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Old 06-11-2019, 11:29 AM   #1 (permalink)
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Front 4 Link Setup

I've got a '92 YJ with a trussed Dana 60 front, gonna run 16" ORI's, full hydro steering, and I'm setting up my front 4 link right now, and I'm coming up with a few questions. I'm going to have my lowers around 44", and going back and forth between running my uppers at 35" or 43". With the shorter uppers I'll end up having another set of brackets on the framerails, and if I do the longer uppers I can use an all in one 4 link bracket like Genright, Artec, TMR etc makes. Anyway, with putting my numbers in the calculator, my questions I'm looking at and need help with are:

1. Total degrees of horizontal angular separation to locate the axle side to side. Shorter uppers total 56.5*, longer uppers give me a total of 49.2*. And I think more angle will be better, yeah?

2. Pinion angle change throughout travel... The longer upper seems to give more favorable numbers on the pinion pointing at the t-case through travel to avoid ujoint bind. But I think the shorter upper should still be okay within the range to not bind, just gets more ujoint angle at full droop

3. Amount of anti-dive and anti-rise through suspension cycle. I think the shorter upper gives more consistent numbers between full stuff to ride height to full droop, but I don't know how much this set of numbers matters at full droop. How is 71% vs 80% at full droop if my anti-dive number is around 61% at ride height? The shorter uppers are 71% at bump, 60% at ride, and 72% at droop. The longer uppers are 54% at bump, 62% at ride, and 80% at droop

So, in my mind it seems like it's maybe splitting hairs or maybe making a rig that's much more stable and predictable, but I'm not sure. What are your thoughts? What way would you guys go and why?

Thanks for the help!
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92 YJ, stroker, 4-linked, ORIs, full hydro, 60/shaved 14b, detroits, 114", 40" sticky Treps, KMCs
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Old 06-18-2019, 04:57 PM   #2 (permalink)
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Every rig is different but I went with slightly longer uppers than lowers to keep my pinion where I wanted it, still very stable I don't notice any downsides to it, I can post my numbers from the calculator if interested.
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Old 06-19-2019, 01:13 AM   #3 (permalink)
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sure, if your rig rides good and you don't have any complaints about how it rides and performs, go ahead and post your numbers! If you do have things you don't like, let me know what they are and what you may do differently next time to change it.

And I think I may end up mounting my uppers and lowers from a single bracket on each frame rail, so the uppers and lowers will be nearly the same length. Taking into account where the uppers and lowers mount on the axle, I think the uppers will end up being about 1" longer than the lowers. How many of you are running a similar setup? What are your thoughts? I should have about 5" uptravel and just over 12" down with how my shocks will be mounted.
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Old 06-19-2019, 11:04 AM   #4 (permalink)
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The only thing I can think of to consider is as the pinion points towards the t case as suspension droops. The caster goes negative leading to death wobble, wandering, etc. If the rig is not high speed or on road driven no worries.
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Old 06-19-2019, 12:47 PM   #5 (permalink)
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Old 06-19-2019, 01:32 PM   #6 (permalink)
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1. The answer is 40 minimum. Given you mention a single bracket this sounds like a single triangulated and not a double? I'd much rather have double with full hydro so that axle doesnt change its front to back position at the wheel when articulating as much.... anyways, its 40 degrees total separation combined between uppers and lowers to center the axle. The larger this angle the less force has to be applied to the joints on the ends of the links to keep the axle centered.

2. Pinion angle change is always related to the length of the arms. The longer the arms, the less pinion angle change. Second factor is the delta in length between the upper and lower arms. Aka, longer upper and lower reduce pinion angle change, but a really long lower with a really short upper will still cause a lot of pinion angle change. Longer control arms will also reduce front to back position change through up down axle movement, as well as reduce axle steer.... so generally, as long as they are not getting caught on things because you wrecked your break over angle, longer control arms are better for suspension geometry (but as said, they come with trade offs).

3. Anti squat doesn't mean anything on the front axle (unless you're in reverse). Anti-rise and anti-dive do though. Remember too that these numbers only mean anything when you have traction. Anti-dive is just an expression of the twisting force on the axle trying to prevent the frame from diving. Anti-dive is an even more personal thing than anti-squat is in the rear. Some people want low anti-dive because it helps them gauge how hard they are really braking. Some people want it high. My personal opinion on the matter is that I don't like high anti-dive because when going down a hill anti-dive helps keep the center of gravity high and makes it want to go over the center of the axle, where as low anti dive pulls the frame down, lowering the center of gravity and tries to pull it under the center line of the axle, thus making you more stable when braking down hill (this concept can seem counter intuitive).... conversely low anti dive typically means low anti rise, which means the front end will lift on acceleration more (but good anti squat helps compensate and the front is heavier), but when you're climbing this could cause good front end traction to want to lift the front, possibly making it easier to endo backwards. Usually though the front tires don't have good grip when climbing and thus this is less of an issue in my opinion.

General rules of thumb would be (in my opinion), figure out what your limits are to make it fit, give it enough angle to center the axle, within that pick the numbers that give you a roll center closest to your center of gravity that you can and still keep proper roll axis inclination that you want, then minimize for axle steer to be minimized, make sure your pinion angle change is acceptable, THEN worry about anti squat/anti dive. This is of course assuming you can change your castor angle to be right no matter what your pinion angle is.

There are some interesting threads on here where racers have talked about running negative anti squat so that under hard braking their castor angle stays consistent because it got their pinion angle change numbers to be good. On my personal rig I run 15% negative anti squat and have found exactly that to be the case. Keeping castor angle proper while under compression or droop helps give the "bite" to climb something by steering left and right while trying to climb, helps keep braking straight under hard braking, and helps make the steering want to self center for your full hydro (likely you'll be running 9-12 degrees of castor with full hydro... some of the 4400 guys run upwards of 20).

Most of the time people focus so much on anti squat and anti dive when in reality there are other more important numbers to worry about, even though they still are important to think about.

Last edited by Shushikiary; 06-19-2019 at 01:43 PM.
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