Car's Performance on bigger wheels

[nikitopo]

Originally Posted by Yoshoobaroo

Good question! The smaller wheel is better in the end. The extra "inch" of tire is in the sidewall, which is much lighter than the thread area. for most tires the weight difference will be negligible. I just checked on tire rack, 2 tires with ~ the same outside diameter (25.0" and 25.1"), both Michelin pilot sport 4S:

225/40/18: 21lbs
225/45/17: 21lbs

^^^This

Originally Posted by Yoshoobaroo

He's running Pilot Sport 4S tires. Outside of the US they are not available in 17", in fact, we only got them in the US very recently. Michelin might bring the 17s to the rest of the world too, but when setting that car up they just weren't available. That's probably the reason he's not running 17s.

^^^And This


Btw. Tsuchiya's "personal" car is tuned by a company called Tec-Arts. It was requested as a project by his parent company Keiwan Planning Co. Ltd. I doubt if he has ever driven the car apart from some magazine photo shooting and film recording.

http://tecarts.com/86andbrz/index.ht...E_MmjDCpst1XkU

[Seiryuu]

Btw. Tsuchiya's "personal" car is tuned by a company called Tec-Arts. It was requested as a project by his parent company Keiwan Planning Co. Ltd. I doubt if he has ever driven the car apart from some magazine photo shooting and film recording.

http://tecarts.com/86andbrz/index.ht...E_MmjDCpst1XkU

. You're right. For a 2012 86, it has seen only a little over 19000 miles as of this year. It has definitely not lived a normal life.
_____


So some math got done. Imma just present the findings and conclusion.



Stock 17x7.5 PP Wheel + stock size 215/45 MPS4S = 20.8 + 19 = 39.8 lbs.; diameter 24.7"

18x8 wheel (the smallest size of the wheel design I wanted) + 225/40 MPS4S = 71.1 + 21 = 38.2 lbs; diameter 25.1". Even though it is not, let's just assume the wheel design is the same as the 17" for math purposes.



So the 18" setup is 1.6# lighter but diameter is .4" bigger. Even with the lighter weight, the diameter increase will cause a drop of about 2% in effective torque due to added .12kg/m^2 of moment of inertia. The same effect as making the final gear taller.


Again, assuming wheel design is the same, an 18" wheel will have to weigh 13.8# or less to negate the above effect.



Therefore, while the 18" setup is still lighter, performance still suffers due to the increased moment of inertia from the increased diameter. However, I surmise that in actuality, assuming that the aforementioned 18" is "better designed" with improved effective location of mass, it probably shouldn't require the calculated low weight to match the stock 17". This is the conclusion of my understanding.


Again, I would like to reiterate what detrimental difference a reasonable 18" (sub-20#) wheel and tire combo will make, it can be easily made up with driver skills or other mods. So in the end, like some of you said, "just get what you want and enjoy the car".

[Seiryuu]

Originally Posted by Yoshoobaroo

He's running Pilot Sport 4S tires. Outside of the US they are not available in 17", in fact, we only got them in the US very recently. Michelin might bring the 17s to the rest of the world too, but when setting that car up they just weren't available. That's probably the reason he's not running 17s.

We'll probably never know without asking the tuner shop why they went with that 18x8.5 with 235 setup. However, not being able to run 17 MPS4S doesn't seem likely to be the case here due to the aforementioned wheel and tire size. I also looked up the wheels and although I couldn't find the weight on those exact specs, the 18x9.5 weighs 21.9# so I guess his are at least 20# each. With the tires, that's a ~42# setup with +.7" increased diameter! Surely there are lighter 18" setups that are readily available. But again, since Tsuchi pulled off a better time anyway, what little downgrade is all made up by the Driver.

[why?]

Originally Posted by Grady

I guess I am backwards. I always found the tire I wanted then had to find a wheel that would fit it.

This is really how it should be done, but most people care about looks and buy the cheapest tires they can to go with them.

Originally Posted by Seiryuu

No kidding. Not sure if you are referring to cars in general but it is appalling to see economy cars like the Mazda3 comes in 19 (NINTEEN) inch wheels for no good reasons! Even the Corolla comes in 18.

It is absurd. I had a Yaris for 12 years, and I chose to use 14x5.5 Volk ce28n. They weighed 6 pounds each, which is crazy light. You could easily feel the difference. And the cost of the tires was so much less as well. Problem is these modern "economy" cars weigh thousands of pounds more than they did 20 years ago, so they need gigantic brakes to stop them.

[Grady]

Originally Posted by Ohio Enthusiast

Just a nitpick - center of mass of a wheel/tire is always at the very center (unless it's unbalanced). What you are interested is the concentration of mass further from the center. Ideally you'd want the mass as close to the center as possible; sadly this is not how wheels and tires work - most of the mass has to be closer to the edge (barrel of the wheel/tread of the tire).

Yes center of mass was not the correct definition.

I do not have time to do the math but moving the rim out 1/2 from center would not change the performance if the rim area lost X amount of weight. That is why I still think it would be better to worry about total wheel/tire weight.

[daiheadjai]

Originally Posted by Grady

Rim diameter has nothing to do with it. it is the combined weight of wheel/tire combo that you need keep from gaining weight. AKA wide rims and tires will slow you down! Your stock car will do well on good sticky 225's.

What about gearing though?
I've read that a smaller rolling circumference translates into shorter gearing (aiding acceleration) while a larger circumference translates to taller gearing (better MPG, but worse acceleration).

[ZDan]

Radius of gyration is what you mean. 18" wheel will have slightly greater mass than equivalent 17", but more of its mass is further from the center, it has a greater radius of gyration. So its polar moment of inertia is bigger by a greater factor than mass.