I also with with pipework/tubes and fabricate custom stuff – so for example this is my “stunt bike” which is a street-legal(ised) trials bike, but after some painful testing i found I could do with some sort of seat, and I wanted a wheelie bar (12-oclock bar), and somewhere to put a foot for standup wheelies.. so after quite a lot of measuring and head-scratching I came up with this novel subframe.
Because these bikes are somewhat designed to be thrown around and I fall off regularly they have flexibility built into the design. So the subframe needed to work with that and move organically. It’s no good having a flexible bike but then a rigid subframe which will be forced to flex and then either crack welds or rip off bolts the first time you loop it. Not ideal.
It was kinda tricky, especially because there is nothing to this bike – it’s just an engine and wheels and every component tends to serve at least 2 functions, so technically it was quite a challenge. But I came up with this solution and was able to build some flexibility into the design. So the rear section is actually articulated and has some beefy rubber dampers which act as a designated failure point in case of a serious impact, and would just absorb smaller impacts, e.g a harsh scrape… rather than ripping the subframe/bolts off when I stack it.
The metal effectively gets denser around the hinged part but there are no external dampers or levers – it’s solid enough to stand on but flexes up and down an inch or two if you apply a decent amount of force, so won’t rip your the mounting tabs off the frame if you stack it enthusiastically.
The hinged part (tail) is sacrificial so in a medium impact the tail section is designed to be destroyed or bent.. I’d much rather that takes the damage than the main subframe or the actual bike’s subframe, both of which are much harder to repair/straighten. The tail section is deliberately not braced to be too rigid, it’s meant to bend if hit hard enough from the side/diagonal.
Whilst it ended up a little bit boxy and utilitarian looks-wise, I wanted to be able to put a tailbag or something on there too potentially if going further afield, and I wanted a foot slot for stand-up wheelies (not that I can do them yet but that’s the goal), so to a large extent the function dictated the form, since it also had to be as light as possible and not impede movement on the bike since it’s still being used for actual trials/climbing rocks and stuff.
It had to be strong but also very light hence the thin tubing and birdcage/space-frame design. The whole thing including the rear-light assembly, indicators and number plate holder weighs less than 2kg (iirc) and it’s now been “race tested” in that the bike and I have slid down various hills and fallen off various ways and times, including at an actual trials competition… and the subframe was excellent throughout and also provided a good handle to drag me out of various ditches and bushes.
rubber dampers near the hinge mean no external springs or dampers and don’t ruin the lines of the bike too much 
street-riding shoe will fit for standups, motocross boot not so much 
Literally a remake of my BF-400/manches from GTA V 
fresh out of paint – super-hard-wearing finiish 
smoothing round the base welds so we get that nice smooth organic blend into the frame which doesn’t catch your boots. 
rubber dampers as designated failure poinit. excess force up and they’ll rip off or absorb it, excess force down they’ll suffer material collapse and the cross beam would likely bend/break, easy to fix though and likely minimal damage for that scale of impact. 
measuring up
Apart from the indicators which got ripped off in the crashed it’s pretty much unmarked so defintely a win :).
There’s a stainless steel hinge for the number-plate mount which means you can fold the plate out of the way for offroad events and means you can land as hard as you like without chewing the plate up in the back wheel. If you land air with the plate in the “street” position on a tall tyre there’s an immediate £10 fine in that it’ll shatter/destroy/chew the plate…but then you shouldn’t be getting air anywhere you need a number plate anyway, right?
Also the act of doing a standup wheelie and putting your foot in the slot pushes the plate forward out of the way. Since the same applies and you shouldn’t be doing standups anywhere you’d need to be showing your number plate, there is no conflict.
