Unsprung weight and why I want less of it :0 - Subaru WRX Forum
 
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post #1 of 6 Old 09-08-2006, 10:41 PM Thread Starter
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Unsprung weight and why I want less of it :)

You probably hear the term unsprung weight bantered around in suspension circles, what is it and why should I care

What is unsprung weight- well it's weight that is not supported by the springs- that's easy! Sprung weight is your chassis/transmision/engine and other assorted sundries- obvioulsy there is more sprung weight than unsprung weight.

What components comprise the unsprung weight- well we have tires/wheels/lug nuts/rotors/calipers/knuckle/hub assemblies- it also includes a portion (roughly half) the weight of the axles/control arms and struts- these latter components are partially sprung and unsprung.

Is less unsprung weight better than more- yup. Actually any loss of weight is good when it coms to accelerating/braking/cornering- loss of unsprung weight has the added advantage of improving the ability of your suspension keeping the tire to the ground (it's primary job). This is what ultimately makes a car handle well- grip!

The WRX in stock trim has a ratio of ~ 7:1 sprung/unsprung (if anyone is interested in the calculations -basically just determing the unsprung weight/corner by weighing a bunch of parts- I can post them)- this is pretty good, some solid axle cars are in the 3 to 4:1 range- not so good.

What's the easiest way to reduce unsprung weight- well it's not replacing the axles Your wheels are probasbly the easiest place to shed unsprung weight, often tires that are equally grippy sometimes have some differences in weight, the use off aluminum in suspension arms can reduce unsprung weight, aluminum calipers can reduce unsprung weight as can two piece rotors (aluminum hats) and many aftermarket strut assemblies are lighter as well.

You can trim 10+/corner of unsprung weight w/ careful choices of parts, but as you can see you can just as easily add to unsprung weight as well.

You'll notice that many of the unsprung components are also rotational- tires/wheels rotors- having these lighter also enhances acceleration and braking- to a much higher degree than simple loss of non-rotational weight (again all weight loss is good, just some is better)

clif note version- buy them damn lightweight rims you've been drooling over

Last edited by Big Sky; 04-28-2007 at 08:53 PM.
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post #2 of 6 Old 09-09-2006, 02:18 PM
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BigSky, great post! The less the wheels and tires weigh, the better the suspension will work, and the better the car will ride too. When buying aftermarket wheels, try to avoid those that weigh more than the OEM wheel/tire combo. Im the case of the '06 WRX, the stock 17 X 7 wheel with P215/45-17 RE92s weigh about 42 pounds per wheel/tire. The 17 X 8 OZ Ultraleggeras with Kumho P225/45ZR-17 tires I'm now running weigh in at about 41 lbs., so I was able to shave a pound per corner, get a far better performance tire and a much nicer wheel.

Some of those 18-inch wheels on the market weigh well over 20 lbs. each and though they look great, they will degrade the performance/ride of your car.
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post #3 of 6 Old 09-09-2006, 03:51 PM
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great write-up
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post #4 of 6 Old 09-09-2006, 05:19 PM Thread Starter
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I should probably include the weight of the 02-05 WRX wheel/tire- wheel (16x6.5") is 16.5 lbs, tire (205/55/16 RE92) 22.5 lbs- 39 lbs total

It's a great number to shoot for (or under if you can ) when going w/ uprated wheels/tires.
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post #5 of 6 Old 09-09-2006, 05:48 PM
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well done. good info.
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post #6 of 6 Old 02-02-2009, 12:32 PM
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Thanks for the info. WOW the oem wheel is pretty light, eventhough its only a 16in rim

_Ryan_
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http://www.misubie.com/
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