I've done a lot of chasing up on this subject - one of the reasons I bought the TBA over a SM was the spoked wheels but I'd really like twin disks on the front.
The Triumph Thunderbird Sport has spoked wheels with twin 310mm disks (same as ours), and I'm told the hub's the same as ours, however the rim's smaller than ours - 17" instead of 18". Both bikes also run 36 spokes so you should be able to lace your standard TBA rim onto the TBird's hub. There's no reason you couldn't use it with the smaller rim (maybe with a higher profile tyre to fill out the guard) and if you're into whitewalls there's more options for the 17" one.
Once you've got the wheel sorted you'd also need the right lower leg from an SM, a right hand caliper, and an extra brake line and fittings. I don't much like the way the SM's have the plumbing for the extra caliper running over the top of the front guard, so I'd either use a splitter under the lower yoke (stolen from a virago maybe) or just run a 2nd line from the master cylinder with a dual banjo bolt.
The bad news is that so far I've completely failed to find anyone who's ever wrecked one of these bikes in Brisbane - I'm not even sure if the Sport was imported into Australia as I've only ever seen the standard TBird here. It might end up that it's easier to buy the hub new from Triumph, or maybe purchase a 2nd hand wheel overseas. If that's the case you then have to find out if the America/Speedmaster rotors will bolt to that hub - you may end up having to buy a pair of TBird rotors, or just one TBA rotor to complete the set.
Let me know if you have any more luck or info on this, as it's something I'd really like to do. Don't know why Triumph didn't do it in the first place.
Matt
