Suspension Tech Q's

[Ando_13]

I don't have to much mechanical knowledge (learning heaps on here) but would like a few Q's answered about suspension components, here I go:

1. What is the Rear track bar and what/why are mods needed to be done to it when lifting (ie: relocating and adjustable track bars?)?
2. I sort of know what the control arms are (big levers connected to axle houseing FR and RR) but I don't know why these need to be adjusted/adjustable when lifting (and at what height do they need to be adjustable?)? And is it these that are changed in a long arm kit and how are these steel monsters adjustable?

Asking all these Q's as prob going to 3.5" lift and looking at things that might need doing if weird handling probs occur??

[Wooders]

1. What is the Rear track bar and what/why are mods needed to be done to it when lifting (ie: relocating and adjustable track bars?)?
The rear track bar (also called a PanaHard Rod) locates the axle in a lateral position - in effect preventing side to side movement relative to the chassis. one end attaches to the axle, the other to the chassis.
As the axle drops away from the chassis, the trackbar travels through an arch. So when you lift the suspension the axle is effectively further away. BUT because of the fixed length of the trackbar and it's circular travel range, the axle is ineffect pulled to one side more relative to the centre of the chassis.
The centre line of the chassis/axles determines what is called your Thrust angle in wheel alignment terms. This should be =0, if unaddressed the vehicle will sort of "crab walk".
Either adjusting the brackets or lengthening the trackbar will help resolve this by centering the axle again. HOWEVER you must be aware in the front of the vehcile the travel arch of the trackbar is related to the travel arch of the drag link. Ideally the track bar and drag link should be parrallel when static and should be as close as possible in overal length. Otherwise adverse steering will be the result (eg bumpsteer)....
It's not a simple thing because then we start getting into steering geometry....

2. I sort of know what the control arms are (big levers connected to axle houseing FR and RR) but I don't know why these need to be adjusted/adjustable when lifting (and at what height do they need to be adjustable?)? And is it these that are changed in a long arm kit and how are these steel monsters adjustable?
Control arms do a similar job to the track bar, but work to locate the axle in a fore/aft position relative to the chassis, and the upper/lower links work to prevent axle twist.
For a small lift they don't really need to be adjusted, but if you start looking at a SYE the you change the style of driveline joint and you need to change the pinion angle.
Also you need to be aware that the shorter the link (control arm or trackbar), the greator the effect since they operate on a smaller arch. in the case of short armed TJ's with loads of travel this can result in things like rear steer - where the arch of one side pulls that sides axle towards the front of the vehicle, whilst the opposite side pushes the axle back = the rear axle is effectively steering you offline.
also the bigger the lift the greator the change ing operating angle of the arms - this can result in a much harsher ride as the arms move more towards a vertical position they are not allowing the axle to move up & down as freely and they start to "locate" the axle in a vertical plane - which is NOT what you want to achieve.
It is for this reason most people find approximately 4½" of suspension lift is about as far as you an comfortably go whilst retaining short arms......

Again there's a LOT to these things - but it helps to think in terms of geometric shapes etc.....fire away with more questions.....

[Jeff]

wow.... my brain hurts after reading that.... makes sense tho.

Ando, I'm running 33s on 4" of lift with stock control arms. It works okay now, but my on-road handling is nowehere near as smooth as it was on 2" & 31s.

As with a most mods you probably can go without them, but you'll improve your vehicle if you spend more money on it :mrgreen:

[Ando_13]

Thanks for your excellant explaination Wooders :D

I bought my TJ with the current lift in place, so in theory it should have a track bar relocating bracket installed (fingers crossed). If I go to 3.5" lift with SYE what needs to be adjusted to change the pinion angle as ref'd?

For a small lift they don't really need to be adjusted, but if you start looking at a SYE the you change the style of driveline joint and you need to change the pinion angle.


So an adjustable rear track bar is a good investment when going bigger than 2" lift, right? Or is this just speculation since each Jeep is diff?