So, as you can see from the picture on the right, this is the chassis before I have added the spider in – if you like this is the before shot. I’m adding the spider in place to provide a virtual cross member through the engine to increase the structural rigidity of the engine bay and also increase the tortional rigidity.
I had several requirements putting this in place. Firstly it needed to be demountable, and secondly it needed to be easy to manufacture and locate. To this end, I went to McGill Motorsports and bought some left hand and right hand 1/2″ rose joints, threaded bungs and 1″ wishbone tube (4m of it just to have lots of spare). All in all, I got 10 rose joints (5 left hand, 5 right), 11 bungs, lock nuts and 4m of tube for about £110 all in, which I think was great value. This kit form approach is way easier than trying to get really accurate location on machined parts. The picture shows the bearing-bung-tube arrangement, just in case you were wondering what I was talking about.
I also needed to manufacture some brackets to attach it all to the chassis, which were cut from 1.5mm mild steel, with the holes reinforced with 3.5mm mild steel plates to be sure the bolt (actually a cap screw) shaft had enough support, so the reciprocation of the application doesn’t wear oval holes in the bracket plate, rendering it useless. These were easy enough to make – weld one to the other, and make 6 brackets the same. If I made 8 the same then I wouldn’t be able to demount the spider. I say 8 brackets in total because each rose-joint will be supported above and below. Remember the golden rule – always support a rose joint in double-shear.
I first of all made a turn-buckle to go from bottom left to top right, as shown in the picture. A turn-buckle is a tube with opposing threaded ends attached. If you turn the tube one way, the whole arrangement expands, and the other causes it to contract. This made it easy-peasy to locate and get the length correct for the first cross-member. The brackets are bolted to the turn-buckle first and then welded into place, ensuring a good fit. Next, on went the tube from bottom right to the existing cross-member. I used hole-cutting software to get this right, and the adjustability in the rose-joint to be sure of location.
Next came the tricky design part because it was becoming clear to me now that I may be welding myself into a corner when it came to making it demountable. as it stands in the picture above, you can see how there’s no bracketry for the top left tube, so the whole arrangement has enough pivot room (pivoting from the top right rose joint) to come loose. If I welded in the fourth bracket then there would be no room to get it out.
I hummed and hahed, and thought of making the top left and bottom right bracket-pairs vertical, but that would mean firstly cutting them out, and secondly losing the extra gusseting support they give. It was only when I was yanking it out to show to a friend did I realise that all it needed was a different bracket-shape that didn’t span the entire angle if a simple gusset were used. It still transfers the load into the focus-node of the space frame in a straight line, so I think I’ve ended up with an elegant, if asymmetric solution.
This whole arrangement is strong – really strong. I’ve tested it by standing on it where all four tubes join and there was no give whatsoever. And that’s loading it in a dimension it’s totally not designed for. I think it’s now strong enough for the job in hand, namely keeping the front of the car rigid enough for the suspension to work well, as well as keeping the engine bay strong enough to cope with the torque reaction of 215bhp.
Happy with that.