RTAB preload question

[cpandrewschmidt]

Hi all, quick question. I replaced my RTABs this weekend. After I got done I discovered some threads talking about preloading them. I understand the concept and why to do it. Now I just have to figure out the proper way with out the BMW tool. Only it says use a straight edge and measure 8mm above the center of your hub. My questions are:

1. my car is lowered. Should i error slightly more than 8mm?

2. do you measure with the car fully jacked up? won't different cars with different suspensions sag more or less than others, throwing off that 8mm measurement?

3. do you measure for below or above the center of the hub?

I attached some pictures for reference of what I was doing. I was going to measure above center of hub, but still unsure of question #2. I think i tighten it too far off 8mm trying to account for my drop in suspension. I was getting a bad shake this AM from the one side i messed with. Will try to correct tonight.

Thanks for the help for anyone who's done this.

rtab1.jpg rtab2.jpg

[tjm3]

(from the BMW tech info)

7. To install without special tool, align bearing block on center point of wheel with an 8 mm diameter rod. See Fig 7 .

G00165452.gif

For stock, any offset/preload is built into the procedure. Flange 'points' to the center point of the axle or wheel. You can use any straight-edge but I think they picked 8mm because it's
common and keeps flexing to a minimum.
Non-stock suspension requires a measurement before dissassembly, your call on how you do that.

[cpandrewschmidt]

what if i didn't measure anything before I took the old ones off?

[tjm3]

what if i didn't measure anything before I took the old ones off?

Then your stuck with a 'guestimate' of how much offset from the flange to the center of the wheel/axle you need for any non-stock spring height. It's possible to get very close and a little extra preload may shorten the life of the bushing a bit but you may never notice it. You can also re-assemble and lower the car with the RTAB bolt loose then get under there an find a way to measure something that you can match with the arm detached from the body later for zero preload, but there's not a lot of working room there.

[cpandrewschmidt]

thanks Tony. I'll probably just error slightly more than the stock 8mm setting and hope for the best.

[bluptgm3]

The measurement described in the Bentley and possibly the TIS is 8mm above the top of one of three cast lugs on the trailing arm, which cannot be seen with rotor and dust shield/backing plate installed.

https://www.bimmerforums.com/forum/s...4#post25101154


What I've done in the past is -
1) With weight on wheels - Measure/record distance from the ground to the highest point of the wheel arch opening.
2) Pull the wheel by jacking @ the shock mount.
3) Lower the car and raise car back up till I achieve the same dimension to the wheel arch, from step one
4) Use a straight edge on the bottom of the 'body bracket' ('BB') and scribe a line w/sharpie marker on the rotor.
5) Re-install wheel, lower car, jacking car from differential, place on Jack Stands.
6) Remove wheels, release E-Brake, place gear selector in neutral.
7) Clean around bold heads and Mark around 'body bracket' attach fastener heads and remove 'body bracket' from pocket.
8) Remove 'BB' from RTA.
9) Remove RTAB. Install new RTAB.
10) Re-install 'BB' using straight edge and line on rotor (from step 4) to align and torque 'BB' on RTA to 81 ft-lbs..
(This step requires some realignment of scribe mark on rotor to straight edge. You have to rotate the rotor slightly to realign the straight edge to the mark on the rotor, but it will align with to the straight edge.)
11) Re-install 'BB' in pocket, align 'BB' to fastener/fastener marks on 'BB' from above and torque to 57 ft-lbs..

RTA Bushing is now in 'neutral' rotational position @ nominal ride height. I've accomplished 27+ sets this way with only one set failure. In this case I identified leaking rear shock that needed to be replaced. Owner did not, relying on the RTAB for sole damping of rear suspension travel.


BMW alignment tool p/n 83300493688, currently (01/2018) available at ECS Tuning.

Sent from my iPhone using Tapatalk

[M52 POWER!]

When I did mine all I did was this:

Don't fully tighten the bushings bolt.
Install RTAB bracket to car, install wheel, lower car to ground.
Drive the car back/forward 10ft or so so the suspension can settle.
With a marker (pref a paint marker) reach into the RTAB bracket area and draw a line across the bracket relative to the trailing arm.

Put the car back in the air, remove RTAB bracket and you should now be able to line the RTAB bracket to the line you just drew. Now tighten and torque the bushing bolt. Tada!

[cpandrewschmidt]

When I did mine all I did was this:

Don't fully tighten the bushings bolt.
Install RTAB bracket to car, install wheel, lower car to ground.
Drive the car back/forward 10ft or so so the suspension can settle.
With a marker (pref a paint marker) reach into the RTAB bracket area and draw a line across the bracket relative to the trailing arm.

Put the car back in the air, remove RTAB bracket and you should now be able to line the RTAB bracket to the line you just drew. Now tighten and torque the bushing bolt. Tada!

Perfect, makes sense!

[TostitoBandito]

Yeah, there's lots of ways to do the same thing, some more complicated than others. Like any rubber suspension bushing, you just want to tighten it while it's as close to its normal operating position as possible. You can use the fancy tool, or just simulate suspension load with a jack until it matches your normal ride height on that side.