Ok, this has been bothering me. This it text from another thread. Do lighter wheels make the car faster regardless of diameter or what?

Thanks!



Someone elses comments:

Lower reciprocating weight it's 8 times more effective
It's better to lower reciprocating weight. I might of messed the spelling up on that, but every pound you take off in reciprocating weight is equal to 8 pounds of dead weight. If you get rims that are 4 pounds lighter than stock. Thats 4 pounds per wheel. 16X8 = 128lbs. I think lowering reciprocating weight is just as important as lowering dead weight. Ohh yeah and removing all the stuff from the trunk is just gonna make your weight distribution worse.

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Good point. But you also have to consider that a bigger diameter rim will have higher reciprocating/unsprung (?) weight. Meaning that a 19inch rim weighing only 18lbs could add more unsprung weight to your car than stock 16inch rims weighing 25lbs or whatever.

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Do the math. (Mass)*(Radius)*(Radius). Turns out that even though the CenterLine wheels are only 14.89 lbs., the fact that they are 19" means that they have so much rotational mass that their weight loss is ineffective.

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Actually, they probably will result in lower unsprung weight.

25.00lb x 17in x 17in = 7225 lb-in^2
14.89lb x 19in x 19in = 5375 lb-in^2

I don't know if the units are right, but the "index" number for the light 19 inches is significantly lower...(assuming the 2KSE wheels are 17 inches). Is this right??? It's probably much more complicated because of WHERE the weight on the rims is. Most of it is on the outside, I would think. So that might make the 19 inches effectively heavier.

Another thing, which is supposedly even more of a factor than wheel weight is tire weight. The tires are at the outside (and they're filled with compressed air, which weighs something too) so they have a big impact on unsprung weight.

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- After having experienced a lighter wheel/tire combo recently, I completely agree. My car accelerated considerably faster with my race wheels/tires. The only problem was that the smaller diameter tires only served to punctuate the already large stock fender gap. Eeek.

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*confused at this point






Thanks!

 

Dooh. Wrong forum.

 

A wheel isnt reciprocating mass... it's rotating mass.

 

Yeah, mzmtg is right!

You would want to consider the moment of inertia for the wheels instead of reciprocating (alternating back and forth) mass.

For this case it would be for a solid disk or cylinder type object:

I=(1/2)MR^2

Where R=radius of the disk and M=mass.

Then I (the moment of inertia) is related to kinetic energy through:

K=(1/2)Iw^2

Where w=radians/sec (which is angular velocity).

This is the amount of energy that would take the car in rotating the wheels (roughly, not counting friction and other quantities).

 

Lighter is always better. Until it breaks that is...

Overall tire+wheel diameter just changes the overall gearing with the same usual effects....


-RMB

 

you guys are just killing me with all this physics stuff...how do you remember all this stuff and actually remember how to use it...man, i'm just tired at looking at all of those formulas...

 

Dont worry about the physics. Just remeber that mass is the enemy of all types of vehicle performance. Lighter is always better.

Except when it isnt strong enough. And brake rotors need to have some mass to absorb and dissipate heat correctly...but other than that...

 

[QUOTE]Originally posted by exilefromhell
[B]Ok, this has been bothering me. This it text from another thread. Do lighter wheels make the car faster regardless of diameter or what?
Thanks!

Someone elses comments:
Ohh yeah and removing all the stuff from the trunk is just gonna make your weight distribution worse.

For drag runs - Always remove the spare and gas from the trunk. You want all the weight up front on the drive tires.

For auto-x and balance - Always keep the spare and a full tank of gas. You want to get closer to 60/40 if possible.

 

Unsprung weight is the weight of the suspension components. The unsprung weight is your wheels, the springs/shocks, control arms etc. The "sprung" weight is everything that the suspension is holding up, like the body, engine etc. Sprung/unsprung weight really affects suspension dynamics. How hard/softyour ride is etc.

DW