The Citroen BX 16v bumper bracket is a well-known weak point on the Ph2 BXs. Unfortunately, a lack of replacements has led to a holdup in the restoration of ‘XPO’. While a UK specialist had previously remanufactured these, the bumper bracket is no longer available. So, what do you do when parts aren’t available? Reproduce them, of course!
Why does the Ph2 BX 16v have metal bumper brackets?
When the original BX 16 Valve was released, the body kit was relatively subtle. Although some GFRP skirts were added, the Phase 1 (Ph1) BX 16v bumpers were made of the same ABS plastic as the 8V GTI. When the second version of the BX 16 Valve was launched, it came with a much more aggressive body kit and GFRP front and rear bumpers.
Beyond being rather heavy, these GFRP bumpers didn’t lend themselves to the existing holes in the body for mounting. They needed a metal bumper bracket to use the same plastic adapters as the ABS bumpers. This bracket assembly is highlighted in green in the image above. Critically, perhaps, the metal adapter was only available to purchase with the entire bumper!
Unfortunately, the original metal brackets were badly powder-coated. Beyond being too thin, the coating was not very well bonded to the metal below. As soon as the plastic finish is penetrated, water enters, and the metal rots inside the plastic wrapper. The picture above shows all that remains of the bracket that came with XPO.
How are the 16v bumper brackets attached?
When new, the brackets were attached to the bumper with little more than a healthy dose of glue. Some sort of flexible polyurethane body work adhesive like Sikaflex
The metal sits on a bed of this adhesive, with excess oozing through holes in the base of the bumper bracket. Often, this oozing, curing as oversized balls, is all that is left holding the brackets to the bumper. The image above is from 2010, when I made some brackets for Jazz in 2mm galvanised steel by hand.
I do ask myself why I didn’t just hand-cut some more brackets for XPO. However, the BX community is such a supportive one that I’m happy to reproduce parts in quantities that are more than just one-offs. There’s no real profit in the reproduction for me, but there is satisfaction in helping keep these cars on the road. The image above shows the plastic adapter on the metal 16v bumper bracket, all bonded to the GFRP bumper.
Where do we start reproducing the bumper bracket?
The starting point for reproducing any part is always measurement. I utilise a number of methods, but the most common are taking a slice and photographing it, like the bumper trim, or using a 3D scanner (more to come). In this case, the simple metal shape is easy enough to measure with a set of callipers.
That said, with so little left of the original bracket and no other reference, getting good measurements is going to be tricky. Some head-scratching, double measurements, and guesswork later, I have a fairly complete set of numbers.
These roughly sketched numbers are soon turned into a dimensionally accurate drawing in Autodesk Fusion 360.
Modelling the bumper bracket in CAD
Once the 2D sketch is made, I can start turning it into a virtual model. In this case, creating the material from the sketch is a pretty simple affair. However, creating the part in CAD means that features like bending radius and bend lines can be calculated.
I’ve taken a few liberties with the bracket design compared to the original. Rounding the incredibly sharp corners should save me from the lacerations caused by the originals. This is a pretty easy addition when the brackets will be laser cut. I’ve also added folding reliefs to the design. These small notches help the bends form correctly and give me a marker to line up in the bending machine.
Fusion 360 also has a sheet metal tool for calculating bend radius. Allowing for the thickness of the metal is critical to getting the plastic adapter brackets in just the right place. Okay, in this case, a few millimetres either way probably won’t make much difference, but it’s the thought that counts.
Exportable bracket design for laser cutting
From the Fusion 360 sheet metal design, it is possible to export a special file containing the instructions for the laser cutting machine. In this case, the file is a DXF (Drawing Exchange Format), which includes instructions on how deep the laser should cut based on the material thickness.
I did go through a little bit of a learning curve with this file type for laser cutting. I use DXF files with a CNC router; however, I don’t usually have aesthetic markings like the text or bend lines shown in the image above. Neither of these had depth details in the file, so the laser didn’t know how far to cut!
Prototype laser cut bumper bracket blank
I’ve been a little concerned about the dimensions I’ve taken for the bumper bracket. The original was so badly corroded that I’m not confident that the dimensions I’ve taken will be perfect. To avoid the risk of making 20 sets of parts that don’t fit, I’ll start with a prototype.
To get the blanks laser cut, I wanted to find a local company that I could visit and speak to and potentially get some guidance. After contacting several suppliers, the only one that actually came back with a price has also been incredibly supportive. Area Welding Services near Nantwich has an amazing laser cutting machine and is happy with mass production as they are one-offs. I collected the blanks a few hours after receiving and paying the invoice. How’s that for service!
A quick trip into my metal folder, and I had a functional bracket. The dimensions were a little out, and the fit between the metal and plastic parts was a little loose. The ‘excess clearance’ is due to my lack of understanding of the CNC laser process. The laser head has a 0.2mm cutting width and follows the centre of the lines in the DXF file. That means the holes are all 0.2mm oversized, hence the sloppy mating.
Cut, bent and plated brackets return.
A couple of tweaks to the design and an order was placed for 40 brackets, 20 sets, to be made up. And this time, it wasn’t just the blanks that would be provided. To help the brackets outlast the cars, they also get a plating. I really wanted a yellowed zinc finish, but the chemicals that create the yellowing have been banned in the UK, so a straight silver is all they get. However, the blanks needed to be folded first to get the plating. It’s a small extra cost, but at least this way, they will all be perfectly formed!
I placed an order, and a few days later, I had a box full of shiny metal. There is something perpetually satisfying about designing a part and then having it manufactured. While this front bumper bracket is only the second BX part I’ve reproduced, I’ve had other parts remade in the past. For me, this box of expensive metal brings a huge smile.
Better still, the metal bumper bracket and the plastic block fit together really well. Better than any brackets I’ve had in the past. Hopefully they aren’t too tight!
One other change I made to the originals, in addition to removing the super-sharp edges, was the bend angle. The original parts were probably set to 90 degrees; however, the flanges on the new part are set to 88 degrees to keep them a little tighter to the plastic block. This overfolding was a little bit of a gamble, as the bumper brackets are a 1.5mm steel sheet, meaning they are stiff with little play. But they fit just perfectly!
Risk and reward
Investing not insignificant time and cash into the reproduction of parts is always a financial risk. The sensible thing to do would be just to make up the parts that I need. But, like with the bumper trim, I won’t be the only BX owner who needs these parts now or in the future. So why do I take the risk for the BX community? Simply because, over the last twenty years, I’ve always found the people involved with BX to be incredibly supportive. I find sufficient reward in returning something to those who have helped such that the financial risk is offset.
Finally, the short run of brackets is available directly from BXProject. Just email admin@bxproject.co.uk if you want a set. Price-wise, these new BX 16v bumper brackets will be £35 for a pair, plus postage. This is quite expensive for a couple of bent pieces of metal, and all the dimensions are above if you want to make your own. However, this pricing will only bring any positive return once thirty-seven sets of forty are sold. Certainly not a ‘for profit’ enterprise!
I want to set up a BXProject eBay page in the next few weeks to facilitate shipping to the EU. However, the price will always be lower when bought directly, so drop me a line!
M
PREV – BX 16v Bumper Trim Reproduction
More Reproduction Articles
Back to Citroen Articles
Back to Automotive Articles
Pingback: Citroën BX - Reproduction - 16V Bumper Trim - BX Project
Nicely done 👍🏼