46 BMW 1983 R100RS Repair Broken Sub-Frame Side Cover Tabs

The 1983 sub-frame has long tabs welded on each leg that help hold the side covers on. But, on one leg, both the top and bottom tabs are broken off. Before I have the sub-frame powder coated I repair these.

Sub-Frame Leg With Original Brackets

Right Sub-Frame Leg With Original Brackets

Sub-Frame Leg With Both Brackets Broken Off

Left Sub-Frame Leg With Both Brackets Broken Off

The profile of the tab has two curved sections that join to a straight section in the middle and I want to duplicate that profile.

Profile of Side Cover Bracket-Two Curves Joined To A Straight

Profile of Side Cover Bracket-Two Curves Joined To A Straight

Tab Dimensions

The original steel tab is 1 mm thick (about 20 gauge) and 10 mm wide (about 3/8 inches).

Bracket Thickness - 1 mm

Bracket Thickness – 1 mm

Width of Bracket - 10 mm

Width of Bracket – 10 mm

Materials

I use some scrap steel grass edging i have laying around. It’s 1 mm thick and made from mild steel. I cut a 10 mm wide strip about 6 inches long using my Dremel with a cut-off wheel. Six inches is long enough to make two tabs.

Scrap Steel Lawn Edging

Scrap Steel Lawn Edging

Lawn Edging Thickness - 1 mm

Lawn Edging Thickness – 1 mm

6 inch x 10 mm Strip Cut From Edging

6 inch Long  x 10 mm Wide Strip Cut From Edging

Forming A Tab

I shape the strip to match the profile of the existing tabs. I can bend it by hand around the existing tab where it is welded to the sub-frame tube to start forming the larger curve that is welded to the tube.

Bend Strap By Hand Over Existing Bracket To Start Round End

Bend Strap By Hand Over Existing Bracket To Start Rounding the End

To finish that curve, I use a 12 mm socket and continue to bend the strip using my fingers until it gets too hard to bend. I finish up using a hammer to complete the curve pounding the strip around the socket while resting it on the flat anvil surface of my bench vice.

12 mm Socket For Finishing Round End Radius

12 mm Socket For Finishing Round End Radius

Next, I mark the start of the straight section using the original tab as a template and bend the strip to the same angle starting with my vice and finishing with my fingers. I use a large diameter drift and a hammer to flatten out the straight section on the flat of my bench vise.

Round End Done & Marked for Start of Straight

Round End Done & Marked for Start of Straight

Use Vice For Bending Transition to Straight Section

Use Vice For Bending Transition to Straight Section

Straight Section Matches Original

Straight Section Matches Original

I mark the other end of the straight just before the beginning of the flare with the slight curve in it. I bend this section using a drift and my vice to get the curve in the end of the strip.

Bend Line Marked For Start of Second Curve

Bend Line Marked For Start of Second Curve

Use Drift in Vice To Form Radius At End of Bracket

Use Drift in Vice To Form Radius At End of Bracket

When I get the curve to match the original, I cut off the excess metal on the end of the strip with the Dremel cutoff wheel. I use a Dremel grinding stone (you could you use a flat file) to round of the end of the tab.

Looking Good Just Need To Trim and Round The Edge

Looking Good Just Need To Trim and Round The Edge

Rounding End with Dremel Stone

Rounding End with Dremel Grinding Stone

Rounding End with Dremel Stone

Rounding End with Dremel Grinding Stone

I use my flat file to dress the strip and remove the sharp edges so it won’t cut me.

Bottom Bracket is a Good Match

Bottom Bracket is a Good Match

The profile for the bottom tab seems a bit different than for the top, so I used the bottom tab as a template for the bottom strip. When I finished, I marked them “T” and “B” so they will be welded in the proper location.

NOTE:
I suspect the tabs were originally identical and they have been bent a bit with use over the years. But, to avoid a surprise should they have been different, I made each replacement based on the profile of the top and bottom original tabs as it’s not any more work and removes a possibility for error.

Top Bracket is a Good Match

Top Bracket is a Good Match

Brackets Ready For Welding

Brackets Ready For Welding

Welding Tabs To The Sub-frame

My son, Branden, is a pipe fitter. I took the sub-frame and tabs to him to weld. We started by marking the location of the original tabs and then ground them off.

Location Marks Ground On Sides of Tab

Location Marks Ground On Sides of Tab

Grind Off Old Tab

Grind Off Old Tab

Location Marks Still Show After Grinding Down Old Tab

Location Marks Still Show After Grinding Down Old Tab

We aligned the tabs with the marks and clamped them to the sub-frame.

Holding Tab for Welding

Holding Tab for Welding

Branden spot welded them using a MIG welder, but a TIG welder would have been the better choice. Unfortunately the TIG welder was not working. Here is the final result.

New Tabs-Just Like The Original

New Tabs Look Just Like The Original

New Tabs-Just Like The Original

New Tabs Look Just Like The Original

All set to take this for powder coating.

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