Trying to fix "first-build" mistakes

Offset is more critical for steering than just for fitting the tires. If the axis through the ball joints is not near the center of the wheel at the road surface, you can easily imagine how hitting a bump in the road will induce a steering force.

Jeep OEM wheels have offset differences as little as about 10mm depending on the different stock tire sizes.

I don't know how to draw a straight line on the photo so I used the ruler function and screenshot it.

D8ECE30B-0245-4005-960F-5D4859527F67.png
 
Right, I have 3.5 backspace that doesn’t help driveability …. But it has been the way to go to prevent lower control arms or swaybar rubbing 🤷‍♂️
 
Right, I have 3.5 backspace that doesn’t help driveability …. But it has been the way to go to prevent lower control arms or swaybar rubbing 🤷‍♂️

I created a chart by extrapolating the factory tire sizes and offsets. Not really scientific or exact but I'm coming up with about -15 as ideal for 35's.

And if anyone is wondering about 33's that would be 0 offset...which conincidentally just so happens to be an 8" wheel with 4" backspacing - a very common wheel for TJs running 33's.

I think that is why you noticed the -19 wheels handled better and as you get further away from that, the bumpsteer and handling get worse.

Maybe better to prioritize offset for the steering/tire size and adjust steering stops to prevent the swaybar rubbing or just don't turn it lock to lock
 
I would also look to replacing the front track bar at some point and pick up a replacement tie rod end for it until you do.
 
And if anyone is wondering about 33's that would be 0 offset...which conincidentally just so happens to be an 8" wheel with 4" backspacing - a very common wheel for TJs running 33's.
An 8” wheel with 4” backspace has a -12 offset.

0 offset would be 4.5” backspace.
 
I created a chart by extrapolating the factory tire sizes and offsets. Not really scientific or exact but I'm coming up with about -15 as ideal for 35's.

And if anyone is wondering about 33's that would be 0 offset...which conincidentally just so happens to be an 8" wheel with 4" backspacing - a very common wheel for TJs running 33's.

An 8” wheel with 4” backspace has a -12 offset.

0 offset would be 4.5” backspace.

Yep...backspacing includes the thickness of the rim from the tire bead to the actual outside edge of the wheel.

I think 4.5" is good for 33s and 4" is good for 35s in terms of best compromise between clearance and scrub radius. That aligns with 0 offset for 33s and -12 for 35s. -15 would be 3.875 but I can't recall seeing a wheel with that backspacing.
 
Yep...backspacing includes the thickness of the rim from the tire bead to the actual outside edge of the wheel.
Far too many rim edge variations to say that X amount of backspacing = X amount of offset. I've seen rims that stick past the inner face of the tire bead by little more than 3/16" and some that are well over 1/2".
 
  • Like
Reactions: freedom_in_4low
Dear All,

finally currectlynk has arrived! Today I started working on the TJ to check suspension geometry. First thing first, I have fixed the passenger side upper control arm bracket (maybe "fixed" Is a big word... but now the bushing is well seated). Anyway, I do not realize how this bracket has bent...Basically seems someone squeezed it in the middle and only in the front section. I have rotated the driver-side front spring (now it is sitting on the well located on the bottom). By means of laser, I measured the position of the front axle... it was not square to the frame (right and left front upper and lower control arms have had the same measure....). I started with the length suggested by RockJock for its 4' lift kit and adjusted the opposite side accordingly for maintaining the front axle square to the frame. At the moment the lower driver control arm is longer than the passenger one by about 1.5 cm. The caster angle was about 4°, I would like to adjust it to 7° but this led to a not optimal pinion angle (I have a stock DANA30 front axle). So, I need some suggestions from you: With a 4' RockJock lift how large can I set the caster angle without issue with the pinion? In other words, what's the minimum acceptable value for the front pinion angle? Please take into account that I do not have selectable wheel hubs.
Thank you again for your help and attention.
 
A couple of things:

First, squaring your axles to the frame sounds logical, but I think they should be parallel and square to one another at ride height. They are parallel if the front to back measurements are equal left and right. They are square if the right-front to left-rear and the left-front to right-rear measurements are equal. You set those by adjusting your lower control arm lengths and track bar lengths at ride height.

Next, Caster (measured relative to the ground) is better as you said around 7 degrees (that is actually the Jeep spec +/- 1 degree according to my local dealer). 4 degrees will give you twitchy steering. How much you can dial in by adjusting your upper control arms depends on how much variation between pinion angle and driveshaft angle your rig will tolerate without vibration. Mine is happy with the pinion 1.5-2 degrees below the front driveshaft which allows me 7 degrees of caster. For reference, I have a 3” spring lift and a 1.5” tuck which may yield driveline geometry similar to yours with 4” springs, assuming you have stock t-case height.

Don‘t forget to cycle your suspension once it’s all set to make sure you haven’t created clearance issues.

hope this helps…

edit: larger diameter tires increase your trail which is stabilizing to your steering, so you may able to tolerate a little less than stock caster.
 
  • Like
Reactions: fortuso
I’m pretty sure when we’re getting to these size lifts, there isn’t a numerical answer to caster.

Pretty sure I read here by some experienced folks, your caster needs to be what your rig will tolerate without creating vibes with the pinion. So 7 ish is what some folks do, so if you’re at 4 and not happy, start dialing and test driving. Yours may be 6 and drive perfectly, and you’re set. But 6-7 depending on what your rig will handle is a ballpark ish.
 
  • Like
Reactions: fortuso
A very basic report.... Finally, I have installed Currie's Currectlynk replacing the previous reverse T-shape steering system. I have modified the previous control arms setting (made by a shop) following RockJock suggestion related to their 4' lift kit. Basically, I have had to short front control arms by a little bit more than 1'. Due to my Teraflex high clearance skid plate, front upper control arms are a bit longer than Currie's suggestion so I been able to have 5.5° of caster and 2° of drop pinion angle (with respect to the front drive shaft. In the rear less modification has been done: I shorted lower control arms by 1/2' obtaining 1° drop pinion angle. I tried to fix the passenger side upper control arm bracket and now the bushing is well sitting. The driver-side front spring has been rotated as suggested by mrblaine. I have verified axles position (square one each other and with respect to the frame) and cycled them to check clearance issue (nothing). Toe has been regulated at about 7/16 out (Currie's instruction). That said, I'm back from a test drive (without steering stabilizer just to have an unfiltered feedback) the result is unbelievable! Night and day! Now my tj drive at 80 mph with no issue at all. Of course, mud terrain tire I have can not be as comfortable as AT or on-road only tires but the TJ run incredibly better than before. Tomorrow I'll go to a tire shop just to verify the alignment (I have done everything alone by using tape measure, lasers and angle finder...) but I'm already satisfied. I really don't know how to thank all of you for your suggestion and help.

IMG_0139.JPG