Which performs best, a larger wheel or smaller wheel?

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I've been told that the best performance comes when you minimize unsprung weight. But then why do BMWs and other performance vehicles come with bigger wheels and lower profile tires? Will larger wheels yeild better performance? I am assuming smaller wheels will reduce unsprung weight.

All comments welcome.
 
RE: Which performs best, a larger wheel or smaller wheel?

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I've been told that the best performance comes when you minimize unsprung weight. But then why do BMWs and other performance vehicles come with bigger wheels and lower profile tires? Will larger wheels yeild better performance? I am assuming smaller wheels will reduce unsprung weight.All comments welcome.
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Obviously... less weight in the wheels equals better acceleration. Less rotational mass. So yes, smaller wheels reduce unspring weight. With today's alloys though, racing wheels that are 17" only weigh what, 20lb each? Large wheels became popular just a few years ago, when car companies started to mimic high end racing models' need for bigger brakes. Off the top of my head, a rotor a little less than 12" requires at least 17" wheels. I gotta admit, large wheels do a LOT for the look of a car.. and thats also probably the real reason why they are popular like you said.
 
RE: Which performs best, a larger wheel or smaller wheel?

Bigger wheels and lower profile tires do not necessarily equate to more unsprung weight. Additionally, it is rotational mass that is more predominant in factoring performance when it comes to wheels and tires.

The wheel must be looked at in totality as a combination of the tire and the rim. Gross weight is one attribute, but distribution of mass is another. Given the same size wheel, a lighter tire is better to minimize rotational mass. Given the same size tire, a lighter wheel is better overall, but wheel mass concentrated at the outer rim is worse than the same mass concentrated towards the hub. In comparison, two dimensionally equivalent wheels with the same weight could be very different in rotational mass dependant upon design.

Component selection is also a variable as nylon or kevlar cords in tires are lighter than steel. Sidewall composition varies greatly and is a large contributor to tire weight. Large racing tires are 'shaved' to minimize rotational mass at the expense of tread thickness. Likewise, rim design and material is obviously important since steel wheels are heavier compared to aluminum and alloy wheels. Finally, one can select polished or painted versions of the same basic rim design and save the additional weight of chrome plating.
 
RE: Which performs best, a larger wheel or smaller wheel?

Driller said it very well.

It basically depends on what you want to do.

There are three things in play here, if you neglect tire construction.

One is unsprung mass. The greater the unsprung mass, the harder the suspension has...

oh, damn. rodney dangerfield died today, at age 82. just heard it on the news. What a bummer, he was a good comedian. Too bad, but i guess we all have to go sometime, and he lived a good life.

Anyway. Unsprung mass is the enemy of handling. The suspension has to work harder to deal with it.

Second is rotational inertia. This is primarily important in accelleration and braking.

Third is contact patch. A larger diameter or a greater width will give a larger contact patch. A larger contact patch will give better traction but also greater rolling resistance (usually a negligable factor).

There is no one answer to your question. It's all about optimization for what you want to do. But if you want performance, and you want rims, shop for the lightest wheels you can buy. The term "mag wheels" stems from that. Racing wheels are often made of magnesium because it is very lightweight. However it is not that strong and is somewhat brittle, so it isn't the greatest for street cars.
 
RE: Which performs best, a larger wheel or smaller wheel?

You also need to consider the OVERALL diameter of the wheel/tire combo. A smaller diameter will be better for acceleration because it effectively lowers the gear ratio (numerically higher).

For instance, I figured my stock 3.08 ratio was more like 3.20 with a 245/50/16. Conversely, my 245/40/20s are about an inch larger in diameter, dropping the gear ratio to around 2.90 (IIRC). Furthermore, wheel weight has increased approximately 12 lbs each so I have increased unsprung weight AND increased tire diameter. Acceleration has suffered and I CAN tell the difference. Although, the car has no problem losing traction at the 1-2 shift, even with the bigger tires. It's only at low RPMs that I notice a real diffence.

Kale
 
RE: Which performs best, a larger wheel or smaller wheel?

Thanks all. Since I am looking strictly at cornering and handling performance, I gather from all of your comments that sticking with the stock 16" wheels and some z rated tires would turn out best.

Is this a valid conclusion?
 
RE: Which performs best, a larger wheel or smaller wheel?

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Thanks all. Since I am looking strictly at cornering and handling performance, I gather from all of your comments that sticking with the stock 16" wheels and some z rated tires would turn out best.Is this a valid conclusion?
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Stock diameter, yes. Z rating on tires implies the speed rating, but the load rating via sidewall construction and subsequent handling is also factored in the performance ratings.

http://www.nhtsa.dot.gov/cars/rules/TireSafety/ridesonit/brochure.html#dotcode
 
RE: Which performs best, a larger wheel or smaller wheel?

Going to 17's increased the handling response a lot. I couldn't believe how much the car changed with only one inch in rim diameter chance. I was also running high performance tires on my 16in rims and the 17's are just hands down much better for conering. I'm running yokohama es100 245/45/17 and 275/40/17 tires. Only my rear tires are wider then with the 16in rims and that shouldn't make too much of a difference. Go with 17 or 18's if you want a better handling car.

PS I also went with the shorter tires to increase my effective gear ratio from 3.73 to like 3.8X.
 
RE: Which performs best, a larger wheel or smaller wheel?

When going to a larger size, caution must be used comparing handling if the tires are not the same make.

I had a set of no-name tires that came with the rims I bought. Eager to install the rims(stock size mind you), I did not swap out the Michelin Pilots I had. The ride and handling suffered dramatically! Burnouts were a lot more fun though! }(
 
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