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I read somewhere that going from 17" to 18" rims will properly decrease performance rather than increase because the greater unsprung weight and greater inertia. Are there any other positve or negative aspects of upgrading 17" to 18"?
 

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2001 M3:Stick, what else?
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ak330i said:
I read somewhere that going from 17" to 18" rims will properly decrease performance rather than increase because the greater unsprung weight and greater inertia. Are there any other positve or negative aspects of upgrading 17" to 18"?
The larger the wheel, the less rubber there is protecting it therefore more prone to damage. Also there will be less absorption of bumps so the ride will be worse.
 

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OMGWTFBBQ
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ak330i said:
I read somewhere that going from 17" to 18" rims will properly decrease performance rather than increase because the greater unsprung weight and greater inertia. Are there any other positve or negative aspects of upgrading 17" to 18"?
Most 18 wheels have more mass than 17s. The mass of 18 wheels is also genereally concentrated further away from the wheel's centerpoint. What follows assumes that the width of the wheel and the overall diamater of the wheel/tire combination remains the same as with the 17" wheel.

A greater amount of mass will generate greater inertia when moving in the vertical (caused by road irregularities and/or vehicle maneuvers). This requires a greater amount of force to act up the inertia to first stop and then return the mass to its neutral position. In a practical application, if your shocks and springs aren't upgraded along with the wheel size increase, it will take more time for your current suspension equipment to apply the required amount of force on the wheel's vertical inertia.

The second consideration applies to rolling inertia. The greater the mass, the more force required to change the rolling speed of the mass. What this means in a practical sense is that the engine has to work harder to get the wheels to start rolling from a standstill or to increase their rolling speed after you are already under way. The translation is that the car will accelerate more slowly with a given amount of throttle with heavier wheels. So, with heavier wheels, to match the same acceleration as you had with the lighter wheels, you will have to use a greater amount of throttle to achieve it which will burn more gas, produce more emmisions and speed up engine wear...and at full throttle, you will still be slower. That all assumes that the mass is concentrated precisely at the wheel's centerpoint. The greater the distance from the centerpoint, the greater the effect.

When you use the brakes, the brakes have to apply more force to slow/stop the rolling inertia. This means that you have stand on the brakes harder to achieve the same rate of decceleration. Using more braking force comes with increased heat generated with the brakes which leads to more rapid brake wear. Rolling inertia won't affect the car's maximum braking performance (distance to desired speed) because that is a function of the tire's grip, but the increase in mass will be added to the total mass of the vehicle and have a corrosponding negative influence on braking performance. Again, this assumes that the increased mass is located directly at the wheel's centerpoint. The greater the distance, the greater the effect.

18" positives...

Shorter tire sidewall. When you change the direction of a wheel, there is a delay before the tire follows. Generally, the taller the sidewall, the greater the delay. Going from 17 to 18" wheels but keeping the overall tire diameter the same reduces the tire's sidewall height which reduces the delay after changing the wheel's direction. This is felt as a crisper, sharper or faster turn in.

Allow larger brakes to be installed.

Not much else that could qualify as objective. The prime subjective positive quality is aesthetics.

Further negatives...

While the shorter tire sidewall gives a crisper turn in, it has a number of drawbacks. A tire with a shorter sidewall can not absorb impacts from the road as well as a taller sidewall. This results in a greater force applied to the wheel (which makes the wheels easier to damage) which increases the transmission of road irregularities to the cabin (decreased ride qaulity). Shorter sidewalled tires also tend to be more suspectible to tramlining and more sensitve to the road's camber for the same reasons.

18" wheels also tend to cost more.
 

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Discussion Starter #5
more

my car is a 2002 330i SP PP. i noticed that BMW has a 18" option. Is it correct to assume the car can handle the 18" while still be safe?
 

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I'm thinking about upgrading my rims from the stock 17's to some nice 19's but I want to make sure I get good quality light racing rims so it won't decrease performance. Any ideas guys?

thanks,

jk
 

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Re: more

ak330i said:
my car is a 2002 330i SP PP. i noticed that BMW has a 18" option. Is it correct to assume the car can handle the 18" while still be safe?
The car is safe with the 18" rims, but the points that [email protected] made above are true. BMW is not going to offer the wheels and tires as an option if they are not safe, but some caution must be exercised as the wheel/tire combo has a lower profile making it easier to scratch on curbs and if you should slam into a bad pot hole there is less tire to absorb the shock meaning a greater chance of damaging the rim. Keep in mind that the difference between the 17" and 18" rims do make a difference but we're not talking night and and day in this regard. I ordered the 18" (Style 72) rims and plan to be careful best I can, but I'd do that with 17" as well.
 
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