Not too long ago there was a company called Nth Degree Mobility. You probably know who I’m talking about. The guy that ran the company worked at Chrysler for 9 years as a suspension engineer, and has two engineering degrees. Suffice to say he knows a little about our Jeeps. Despite the credentials and the amazing products they made, the company did not do well, and their designs/products were purchased by American Expedition Vehicles (AEV) shortly after they went out of business. You can find a lot of Nth’s products on the AEV site now. Anyway, the point to all this is this. There is a little place on the internet called the Way Back Machine. It’s an archive of almost any site you can imagine that ever was. I looked up the Nth Degree site and came upon their old FAQ section. There is some pretty awesome tech in there so I figured I would share it with you guys. Here is a direct like to the archive of their FAQ. Here’s some of the more interesting stuff I found:

Vibrations on an 03+ automatic trans TJ

There are several ‘built in’ vibration problems with ‘03+ TJ’s – though ironically the biggest one is usually solved by ditching the factory center skid because it ‘drums’ in response to normal powertrain vibes (which the engine/trans mounts usually absorb). If you have an automatic trans, there is a wavering ‘droning’ sound that happens whenever the trans is in OD – because the .69:1 ratio aligns with the third-order firing frequency of the I-6 engine – you can’t escape this issue except to use the OD lockout button to keep the trans from shifting into OD so prematurely. If this sounds like your issue, try hitting the OD lockout button while driving between 45-55mph and see if it makes the issue go away – then you know what it is.

What’s the deal with death wobble / shimmy?

A doctoral dissertation could be done on shimmy (aka ‘Death Wobble’)…It is not so much a ‘design flaw’ but rather an engineering challenge that has yet to be absolutely overcome in suspension design…and we off-roaders just have to live with it more than the rest due to the fact that we do “all the wrong things” to our rigs and basically invite shimmy to happen as a result. Shimmy continues to defeat even the best full-compliance vehicle dynamics modeling programs (Jeep uses ADAMS). The phenomenon is pretty well researched, but it plagues even the factory at times…Jeep almost had to delay/cancel one model launch because it developed a shimmy problem during the last run of prototype parts – only 3 months before launch. We had to ‘tweak’ four things about the suspension to make it go away: reduced lift height to +0.6” from the traditional 1.0”, change durometer of front LCA bushings, re-valve steering damper, and change nominal caster spec…”traditionally”, the last two are the primary things that affect the risk of shimmy, but another major player is front tire balance…they must be *dynamically* balanced (i.e. weights on both rim edges, so the wheel/tire assy is balanced in all planes, not just one).

So here’s the ‘techno-speak’ dissertation on shimmy: It is an imbalance of the gyroscopic forces generated by the spinning front tire/wheel assys…when the ‘setup’ of the front end is ‘vulnerable’ to starting shimmy, it happens due to an asymmetrical force (i.e. one tire hits a bump that the other doesn’t) or also it can happen when the bump-force is not even side-side (in this case it usually happens at higher speed due to say a ‘swell’ on the highway). The problem is worst with wheel-wheel tie-rods (which is why the TJ and most late Jeeps use the Haltenberger linkage – aka ‘inverted Y’ design) because the tires can ‘crosstalk’ directly to each other. The larger/heavier the tire/wheel assy’s are, the stronger the inertia and thus the worse the shimmy can be if it starts….of course a damper is the most direct defense once shimmy starts, but what you *really* want is for it to not start in the first place. For that, you have to look at the ‘free-body diagram’ of the forces that can influence the stability of a spinning front tire/wheel assy. The key for shimmy issues is the distance between the ‘center of tire contact pressure (CTCP, which is not directly below the hub due to ‘pneumatic trail’ – it’s actually further back.) This is the effective center of downforce of the tire’s contact patch when it’s moving/rolling) from the ‘steering point’ (the point where the steering axis intersects the ground plane). If you leave the caster setting at the factory spec of 6-8 degrees while increasing tire diameter from 27-28” to 33-35”, then you’ve effectively *increased* this distance (and therefore the imbalance force it can generate), while at the same time you’ve increased the mass that drives this force (the larger/heavier tire/wheel assy), so when things get to shimmying, they REALLY shimmy! So…what to do? If you’ve followed this far, the answers are easy to ‘guess’, but should be divided into ‘preventative’ and ‘band-aid’ fixes:

Preventative (i.e. will reduce the likelihood of shimmy, in guessed order of empirical relative effectiveness):

1) Keep wheel/tire mass low (i.e. run stock-sized-or-close tires and/or aluminum rims – both of which are likely unacceptable to a real ‘wheeler)

2) Keep CTCP – to – steering point distance short by running stock-ish diameter tires (also not acceptable)

3) Reduce the CTCP – to – steering point distance by reducing caster (moves steering point back towards CTCP)

4) Run radial tires!

5) Dynamic balance the tire/wheel assy’s (weights on both rim edges)

6) Avoid direct wheel-to-wheel tie rods, or if so, have deliberate ‘compliance’ in the tie rod (i.e. not too stiff, but steering feel/precision will suffer)

7) Maintain high lateral stiffness via proper trackbar design and bushing rates (i.e. no non-preloaded urethane bushings and hollow-tube t-bars!)

8) Tune front control arm bushings on the ‘stiff side’ (i.e. high-ish durometer…TJ uses stiffer LCA bushings than XJ for example, due to trackbar bracket stiffness differences, etc.)

9) Drive slow and/or don’t hit any bumps!

Band-aid-type fixes:

1) Heavy-valved steering damper (helps a lot if tire/wheel mass is 100# per assy or less. Multiple dampers will help with heavier setups, but too much damping will limit steering response time)

2) Match all compliances together to allow drastic violation of above guidelines (i.e. apply a lot of engineering to the bushings and linkage stiffnesses while screwing up the other engineering parameters…)

…so what’s the “Bottom Line”? It’s this: the stock caster spec is NOT the appropriate spec for your lifted Jeep running bigger tires. For 33” tires, I recommend about 5.0 degrees, and for 35” – about 4.0 degrees.

Ultimately your issues (or not) with shimmy will be determined by how well your rig is ‘set up’ either by you or your shop…so the main thing we’re doing is putting some watered-down version of the above in our product instructions (i.e. a new caster spec based on tire size), but also we’ve made sure that the lower caster setting is in the middle of the adjustment range so you have a chance of setting proper caster (which I’ve found isn’t possible with some lifts), and this will help you understand why we’ve designed the front trackbar as a factory-type one-piece solid forging with high-durometer, rate-plated bushings and a stiff trackbar bracket/brace.

TJ vibration around 30mph

If the vibration starts at about 30mph and increases linearly with speed, then it may be driveshaft balance

Vibrations at start up

Momentary vibes at start up come from one of three sources (all the usual vibration-producing ones):

1) The engine. If you gearing isn’t as low as it ‘should be’ for the tire size you’re running, you may be launching at too low an RPM, causing the engine to ‘lug’ which will produce a stronger-than-normal vibe until the engine ‘spools up’ to it’s preferred operating range.

2) Trans. If you have an automatic, the new 42RLE 4-speed has lots of vibration issues that the factory failed to overcome since they are inherent in the fact that that trans is supposed to be Chrysler Cirrus FWD transaxle (that’s what they derived it from). The torque converter on that trans is stupid-small and could ‘shudder’ at launch for the same reason as #1 – gearing not low enough. BTW: “low enough” is not just the mathematical correction for tire size, you also have to factor in added rolling resistance for knobby tires, etc. Ironically, with this auto tranny’s super-tall OD gear (.69:1), you want to run 4.88′s with just 33″ tires, running with more means your essentially ‘undergeared’

3) Driveshaft Angles. I assume you have a double-Cardan (aka ‘CV’) rear driveshaft. Even so, if you have substantial angle at that rear DC joint, it *can* create vibes under load since it’s not really a ‘Constant Velocity’ joint – typically these non-constant-effects are subtle and you don’t notice them, but at 15+ degree operating angles under heavy load, you might.

Shock length vs. proper bump stop spacing

Everybody wants to run the longest possible shock, but this is not the only key to ‘maximum travel’, and running too-long shocks comes with several penalties/side effects. Shocks (and the brackets they’re mounted to) are not designed to be your up-travel (compression) limiter – your jounce bumpers are (aka ‘bump stops’). Your lift kit should have included bump stop spacers – but probably not enough…most lift kits don’t come with the appropriate amount of spacing…if you have a 4” lift, you need roughly 4” of spacers! Proper matching of suspension travel and shock minimum and maximum length is a basic requirement for a proper-functioning suspension that most suspension companies leave to you!

You must check your TJ to make sure the shocks do not bottom before the bumpstops are compressed (the jounce bumper can be completely compressed within the cup it mounts into). A reasonable method for determining whether your shocks are too long is to flex your TJ until a wheel is in the air while the vehicle is going ‘up hill’ (forward on an RTI ramp) – then check to see that there is some compression travel still remaining in the ‘stuffed side’ rear shock. If you can’t see the shaft due to a dust boot, unbolt the lower end while flexed and see how far up you can push it. If the bumpstops aren’t fully compressed (or don’t even touch!) before the shock bottoms out and you do not correct this with longer bumpstop extensions or shorter shocks, you WILL have problems with your shock mounts – whether using the Shock Shifter or not.

Since most aftermarket off-road shocks are overbuilt with 18mm rods, etc. the problem usually is a failure of the mounting brackets, not the shock. With the stock shock mounts, this mis-match of shock vs. axle travel will eventually cause the shock to rip the stock bracket off the axle tube (which is often a reason for buying the Shock Shifter). When the Shock Shifter is added, but the mis-match problem is not corrected, you will bend and eventually break the SS upper bracket(s). This is not a design flaw with the Shock Shifter and is not warranted! – it means you have the mis-match problem and must correct it. Nth will sell you a new bracket but you must correct the problem or it will simply happen again. With the SS is installed properly along with proper bumpstop spacing, shocks that fit when mounted in the stock locations will also fit when in the SS position and give roughly the same articulation.

That’s all for now. There is more on the site, so you may want to check it out if you found some of this interesting. Happy trails!

-Davin (Raskull)