BMW E30
Rear Subframe Modification
By: Roberte
Page 1 of 2
If you own an '80s vintage BMW and if you ever have the desire to try and modify the ride height of the vehicle, you're going to find out very quickly that the factory alignment settings for the rear of the car are fixed from the factory and that BMW never intended for you to modify or adjust the factor settings. Hmmm, you say, What's a person to do about it? Well, a couple of options are available to you to perform the needed adjustment to your suspension.
The first thing you need to determine is how much and what type of adjustment do you really need. If you only need to adjust the toe in value, then BMW itself makes an adjustable rear trailing arm bushing (RTAB) specifically designed for this purpose. Essentially it's a rear trailing arm bushing with an offset hole in it. Simply by installing it at a "+" or "-" orientation will add or subtract the amount of toe needed on that particular wheel. I believe the amount of adjustment is 0.5 degress, plus or minus, but I could be wrong. I do know that if your vehicle has more or less toe than this amount, you probably have some severe problems with your suspension or possibly even frame damage. Also, keep in mind that BMW only recommends this adjustable RTAB for toe adjustments only, not for camber. Why I don't know, because if you installed them inboard you get some camber adjustment...Because I guess. The other thing to keep in mind is that more than likely you're gonna want to have this installed by a shop that has done this modification before and access to an alignment machine. This is because that there is no adjustment to this bushing once it's installed other than to remove it and repress it back into the trailing arm. The good thing about it is that once it's set, you don't have to worry about it again.
If you have decided that you want to lower your car, the first thing you're going to notice is that the more you lower it, the more negative camber you're going to get. (Note: Camber is the angle of the tire when compared to a line that is drawn perpendicular (90 degrees) to a level surface (the ground dummy). The more the top of the tire "leans" in towards the vehicle, the more "negative" camber you have. Conversly, the more it leans away from the vehicle, the more "positive" camber you have).
The second type of adjusters is a similar setup to the first except you are able to adjust for both toe and camber. These are produced by a company called KMAC and are typically sold thru vendors such as Bavarian Auto Sport or BMP Design. It works similarly as the factory eccentric RTABs except that they can be adjusted while they are on the car. The other advantage to this setup is that they are fairly easy to install. Downside to this product is that like the factory eccentrics, you are limited in the amount of adjustment that you have. Again, I believe that the amount of travel is only about 0.5 degrees....(I think, please don't quote me on this). Bottom line, not enough adjustment to correct for either sport springs or cup kits if they are installed on your car. The other thing you have to look for is that since they are adjustable, they can come loose and go out of adjustment. Depending on how you drive your car, you may need to have the alignment checked much more periodically that you would if you have factory parts.
The last type of adjusters you can have requires modification of the subframe itself, but the benefits include a larger range of adjustment (1.25 degrees +/- for toe and 2.5 degrees for camber depending how its installed), can be adjusted while on the car, and they do not have a tendancy to loose adjustment thru use. The particular set of adjusters I installed were from Ireland Engineering, but Dinan uses a similar setup in their product lines.
Keep in mind that this modification was done on an E30, but similar mods can be done on '82+ E28 5 series and '83+ E24 6 series vehicles as they have essentially the same type of rear subframes.
Whew! Enough of the overview and theory, lets get down to the fun part.... the actual modification. First of all, I'm not going to try and kid any of you. In order to perform this modification, you (or your really close friend) has to know how to weld and you to have a welder available for use. Otherwise, find a friendly local machine shop who will do this for you and still only charge you a reasonable amount! Good luck in your seach! However, if your feelin' froggy, personally I recommend a 110V Mig type welder for this mod. Here's what I have:
Other tools such as a die grinder, angle grinder, drill, etc are needed too.
Secondly, unless you don't mind you're car being down for while, I would recommend getting a second subframe to modify. That way you can take your time on it and still drive your car. For E30s, pretty much all models' subframes are interchangeable with the exception of the M3 (of course since that's what I have). The only difference w/ the M3 is that it has exhaust hangers on it, so you're not gonna benefit from one of these if you have a standard E30... save your money for something else! Anyway here's an M3 subframe that I picked up from a guy in Chicago... Eeeeeewwww! Rusty! Oh well, us M3 guys can't be so choosy since there's a whole bunch less parts available.
First thing you're gonna want to do is have the subframe bushings pressed out in order to get them out of the way (thanks Bert!). The other reason and more importantly is that the top of the subframe bushing mount will be used as the reference for ALL the modifications. In this instance, I am assuming that the top of the subframe runs parallel to the toe direction of the can and perpendicular to the travel of the camber. This also provides your reference to both sides of the vehicle and each other.
