Unsprung Weight

phorensic2k

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Yeah so I have been doing a lot of planning on my truck lately with my fabricator and he suggested I go IRS for various reasons I won't go into. I was wondering, however, out of the possible rear end configurations, which has the lightest unsprung weight.
Here is a list of configurations that don't necessarily equal performance or correct geometry. Just trying to get an idea on weights.

Stock leaf-spring setup
Stock leaf-spring with track-bar
3-Link
4-Link
IRS
Cantilever (including 3 or 4 link to locate axle)
anything else you can think of

Awww yeah, you know you like the cantilever, hehe.
 

farmboy

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Quater elipticals, with a three or four link of course.

Boy, Farm
 

drtdevil93

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i think the lightest possible would be irs with a torsion bar and a cantilevered shock. i believe half of the shocks weight is unsprung, torsion would eliminate the coilover, cantilever should eliminate the bypass shock weight from being unsprung. (half of the connecting link would be unsprung, i think???). or like talked about in another post you could do some work with the motion ratio and eliminate those heavy bypass tubes (every ounce counts!!!)

erik
 

FABRICATOR

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phorensic2k,
Nothing even comes close to IRS. Solid rear axle setups for racing range from around 350 to 600 pounds of unsprung weight. This is 150 to 300 pounds per corner. IRS is around 90 to 150 pounds per corner. This range is from buggies or class 7, up to the heavyweights. A heavy vehicle can get away with high unsprung weight, but lower is better. A good target is no more than 10% of vehicle weight per end or 5% per corner. This pretty much applies clear up to Formula 1. With this it's easy to see why a 6000 pound TT can still go fast in the rough and why a light vehicle with a solid rear axle does not.

drtdevil93, you're on the right track...less than half the shock is unsprung because you are only moving the shaft and piston. You don't eliminate just the by-pass tubes, the whole shock and all the related beef goes!

<font color=orange>The best ideas are the ones that look obvious to the casual observer.</font color=orange>
 

SJP12

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Is a "IRS" an independent rear suspension? Are you putting this on a truck or a buggy?

I LIVE MY LIFE 1 SET OF WHOOPS AT A TIME!!!
 

tkr

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ok, time for a stupid question....what is sprung and unsprung weight? Is unsprung just what moves with the suspension?

Matt Nelson
Team Kwik Racing
 

phorensic2k

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Putting this on a truck, GMC Sonono to be exact. It will be tube framed and totally custom suspension for the front, still deciding on the rear.

To the person that stated that IRS is the lightest: are you including axle weights as well?? Also, these are going to be some long freakin arms too, center mounted (well, as close to the center as possible) with a track width close to 6" wider than stock. I had a pretty good feeling that solid axles were heavy but I didn't know how much compared to a full on race A-arm setup.
 

drtdevil93

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fabricator: are you saying to have no shock at all? i was saying with a good cantilever, a standard (non-bypass, non-coilover) shock would be all thats necessary.
ive actually designed (if a picture on scratch paper with a couple numbers around it can be called designing) a a-arm setup sprung by a transverse leaf spring. looks like it would work. not only that, but with leaf springs and a-arms both having a highly progressive nature, a bypass shock or cantilevered setup would probably be unnecessary. might be my first big project when i get a shop setup at my house.....

erik
 

V8Ranger

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I understand the concept of sprung/unsprung weight but it seems to me that you would also want to take into account of where the mass is on the suspension component. Mass closest to the pivot point on a suspension arm is going to have less effect on handling characteristics than a tire which is mounted at the furthest location from the pivot point.
 

1992f150

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on a 3/4 link setup the driveshaft would be part of the unsprung weight right? And for IRS with a front engine configuration, the differential doesnt move correct? So it shouldnt be part of the unsprung weight?

Azusa: shame of the foothills
 

ntsqd

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If it moves with the tires it's at least partly, if not all, unsprung weight.

If it moves with the chassis it's sprung weight.

Reducing unsprung weight is like finding hidden HP, you will go faster.

Mass at the axle end of a link or arm is more significant than mass near the chassis pivot, but that's Moment of Inertia making it so, not just unsprung weight.

Where things get tricky is in stuff like A-arms or links. One end moves with the tires. The other moves with the chassis. If the arm is exactly symetric and there aren't any other influences, then you can safely say that half of it's mass is unsprung. As soon as they aren't symetric or you mount a spring or damper to the link or both, then things get funky pretty fast. Figuring out what your exact unsprung weight is, is pointless. Make things as light as will live thru the punishment you intend to give it.

Colin Chapman, of Lotus Cars, was quoted as saying the perfect race car disintegrates at the finish line. That might be a little hard on drivers, but it would be a sure indicator of everything being absolutely as light as was needed to do the job.

TS

I used swerve around my halucinations, now I drive right thru them.
 

drtdevil93

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isnt chapman the same guy that said every part on a race car should have 3 functions or it should be eliminated. that one continues to blow my mind.

erik
 

ntsqd

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That's the guy. Try designing and fabbing parts like that. That will really blow your mind. "So let's see, the shock hoop is also the engine mount crossmember and the upper control arm rearward mounting point, where does the steering box go......" Sometimes when I'm doing really good I can make one part do two things.....

TS

I used swerve around my halucinations, now I drive right thru them.
 

drtdevil93

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i do that a lot of times. i like to play mind games with it. i always end up cheating though. like the forward j-arm mount, which is also a radiator mount, and umm... headlight mount. kinda like cheating at solitaire.

erik
 

tkr

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Thanks Thom, good explaination.

Matt Nelson
Team Kwik Racing
 

FABRICATOR

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phorensic2k
When comparing IRS to solid axle, yes it includes 100% of axle weight. A-arms are not the best idea in the rear. There are other and better choices.

drtdevil93
I was talking about no by-pass shock, just using a coil over. Again, A-arms, are probably not the best choice in the rear. It's been tried many times. The rears hit rocks more than the front, and A-arms back there tend to get detached from the car easier. A transverse leaf spring could work, as it's not much different than a pair of quarter eliptic springs turned sideways. Hmmm. A single element leaf spring is much more efficient than a stack of leaves, but it would be one busy spring. A transverse spring offers little for attaching the axle to the chassis, overall advantages would be questionable.

V8ranger
Where the mass is on the suspension makes a big difference. It must be kept in mind that straight line handling of the bumps is the highest priority (If not, the vehicle shouldn't be out there). In this situation, the entire solid axle, wheels, brakes, etc., together make up a single component. Again, in this situation, that component is usually 100% unsprung weight, and is the farthest one from the pivot point. At least part of the weight of everything else that moves is added to that.

<font color=orange>The best ideas are the ones that look obvious to the casual observer.</font color=orange>
 

elcaprerunner

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I don't understand. Why not use a bypass shock? What about using Torsion bars and bypass shocks like the Baldwin's? What is the problem with having a coilover and bypass shock. It always seems like a truck that already has a coilover and a bypass is added, it always seems like the truck does way better when the bypass is added. It also seems that a truck with coil springs or torsion bars and a bypass does better than a coilover setup (in the front). So why not just a bypass shock and no coilover? It always looks like that works so much better. I have experience riding in trucks with both setups, so this is why I am asking. When you say just a coilover, do you mean say......A Robby Gordon shock? Those are internal bypass. I agree with not doing independant suspension in the back. I saw Letner's Trophy Truck perform this weekend at the MDR race and even though it did awesome, it seemed very springy and bouncy in the back.

FABRICATOR- What do you think of Kyle Taylor's new trophy truck? The suspension setup in the back (cantiliever)? Do you think that would be better with just a coilover? I don't think so, I think it will do better with coilover and bypasses, as would alot of trucks.

DIRT'S FOR RACING, PRERUNNERS ARE FOR GETTING THERE!

H.O.R.E.
Hemet Off-Road Enthusiests
 

FABRICATOR

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It is well proven that all of these combinations can work very well. None have consistently proven to be superior to the others. Most of what is being built is more a matter of convenience and a genuine lack of engineering. It is fairly easy to build a nice riding system with coilover and bypass shocks together. It is a bit more difficult to have a perfect torsion bar and bypass setup. It is still more difficult (technically) to have a linkage setup to provide the desired rates of damping and springing. The one that can offer the biggest advantage is the linkage setup. The fact that it can outperform the others is only one advantage. It can also eliminate hundreds of pounds of sprung and unsprung weight from something like a Trophy-Truck. Bypass shocks are hard on components, linkage is not. This and other factors have been explained numerous times. IMHO this is the next generation of speedy vehicles, and I don't agree with not using IRS. It all applies to the overall package. If you're going to run a 5 to 7 thousand pound vehicle with a solid rear axle why bother with technicalities. It already weighs up to double what it could, and has to stay that way to work it's best.

<font color=orange>The best ideas are the ones that look obvious to the casual observer.</font color=orange>
 

JrSyko

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Elcap -
IRS can work very well if done properly, just look at Ivan's old PPI TT. It was IRS and worked killer.

See ya in the dirt!
 
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