Solid Axle Knuckle/Trunion Bearing Shim/Preload Question

[toyotaboy80]

Hi.

I am in the middle of rebuilding a '85 axle and I have a question regarding getting a heavier pre-load out of my knuckles.

As far as I know my axle is UN-molested and when I took my axle apart there was a .039" shim on the top, bottom, left and right. To my understanding, the bottom shim centers the knuckle and the top shim sets the pre-load.

I have read countless times of people saying anything that is done to the top must be accounted for in the bottom, and vice versa. To me that doesn't make sense. If the bottom shim centers the knuckle, then that shim would stay constant and the only variable in shimming the knuckle would be the top shim: thinner for more pre-load, thicker for less pre-load. If the bottom centers the knuckle and I take .010" off the top to get more pre-load and add it to the bottom, that would then put the bottom off center by .010" and not change the pre-load because there would be the same shim thickness overall.

I didn't measure the knuckle break free pull load before I took it apart but hypothetically say I throw it all back together and with the .039" shims all around I get a 8lb pull. Say I wanted a 14lb pull. Can I shim down the top until I get a 14lb pull without touching the bottom? I don't have a knuckle SST, but if I did and if the bottom one does in fact center the knuckle and the top loads the knuckle, that would mean if I took a measurement at 8lbs with the tool, the bottom shim would calculate out to .039" and the top would calculate out to .039". And if say I set the pre-load to 15lbs with the tool, the bottom would still calculate out to .039" and the top shim calculation would be less than what it was for the 8lb pull. Is this making sense?

 

[Mudder]

Bottom shims only center the housing. Tops set the preload. So only mess with the top if you want the preload adjusted.

 

[ntsqd]

The simple way to look at it is that posted above.

The reality is that both sets of shims (top and bottom) affect both the pre-loading on the bearings and the centering of the knuckle. Nothing there is perfectly rigid, so a change in either shim affects both due to distortion of both the knuckle and the Birfield ball. The equal reality is that those distortions in a small change are irrelevant and the simple view gets the job done.

Why do you want to increase the pre-load so much? My FSM calls for 6.6lbs. to 13 lbs pre-load.

 

[toyotaboy80]

After looking at the instructions again and using a hypthetical calculation, the bottom shim would remain constant to the particular axle and knuckle that the calculation was taken on. Putting the tool in the axle and scribing a line and then putting the tool in the knuckle and scribing a line finds the centers of the axle and knuckle. D(the width between the scribed lines)-3mm finds the bottom shim. D is constant to a particular axle and knuckle pair. So if the measurment for 8lbs preload was a 2mm shim stack, D-3mm would give the bottom shim stack and the bottom shim stack subtracted from the total 2mm would be the top. If the measurement for a 15lb preload was 1.8, D-3mm would still be the same and subtracting that from the total 1.8 would be the top shim stack.

Looking at the math, the bottom shim doesn't change because that is what center is derived from. Looking at the axle in motion, the knuckle does float, but the way I am looking at it, a correctly calculated bottom shim allows for equal measurments of float. If the shim on the bottom were thinner or thicker than centerered, the knuckle could float more one way than the other. At the end of the day, it depends on the application and how much .010" means I guess. If the axle floats say .010" in either direction, and thats what the seal and CV can account for, then having a centered knuckle would be a precise fit. While having the bottom shim out by .005", the knuckle still floats a total of .020", but .015" one direction and .005" in the other.

Maybe I should just slap my S*** together and call it a day... it probably isn't as precise as I am thinking due to the CV's ability to make an angle.

Going heavier(15ish lbs) seems to be the concensus with a larger tire. I would think 6-13 was calculated with a stock size tire. If anything, I would like to be in the upper range of the FSM recommendation if not a little above due to not running stock size tire.

 

[BioTex]

Are you running a steering stabilizer? I find that with larger tires a good steering stabilizer really helps to keep the tires rotational mass on plane.
I would worry about increasing the preload over the recommended amount and causing premature bearing failure. I've never seen it happen, but that's just me worrying about it anyway.
I agree you should be on the recommended higher side of the measurement, but I wouldn't go over it by much if any.

 

[ntsqd]

I've got about 130,000 miles on my truck running 33-9.50's & 33-10.50's. I don't crawl the truck exclusively, it's a partial DD, occasional crawler, annual quasi-"expedition" rig, and frequent desert race chase truck. I pound divots into the bottom trunnion bearings long before anything else goes bad. I've never strayed outside of the FSM's recommendations as more pre-load won't help.

Either shim can center the knuckle, it matters not which one. Just have to pick one to start with and its easier to fix the bottom one. When the pre-load shim is selected and torqued into place the knuckle will no longer be centered due to distortion, but the off-centeredness is so slight that it isn't worth worrying about. However, the more pre-load put into the system, the more off-center it will be. This is really only important if it is so off center that the axle splines at the diff can't function as intended or the inner seal is caused to gap.