Control arm?

[C.Tree]

Are all Impreza's(NA's) lower control arms aluminum or only WRX/STI? Thanks

[PhyrraM]

For the North American market:

Only STIs and '06 WRX sedans.

EDIT: talking fronts only. I'm not sure what cars had steel vs. aluminum rear lateral links.

[fateo66]

For the North American market:

Only STIs and '06 WRX sedans.

07 WRX sedans have them aswell

[slopoke]

good to know ... thanks

[Mechie3]

07 WRX sedans have them aswell

No. 07 Wrx are steel. 06 Wrx is the only Wrx to get aluminum arms unless you count jdm models. STI arms are aluminum but add caster vs the aluminum 06 arms and all steel arms.

http://forums.nasioc.com/forums/showthread.php?t=1735980

[PhyrraM]

The *rumor* has it that a few of the early '07s may have had the aluminum arms to finish off factory stock. Not sure I go for that because generally, the North American cars are not the first models to run down the assembly line over in Japan.

The added castor of STI arms shouldn't matter on the 818. If the upper arms on the production 818 stay as adjustable as the ones on the prototype chassis seem to be then dialing in additional castor on any lower control arm should be easy.

I'm guessing that the added strength of the aluminum arms, while pretty (and of course bragging rights), will wreak havok on the 818 chassis when under duress. The steel arms sound like the ticket for saving the tube frame chassis from undue carnage in the event of a front wheel hit.

Edit: Forgot that the aluminum arms are also a bit stiffer, but I wonder if that will manifest on a car 1000 pounds lighter.

[C.Tree]

Is the alum lighter than steel? reducing unsprung(is that the right spelling?) weight with overall better handling? Thanks

[Mechie3]

Im wondering how the bolt on shock mount will affect the steel piece and how its placement will vary vs the aluminum one.

[PhyrraM]

Is the alum lighter than steel? reducing unsprung(is that the right spelling?) weight with overall better handling? Thanks

IIRC, about 1.5-2.0 pounds per arm, most of that sprung. I would guess only a few oz. difference in unsprung. The spring mounts virtually at the end, so most of the weight is sprung (as long as my logic holds).

[Xusia]

Aluminum is less dense than steel, so an object of equal size & shape will weigh less if made from aluminum as opposed to steel. Steel is a stronger metal, but remember it's an alloy - not an element like aluminum - so unlike aluminum there are various grades & mixtures of steel. Also, the manufacturing process is can be more significant in terms of the strength of a part than the metal it's made from. For instance, a part forged from aluminum can be stronger than a part cast from mild steel (assuming the same size/shape - you can always make an aluminum part thicker, bigger, or whatever to achieve whatever strength is required).

[RM1SepEx]

We always use aluminum alloys, not pure elemental aluminum, usually with some sort of heat treating

basically aluminum weighs 1/3 as much as steel and is about 1/3 as strong (yield stregth). The key is how it is designed/implemented taking advantage of its properties.

steel fails by stretching and distorting before ultimate failure, aluminum tends to fail terribly

So you can't / don't just make a steel part in Aluminum, you design based on the material properties

steel is about 1/3 cheaper too

all properties are for "general" properties (base Al alloy 1100, mild steel) , heat treating, exotic alloys, forging can all result in different numbers due to the alloy or microscopic material crystal structure

and yes I'm a ME geek, materials were some of my favorite engineering classes.... Questions like I need a material to do X, tell me what you do to obtain those properties... describe the heat treating process, microscopic grain structure, etc were fun tests for me!

Side note: some of the best blades made eons ago were heat treated including a quench through the belly of a nubian slave! The temperture heated to and rate of cooling has a direct effect on crystal structure

[Mechie3]

Yes aluminum is less dense than steel (though we as stated, alloys are always used) but geometry plays a big part. A forged, cast, or machined aluminum part generally follows the structure of I and H beam cross sections. A weldment of steel can be formed into a box section given a higher moment of inertia with less weight.

[RM1SepEx]

A weldment of steel can be formed into a box section given a higher moment of inertia with less weight.
???

we usually care about moment of interia for rotation... not a big issue for a suspension control arm.

The welded steel part ends up made from simple stampings, welded together, focus is on less $ to mfg. not the minimum weight.

The aluminum part is always made to minimize material since it is so expensive and is designed to maximize the material's properties/performance.

[Mechie3]

Inertia is also used in equations for bending or torsion to determine the strength of a particular cross section. It's why a large diameter thin walled tube can be stronger and lighter than a thicker walled small diameter tube. The large box section has a higher moment of inertia that the flatter aluminum arm which is why even though steel is heavier as a material the two control arms have similar weights. The steel one is thin walled and has less material.

For torsion the equation is TL/J the equation for bending is MC/I where J and I are moment of inertia about a particular axis in the direction of twist or bending. It's why builders use I beams instead of flat plates. Much stronger for the same amount of material.

[Xusia]

Holy crap, we have some SMART people on this forum!

[Mechie3]

To be fair, I went to school to be a mechanical engineer. :)

Ask me something about chemistry and I'll nod and smile and assume you're correct. I work at a medical device company with a huge chemistry department and work closely with them. I just make mechanical things do what they want them to and leave the chemistry to them.

Smart is all relative. :)

[SkiRideDrive]

IIRC, about 1.5-2.0 pounds per arm, most of that sprung. I would guess only a few oz. difference in unsprung. The spring mounts virtually at the end, so most of the weight is sprung (as long as my logic holds).

The portion of the control arm which is sprung weight and the portion which is unsprung does not depend on the location of the shock mount. The entire arm does move when the wheel moves up and down but it moves in a rotational motion, therefore it does not move vertically in the same manner as the wheel assembly and is not entirely unsprung. You can find the CG of the arm and it's distance from the pivot point and then calculate the corresponding effective unsprung weight of the arm but I haven't done that in a few years and also don't have a control arm in front of me. After looking at a picture of the control arm I would assume that approximately 1/3 of the arm's weight would be effectively unsprung.

-full disclosure: was also a ME major, focused on racecar suspension design and tuning (FSAE).

[slopoke]

... racing team for Thunderhill??? ... now THAT'S pure speculation ... LOL

[RM1SepEx]

Inertia is also used in equations for bending or torsion to determine the strength of a particular cross section. It's why a large diameter thin walled tube can be stronger and lighter than a thicker walled small diameter tube. The large box section has a higher moment of inertia that the flatter aluminum arm which is why even though steel is heavier as a material the two control arms have similar weights. The steel one is thin walled and has less material.

For torsion the equation is TL/J the equation for bending is MC/I where J and I are moment of inertia about a particular axis in the direction of twist or bending. It's why builders use I beams instead of flat plates. Much stronger for the same amount of material.

correct, the strength is due to the distribution of the material away from the center line of the stress under bending... a 2 x 12 is stronger than a 4x6... even though they have the same cross sectional area. It's more than that with the design of those suspension arms... The steel one actually has a deeper cross section and more material/strength away from the center line but it is distributed so that it can flex too much. Also a weldment is only as strong as the welds... I think you would find that the aluminum arm has the higher moment of inertia (I) value...

It's important to indicate that it is moment of inertia (I) vs just inertia... two different things...

Been too long since I had to use my old Mark's Handbook or HP calculator. Perhaps my PE test in 1986 or 87... I actually worked more with materials like silicon and gold for parts in fighter jets, missiles, and space ships than structural beams...

My 05 WRX arm was damaged and the welds torn apoart and the steel "tube" crushed and distorted. The aluminum arms should be much stronger. I'll be ordering up a pair since I need one new one anyway, and they are much prettier... do I polish them and clear powder coat them?

yup, another Mechanical Engineer on the forum

[Mechie3]

Hooray for MEs. Lol.