Homemade-Bonnet Frame MKII

Bonnet strengthening brace

My first attempt at a homemade bonnet frame was a disaster. Although each rib was very strong in one direction, once assembled it was hopeless. It flopped around like well organised spaghetti. There was absolutely no way I could use it as to bend steel around as planned. It would have worked to strengthen a bonnet that was already bent into shape, but I didn’t have one of those.

It’s been a slow month, as unemployed again and pennies are tight.

When I made my first attempt, I tried to use identical profile ribs front to rear. With hindsight I can see that this would never have worked. I’d also found my initial wireframe templates weren’t nearly accurate enough.

I could have modelled them in CAD but I took the quick route by doing it in Excel. By carefully measuring the slightly wonky bottom edge of my MG Midget Windscreen, I could scale the height and width until I had enough measurements for 3 totally solid supporting ribs.

Excel Bonnet Profile

Transferring the designs to steel

The calculated lines were carefully plotted onto paper templates. Using these I could scribe the steel using engineers blue for clarity. You can probably see the top curve is a different profile to the bottom one as it is closer to the rear of the bonnet.

Paper TemplateThe new braces however used way more steel than the old ones. Without lightening, these braces would weigh a ton so, I used my hydraulic punch to fill them full of holes.

Bonnet Brace - Lightened

To tie in with the cars eventual theme, I flared the holes with dimple dies. These holes nearly halved the weight and add loads of stiffness. Normally, I’d do the dimple dies with the hydraulic press but I had to do them all by hand, so they aren’t so crisp as I’d like.

Bonnet Brace - Dimple Die

I had to make each rib in two halves as the were too wide to fit in my 680mm folder, but that was alright as I added a central rib to hide the join.

Bonnet strengthening brace

In the photo below, the braces are sitting on top of the bonnet sides. I still needed to cut recesses into the braces for these bonnet edges to slot down into. With them recessed in, I could then turn my attention to the rear hinge mounts and the nose cone.

Bonnet strengthening brace

More Dimples Than a Herd of Teenagers

There are currently 82 Dimple Die holes of various diameter in the internal bracing. By the time the hinge mount and the slam panel are done, I’m sure there will be well over 100.

Bonnet strengthening brace

I rescued 2 of my previous braces as the rear one and yes, they’ll get dimple die’d latter.  Except for the rear 10″, I’m going to re-do the sides a little wider.

Bonnet strengthening brace

Now I have the braces welded together, I can place a flat bar over the top and it should touch all 6 vertical supports. If any are slightly off, I can file them or twist and tweak. Twisting and tweaking now takes a fair bit of body weight, but it’s still possible.

Solid BMW e46 hinge assembly

It was at this point I worked out I’d mounted the hinges too high. I wanted the back of the bonnet flex free and self opening on gas rams. To make this possible I’d joined them together with 25mm box. Dimple die steel sheet is good but I wanted something even stiffer.

When I made the box section, the bulkhead cut-outs and the mounts for the hinges, I messed up big style. I hadn’t allowed for the fact the bonnet slopes down hill and tapers in.

Bonnet strengthening brace

You might be able to see in the shot below that the outer corners are either 20mm too high or 60mm too wide. The box section is poking through the currently imaginary bonnet sides.

Bonnet strengthening brace
I had to cut out the bulkhead and recess the hinges by another 35mm and drop them 25mm. It actual helps to hide them a little bit, so all is good.

The cutting process did mean I had to remove about 30 Cleco’s and disassemble virtually everything. You might think that this simple structure would simple go straight back to together like NASA designed Lego and all my measurements would be bang on again.
I spent 4 confusing hours with crow bars and long lengths of angle iron levering and swearing, watching Cleco’s pop out as I got everything back where I wanted it again (well close enough for now).

To make that frustrating process worse, I had 6 hours in A&E and another hour in surgery having a shattered cutting disc removed from my eye. Ooops, I should have been wearing proper googles not just safety glasses.

Bonnet strengthening brace

At over 3Kg, it does come at a weight penalty, but the strength over the MKI version has increased considerably.

The Art of Metal Craft

I truly believe, with this hands-on metalworking game, it is important not to be afraid of failure. If you are like me, learning from YouTube and magazines rather than careful direction from an experienced teacher, gaining hands on skills takes plenty of experimentation and practice.

I used to teach Martial Arts. Occasionally, a newbie would come in, watch a spinning hook kick then execute a perfect first attempt. However, 99% of all students learnt these physical skills by repetition. Personal tuition would accelerate the learning process dramatically, but essentially it all came down to repeated failure, constant practice and time. Similarly, hand-craftsmanship cannot be researched or learnt from a book, it takes determination, persistence and of course hard work.

There are those of you that will look at these pictures and see wonky edges and dented panels and that’s alright. I’m learning and maybe my next attempt might be better. I recently saw lots of early 90+ year old brass-craft cars and the hand craftsmanship was fascinating; as were all the flaws.


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