I’m fortunate enough to have buddies who graduated mech engineering with me who have their own mill/lathe/router to help me out, I could use a dedicated garage though!
How does a mid-arm suspension improve the climbing capabilities from the drivers seat?
- Thread starter mattcogdell
- Start date
You are taking a snippet of info and using that to build a mountain of bad information. The cheating wheelbase came from folks wanting to chase wheelbase in small increments by just getting arms a couple of inches longer which is bullshit. Really, who has been on a difficult obstacle and uttered the words-"If I just had 1 more inch of wheelbase, I'd be a hero right now". That topic came up over and over and it is like everything else out there, as soon as you tell them it makes little difference and therefore is not a sound idea then the reaction is "well, if I can't fuck with the wheelbase, I don't want it.".
That and I fully despise hacking into the wheel arch to slide a spring perch back, it's gross, it's hack, and disgusting. I have built rigs around the Savvy Mid Arm from 101 to 114" in a TJ Unlimited on 40's and they all work great.
And like most things, the discussion winds up being about wheelbase and not geometry. How do you make the front arms longer without affecting the front geometry?
Brianj5600
TJ Expert
Can you expand on this? IC can vary greatly in length and height without changing AS. Keeping it simple, let's use 100% AS and move the IC along the AS line. How will the Jeep behave with a short,low IC? As the IC moves longer and higher along the AS line, how will the suspension act differently?
sorry i didn't read all 4 pages, and pulling my misinformation from the thread, sorry.
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I can mostly wrap my head around the height of the IC. What I don't really grasp is the full significance of the IC along the x axis. For example, an IC that is near the AS line vs an IC that is infinitely far away. Some distance away from the AS line seems to be desirable. But can too far be too much? Ultimately, I feel like that is an academic question that doesn't concern most of us on a practical level, with one possible exception sort of described below.
When looking at the calculators and for the propose of understanding these points in space, imaginary lines and measurements — 100% antisquat is the same as neutral antisquat or no squat/no rise when the suspension is loaded (I really wish the terminology was different).
Relative to the imaginary 100% antisquat line (as partially defined by the center of gravity) a high instant center can cause the loaded suspension to rise. This can result in the uncontrolled hopping phenomena as the rear stops rising, loses traction, falls, regains traction, rises, falls, over and over again.
Relative to the imaginary 100% antisquat line, a low instant center can causes the rear to sink. This can result in the the Jeep shifting it's weight backwards to the point that the the front wheels lift off the ground and eventually to a backwards roll.
A stock, unlifted TJ has a fairly neutral squat. Meaning it is incredibly stable in a steep climb. But then we start adding larger tires and taller springs to get the axles and frame out of the rocks. This screws up the stability of the stock suspension by raising the instant center higher and higher above the neutral antisquat line. The result is the hop we sometimes see on a lifted short arm.
The significance of screwing up the stock suspension geometry generally doesn't matter until people start pushing the limits of the Jeep. Meaning that the problems of lifted short arm geometry doesn't matter until it matters. It simply doesn't matter for most (me included).
A long arm where the rear upper mounts are close together will both significantly lower the instant center and also pull it in close to the AS line. While this will reduce and even eliminate the hopping, this change to the IC is too much. This is where a long arm can potentially lead to a backwards somersault. But, just like the problems of a lifted short arm, the problems of a lifted long arm don't matter until they matter. In effect, one problem got changed to a different problem. And neither one matters until it does.
Where these hops and squats are significant enough to matter and are worthy of fixing, the goal is to restore the lifted suspension geometry to something closer to what a stock TJ has. This means moving the mounts to reposition the instant center back to a more neutral antisquat. These mount positions also take into consideration other practicalities like strength and clearances. To achieve that, the mounts will end up in a place that looks like what the Savvy mid arm looks like. The mounts are placed in a particular place to position the instant center. The frame and axle mounts are connected together with control arms. And the arms end up being a length that is neither short nor long.
To circle back to the 4 link calculators we see on places like Pirate and JeepForum, they are only useful in learning to understand how changes to the mounts and vehicle weight effect the geometry. They cannot design an ideal suspension, unless that vehicle never moves and never leaves flat ground.
Glad we see eye to eye. I was referencing the mid arm at the same build level as the genright ext kit (coil overs and stretch). I’m not talking about LJs, TJs only. The only TJ I know of with a mid arm that has been stretched and has 37s, the owner had to lengthen the frame in-front of the rear wheel, chop of the back of the frame and run a fuel cell. I do not want a fuel cell.
i'm sorry, guess i'm lookin from a different angle. but think i was trying to say the same thing.
just the wrong way.
just the wrong way.
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Instant center and travel arc describe two completely different things. Looking alone at the travel arc of the wheel tells nothing about the instant center.
Genright offers several tanks that can stay below and offer rear stretch possibilities. i've looked at so many tanks/cells lately i cannot remember, but some require the stock spring shock combo to be replaced.
a back 1/2 kit would certainly make it a lot easier to fit fat CO's, and likely a reason to not be able to use some underside tanks.
a back 1/2 kit would certainly make it a lot easier to fit fat CO's, and likely a reason to not be able to use some underside tanks.
The tj I’m referring to had to have the rear of the frame removed hints the reason he went to a fuel cell
Still not the same way to get to the same point. The upper arm needs to be considered. And a radius arm can have a very similar travel arc as a two link arm, but the IC is in two vastly different places with two completely different behaviors.
I'm giving you a hard time because this stuff is already difficult enough as is without conflating two different things.
the alien gives the most room but least capacity, i think it allowed up to 9"' with Dana 44.
9" is not shabby and might put a 37 beyond my bumper. but may not be a great tank for long highway runs. it appears to hang just as low as stock but would be between the wheels now not behind them. stays below, factory pump, just gotta eat the tag.
9" is not shabby and might put a 37 beyond my bumper. but may not be a great tank for long highway runs. it appears to hang just as low as stock but would be between the wheels now not behind them. stays below, factory pump, just gotta eat the tag.
I don't believe 9" is possible nor correct unless you mean stretch from the wheelbase at 4" of ride height then maybe. Most 4" lifted rigs have 92" of wheelbase so plus 9 would put it at 101. It takes about 104 to get 37's past rear bumper. I doubt you will get 9" from stock or 103 and get any tank under the back of a TJ unless it is very small and very specialized.
That is during the cutting process to get one side stuffed.
Not much of a rear bumper to get past.
104 on 37's with Rock Jock iron 60's.
LONGJP2
TJ Expert
I enjoy reading these setup/geometry threads. However, I feel most of it is lost on the majority. Those that never push their rigs to the limit; needing every bit of suspension advantage they can get to clear an obstacle. Not to mention the cost of customization.
Even worse, it seems the new generation thinks V8s and 40s are the answer to everything.
Even worse, it seems the new generation thinks V8s and 40s are the answer to everything.
Speaking as one from a generation or two back, they're not?
