E46 BMW Rear Subframe Reinforcement

 

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Similar to the E36 BMW the E46 series chassis was cursed with thin sheet metal in some vital areas. The rear subframe, particularly on the early produced non-M 3 Series, often rips from the mounting points over time. While BMW made a recall kit for the E36 chassis they didn’t do so for the E46 so we are left to our own devices to reinforce it. Typically what is done is 4 plates are welded on the bottom side of the chassis.IMG_20151013_220941_zpse7ta1rdx[1] There are commonly available, simple design plate kits available from places like Turner Motorsport while other companies have made more elaborate plates like the ones available from Reddish. When we do repairs in house we provide our own special plates which are a larger version of the commonly available plates and are similar in design to the Reddish version. In most circumstances our plates will reinforce the car sufficiently to prevent any future issues. Whenever we take on repairs in our shop we always provide a lifetime warranty against future cracking, that is how confident we our in our service. On cars that have cracks or ripped points already it is necessary to take some extra time and making additional plates to reinforce the areas that already have damage. In addition to customized plates for each car we also repair and reinforce any specific damage found on each car, these failures happen in different ways and having a competent fabricator repair your chassis is crucial. When doing these repairs it is important to understand the underlying structure of the unibody chassis so that the repair can be done by attaching the plates to structural parts of the car and not just weak sheet metal like many repair shops often do. Since we have experience in doing this repair many times and our experience in building roll cages that tie into these structural areas, we know how to properly brace the subframe mounting points to other parts of the chassis for each individual situation.

To have sufficient room to weld in even the smallest plates the entire rear subframe of the car must be dropped. This requires disconnecting emergency brake cables, disconnecting the rear brakes and electrical connections, then fully dropping the rear sub frame. If considering doing this job, it is wise to consider replacing differential, subframe, and trailing arm bushings while the sub frame is down and everything is easily accessible. This is also an excellent time to fix any differential leaks.

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At our facility we TIG weld the reinforcement plates on the bottom side. TIG welding (as opposed to MIG) allows us to better control heat put into the car to reduce chassis distortion and weakening of the chassis steel. TIG welding requires very clean metal which can be difficult on the chassis so we clean extensively before any welding starts. The welding process takes hours, sometimes days if the damage is extensive. This is the only way for us to ensure that the unibody isn’t damaged by the heat of the welding process while still adding significant strength to the subframe mounting points with enough welds. Many shops doing this repair don’t realize that the outer sheet metal that is easily visible is actually quite thin and not all that structural. So a very important part of this repair is to attach the thick reinforcement plates to the spot welds which connect the outer thin sheet metal to the thicker base metal underneath it. We do this by using a spot weld drill to cut a small opening in the thin sheet metal and exposing the underlying structure. We then rosette weld the base metal to the plate. How many places we do this will vary from car to car. The earlier chassis from 1999 to 2003 had less spot welds in the area (which is why these chassis are more prone to failure than the 2004 and newer cars). While many people think the M3 has the most failures because of the higher torque the engines produce the early 3 series actually fails more frequently at the left rear subframe mounting point because of the differential design. We have found that cars with more significant damage also have failed differential or subframe bushings.

 

Once those plates are installed the trunk is opened and the liner is removed. The sheet metal covering the top side of the rear subframe mounts must be cut to reveal the 3 spot welds that hold the top side of the subframe mount in place. Often times during manufacturing these three spot welds miss their structural target and this results in eventual cracking in this area. Our method is to connect the 3 spot welds together to strengthen the area. When necessary, if the welds are cracked, we will add a plate in the area to spread the load from the plate.

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Once all the welding is done the car is treated Wurth self etching primer and allowed to dry. This primer makes it very easy to apply the sprayable seam sealer also provided by Wurth. By using the sprayable seam sealer we are able to very closely replicate the OEM underbody look on a BMW. After allowing 12 hours for the seam sealer to cure we also apply a Wurth underbody spray which provides a final layer of protection against road debri and moisture.

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Since the rear subframe was removed and the suspension geometry was modified by adding the plate thickness we must then do an alignment on the car to put it back into OEM spec. This is a great time to have us do a custom alignment or corner balance on your car if it sees track use.

If you have an E46 chassis that has not had the reinforcement done on the subframe we encourage you to give us a call for a quotation on this important preventative maintenance item before a relatively straightforward repair becomes a difficult one.

 

Update: E46 Rear Subframe Reinforcement Plates Now Available for Purchase

We’ve often received requests for our reinforcement plates from clients who are our of state or even out of the country and can not bring their car in for us to do the repair

We are happy to offer our E46 Rear Subframe Reinforcement plates. Please feel free to reach out if you have any questions – otherwise, check it out on our store page.