we all expirience it sooner or later, while surfing across the net i found this. It was writtne by Jim frens the owner of nth degree mobility (now works for AEV) he was a engineer at chrysler and knows his isht.
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!
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.