I've designed and made side and centre stands for bikes that never had them. It's something I've developed into a kind of specialty over the years, and have done a couple of custom stands for mates. If there's nowhere suitable on the frame to fit a side stand, you have to get creative. Any design has to consider ease of operation, stability of the bike when on the stand, cornering clearance and visual impact. Sometimes it's a challenge to create a stand that'll tuck away nicely and not look out of place on the bike. I've had a couple of difficult ones, but so far not had a side or centre stand job that's defeated me. There's always a way to do it.
For maximum stability, a side stand needs to be placed near the centre of the bike. Those factory Laverda triple side stands that attach to the left front engine mount are pretty crap. My first Laverda (a 1200) had one of those and I hated it.
The answer is normally a sub-frame of some kind that's either bolted or welded in place. Bolt on solutions are sometimes preferred if you don't want to make a permanent alteration to an original classic, or if a welded structure would get in the way of future engine removal. These fixtures can take the shape of the plate typically used for the Laverda triples or a sub-frame fitted between the lower frame rails. I used a bolt-on plate similar to the Laverda solution for my MHR Ducati.
I applied a bolt-on subframe solution for the centre stand that I made for my Benelli Tre-K. The sub-frame attaches via the footrest mounting bracket bolts and carries the centre stand pivot. As far as I'm aware, it's the only Benelli Tre-K on the planet with a centre stand.
A Laverda 750 could be an interesting challenge for a side stand because there's no frame under the engine to fix stuff onto. The solution might be something that attaches to footrest mounts, rear engine mounts, or perhaps somewhere else. I might go and visit a mate who has a 750 and have a grovel around underneath to see what possibilities present themselves.
Your idea of attaching a side stand to the left leg of the centre stand is one I hadn't considered before. It has merit in that it requires no modification to the bike's frame. There are a couple of things you'd need to consider though. There's a potential source of instability because the centre stand's pivot could allow the bike to roll backwards while on the side stand, pivoting the whole centre + side stand assembly forward. You'd also need to make sure the centre stand springs can support the additional weight of a side stand hanging off it.
Maybe a more workable idea would be to mount the side stand on the same pivot as the centre stand, using an extended centre stand pivot bolt to carry the side stand in a cantilever fashion. A cantilever side stand pivot is not unprecedented. My Benelli Tre-K has one. Apparently early versions would occasionally snap off, dumping the bike on the ground, but they improved the design with a larger diameter pivot bolt that seems have cured it.
The most recent job I did was a side stand for a mate's "Bural" (Ural with BMW engine) last year. I opted for a bolt-on sub-frame between the frame rails to mount the side stand pivot on. It wasn't a particularly neat looking job because a clunky-looking fitting suited the bike's "steam punk" character. I actually used a hex head bolt as the stand's toe lever just to accentuate the clunky aesthetic. I've attached some pictures below for your amusement. I don't remember what the stand itself originally came off. It's one I'd had hanging around in my spare parts junk heap for years. I don't even remember how or when I acquired it. I'd guess it's off a Japanese bike of some sort and was discarded because the pivot was flogged out. I had to make up a new bearing bolt and then weld up the pivot hole and re-drill it to suit the new bolt.
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