finding a bike trans to reverse-engineer...

...where and how might it be accomplished?

I've got a personal design project (downhill mountain bike with a 6 speed gearbox that can use a standard Sram trigger shifter, for anyone interested) and while gearbox bikes have been done and done and done and done over, they all require either a grip shifter with two cables, and I don't believe any of them use the shift barrel setup like a motorcycle sequential box does. I've got a feeling that by adding a pulley of a specific size with a return spring (set to return the gearbox to the highest gear if there's no cable resisting it), getting the cable pull to match up with the shifter, you could use an off-the-shelf shifter and have a nice, compact gearbox mounted way down for a crazy low CG.

the biggest snag with this design project is I really don't understand how a bike sequential gearbox works, so I can't even begin to draw one up, and it's a lot easier for me to understand something if I can hold it in my hands, shift it by hand to see what happens, etc. anyone know where I can pick up a used motorcycle gearbox for relatively cheap? preferably non-unit (don't want an engine to futz around with because then I'll be tempted to stick it into something and hoon around )

thanks in advance

In reply to Slyp_Dawg:

Google some large motorcycle dealers and ask them if they have something available. Or a used motorcycle parts co.

At this point, any pre-unit motorcycle transmissions are probably collector items (old Triumphs, BSAs and so forth.) I'd watch the local Craigslist for someone parting out a small Japanese two stroke dirt bike from the 1970s or so - I see them all the time, selling for little money. The transmissions are integral but the engines are small enough you can still toss the whole thing up on the bench.

I just checked my local Clist, and someone is selling a complete 750cc Honda engine/transmission for $125, you should be able to find a little dirt bike engine for a lot less.

thanks for the advice, I didn't realize that non-unit transmissions were that vintage, I had seen a Harley transmission (no engine hanging off of it) that looked relatively new a few years ago, for about $125. I'll check craigslist and if I can find a bike-equivalent to a pick-n-pull, I'll check those as well

Ah... an MC-derived DH bike gearbox... That idea takes me back a decade or so...

The only relatively recently produced non unit motorcycle gearboxes that you are likely to find in the US are from HD big twins. As has already been mentioned, find a small displacement dirtbike engine and figure out what to do with the parts you don't need.