are drop links unsprung?

[bkreigsr]

I'm thinking of fabing a set of sway bar drop links out of unobtainum (or something light) and was wondering if I will be getting a reasonable bang for the buck.
or are the wheels, and everything attached to the spindles (tires, rotors, lugs, bearings, washers, retaining nut) the only unsprung weight?
calipers definitely not, right?
thanks
Bill K

[Brant]

The calipers have to move up and down with the wheel.
(granted they don't have to be acclerated like the wheel, but that doesn't matter for the definition of sprung or unsprung)

[Wil Ferch]

I think you're asking two questions:
1.) would heavy or light drop links make any difference?....answer..probably not ...in anything you can feel or measure if made of both ends of the possible weight spectrum...say, steel or titanium.
2.) Is a drop link considered part of the sprung or unsprung weight of the car? Answer...interesting question..let's anaylze:

The car "chassis" is considered the sprung weight. The "unsprung" weight would be the wheels. ( Requires you to imagine the wheels solidly on the ground and the car body "bobs".)

It gets goofy on pieces that directly connect to "both sides" of the issue, like "A" arms. I would guess that 1/2 of the weight of an "A"-arm is unsprung and the other half is sprung. A convincing argument can be made, I guess... to call this all unsprung since it is part of the "wheel system". Using the first assumption...the anti-roll bar is connected ( basically) to the car and would be considered sprung weight. The drop links are most likely considered unsprung as I can argue that they will move in much the same way as a wheel would... relative to the body.

IMHO...what we have here is an interesting academic exercise that ( no matter how you identify it..as sprung or unsprung)...proll'y won't make a hills bean of difference if a few ounces heavier or lighter.

Of course...I could be all wrong on this...love to hear other's opinions...

Wil

[Brant]

only a layman's perspective....
but anything that moves is a little bit Unsprung.

so most of the sway arm
all of the wheel, tire, lug nuts, bearings
all of the brakes
all of a coil spring.
half of a torsion bar
half of a drop link.
a tiny bit of the axle.. (although the axle has to be spun or accelerated which is a whole nother catagory of weight type on a car)

I think of sprung versus unsprung in a vertical plane and as weight that has to move up and down with the suspension.
brant

[Wil Ferch]

Why "all" of a coil spring but only "1/2" of a torsion bar ???

One "end" of the coil spring is connected to the chassis, and moves with it, the other "half" moves furiously with the wheel....

Wil

[bkreigsr]

don't you think that just knowing those ti drop links are there is worth a couple of tenth's per lap?
so, you could get a double bang for the buck with lw half shafts, since they are 1/2 (not sure I totally understand that) unsprung and have to be spun also.
how about the cv jounts, lw on the wheel end, and doesn't matter on the trans end?


Bill K

[304065]

Well, if you were willing to fabricate halfshafts from Titanium, you could build them bigger and stronger, without increasing the mass. This is, in fact, what Porsche did with the 935.

If you wanted to fab some "cheater" halfshafts that were titanium, but painted to look like steel, and go to the trouble of making various CV components out of titanium, you could definitely effect a weight reduction. I wouldn't be surprised if there were just such a thing out there in PCA stock class club racing.

However, you would have to consider, in addition to the absurd cost, the particular properties of titanium that make it unsuitable for use in such a part-- titanium in contact with other metals will gall, and makes a lousy friction surface. So if you built droplinks out of the stuff, you'd have to use some kind of a bushing at either end to make it work, thereby increasing the weight (both sprung and unsprung) of the part. Back to square one.

Also, droplinks are a wear item, and you should definitely have a few in the spares box for every weekend. I'm not sure that given the size of the average link, about eight inches in the front and four in the rear, that the weight differential would be sufficient to result in a lower laptime, nor justify the cost difference.

Put another way, wouldn't you rather tear the engine down for a maintenance rebuild halfway through the season, vs. having titanium droplinks? I would suspect that that would give you more bang for the buck.

[Brant]

Ok...
I'm certainly open to hearing from all the experts.
perhaps the coil is less than "all sprung".

but more of a torsion bar is connected to the chassis than comparison to a coil. I think a higher percentage of a coil type spring is moving up and down in comparison to the percentage of a torsion spring moving up and down. (one of the beautiful things about a torsion bar suspension design.)

The axle is not moving up and down much with the suspension (transmission is rigid). Only the outboard side of the axle compresses and rebounds with the suspension.

The thing about rotating mass, is that due to centrifugal weight anything that spins in a small circle (ie: axles) is not contributing as greatly as things that spin in a large circle (ie: the outside of a flywheel or a tire). You might find that carbon fiber brake discs or even titanium lug nuts are more beneficial than titanium axles for rotating mass.

So when considering $ saved on rotating mass, you'll get more bang for the buck on weight that spins in a larger diameter.

We built our current car with a simple rule about weight reduction.
any (legal) weight that was over $50 per a pound was too expensive.

any (legal) weight that was under $50/lb was worth trying
(until we ended up running out of money)

[Nine9six]

My understanding is if the suspension supports it; it is sprung weight. If not, its unsprung.

[911pcars]

Given the choice of two materials with equal strength, I'd go with the lighter one regardless of whether it's sprung or unsprung weight. Whether the cost of a ti piece is worth it would be a part of the bigger weight loss picture. On a street-based car with other creature comforts, I'd say the advantage would be bragging rights and little advantage by itself. If it's part of a larger weight loss program, then I guess you remove/replace parts while observing your bank account.

But if I were to take sides, anything attached to the suspension that adds to its mass would be considered unsprung weight. I'd consider a portion of a drop link as unsprung weight.

A titanium tube with ti nuts welded on each end would make a swell drop link. If it were affordable that would be a plus. I wouldn't worry about thread galling as there are antiseize materials to coat the requisite threaded rod ends.

Sherwood

[Wil Ferch]

nine9six:

You're correct...the same as being able to accurately describe night vs day.

However, we're talkin' dusk and dawn here... a "door" on a car is sprung weight....a "wheel" is unsprung weight. Now look at a coil spring , a drop link and a torsion bar, where one end is "one way"...and the other end is "the other". It's these gray areas that we're discussing...

- Wil

[autobonrun]

You guys are correct that a percentage of the suspension components are counted as unsprung. I've seen 50% used in some discussions. And as Wil stated, since the drop link weight constitutes such a small portion of the total unsprung weight, changes in its weight should have minimal if any effect on the dynamics. IMO

http://en.wikipedia.org/wiki/Unsprung_weight

http://www.musclecarclub.com/library/tech/weight.shtml

The end of the following article is very interesting and although it isn't written around automobiles, the concepts are directly applicable. It discusses the impact of unsprung weight on rolling resistance.


http://www.fitwerx.com/NewFiles/Tech%20Center/BicycleRollingResistance.html

[Zeke]

Originally posted by Brant
Ok...
I'm certainly open to hearing from all the experts.
perhaps the coil is less than "all sprung".

but more of a torsion bar is connected to the chassis than comparison to a coil. I think a higher percentage of a coil type spring is moving up and down in comparison to the percentage of a torsion spring moving up and down. (one of the beautiful things about a torsion bar suspension design.)

I like your point. The spring has the energy that is doing the work. It is not dead weight. How does that figure in?

Again, not that it makes a lot of difference. I like Sherman's point, too. Just bling is good enough to do TI drop links. Hey, no rust.

I look at (and carry from one place in the garage to another ) my AL trailing arms complete. Yes, they are beastly heavy. Get the calipers and rotors off and they are significant ly lighter. The e-brake ass'y adds to the problem. Why didn't Porsche consider (maybe they did) inboard discs? That would have been really neat.

Which Porsche race cars had inboard rear brakes? Nearly all of them? Sorry to have drifted away from the subject, but I see the rotors and calipers as the biggest offenders that certainly could have been moved elsewhere. Next in line would be the CV, stub axle and hub. A lot there for a simple job, don't you think?

[Wil Ferch]

As a "build" on autobonruns' excellent links....think of the time you saw early 4WD trucks and Blazers (and such) on the road...and they would "hobby horse" over pavement irregularities. The car would pitch forward and back and the two live axles seemed not to act as "suspension" at all. That would be a good example of (relatively) high unsprung weight.

Wil

[911pcars]

"...The e-brake ass'y adds to the problem. Why didn't Porsche consider (maybe they did) inboard discs? That would have been really neat."

Zeke,
Inboard-mounted stuff, while light on the unsprung side, runs hotter due to their location out of airflow range. I think e-type Jags have inboard rear disk brakes. Not sure how long they lived. However, it would have been interesting if Porsche mounted even the e-brakes inboard. The unsprung weight difference might be the same between a Fuchs wheel and an aftermarket replica.

Ti spring plates anyone? Anyone interesting in reshaping the al. control arm to lose a pound?

Sherwood

[Brant]

Milt,

you need to get another teener then...

I went from a vented rear set up on my old car, to a solid rear set up on my new car (with no parking brake what-so-ever. The teener rear rotor, doesn't have the heavy drum style parking brake.)

I shed around ten pounds from my Before and After brake weight, with no loss of braking ability or heat sink ability. (cooling's one's brakes weighs A-LOT less than increasing one's heat sink!)

brant

[Zeke]

Originally posted by Brant
Milt,



I went from a vented rear set up on my old car, to a solid rear set up on my new car (with no parking brake what-so-ever. The teener rear rotor, doesn't have the heavy drum style parking brake.)

I shed around ten pounds from my Before and After brake weight, with no loss of braking ability or heat sink ability. (cooling's one's brakes weighs A-LOT less than increasing one's heat sink!)

brant

I like that, too! Thinking outside of the box is great! I put ventilated rotors on the rear of the 914 and I knew from the beginning I didn't need them. I just did it to do it....

[bkreigsr]

nice stuff guys
good winter shop class wonderings
is there something between steel and ti, say, one of the al alloys that could also fill the bill? high strength plastics? the drop links on the rear of my camry are delrin.
didn't the 917 series have inboard rear brakes?, maybe from the 910 on up?

[Wil Ferch]

No.... no prototype ( or even production-based) Porsche had inboard brakes...AFAIK ......

-Wil

[Brant]

we put a lot of consideration into sprung/unsprung when we built our car.

Here is our inverted rear aluminum shock.
(theoretically moves a little bit of the weight off of the suspension)