Nathan's proposals mostly look pretty reasonable to me. As an interested bystander (since I'm unlikely to ever get Down Under to race) I'm puzzled by the insistence on stock or OEM specifications on the bikes. Stock motorcycles are boring, who wants to bother with them when you can have something really cool?
Here in the USA when LT suspension came in there were lots of people who were modifying their bikes. Moved up dampers, laid down dampers, Wheelsmith slider extensions on Maico forks, Competition Dynamics kits to monoshock your twinshock, bent swing arms, braced swing arms, custom swing arms, custom frames and on and on and on. The magazines had multiple articles on the conversion processes, including on trials bikes like the Bultaco Sherpa T. You could buy different seats, tanks, fenders, airboxes etc so bikes that didn't look very stock were pretty common.
There were also a fair number of people who had access to machine tools at work or had well-equipped shops at home (look at people like Harry Hindall and others in the SoCal aerospace industry areas or Joe Bolger) who would make very cool/trick one-offs. Make your own frame, make your own wheel hubs, graft on outboard motor reed valves, borrow dampers from race cars, wrap a water jacket around a cylinder or head, may not have been common, but it certainly happened.
It may make sense to exclude some
obviously later out-of-period parts like more modern and effective large diameter teleforks, but I can't see why any small TLS front drum should be a problem -- any MX drum brake is pretty much functionally the same as another as there's only so much improvement that can be gotten out of them and if you look around there were some small TLS (or even 4LS) brakes available in the 60s and 70s. Van Tech sold their own floating backing plate assemblies for OEM hubs and I think Wheelsmith may have too. It wouldn't be an insurmountable task for someone to grab cams/levers from a couple of SLS brakes and make their own TLS backing plate.
While I have my gripes with some of the AHRMA rules it does seem to work pretty well to have a maximum travel rule for the different period classes, and how a bike achieves that is largely immaterial as it is likely that someone tried just about everything back in the day. If someone can make a Skunk Works link and some moved up damper mounts work for them within the allowed travel, more power to them. Once you start saying "OEM this or that" you start opening up a can of enforcement worms. This is racing, not a concours d'elegance where people lose points for using the left-handed prawn nut on their bike made on 02 May 1965 when everyone knows that was only fitted to bikes made on 30 April 1965.
Perhaps if you required "any technology available in the period, any drum brake wheel assembly accepted, max suspension travel XXX, max stanchion OD on RSU telefork of XXX" and left it at that you'd have fewer people wondering about the legality of their particular cool part. You could also have "must present a general appearance evocative of the period" to preclude people putting a 1985 "snail" expansion chamber on their Greeves Challenger. I think most people can agree about "that looks appropriate" issues that are based on cosmetic appearances and not technical issues. If the bike looks good to Joe or Jane Average Person from 20 feet, it is good.
Keep the rules as simple and broad as you can and you'll cut down the grumbling and complaints.
cheers,
Michael
