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Old 07-25-2009, 02:59 AM   #1
kman25i
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Default heavier wheels/tires decrease hp?

Can someone answer this for me?

Apparently the stock 16" steel wheels that come on the 2.5i impreza are 16.5lbs. The stock tires are 21lbs according to tire rack. The total weight would then be 37.5lbs.

Now if I was to go to a wheel/tire combo that weighed 18.5lbs and 24lbs resprectively the total weight would be 42.5lbs.

That's an extra 5 lbs per wheel/tire. I read somewhere that each pound of unsprung weight is equal to 7lbs sprung weight and going by that am I correct to assume that going with the new wheel/tire combo would amount to the same as driving around with an extra 140lbs in the car? That's kind of insane and makes me not want to get 17" wheels...

Edit for bad math at 2am
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Old 07-25-2009, 10:21 AM   #2
UkNuck
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Are the stock steel wheels really 16.5lbs? That's about right for the OEM 16x6.5in alloy wheels (the ones on the 02-05 WRX) and I have both those and a set of 16x6.5in Subaru steelies and I'm pretty sure the steelies are heavier.

Anyhow from a straight line performance POV what you really need to compare is the rotational moment of inertia which depends on the radial distribution of mass - so for example a heavier tire with a lighter wheel will be 'worse' than a lighter tire with a heavier wheel. Almost no one goes to those lengths in practice - most people would rather have a tire with decent stiff sidewalls even if that makes it a pound or two heavier. The sprung versus unsprung weight is a slightly different issue.

Anyhow I wouldn't obsess too much unless you are looking for that last 1/10 - 18.5lbs is actually pretty good for a 17in wheel - you only get lighter than that with top end forged wheels or by giving up strength. As they say

- cheap
- strong
- light

pick any two
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Old 07-25-2009, 10:46 AM   #3
UpSideDownDesi
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Default

no loss in HP, more rotaional mass you put on more slugish your car will feel. It will rev up tad bit slower, but no loss in HP.
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Old 07-25-2009, 11:30 AM   #4
dbrier
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Less HP to the ground, yes.
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Old 07-25-2009, 11:45 AM   #5
Wovencoils
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Default 16.5!?

16.5lbs huh? In a thread months ago they were weighed with the stock tires by someone on here, and he said the wheels were much closer to 20lbs. each. Are you sure on the 16.5lbs? This is pretty significant info, and means whenever I get new wheels and ditch the stock ones, i wont settle for anything that isn't lighter than 16.5. lolz.
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Old 07-25-2009, 12:29 PM   #6
FLZ_Boy
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Straight line might be a tad slower. If you use a wider tire combo, you will probably be quicker on the lap times on a roadcourse.
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Old 07-25-2009, 12:58 PM   #7
stanjk3
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The stock 5-spoke wheels on the 05-06 LGT and 06-09 Legacy 2.5i are 20 lbs each.

If you put on a set of like 20" dubs you'd notice the extra weight, but 1-2 lbs probably not.
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Old 07-25-2009, 01:14 PM   #8
ultrasonic
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Default

That's the concept: more weight factors in to lower your power to weight ratio. A very light car can be blazing fast with low horsepower. A heavier car will require more horsepower to be as fast.

Weight being unsprung also impacts handling more than sprung weight does.

Also, with wheels you have to consider where the weight is. With a rotational mass, like a wheel with tire, weight towards the center is less detrimental than weight closer to the edge. This is due to momentum.

But bigger wheels do look cool.
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Old 07-25-2009, 01:55 PM   #9
txl146
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that's the concept also, but does it always translate into something you can really notice?

for example, I saved ton of unsprung weight by going with 4 pot front setup (-10lbs lighter than LGT set up just on rotor alone), USDM BBS (-3.5 lbs lighter than LGT wheel), PS2 tires (-3lbs lighter), which saved over >15lbs per corner. However, I really couldn't feel the noticeable difference in acceleration.

1/4 time also hasn't improved at all. Perhaps I suck at driving.
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Old 07-25-2009, 02:33 PM   #10
Uncle Scotty
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Default

all
ALL
the
stock
wheels
weigh
DIFFERENT
and
any
morons
wanting
to
compare
weight
of
different
wheels
will
be


why
does
this
keep
happening
here
?
?
?

oh
I
forgot

NOBODY
pays
ANY
attention
to
what
****ing
YEAR
car
is
being
discussed
and
they
just
throw
their
****
in
and
confuse
the
****ing
thread

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Old 07-25-2009, 03:28 PM   #11
kman25i
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Default

I'm talking about the 08,09,10 2.5i impreza stock steel wheels with the wheel covers. I had ordered 17" wheels with tires but canceled my order after reading (while using the search function and searching for "oem steel wheels") that the stock wheels were only 16.5lbs and tire rack says the oem tires are 21" lbs.

The 2.5i only has 170hp as is and driving as if there was another 140lbs in the car isn't very appealing.

Can someone verify the weight of the stock oem steel wheels with covers on a 2008, 2009 or 2010 impreza 2.5i, if you have one yet (prolly don't)? I want to buy alloy wheels and UHP summer tires and use the steelies for winter tires.
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Old 07-25-2009, 07:34 PM   #12
Uncle Scotty
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Quote:
I want to buy alloy wheels and UHP summer tires
there are PLENTY of light 17's available

a 17x7 with a 215/45 will be great and you wont notice the 2-3 pounds/corner
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Old 07-28-2009, 02:06 PM   #13
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Default

Quote:
Originally Posted by dbrier View Post
Less HP to the ground, yes.
Absolutely right.

I think it was Sport Compact a few years ago who did an article where they dyno'd a car before and after changing to a heavier wheel/tire package and it put down less power.

Heavy wheels will also increase brake temps with all else being equal. They will obviously increase overall weight of the vehicle and will increase unsprung weight which will hurt handling slightly over bumpy surfaces since the spring/damper combo will have to work harder to control them.

-Dave
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Old 07-28-2009, 02:27 PM   #14
Ty B
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Quote:
Originally Posted by txl146 View Post
that's the concept also, but does it always translate into something you can really notice?

for example, I saved ton of unsprung weight by going with 4 pot front setup (-10lbs lighter than LGT set up just on rotor alone), USDM BBS (-3.5 lbs lighter than LGT wheel), PS2 tires (-3lbs lighter), which saved over >15lbs per corner. However, I really couldn't feel the noticeable difference in acceleration.

1/4 time also hasn't improved at all. Perhaps I suck at driving.
No, you suck at physics. Look up rotational inertia, look at the equation. A mass reduction only has a one to one impact, while a reduction in Diameter would be ^3, power of 3 change to the inertia. Did the diameter of the brakes change, no or very little. You are expecting something for nothing.

Quote:
Originally Posted by OAKOS Automotive View Post
Absolutely right.

I think it was Sport Compact a few years ago who did an article where they dyno'd a car before and after changing to a heavier wheel/tire package and it put down less power.

Heavy wheels will also increase brake temps with all else being equal. They will obviously increase overall weight of the vehicle and will increase unsprung weight which will hurt handling slightly over bumpy surfaces since the spring/damper combo will have to work harder to control them.
-Dave
Hey Dave, where did the HP go? Did it just vanish? Was it the larger diameter, or the larger wheel width? Two different things here.

By implication are you gonna say that if we all throw on 13 inch tire/wheel combo's that we are absolutely going to gain HP?
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Old 07-28-2009, 02:45 PM   #15
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Default

Quote:
Originally Posted by Ty B View Post
No, you suck at physics. Look up rotational inertia, look at the equation. A mass reduction only has a one to one impact, while a reduction in Diameter would be ^3, power of 3 change to the inertia. Did the diameter of the brakes change, no or very little. You are expecting something for nothing.



Hey Dave, where did the HP go? Did it just vanish? Was it the larger diameter, or the larger wheel width? Two different things here.

By implication are you gonna say that if we all throw on 13 inch tire/wheel combo's that we are absolutely going to gain HP?
It's been a while since I read that article but I'm sure they kept the overall diameter as close as possible between the two to keep mechanical advantage (gearing) out of the equation. The point of the article was to compare changes in rotational mass to power output. I've heard of similar results on a dyno with CF driveshafts and lightweight flywheels and those would have zero affect on gearing or mechanical advantage. We all know that dyno results aren't the be all, end all indicator of real world performance, but in this case it sure seems to indicate something.

-Dave
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Old 07-30-2009, 08:24 AM   #16
dbrier
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To exaggerate the point of heavier wheels, roll a basketball and a bowling ball on the floor. The bowling ball takes more energy to get moving, then once it's moving takes more energy to stop.

The HP lost is making the wheel spin.

Lightened flywheels, pulleys, and wheels all have the same effect.
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