Identifying the pivot point is easy - look at where the big pivot holding the swingarm is. This only works with a single pivot, so there's no trying to work out where the instant centre or centre of curvature is. I've done this to 3 frames - Scott Gambler, Scott Voltage and Evil Undead, and I picked these frames because they fit the following criteria: 1. Single pivot. 2. The pivot is nice and chunky, able to fit some kind of axle through it without frame modification. 3. Has space between the BB and pivot for clearance for two cogs. 4. Has the required space for the new chainline, so the position of the chain stays is key here. Making the jackshaft is the easiest bit, it's just an axle of some sort, and the cog carrier is just a freehub with the pawl section cut off and a circlip groove cut in to hold it in place. The right freehub is key as it needs to have bearings in the correct places that won't get cut off. You need to use an eccentric BB to get the chain tension in the primary chainline. Once you have the right frame there is a mountain of gear ratios that all come with their own inherent issues... Direct mount chainrings are often required due to their availability in smaller sizes (otherwise you're limited to 30/32), but direct mount chainrings only come in even numbers, and also only narrow/wide which means you can't use a half link chain to get the right length (The EBB only has about 6mm of throw which isn't enough for all options). BMX cranks give you more sprocket options, but they're thicker and don't work with narrow chains. Different cog combos came with other clearance and tension issues. There were over 3000 gear combinations so I created some spreadsheets and formulas to narrow it down based on my measurements to then manually try the few options. And I still only ended up with about 5 usable options that fit my gear requirement. And lastly, I modified and reversed an SB One Boner tensioner to tuck in at the back and tension the secondary chainline. This doesn't put pressure on the suspension though, because this chainline doesn't experience chain growth.
I’m hoping this’ll get me out and about in the evenings or before work in the mornings. The sand and grit around here destroys brakes / gears / rims hence tip parts.
