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Lift Theory

Fish'nCarz

NAXJA Forum User
Location
Victor, ID
I know I'm gonna get roasted for this, but I just went through 34 pages of archives and can't find an explaination of how all the parts need to work toghether when you start adding inches.

For all you search wizards, is there anything in here that tells you how lifting effects the geometry of the drive line and steering, and what you need to do to make everything work and play well together? I want theory and math, not just, "You should buy one of Jimmy's lift kits, they rock."

If it doesn't exist, should we make one, sorta like a FAQ?

Okay, tell me why I'm stupid.
 
Like why longer coils are needed? Because they provide the lift up front.

Longer control arms? Because the stock ones are super-flimsy and will also pull the tire back while tilting the pinion down. Longer arms correct that.

Longer brake lines? So the axle has enough room to flex.

Trackbar? The stock bar doesn't allow for much downtravel. When lifted, it will pull the axle to the driver's side as well.

Rear springs? Longer and provide lift.

Shackles? Often at least slightly longer because they help the spring move fully.

That's the short n'sweet...
 
Yucca-Man said:
Longer control arms? Because the stock ones are super-flimsy and will also pull the tire back while tilting the pinion down. Longer arms correct that.
Very true. There is also the option of relocating the LCA mounts @ the axle up behind the axle tube. The UCA mounts will also need to be moved up the same distance. Without enough lift, however , this could cause some issues with clearance above the axle. The issue of the tires drooping and folding under can still exist if the suspension has a large amount of down travel, aka droop. Keep in mind that when this happess your wheel base can shrink several inches, 6" - 8". when this happens, things can get pretty damned tippy; waaaaaay too tippy. :D
 
Dependent variables. You change the suspension height and you may need to adjust a number of other parameters so that things physically work properly. If you dont feel confident in doing things yourself, buy one of Jimmy's lift kits and hope he's already figured out the important things for you. NAXJA is a great site for fellow enthusiasts to discuss XJ related topics such as what lift kit worked well for them. If you really want to see theory, read some books - Milliken's "Race Car Vehicle Dynamics" and Gillespie "Fundamentals of Vehicle Dynamics" are a few that come to mind
 
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Are there some other books that concetrate on 4X4s, maybe even XJs?
I'll go search 'books."
 
Simo said:
Dependent variables. You change the suspension height and you may need to adjust a number of other parameters so that things physically work properly. If you dont feel confident in doing things yourself, buy one of Jimmy's lift kits and hope he's already figured out the important things for you. NAXJA is a great site for fellow enthusiasts to discuss XJ related topics such as what lift kit worked well for them. If you really want to see theory, read some books - Milliken's "Race Car Vehicle Dynamics" and Gillespie "Fundamentals of Vehicle Dynamics" are a few that come to mind

Only book I could find were these, posted by you!
 
If you really want to fully understand all the ins and outs of steering and suspension girometic (<-spelling.) and there interrelationship (<- that can't be right) get you self an engineering degree. Then work for a major auto manufactory for a few year. developing same. Then come and explain it all to us. Some of the dudes in "AD FAB" punch some number into a computer but it still comes down to there best educated stab in the dark. LUCK.
 
Fish'nCarz said:
I know I'm gonna get roasted for this, but I just went through 34 pages of archives and can't find an explaination of how all the parts need to work toghether when you start adding inches.

For all you search wizards, is there anything in here that tells you how lifting effects the geometry of the drive line and steering, and what you need to do to make everything work and play well together? I want theory and math, not just, "You should buy one of Jimmy's lift kits, they rock."

If it doesn't exist, should we make one, sorta like a FAQ?

Okay, tell me why I'm stupid.
You're not stupid, I think you might just be overthinking the issue. This isn't rocket science and you don't need a bunch of difficult math to figure it out. Insofar as the theory goes, Yucca-Man detailed it very well above. What exactly are you looking for? There are plenty of pages out there that delineate how and why to lift that don't get into much math at all. Its actually a pretty simple process to lift a vehicle. The difficult math occurs when you want to lift high, and that math is only difficult because its all in the money... :D
 
There are trade offs to any aspect of modifiying anything. 1st of all you need to figure out what you want to do with your jeep, then figure out what will work for that the best. More than anything its trial and error with lots of research to go along with that to find out what works best for you. For the most part what you read is an opinion and not an across the board fact. A radius style long-arm front will be the best thing in the world for some while others wouldnt touch it. It really comes down to what quirks you notice. The suspension calculators help you figure out what youve got and how to change it for your likings but it still takes knowledge and experiance to modify it for you.

On the simple side of things, if youre looking for a "lift kit" look for one with enough adjustability to get your steering geometry back in check. Then decide what you dont like about it, there will always be something...
 
Fish'nCarz said:
Are there some other books that concetrate on 4X4s, maybe even XJs?
I'll go search 'books."

It doesnt necessarily have to be XJ specific, for any vehicle you can make generalizations concerning "Adding Inches" to driveshaft, pinion angle, steering, and alignment issues. Most of these are pretty simple and can be remedied by making the appropriate adjustments. The lifted vehicle will also have altered handling characteristics to resist dive, lift, squat, and roll under braking, acceleration, and cornering maneuvers. You can change the mounting positions and/or link length on your front suspension to adjust both static features like pinion angle and caster, as well as handling (instant center, dive, lift, roll axis, COG, etc) Link geometry has become a popular topic of mixed quality on some 4x4forums so you can continue to read about why things act the way they do. It can be a large undertaking to redesign your suspension and not really necessary on most XJ applications but since you are asking WHY you can read and be more informed to decide what you want to change as your vehicle progresses. Enjoy.
 
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Beej said:
You're not stupid, I think you might just be overthinking the issue. This isn't rocket science and you don't need a bunch of difficult math to figure it out. Insofar as the theory goes, Yucca-Man detailed it very well above. What exactly are you looking for? There are plenty of pages out there that delineate how and why to lift that don't get into much math at all. Its actually a pretty simple process to lift a vehicle. The difficult math occurs when you want to lift high, and that math is only difficult because its all in the money... :D

It's just that I want to know why things work the way they do, not just that they work. What causes the nose to dive on a poorly sprung vehicle? What's the point where that doesn't happen? What's the point where the opposite happens? What angles will cause DW and what ones won't? There seems to be a lot of variability in practice, but I'm thinking that if you measued everything on the DW vs non-DW trucks you'd see an immediate correlation.

Yeah, I know that there really isn't any need to know all this stuff. But there's really no need for most of what we do on here. It's just like fishin'. Either you like it or you don't.

I knew I was gonna catch sh*t for this, but I still want to understand what's goin' on.
 
no ones hassling you, in fact it is good to ponder things. Only dont approach it with a pessimistic attitude. Enjoy the subject, continue to read, and you may be able to explain the 4wheelin phenomena experienced out on the trail... ie: why a radius arm front suspension might unload on climbs or why one vehicle hops when spinning tires instead of remaining planted grabbing traction ...
You may find some discussion of brake dive in this thread http://www.naxja.org/forum/showthread.php?t=15764&highlight=brake+dive
DW Death wobble is a chaotic vibrations problem that can happen on any vehicle but its definitely more prone on one running heavy out of balance tires with the slightest misaligned abused worn out sloppy steering/tracking parts.
 
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Simo said:
no ones hassling you, in fact it is good to ponder things. Only dont approach it with a pessimistic attitude. Enjoy the subject, continue to read, and you may be able to explain the 4wheelin phenomena experienced out on the trail... ie: why a radius arm front suspension might unload on climbs or why one vehicle hops when spinning tires instead of remaining planted grabbing traction ...
You may find some discussion of brake dive in this thread http://www.naxja.org/forum/showthread.php?t=15764&highlight=brake+dive
DW Death wobble is a chaotic vibrations problem that can happen on any vehicle but its definitely more prone on one running heavy out of balance tires with the slightest misaligned abused worn out sloppy steering/tracking parts.

Pessimistic?!?!? I'm a freakin' fly fisherman and bamboo fly rod maker, for cyin' out loud!! I don't believe in pessimism!
 
your assumption that we will hassle you because you asked a valid question is maybe why Simo felt pessimistic vibes. The best advice I can give you is, whatever you do, make it quality. Being a fly rod maker, you know this. If you can make fly rods, you can definately make a suspension system on a Jeep stronger.
 
Get this book........
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I bought it here http://www.amazon.com/High-Performance-Cherokee-Builders-1984-2001-Design/dp/1932494146/sr=8-1/qid=1170904847/ref=pd_bbs_sr_1/103-1539876-8282243?ie=UTF8&s=books


I love it. Ive been reading it since Christmas.
 
Think of lifting your Jeep this way.....It came from the factory as a working system. Everything works in relation to something else. You change one part of that system, it creates problems for everything else until you've replaced the entire system proportionally. You can't just throw springs and larger tires on it and go....You have to address every other part under the frame as well, to include axles and gears. So to answer your question would be very difficult here, but a tip would be, try searching about specific parts, ie, track bar, driveline angles, ect. And as much as everyone hates to hear it, NAXJA isn't the only site on the interweb that applies to us. You'd be surprised what Google and Yahoo can bring up.
 
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