Un-sprung vs. Sprung Weight

RSTPerformance

RSTPerformance

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What are the pro's / cons to sprung vs. unsprung weight?

I have my own thoughts, but would like the real answer!!!

After discovering the damages that were done to the rear Axle in the Audi from years of fatiuge I have finally convinced myself to build a sway bar. I am not so convinced how much it will change the handling of the car no matter how stiff it is thus my mind is working overdrive thinking about this.

I have to add about 25 - 50lbs of balast in the front seat area. The car currently weights 850 on each front corner and about 425 on each rear corner. My thought is to make a super heavy duty rear sway bar and not worry about weight even if the thing weighted 50lbs because I could just take out the balast.

I don't really want to debate the "using one rule to cheet another rule" because that is NOT my intention, my intention is to build a sway bar that will be basically solid, and I do not want to worry about weight, however if unsprung weight is a "bad" thing then I might have to reconsider my thoughts.

Raymond "Trying to understand the difference" Blethen
 
http://www.kakashiracing.com/swaybar.html

I'm not sure you understand the definition of "unsprung weight". That term refer to mass that is not supported by the suspension springs and thus must be controlled by shocks. The car's body, for example, is sprung weight, whereas wheels are unsprung weight. The top hat of the springs is sprung (har-de-har) whereas the brake caliper is not. The struts and springs themselves will be generally considered 50/50 for calculation purposes. A general rule of thumb is that in its affect on suspension performance, a loss of 1 pound of unsprung weight is roughly equivalent to losing 10 pounds of sprung weight; thus the high value placed on lightweight wheels and tires.

In regards to a swaybar, in my example pretty much the whole dang thing is sprung weight, except for a small amount of the weight of the end links.

So, I'm guessing that's not what you intended to talk about...are you maybe asking about springs versus swaybars, a whole 'nother discussion in itself? - GA
 
Actually it depends on the type of sway bar/suspension setup you have. For example, I have a twist beam rear axle, and the sway bar is part of the axle, making it mostly unsprung weight. (IOW, I have a very stiff, but very heavy twist beam rear suspension) Now, if I move the sway bar onto the body frame rails, and use links to attach to the twist beam, I still have the same total weight, but with very little unsprung weight. Unfortunately, I've never done this comparison as I have exhaust, wheel tub, and tow hook in the way. But, as GregAmy stated, you are better off with more sprung weight than unsprung weight because sprung weight can be controled/manipulated with the springs and shocks.
 
John-

Your description of what I am doing is much better than mine as far as the sway bar... It would not attach to the body at any spot, thus it would all be "unsprung weight" as is the rear axle (solid rear axle - typical on older VW Audi's). Thanks for your reply!!!

now back to the question at hand... Greg you better reply ;)

Why are you are better off with more sprung weight than unsprung weight, only because sprung weight can be controled/manipulated with the springs and shocks? If so how much do you really think this is a benefit? or how can we prove this is a benefit?

Greg-

Thanks for the description, I did understand that much, but I think it was helpful to anyone else who might read the thread. Also thanks for the link with all the pictures... Looks like you have one heck of a solid rear swaybar!!! I still have a hard time understanding how that all works, and if you don't mind at Lime Rock I would like to come get a personal look and explanation from you on your set up? Let me know!!!

As for wheels/tires I thought that we actually cared about the rotating mass. IE: The heavier the wheel/tire combo the more HP you steal trying to turn them?

Thanks for the input!!!

Raymond
 
John, you come out of retirement already?
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Probably not. Between a MAJOR house refurbishment, car still in pieces, work, and going back to school (the owner of the company I work for made me an offer I couldn't refuse :023:), it's not looking likely. BUT, maybe, just maybe I'll make Grattan over Labor Day.
 
John, agreed, except I'd argue the twist beam rear axle (kinda like the Rabbit, right?) is probably 50/50 sprung/unsprung. You gotta figure at least half that weight is supported by the pivot bushings to the chassis.

>>> Why are you are better off with more sprung weight than unsprung weight, only because sprung weight can be controled/manipulated with the springs and shocks?

Not exactly. You're better off with LESS UNsprung weight because unsprung weight must be controlled by dampers (shocks). Springs support the chassis, dampers control both springs and unsprung weight.

>>> ...how can we prove this is a benefit?

Kinda hard to "prove" without testing, it's one of those things you take on faith. But to understand something, take it to the idiot extremes: think of what kind of shocks you need if you bolted 50# of lead onto the bottom of each strut...then think about what kind of shocks you'd need if you removed virtually all weight from the suspension entirely (e.g., made everything of carbon fiber). It's pretty significant, thus the 10:1 accepted factor.

>>> ...I would like to come get a personal look and explanation from you on your set up?

Any time. Anyone and everyone is always welcome to come sniffing around our paddock. Never feel timid about looking in, over, around, through, whatever you want to see.

>>> As for wheels/tires I thought that we actually cared about the rotating mass.

That, too! But that's only a factor in the acceleration/deceleration of the rotating mass (and the resulting acceleration/deceleration of the vehicle), not in consideration of dampers (and cornering forces and control).

GA
 
John, agreed, except I'd argue the twist beam rear axle (kinda like the Rabbit, right?) is probably 50/50 sprung/unsprung. You gotta figure at least half that weight is supported by the pivot bushings to the chassis.
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No argument here. Ultimately it's going to depend on exactly how the beam/sway bar are designed. But for the sake of the discussion, its pobably closer to the 50/50 then 90/10.
 
What are the pro's / cons to sprung vs. unsprung weight?

I have my own thoughts, but would like the real answer!!!

After discovering the damages that were done to the rear Axle in the Audi from years of fatiuge I have finally convinced myself to build a sway bar. I am not so convinced how much it will change the handling of the car no matter how stiff it is thus my mind is working overdrive thinking about this.

I have to add about 25 - 50lbs of balast in the front seat area. The car currently weights 850 on each front corner and about 425 on each rear corner. My thought is to make a super heavy duty rear sway bar and not worry about weight even if the thing weighted 50lbs because I could just take out the balast.

I don't really want to debate the "using one rule to cheet another rule" because that is NOT my intention, my intention is to build a sway bar that will be basically solid, and I do not want to worry about weight, however if unsprung weight is a "bad" thing then I might have to reconsider my thoughts.

Raymond "Trying to understand the difference" Blethen
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throw a 40lb sprung fire extinguisher in the trunk :rolleyes: and add your 10lb unsprung swaybar?
 
Greg - All I can say is: Gorgeous. Nice work.

Raymond,

I tend to always try to 'turn it around' to force people to think fresh, so here goes:

The goal of your suspension is to always keep your tire on the ground with a nice, constant load (and hence, contact patch). Your spring pushes on the suspension, but it only pushes down as hard as it is pushing up on the rest of the chassis (action = reaction, right?) SO - you hit a bump - and your wheel starts going up. You need the spring to push it back down. If the corner weight is really high, you need a bigger spring to fight that inertia, right? Sprung weight = bad, because you need a stiffer spring to keep that corner on the ground after a bump. That stiffer spring pushes up on the chassis, lifting it. You have to wait for the CG of the chassis to come back down, then, for the car to settle. On a formula car, this ratio is literally doing just that - lifting the CG of the whole car over a bump instead of just lifting the one corner. On a sedan, not as much, but think more about what happens AFTER the bump - how can I get the tire back down on the ground. A lighter corner will get there faster with the same spring rate.

So - mount the drop links on your suspension, and the bar bushings on your chassis, keeping as much of your new bar as 'sprung' weight. Those bushings are creating your force transfer, so make sure they are tied to stiff areas of your structure.
 
Greg - All I can say is: Gorgeous. Nice work.

...... A lighter corner will get there faster with the same spring rate.

So - mount the drop links on your suspension, and the bar bushings on your chassis, keeping as much of your new bar as 'sprung' weight. Those bushings are creating your force transfer, so make sure they are tied to stiff areas of your structure.
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FYI guys, Mr Youngren is employed by a large unnamed automotive manufacturer as an engineer, so he's not just talking after staying in a Holiday Inn Express last night....like me!

Huge unsprung weight is one of the reasons trucks ride and handle like crap, and a great reason to NOT get the 4 wheel drive version if you don't need it. (Along with wasted energy, wear and tear and so on.)
 
You comment that its function is questionable, so if your goal is to simply add balast in a stretegic (sp?) location, why not make it weigh what ever you want, then fire the mechanic who just happened to leave it unhooked?

Either that or build your nice sway bar that works but make the mounts super heavy and use lead sleaves for bushings?

Spare tires? sch-80 exhaust? Screw bondo, use lead like in the old days (this might actually be closer to the requirement to replicate the way the factory would make a repair)? Hook up that cool suit to the BIG cooler full of ice? Adding weight is easy.

You drive the car at 10 tenths, why would you read the rule book with anything but a 10/10ths attitude?

Matt
 
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