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Old 04-20-2017, 05:30 PM
Fair Fair is offline
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continued from above

SQUARING UP THE CHASSIS

The two main series that this car will be competing in don't require a roll cage, but its a damned good idea when you are planning something this fast. Before you want to ever put an elaborate, welded roll cage structure into any car you want to make sure the chassis is square and true. Nothing worse than caging a bent chassis - you just wasted a lot of time and ruined the tub.

We knew this car had a light front hit at some point, as it was missing most of the front sheet metal that bolted on: bumper, bumper cover, lights, radiator support, plus and the hood & front fenders were a bit munched. Otherwise it looked like a solid, rust free M3 chassis with the M3 specific front spindles, rear subframe and rear fenders.



In late summer 2016 I delivered this Coupe chassis to our friends at Heritage Collision Center in Sherman, Texas for an inspection and squaring up. I also brought them a new factory front radiator support to install and align into the "front doghouse" of this car.



They put it on their frame table and measured diagonals on known chassis points against BMW factory data. After a few tugs it was square and ready for the front radiator support, which they installed and trued up as well. I picked it up and brought it back to Vorshlag so we could begin.

REAR SUBFRAME REINFORCEMENT

Once the chassis was trued up we had a bit of a wait while the schedule cleared up and the owner was ready to really begin this project. We did sneak in the rear subframe reinforcement work early on before we "officially" began. This is where we beef up the rear subframe mounts at the chassis, which is a factory approved fix for an issue they acknowledged on the E46 chassis. We did this work when we had a tech "under tasked" for a couple of days, back in the Summer of 2016.



The BMW E46 chassis' primary weakness is in the trunk floor right where the rear subframe mounts. There is a lot of flex in the rubber subframe bushings which allows for constant movement. Over time this slide-hammer like movement can cause fatigue cracks in the sheet metal mounts. If the cracks are ignored long enough it can cause failure of the mounting areas.



Donnie put the M3 on a 2-post lift and removed the rear subframe assembly, which holds the differential housing and the rear suspension. Normally when doing this E46 reinforcement job we would remove the sloppy rubber factory mounting bushings for the subframe and diff and replace them with aluminum or polyurethane. We actually procured a second complete E46 M3 subframe for this car, since we had major modifications in mind for those bits. So the stock rubber bushings were untouched at this point.



With the subframe assembly removed we can finally inspect the trunk floor mounting points. Then the paint, seam sealer, and undercoating materials are removed from these areas where the plates will be welded - our air powered OBX "Crud Buster" tool makes quick work of these coatings. The reinforcement plates are CNC laser cut and bent pieces we source from CSM. These have a much larger footprint than some other reinforcement plates we have used. The CSM plates cover more area, which better covers some of the larger cracks we have seen in these cars.



This time the cracks were visible before removing the paint - they had propagated a good ways. Luckily we only found cracks in one of the four mounting positions, which is pretty good for an E46 of this age. Often we see cracks are all 4 points of contact. In the dozens of E46 cars we have done this work to only once did we find a car without any cracking. Like I said, its the primary weakness of this car, but easily fixed and reinforced.



Before the reinforcement plates are welded on we have to "drill stop" the cracks and TIG weld them for a proper repair. The small drilled holes at the end of the cracks removes stress from the sheet metal and keeps the cracks from propagating while they are being welded up.



Once all of the cracks were repaired, then all four plates were TIG welded to the chassis, it was time for seam sealer, primer and some paint. Once the paint dried the stock rear subframe assembly was bolted back in place, to allow the car to roll around. In the end it took about 11 hours + $250 in materials to get these areas properly repair and reinforced, but they should be good for life now. We had big upgrade plans for the second M3 subframe we purchased for this car, which I will show in a later post.

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