How much difference will -5lbs make?

[Miniata]

Originally Posted by dradernh

So if I drop 15# per corner, that's an effective 1200# total reduction for the car, and I now have, effectively, a 1550# car...instead of a 2750# car. Gadzooks!

I did drop 12lbs per corner of wheel/tire weight on our Forester XT going from stock wheels with heavier than stock tires to the lightest wheels and tires I could find for it (without breaking the bank). Made a dramatic difference in acceleration and ride quality, but unfortunately the tires were a low rolling resistance with a mushy sidewall. In retrospect, I would have rather given up a couple of pounds of weight on the tires to get a higher performance, better handling tire.


Nowhere near an effective 20lbs per corner though, as that would have meant close to 1000 lbs of effective weight reduction. I have done 500 lbs of weight reduction on another car in the past (most of that not unsprung weight) and it wasn't that dramatic. It did make 5th gear acceleration with the lighter wheel package feel like 4th gear acceleration with the heavier one on the Forester.


It is difficult and costly to remove that much wheel/tire weight from the twins though, unfortunately. Lightest reasonably priced 17" wheel/tire combo I found on Tire Rack is 8 lbs per corner lighter than stock (Kosei K4R/Hankook V12evo2). Stepping up to a much more expensive wheel like a Volk CE28 can drop the weight lost to 10 lbs/corner. Going down to a 16" wheel/tire combo with Volks makes a 12 lb weight loss per corner possible with shaved tires.

[troek]

@Yui-Chan ill take some pics later today.

[Caspeed]

The rule of thumb is 1 Lb of rotational mass in a tire /wheel/brake equals 10 Lbs of sprung weight.

So removing 5 Lbs per wheel/tire should roughly affect performance as much as lightening the car 200Lbs. 5x4x10=200.

The key is Rotational mass not unsprung weight. Tires, wheels, & rotors do not only accelerate linearly but also rotationaly. The further from the center of the rotation the more critical the weight is. 1 Lb on a tire has a greater effect than 1Lb in a rotor. However they would both affect unsprung weight about the same amount.

[Yui-Chan]

@troek: Sugoi!

Wish 16" RPF1s were available with concave or flat spokes though, like the 17x9 +35. But the low weight is awesome and very attractive!

[troek]

日本語話せますか？ yeah i wish they looked like the 17x9s as well.

[Yui-Chan]

日本語は得意じゃないんで。

[troek]

ぼくも下手:p

[dradernh]

Originally Posted by Miniata

I did drop 12lbs per corner of wheel/tire weight on our Forester XT going from stock wheels with heavier than stock tires to the lightest wheels and tires I could find for it (without breaking the bank). Made a dramatic difference in acceleration and ride quality, but unfortunately the tires were a low rolling resistance with a mushy sidewall. In retrospect, I would have rather given up a couple of pounds of weight on the tires to get a higher performance, better handling tire.


Nowhere near an effective 20lbs per corner though, as that would have meant close to 1000 lbs of effective weight reduction. I have done 500 lbs of weight reduction on another car in the past (most of that not unsprung weight) and it wasn't that dramatic. It did make 5th gear acceleration with the lighter wheel package feel like 4th gear acceleration with the heavier one on the Forester.


It is difficult and costly to remove that much wheel/tire weight from the twins though, unfortunately. Lightest reasonably priced 17" wheel/tire combo I found on Tire Rack is 8 lbs per corner lighter than stock (Kosei K4R/Hankook V12evo2). Stepping up to a much more expensive wheel like a Volk CE28 can drop the weight lost to 10 lbs/corner. Going down to a 16" wheel/tire combo with Volks makes a 12 lb weight loss per corner possible with shaved tires.

Sounds like you've thought about this in depth.

There is one more way to get significant weight off the twins' corners. The Essex Parts AP Racing Sprint Kit will drop 10.8# per corner; list is $2099, and some of the vendors here sell the kit.

http://www.essexparts.com/essex-comp...86-bundle.html

[CSG Mike]

Originally Posted by dradernh

Sounds like you've thought about this in depth.

There is one more way to get significant weight off the twins' corners. The Essex Parts AP Racing Sprint Kit will drop 10.8# per corner; list is $2099, and some of the vendors here sell the kit.

http://www.essexparts.com/essex-comp...86-bundle.html

:raises hand:

[Yui-Chan]

Unsprung-to-sprung weight equivalent comparisons varies so greatly among different people and companies lol... I've seen anywhere from 1:1.7 to 1:20....most popularly, 1:6 ..but I've read about some smart engineer guy doing some intense calculations and figured out that 1:1.7 is the most realistic (which it sounds and feels like in the real world I think).

Of course, like someone mentioned already, it also depends on rotational mass and how much of its' mass is away from center rotation. Less rotational mass and weight = less gyroscope effect = less work the suspension needs to work and compensate.

Also, for weight-reduction to hp equivalent calculations, it's best to use the factor of each car's specific power to weight ratio (gt86 = 2775 / 200bhp = 13.9)

AP Sprint kit = 20lbs weight reduce

20lbs * 1.7 to 20lbs * 6 (depending on which unsprung-to-sprung myth ration you're using, lol) = 34lbs to 120lbs sprung

34lbs / 13.9 to 120lbs / 13.9 = 2.5bhp to 8.6bhp from an AP Sprint kit!

Now, 1:1.7 to 1:6 is a huge difference, but it seems no one really knows what the definite true equivalency ratio to use is (as everyone says different things...anywhere from 1:6-1:20?! lol!). I also don't really know what I am talking about (lol) and I am just a hobby'ist and not an engineer obviously. But, from my ghetto calculations, you can see how weight affect crank bhp ..take it with a grain of salt...as I am just a ricer and not an engineer lol.

[Phil_Marino]

Originally Posted by Cheeseballs

From Enkei themselves:

"Every pound of unsprung weight savings has the effect of reducing overall vehicle weight by 20lbs."

http://www.enkei.com/rpf1.shtml

Well.. It depends on what effects you're considering. As far as acceleration, wheels usually have about a 1.5 mass multiplier. It depends a lot on the tire section height - the closer to the OD of the tire the wheel mass is, the larger the multiplier- but it probably rarely exceeds 1.6.

So, 5# x 4 wheels x 1.5 = 30#. That means, as far as acceleration is concerned, saving 5#/wheel is like reducing total (sprung) vehicle weight by about 30#. Maybe the same as taking our your spare tire.

Handling (and ride) changes are harder to quantify and are probably affected much more than acceleration.

Also, keep in mind that Enkei's goal is to sell their lightweight wheels.

Phil

[Cheeseballs]

Originally Posted by Phil_Marino

Well.. It depends on what effects you're considering. As far as acceleration, wheels usually have about a 1.5 mass multiplier. It depends a lot on the tire section height - the closer to the OD of the tire the wheel mass is, the larger the multiplier- but it probably rarely exceeds 1.6.

So, 5# x 4 wheels x 1.5 = 30#. That means, as far as acceleration is concerned, saving 5#/wheel is like reducing total (sprung) vehicle weight by about 30#. Maybe the same as taking our your spare tire.

Handling (and ride) changes are harder to quantify and are probably affected much more than acceleration.

Also, keep in mind that Enkei's goal is to sell their lightweight wheels.

Phil

Good points you make. I figured Enkei's claim has diminishing returns since it's possible to lose a whole lot of unsprung weight if you've got the determination and $$$ to do so.

[Captain Snooze]

Originally Posted by dradernh

There is one more way to get significant weight off the twins' corners. The Essex Parts AP Racing Sprint Kit will drop 10.8# per corner;

No, only on the front.

[Captain Snooze]

Originally Posted by TM

I don't know too much about it, but they say every 1lb of unsprung weight is equivalent to ~10lbs sprung weight.

Who are they? I love these numbers. They crop a lot in forums with nothing to back up the claim. I am suggesting a 1 pound saving on a 20 pound mass is more significant than on a 30 pound mass of unsprung weight.

Originally Posted by Cheeseballs

From Enkei themselves
"Every pound of unsprung weight savings has the effect of reducing overall vehicle weight by 20lbs."

You mean from Enkei's marketing department. There is no mention of where that mass is taken from. That is,there are no numbers indicating the reduction of angular momentum. Taking 1 pound from the rim is more significant than taking one pound from the hub.