Just writing to describe a situation that occurred with my Brampton damper conversion. I was attending Coburn Benson's memorial service last month and had a wonderful 125 plus mile ride over to Kingfield, Maine and back......until the last 7 miles just outside Monson. After hitting several large bumps my front Thornton shock conversion failed in rather dramatic fashion, spitting the bottom seat and spring out the front of the fork into the fender, denting the bottom of the headlight bucket, popping out the headlight and rim, and slightly crimping the front brake cables when the upper links came to rest against the handlebar. It looks like the rod broke and the lower clevis flipped forward, bending the spring until it came out of the lower spring seat . I found the bent spring alongside the road but not the seat. I made it the rest of the way home in that genuflected position at less than 15 mph on the back road beyond Monson. The bike handled surprisingly well with no front suspension.
I'm not sure why this happened, but the shaft failed at the threads, and possibly stress or embrittlement and heat treating may have been involved or perhaps I did not preload the spring enough, since I did bottom out occasionally. My clevis mounts were free to pivot so nothing was binding. Anyway, I will now go back to the original Brampton spring, at least for a while. It is on the bike now and I have to repair the fender before I take it out again.
I have looked at the failure point and notice that the hole in the clevis is drilled and tapped just a bit off center, (My fault), Less than 1/32 laterally. This slight non concentricity to the spring center might have stressed the threaded joint with repeated flexing. Another cause might be the difference in angle between the fork motion and the damper motion. See post 28 picture. It could be that having two easily pivoted clevises does not support the assembly, and it might be better to support the rod end more. This angle difference may have bowed the spring and stressed the shaft at its weak point at the thread diameter reduction. Other Brampton conversions have hooked the lower damper mount over the original bottom spring hook behind the lower mounting hole and this would have prevented the clevis from flipping or slightly rotating forward and shifted all the necessary joint rotation to the top. That might have worked and if I do decide to repair mine, that will be the way I do it.
I have found a replacement spring on eBay and Carleton Palmer put me in touch with Pierre Vaillancourt, who was associated with his family's original Works Performance company. He was very helpful. Works performance has been out of business for two years but Worx Shocks is still in business and services works performance shocks and makes new parts. The people that work there used to work for Works performance. Same phone number 818-701-1010 Monday through Friday before noon Pacific time. I called and they have the shaft and will make a new spring seat for me. Pierre also suggested that I could machine a new shaft from 1/2" 17-4 ph H900 that would be twice as strong as the original shaft. This material is available from McMaster Carr. Since I would be remaking my own clevis I could machine the rod end to 12mm instead of the original 3/8-16", making it stronger yet.
I have not decided whether to use the Works coil over again, and will ride the original spring for a while to evaluate if the coil over is that much better. Carleton feels that a Thornton on the rear of a B like I have improves the performance of the Bramptons by damping out the bouncing so it is not transmitted to the front. I have never ridden my bike with the original front spring, since I put the front Works coil over on when I finished the restoration in 2015. I will repair the coil over in any case, and if I want to go back to it it will have the stronger shaft and probably a hook over the original spring hook.
Sorry to put a "damper"on enthusiasm for this modification, but I thought I should at least warn others to keep an eye on theirs. I was super pleased with the ride while it lasted, and despite the concerns I have heard about using the headlight mount for the shock mount that held up fine and it was the shock that failed.