Unsprung Weight--I need a tutorial, please

/wwwthreads_images/icons/smile.gif Luckily for all of us, one of my oldest moonguys/wwwthreads_images/icons/crazy.gif was at Indy in the Heyday of the Offy-powered roadsters, and he STILL has "unsprung weight" as a mantra. HIS explanation is that in order for the chassis to "work"....for the suspension to do all that it is supposed to do about cornering, about getting traction, about distributing weight where it is supposed to be.....the weight HAS to be ON the suspension/wwwthreads_images/icons/crazy.gif.....as in compressing the springs, shocks, etc. Otherwise it may as well be on someone else's Jeep. Moonguy's/wwwthreads_images/icons/wink.giftheory is that unsprung weight is bad even in rock crawling because it keeps the chassis from adding weight, just when it is the most critical. That's why the automakers(Hummers included) has tried so darn hard to get the gearboxes off the axles and ON the chassis. They even went to INBOARD brakes to reduce unsprung weight on hot cars that need to corner real good. This IS a difficult concept, especially if you don't have a lot of race track time like mooney here and I have, where you have had a chance to watch the car "work" and try different things. We always refused to run HAlibrand Quick-change rear ends(Man...does THAT date me or WHAT?) because they were so heavy and added so much to the unsprung weight of the car. We opted instead to carry two different transmissions with slightly different gearing and between that and tires, we could usually get what we needed./wwwthreads_images/icons/smile.gif

CJDave
I never believe any statistics unless my moonguys /wwwthreads_images/icons/crazy.gif/wwwthreads_images/icons/wink.gif made 'em up themselves.

I think it works along the lines that mass (or weight) is slow (the heavier something is the more difficult it is to change directions), so a heavy axle will move further up (say) then a light one if the springs/shocks have the same rating. So a bump is "cleared" better with less axle weight, this is one of the advantages of Independant suspension systems where there is less unsprung weight. Although I think the effect is more pronounced at higher speeds rather then at rock-crawling speeds. This also why high speed cars have these ultra low profile tyres, the tire weighs more then the light alloy, so less of one and more of the other reduces the overall weight.

So all those axle trusses and axle skid plates don't make a lot of sense from a weight control point of view.

Hope this helps

BlueJay
93 YJ with upgrades (and it is Blue!)

Grant & CJDave,

You guys are so right. One really should look back to go forward sometimes.
My unsprung weight goes back to dirt bikes. Every pound you put on a bottom
of the front fork was that more leverage you had to deal with. Same on the end
of the swing arm in back. Its like takeing your hand and punching as fast as you can away from yourself and then bring it as fast as you can back to your chest with out stopping back and forth. Them put a 3 LB hammer in you hand and
try doing the same thing as fast as you can.It will be harder to do and take more power and time to move back and forth.
You will turn shocks upside down forks upside down go to disk brakes any thing
to overcome the unsprung weight.
The point on a race car or dirt bike is to keep the tires on the ground.
You cannot get the power on the ground or turn with your tires in the air.
I see this so clear in high speed stuff.
On the slow speed rock crawling I have a hard time seeing it.
BlueJay was a great help here stll have a block here. Don't know if it worth the time and money to worry about?????? Umless we are going on a high speed
blast down the Baja/wwwthreads_images/icons/smile.gif Don't think{I KNOW} this old bod could take it any more. I will warn you younger guys now bumps and jumps use to be smile /wwwthreads_images/icons/smile.gif
makers. Now they just flat hurt./wwwthreads_images/icons/frown.gif.Maybe if I had not smiled so much
when I was young it would not hurt so much now./wwwthreads_images/icons/smile.gif Who knows I would not trade any of it.

any easy way to measure unsprung weight (not 100% accurate) is to weigh everything below the spring (tires, axles), half of the weight of what is attached both to the frame and the axle (ie shocks, springs, drivelines, sway bar links, etc) and add it all up per end. this will give you an idea of how much it makes a difference. to see how unprung weight changes handling, put on a set of BFG ATs on aluminum wheels and drive it, then switch to swampers on steelys and see the difference. for slow speed work it is not as much of an issue, but with a little playing with you can make a big difference. take for example the weight difference between a 9in and a dana 60. Almost 200lbs depending how it is set up. now you know another reason why 9in are used in a lot of racing instead, a little sacrifice in strength for a weight savings.

dan
Good things come to those who do research!!

I've got alittle time working on circle trackers and "unspung weight" is something i think i understand alittle. i always thought that less was better (always) because "lighter suspensions are faster moving and stay in contact with the ground better" ever drive on a "washboard dirt road" and your truck just skips along the tops of all the bumps? it makes it very hard to turn because your tires don't have much contact with the road, (they are just skidding along on top) bla bla bla

i also have a question,
i kind of think that alot of unspung weight combined with a low center of gravity is a good thing for a slow moving "rockcrawler".
CJDave you said "because it keeps the chassis from adding weight, just when it is the most critical." when you were talking about to much unsprung weight, could you (or anyone) please explain your thinking to me. i don't disagree, i just don't understand

3/4tonYJ
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