/45-17 to compare, it only has 7.5" of contact on the same width rim. Theoretical numbers are just that - theory. Reality differs, sometimes significantly.

I am talking about reality, not theory. My SUV has a 225 tire on a 7" rim, because in such cases you need more tire flex. It is not ideal for a modern sports car. Even the BRZ PP uses a 215 tire on a 7.5" rim and not a 225. Check whatever modern sports car you like. The chances are minimal to see something like the following by the factory. It is the size you propose on a 16x7 wheel. Maybe it is nice for some retro car people ...

[venturaII]

I'm not interested in following trends or marketing-dictated styling, which HEAVILY influence what manufacturers produce. If I followed the factory, I'd still have 20 pound anchors for wheels. You're going to have a pretty hard time convincing me that my lap times would drop by going to a 205/55 from a 225/50 on the same wheel. You're basically saying that everyone spooning 245/40s onto 7" rims so they stay in Stock classes would be faster if they kept the original 215s.

[churchx]

venturaII: At auto-x. There, limitations of class rules and importance of grip no matter illeffects dictates everything. Take out those artificial limitations, and get people showing better laptimes in HPDE with 215-225 (though of grippier rubber compound of course).

BTW, regarding 16" wheels .. imho best compromise would be x7. Thought of line going - that if one goes downsize for lightness sake, x6.5 offerings choice is very limited, and much easier to find really lightweight aftermarket ones of more common x7 width, with x6.5 being mostly heavier steelies and OEM wheels. Also if one takes into account advised tire widths for wheel width, 185min-195-205best-215max for x6.5 vs 195min-205-215-225max for x7, then imho 2nd case gives a bit better range for both running sufficiently narrow winter tires and wide enough performance tires for track w/o stretching, if same wheel set gets reused.

[venturaII]

Originally Posted by churchx

venturaII: At auto-x. There, limitations of class rules and importance of grip no matter illeffects dictates everything. Take out those artificial limitations, and get people showing better laptimes in HPDE with 215-225 (though of grippier rubber compound of course).

People are NOT going faster with 215-225/45 on 7" rims than they are when using 245/40 on the same wheel. That combination simply would not exist if it wasn't faster. Don't throw a random caveat in there about "different compounds" because you're comparing apples and oranges at that point. The theoretical benefit of optimal tire width for a given rim size doesn't outweigh (no pun intended..) the advantage of a less optimal combination putting more rubber on the ground, especially when there's almost no weight penalty to go along with it.

[churchx]

NOT? In Auto-X? Maybe. But seen several times mentioned quicker laptimes for less wide tires at HPDE. This won't do?

[venturaII]

My understanding is that is outdated info, as discussed in several other threads. And nowhere do I see anyone going faster on 205 or 215, regardless of wheel width..

[churchx]

My google-fu must be too weak, as i can find mostly posts & threads saying opposite in this forum. At least for NA (which most are). I see just Mike, Dave-ROR, Robispec and so on claiming having better times with narrower for track/hpde. Though i guess it might be not just because of "less rolling resistance/weight/more power to accelerate", but that sometimes too wide might mean too hard to get enough heat in track tires for optimal temps (if NA power/non heavily modified, which are again, most), or less optimal slip angle, or worse gearing.
And then there are practical sidegains to running narrower except just performance points of weight/less sidewall flex/less rolling resistance/quicker to heat up, for example - cheaper cost, more space in car for other things if track set of 4 is in car, less suspect to aquaplane (if tires are dual-use), sometimes better availability of specific tire model, need to spend less on accompanying mods (eg. suspension/brakes/aero/power, to get more out of wide tires).

[venturaII]

Well, I'm not interested enough in turning this into a 245 vs 225 thread to look up specific threads; I'm just going by memory. But my original statement stands that no one is going faster on 205 or even 215 compared to 225 on the same tire, regardless of wheel width.

[why?]

Originally Posted by fatoni

why?

My car is a dd/toy. I don't track it and I don't compete. I also don't give a crap about "ultimate grip," I want to slide around, it is much more entertaining to me. If I could go narrower I would, however the tire selection becomes slim to none, and you just cannot find wheels narrower than 7"s that are reasonably light. 205 fits on the wheel with no stretch, for me it makes all the gauges and speed gauge perfectly correct, and there is enough grip to have fun, but not enough to make it boring.

[fatoni]

Originally Posted by why?

My car is a dd/toy. I don't track it and I don't compete. I also don't give a crap about "ultimate grip," I want to slide around, it is much more entertaining to me. If I could go narrower I would, however the tire selection becomes slim to none, and you just cannot find wheels narrower than 7"s that are reasonably light. 205 fits on the wheel with no stretch, for me it makes all the gauges and speed gauge perfectly correct, and there is enough grip to have fun, but not enough to make it boring.

Cool. It just sounded like you thought 205 was better for a wheel diameter rather then you just prefer 205s.

[OfficeLinebacker]

I'm following this discussion closely, and it seems that the long and the short of it is that tire weights less than rim. (In other words, all other things held constant, a 17" tire and rim combo is going to be heavier than a 16" tire and rim combo and lighter than an 18" tire and rim combo).

So if you're optimizing for weight savings and fuel efficiency, you actually want to downsize to a 16" rim and higher profile tire.

Is this correct?

[why?]

Originally Posted by OfficeLinebacker

I'm following this discussion closely, and it seems that the long and the short of it is that tire weights less than rim. (In other words, all other things held constant, a 17" tire and rim combo is going to be heavier than a 16" tire and rim combo and lighter than an 18" tire and rim combo).

So if you're optimizing for weight savings and fuel efficiency, you actually want to downsize to a 16" rim and higher profile tire.

Is this correct?

Yes. If you don't autocross or track there is no reason for massive wheels or tires. 205/55/16 or 195/55/16 is a perfect size for our vehicles. The RPF01 weighs 13 pounds in16x7, TE37 weighs 11.5, and T66F weigh 10.5 pounds. Although there are people that autocross on 225/16 tires as well.

[churchx]

Hmm. I find few reasons to justify a bit bigger sized wheels even for daily driving. - Steering feel (better performance tire choice & brake clearance probably matters less for DD).
On smaller wheels higher sidewall profile with a bit more slop will still provide good grip, one can still go fast on smaller tires and tires will be cheaper and ride a bit more comfortable/compliant.. but feel, that sharpness of turn-in will be a bit compromised. I love handling and feel of it in twins, thus among my wishes/preferences is to keep a bit more "feel"/enjoyment. This is driver car, for driver enjoyment. I want to retain that enjoyment.
Yes, 18" might be even more in "feel" direction, but imho 17" is good all-rounder compromise, good enough clearance for multiple BBKs, better performance tire selection, while more expensive and less compliant then 16", cheaper & more compliant then 18".
With one exception (and again with that "feel" involved) - winter tires/use. Winter tires will be sloppy no matter what due softer rubber compound, winter tires on 17" on 18" still won't net better handling feel, so this lacking "feel" won't justify some cons of bigger wheels as result pros of smaller wheels & tires going ahead, eg. on worse roads in winter extra compliance & rim protection of higher profile may matter more, cheaper tire cost always is good, and on bigger wheels -winter-tire- selection is worse, with harder to find narrower tires for wheel size, for optimum grip on ice & snow.
So as result i use light 17" set for summer, and have 16" set for winter use.