Hmmm...weight savings worth it?

[KarlC]

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So if you reduce your unsprong weight (like wheel weight) by 1lb what does the equal in terms what you would need to do to match it in over all car weight reduction ?

1lb of unsprong weight = ???

1 x 3 ? 1 x 4 ? 1 x 10 ?

I have read mall of the above, does anyone really know ?

Also how does this change 0-60 ans 1/4 mile times ? (amusing that ALL else is the same)

Thanks
.

[FLYING X5]

Or if your Fat like me, loose 20 pounds, so you don't have to buy expensive wheels... (i'm not really fat, but I have lost 20 pounds in the last year, so that saved me like what, $5k for new lighter wheels?)...

Yes I bring the comedy to the actual serious thread...

[DBFIU]

.
So if you reduce your unsprong weight (like wheel weight) by 1lb what does the equal in terms what you would need to do to match it in over all car weight reduction ?

1lb of unsprong weight = ???

1 x 3 ? 1 x 4 ? 1 x 10 ?

I have read mall of the above, does anyone really know ?

Also how does this change 0-60 ans 1/4 mile times ? (amusing that ALL else is the same)

Thanks
.

It depends on the diameter of where your weight is located with respect to its rotating axis.

[KarlC]

It depends on the diameter of where your weight is located with respect to its rotating axis.

OK I get that but how about an average number ?

[Sticky]

Or if your Fat like me, loose 20 pounds, so you don't have to buy expensive wheels... (i'm not really fat, but I have lost 20 pounds in the last year, so that saved me like what, $5k for new lighter wheels?)...

Yes I bring the comedy to the actual serious thread...

But that wasn't rotational mass.

[inlineS54B32]

OK I get that but how about an average number ?

If we are talking about unsprung weight - and not rotational mass (two different things), read this: http://www.xcceleration.com/wheel.chart.htm

All the way at the bottom. (10 pounds per pound of rotating mass - e.g. tires, wheels)

Regarding unsprung mass, this will help with handling and braking - and of course acceleration... I am not sure what the "number" is, as DBFIU alluded to above, it changes depending on if this mass is rotating, and if so - where the center of mass lies on the "circle".

[DBFIU]

OK I get that but how about an average number ?

There is no average number. A 20 lbs 22" diameter rim is going to perform much worse than a 20 lbs 17" diameter rim. You have to calculate the moment of inertia of both to compare them.

[KarlC]

There is no average number. A 20 lbs 22" diameter rim is going to perform much worse than a 20 lbs 17" diameter rim. You have to calculate the moment of inertia of both to compare them.

But the 20 lbs 22" wheel would have tire that weighed much less that the 20 lbs 17" wheel if both where the same over all height, so less weight at the outer edge right ?

[DBFIU]

But the 20 lbs 22" wheel would have tire that weighed much less that the 20 lbs 17" wheel if both where the same over all height, so less weight at the outer edge right ?

One thing at a time. Good observation though but let me break it down like this.

Lets assume were talking about the rims only and not the tires. A lot of the weight of the rim is concentrated towards the outside of the rim, this is the worst place to concentrate weight as a matter of fact but it has to be done, the tire needs a stiff metal cylindrical surface to mate to.

So the formula for computing moments of inertia for rotating objects requires you to raise the radius of the object to the second power. Even if both rims weighed the same, and im doing this for simplification matters, the 22 inch rim will have a lot more rotational inertia than the 17 inch 20 lbs rim.

You could in fact have a tire for a 17 inch rim be very tall and maybe that can take up some of the difference, but we are talking strictly rim weights. It is safe to assume that extra diameter of the 22 inch rim will only hurt you even with less mass of rubber on the periphery. I can assure you that.

The only advantage a big rim has is you can get a much shorter sidewall tire on there while keeping constant tire diameter. This helps handling, because the sidewalls are stiffer. If you have mucho dinero, you can design the 22 inch rim to have the same rotational inertia as a 17 inch rim, and still have the short sidewalls and good handling characteristics that come with those tires. By the way, exotics such as ferraris and porsches use magnesium rims and they are in the 19-20" size range. That is the only way to combat the huge penalty of rotational inertia observed by increasing rim diameter.

Reducing rim weight alone means nothing, one must reduce rotational inertia to feel the benefits. And you are correct in the assertion that tires have something to do with this, when I shop for tires I take into consideration tire weight as well because that is the outermost extremity of the rotating body which will have a big effect on inertia as well.