FRONT CA length, upper vs lower

T&A-XJ

New member
So Ive got my 3 link (front) designed with proper geometry but have not been able to find the answer to my dilema. the single UCA will be 18-20" long(depending the AD% I like best) and the lower CAs will be 26" long. I would prefer to have the lowers longer @ 29-31" to make mounting much better and a little higher but if the lower must be closer to the upper length the 26" arms will work just not with my prefered bracketry design.

Would there be a problem with running a 20" UCA with 30" lowers in the front suspension? I searched and searched but found nothing on prefered link length FOR THE FRONT. I know the rear suspension works best with apx 25% shorter uppers is this true for the front even with the forces applied backwards?

Thanks in advance, this project might actually get started!
 

MrShoeBoy

New member
Have you plugged your numbers into the 3 link calculator? I have similar questions regarding the front suspension and link lengths and after playing with the calculator, I got numbers that seemed like they would work with the lengths of the uppers and lowers being that different. The only thing I ran into was the vertical seperation at the frame. From the reading I have done here and over on Pirate, the ideal seperation at the frame is 6 to 8" but I can get decient numbers with seperation in the 3 to 4" range. Will this work or will it cause problems? Sorry to steal your thread but I figured its close enough of a subject it wouldnt hurt :)

AARON
 

T&A-XJ

New member
Triaged sig line on the Pirate4x4 board has the 3&4 link calculators.

Yes I have used the calcs to check my paper design. Its hard to tell from the info you have posted but I can say that the more sepa
ration the easier it will be on the upper link and joints. My design surrounds a 80% anti-dive and is adjustable from 40% up to 120%
 

MrShoeBoy

New member
My upper link is 25" long and the lowers are 31.5" long. The vertical seperation at the frame is 5" and 9" at the axle. This gives me a 100.5% anti-dive, roll axis slope of .187, roll axis angle of 10.62, and a roll center height of 24.86. I can adjust the upper link mount at the frame and get the AD numbers to change from 130% with link sepearation at 4" and 84% with link seperation at 6".

So how much extra stress is being placed on the upper link with seperation at 5 to 6"? Is it beyond the capacity of a large RE flex joint or should it be avoided all together? I was planning on using 1.5"x.25" DOM for the upper link with large RE flex joints at each end. My lowers are already 1.75"x.25" DOM and they have survived just fine in the 5 years I have been running them. I have an existing radius arm style long arm setup and have finally gotten annoyed with the handling characteristics of it.

Thanks,

AARON
 

T&A-XJ

New member
If you look at the material section of the calc. you can see the load on each arm. Im not sure how it relates to reality but it can give you a good idea on how much difference there is when moved around.

Even though you are working on the front(steering axle) 10* is alot of roll angle. try to make you lower links closer together at the frame to help with this. mine has 4.5* and if i lower the jeep an inch as i plan on doing it goes down to 2.3* if this is a trail only rig its not a big concern but i am designing mine for long freeway trips. I would say you need as much separation you can get just for your AD%. 130% is almost radius arm territory. From what I have found most people here say to shoot for 70% or so to start with for the front. My theory is leaning more toward 100% so it will be more nuetral (in theory) My current radius arms are about 140% ad and in front wheel only drive I can hit the bumpstops on med-heavy acceleration. I dont want the opposite to happen on a waterfall without having a linked rear to match the geometry. The front end gets light enough for me on some climbs. This is just theory right now which is why I am building in pleanty of adjustability to play with.
 

MrShoeBoy

New member
I have radius arms up front now and the unloading on the front end on climbs is starting to bother me. I have never hit the bump stops in Fwd when accerlating, but it does torque steer real bad. I didnt think it was so bad but now that the XJ has gotten bigger and I have really started to push it harder, its becoming a problem for me. I dont want to move the lower links since they are already in place and a total suspension overhaul is going to wait for the next build. What exactly does the roll angle do to the geometry? The current XJ shell is still an experiment on what works and what doesnt so I figure I can toss on a long upper and see how it goes. Then after I get a feel for how it works, consider building up a new rig that will have all the good stuff from this current one minus all the bugs. Thats the theory anyways....

Thanks for the help and sorry again for stealing your thread,
AARON
 

T&A-XJ

New member
If I can explain correctly, the roll angle is how much axle steer you get with articulation or body roll in a corner. This is especially a concern for rear applications that are street driven. Positive roll angle is oversteer and negative would be understeer. A friend of mine had a short armed ZJ with 6" lift that looked as if it was drifting corners because the body roll would oversteer the rear axle and kick out the rear end even though the tires were not loosing traction. Looked pretty bad ass in the rearview mirror though. A front axle application is much easier to deal with because the wheels are dirrectly steerable.
Though many say it isnt a concern my opinion is that with little to no roll oversteer a vehicle will be much easier to control if you end up sideways or drive up a twisty mountain road. If you have delt with the 10* roll angle and it hasnt been a problem leave it, a 3 link will still be much better than a radius arm suspension.

No problem with hijacking the thread...at least its still up at the top.:speepin:
 

Boostwerks.com

NAXJA Supporting Vendor
Anyone happen to know how the RK 3 link LAs stack up with the calculations? Just something I've been wondering for a while now.
 
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