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Somewhat newb question, but I've been playing the the 4 link calculator, and reading and reading and reading.

Reading here on Pirate indicates that positive roll axis is a poor design, and roll over-steer is to be avoided.

However, looking at pictures of TJ's and also having a buddy have his 4runner's rear 4link built at a shop, I am confused.

The shop that built my buddies rear 4link indicated that the reason for putting parallel lowers on (IE non-triangulated lowers) was because the truck would see street time. I can't remember the length off of the top of my head, but They were not parallel to the ground, and were likely 35 or 40 inches long. Jeep TJ's appear to have a similar set up. Parallel, but shorter, lowers, and a triangulated upper (or uppers, depending on the kit).

So, on to my situation. I attempted to mess around with my 4runner's rear suspension. First I tried radius arms, didn't like them, and modified it to be a 3 link/panhard. I have roll steer when I step on the throttle, bad. Holding the steering wheel straight, the truck will literally pull hard enough to the left to change lanes when starting from a stop at a light. Posted up here, and it was confirmed to be roll steer.

So, I need to change it. I was hoping to at least leave my lowers intact, which, sadly are only 30" long.

They are mounted even with the axle tube and just above the frame. No matter how I play with the uppers, I have positive 9 degrees roll axis. This is what I was expecting from my reading, and was resigned to cut it all off. But before I did, I thought I'd get the following clarified:

I get that parallel lowers that are not parallel to the ground will have the roll over-steer/roll-steer characteristic, but if roll over-steer causes such quirky driving on the road, how do jeeps get away with it? And why did my buddies rig drive so straight when he certainly had to have a positive roll axis?
 

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I have two proposed senarios first is does every one agree ithat if you build the lower links strong enough to resist bending when point loading the center of the lnk that it will be srong enough wth the lower link mounted abve the axle center line to resist the the incresed torque leverage it will experience. Second question is related to setting a fourlink up as a watts linkage being that the axle mounting points at the axle are reversed at the frame. explanation example is the bottom axle mounts to the top axle mounts have a vertical seperation of say 8" when you look at the frame connection point the top bars are oriented lower on the frame than the axle lowers making an x shape looked at from the side this setup has increased lateral holding on race cars with independent supension would the prnciple translate to a fourlink as well.
 

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This thread is a very nice resource, but there's one question I have. It looks like people have mentioned it a couple times already but it never really got a satisfactory answer.

All these calculations are performed with the assumption that you're running a heim or other cartage type joint on both ends of the links, correct? Will adding a polyurethane bushing onto one end cause any dramatic change in the behavior of the suspension (other than acting as sort of a "built in limit strap" because of binding during articulation)?

I'm curious because I know that some people say that you need to have at least one bushing if you want to drive the thing on the road with any regularity. If that's not true then please tell me now. I need my Cherokee as a driver but I can't resist making the thing as off-road capable as I can and still drive it around town (it's a fun challenge if you work at it).

Also, would you want the bushing on the frame end or the axle end? I would assume frame but I don't have any basis for that other than "it seems right".
 

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wow i think that was to much to take in. after the first half i thought i was begining to understand, then the second half started using up the brain space the first half took.

great info. im just a union iron worker with little math skills and decent fab skills. I threw my suspension togeather a week or two ago not knowing any of this stuff and now im scared to even do the math. My plan is just going to be trial and error, id be pissed to see all the numbers where i dont want them to be so im just hoping for now that it works good.

sorry i didnt have any tech to add
great info though and something i want to learn

pics are at almost rear bump. at ride heigth uppers slove up slightly at the front.

how screwed am I?
 

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That doesn't mean you are fawked, I designed my upper to be 12% longer than my lowers to point the pinion up on droop travel. Like I said put your #'s in the 4 link calc and see where you are at.
 

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This thread is a very nice resource, but there's one question I have. It looks like people have mentioned it a couple times already but it never really got a satisfactory answer.

All these calculations are performed with the assumption that you're running a heim or other cartage type joint on both ends of the links, correct? Will adding a polyurethane bushing onto one end cause any dramatic change in the behavior of the suspension (other than acting as sort of a "built in limit strap" because of binding during articulation)?

I'm curious because I know that some people say that you need to have at least one bushing if you want to drive the thing on the road with any regularity. If that's not true then please tell me now. I need my Cherokee as a driver but I can't resist making the thing as off-road capable as I can and still drive it around town (it's a fun challenge if you work at it).

Also, would you want the bushing on the frame end or the axle end? I would assume frame but I don't have any basis for that other than "it seems right".
The calculator doesn't care weather or not your running hiems/flex joints/bushing.

bushings help isolate road noise/vibration from the chassis
they also tend to deflect some when under load which will have a slight effect on your link geometry.

I run bushings at the frame and ballistic joints at the axle. my ballistic joints handle all the mis-alignment, the bushings stay pretty straight through out flex. This helps prevent premature wear on the bushing
 

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I'm setting up my front 3 link, and I had a thought, but wanted to see what others though first:

I'm really tight on space trying to clear the driveshaft etc...(toyota drivetrain in 1st gen 4runner.)

This truck will see street time, and needs to comfortably cruise at 75 on the highway, so I'm building in negative roll axis (or trying my best) by triangulating the lowers as much as I can.

because of the space limitations, I have come up with the following idea:

1. Mount the frameside lowers side by side
2. Run them at 90* from the crossmember, ie: parallel with the frame, or straight front to back, if that makes sense.
3. Run them in triple shear bracket, with a single, 5/8" bolt, as so:
|B|B|

Where the vertical lines represent 1/4" steel brackets, and the B's represent bushings.
4. Notch the tubes at an angle so that when welded to the bushing, they are appropriately angled so that the heims on the axle end of the lower link are straight.

The only downside I see to this is: its a single bolt instead of two, so if it were to fail, both links could potentially have issues.

However, I don't see a single 5/8" bolt as any weaker than two 5/8" bolts, when supported in this fashion. The three brackets simulate double shear for both bushings.

Thoughts?
 

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Hey from down under,

ive been doing some research on link suspension and am looking at doing a dual triangulated 4 link setup to accommodate the rear steer going in.
i have been reading this thread and am hoping you guys can shed some light in my direction.
i have been using the link calculator(metric) and seems ok, but one mate said i might have some issues as both upper and lower chassis mounts are on the same cross member.im not sure if this is going to be a problem.
if you could have a look and see what you think.

here are some link calc pics





http://i58.photobucket.com/albums/g252/ ... setup2.jpg
http://i58.photobucket.com/albums/g252/ ... ksetup.jpg

Cheers

Evan
 

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This thread is all over the place with many people chiming in with different oppinions about all of the various different measurement points. I'm not sure who's advice is the correct one to follow.

Could somebody distill this thread into a few accurate bullet points that will work well repeatedly for any given vehicle?

It's possible that I missed it, but I'd like to know what the starting points are for taking measurements from, to figure out what my suspension already has, and what I would need to do to change it and improve it.

I was pointed to this thread in another forum, and now that I read it I'm even more confused than when I started! :confused:

Maybe I need to break out my crayons and try again? :D
 

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First of all, great thread and info provided. Now for my first post here ill try to explain the way I see this in my head so feel free to correct me if I'm thinking wrong before I screw up my suspension with my way of thinking. A little background first. English is my second language (Yugoslav here :) ) so bear with me as ill use the terms I know to try and explain mysef... on to thinking!

Now I drive an 18wheeler and even fully loaded the damn thing will lift the frame above drive axles up a few inches every time I take of or shift hears or it will suck the frame down when backing up. Now if I think bout my TJ and think about linking the rear I see 2 different outcomes by placing links in different locations on the axle side. The way I think about it is: If I was to somehow prevent tires from moving forward by any means what would happen is once the torque is applied the axle housing would try to rotate about its center line/axle shaft and rotate everything attached to the housing... Now placing the lower links below the axle centerline at an upward angle toward chasis would create a pushing effect on the mounting point at the frame side all the way untill the mount on the axle side reaches the centerline in front of the axle tube. If uppers are mounted behind the center of pumpkin (when viewed from above) also at an upward angle toward frame this would create a pulling back affect. Together the uppers and lowers would want to push and pull on the mounting points lifting the back up before pulling the frameside mounting point in a straight line between the mounting point at the axle side and a point on the vehicle I haven yet figured out but probably somwhere above and in front of the front axle...

Now if lowers are mounted in front of the housing at/above centerline parallel to the ground toward the frame and the uppers at an upward angle towards the frame, when torque is applied and the housing wants to rotate backwards the mounting point of lowers at the axle eng goes straight up and the mounting point of uppers goes straight back pulling the mounting point at the frame side down in line with ??? Once again I'm yet to think that far :)


So basicaly, can a person try to figure things out just by thinking what would happen with the housing, links, the mounting points and angles if the tires/axle centerline were to stay still and everything else spin around backwards ???

I assume the front end of the rig would ask for opposite thinking, no?
 

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I'm setting up my front 3 link, and I had a thought, but wanted to see what others though first:

I'm really tight on space trying to clear the driveshaft etc...(toyota drivetrain in 1st gen 4runner.)

This truck will see street time, and needs to comfortably cruise at 75 on the highway, so I'm building in negative roll axis (or trying my best) by triangulating the lowers as much as I can.

because of the space limitations, I have come up with the following idea:

1. Mount the frameside lowers side by side
2. Run them at 90* from the crossmember, ie: parallel with the frame, or straight front to back, if that makes sense.
3. Run them in triple shear bracket, with a single, 5/8" bolt, as so:
|B|B|

Where the vertical lines represent 1/4" steel brackets, and the B's represent bushings.
4. Notch the tubes at an angle so that when welded to the bushing, they are appropriately angled so that the heims on the axle end of the lower link are straight.

The only downside I see to this is: its a single bolt instead of two, so if it were to fail, both links could potentially have issues.

However, I don't see a single 5/8" bolt as any weaker than two 5/8" bolts, when supported in this fashion. The three brackets simulate double shear for both bushings.

Thoughts?
Interesting idea. Its always good to look at things from a different perspective! I agree that the single 5/8 bolt will not be more likely to fail then two bolts although it maybe more catastrophic if it does fail.

The problem I see is that by angling the tubing on the joint it will continuously side load the joint at what ever angle you tubing is at. I don't think they were designed for this in a constant manner and would probably wear even faster than normal or cause the cartidge retaining mechanism to fail and the joint to come apart (Johnny Joint...)

Only way to know for sure tho is to do it. Is it done yet?
 

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So I was confused beyond all belief by the calcs, terminology, theory behind all this mess. Until I opened up solid works and made models of the links and axles in an assembly. Then made a draft of the assembly and drew in lines for link center lines, CoG, the anti-squat line, roll axis... Now I have an understanding of how moving the different points around effects the numbers. The physics of the 'exception to the rule' rigs is still over my head though.

Now I need to get measurements from my LJ and actually put my model to use.
 

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I don't remember if it is posted earlier in this thread but this is a good how-to on the calculator.
http://www.pirate4x4.com/forum/showpost.php?p=4929623&postcount=21

I have also done the whole solidworks thing but still revert back to the calculator because of how quick I can make the changes. One thing I want to do and haven't is use the calc. like a design table and make changes to the calculator update the SW model. I just use a template I have and type in the numbers though.
 

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The link axis are derived from from a top and side sketch, editing the link mounts is as simple as adjusting the dimensions on one of those sketches. The baseline for the dimensions is the 3D center of the axle. Each link pair is a separate part file. I drew one link, and mirrored it across the origin. I have a top and profile draft of the assembly I use to find the roll axis and IC. It updates automatically with the assembly.
 

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i have read through this whole thread, learned about as much as i could take in within two days of reading and not experiencing it first hand. i decided to play with the numbers on the calc, and seem to have come up with what seemed to be okay, however i dont know, as i have never done this before. im looking to ditch the leafs in the rear, and link it with some tj coils.
im running a 97 ranger ext cab, 4L, m5od, 1354, '79 hp d60, sterling 10.25, 38.5x15 tsl sx's. plans when i link it are 1350/1354 doubler, 5.5" rear tj coils, and a bII tank in the rear.
here are some screen shots of my results that i came up with. and i use the truck to drive to and from the trails, like to hit the faster stuff, and attempt to crawl. so i need it to be stable at speed, and on the road. as stated before i tried to keep the AS% low to keep the wheel hopping down, as i can adjust more into later.

any helpful insight is greatly appreciated, and im looking to have this done when tax season comes around.


http://www.flickr.com/photos/[email protected]/6534565879/http://www.flickr.com/people/[email protected]/

http://www.flickr.com/photos/[email protected]/6534565793/http://www.flickr.com/people/[email protected]/

http://www.flickr.com/photos/[email protected]/6534565691/http://www.flickr.com/people/[email protected]/

http://www.flickr.com/photos/[email protected]/6534565601/http://www.flickr.com/people/[email protected]/

http://www.flickr.com/photos/[email protected]/6534565479/http://www.flickr.com/people/[email protected]/

oh and i got the weight by using my buddys scale at his dads quarry, so im not sure how accurate they are. and i guesstimated the unsprung weights.:D
 
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