My dealer is almost as frustrated as I am. With the exception of the teardown for the engine block hole from second gear fracturing, ALL the other teardowns have shown NO signs of damage, metal fragments, bent forks, broken circlips, busted springs, or any other visible cause of the problem.
On this last repair attempt (hopefully done this week), Triumph confirmed correct assembly orientation of all the parts. The dealer opted to replace a few circlips and replaced the shifter shaft.
I've studied this thing from every photo available. Thanks to all that have done their own rebuilds and documented the effort.
Pages 8.2 thru 8.10 of the service manual will be useful during this discussion.
Based on the assembly details, basic mechanical understanding, and an engineering mind that doesn't know when to leave well enough alone, I have come up with a 4-beer theory for my transmission problems:
1. If second gear or the dog ring is disengaging under acceleration load, then there is a reaction force making this happen PLUS an insufficient amount of opposing force to hold it in engagement.
2. The thrust load imposed from torque on the helical cut gear is the most likely culprit for disengagement force. A secondary factor could be slop in the assembly and/or bad bearings that are exacerbated under acceleration. The thrust load is unpreventable. The cluster gear assembly, forks+guides, and selector drum are all needed to keep the gear engaged in spite of this thrust load. In fact, the upgrade kit attempts to remedy the problem by shimming the clearances in the whole assembly.
3. If the selector drum (the cylinder with the grooves for the fork guides) is indexed to the proper position, then the fork guide will rest in a flat spot on the groove. Thrust load on the shifter forks imposed by the engaged gear will be resisted by the fork and its guide pressing perpendicular to the groove in the shift drum. Things will stay in place and all is well. No rotational force on the selector drum will be present.
4. However, if there is slop between the shifter fork and the selected gear and/or the fork guide is resting in a sloped portion of the groove on the selector drum, then the reaction force will make the gear+fork slide along the path of least resistance. If the shifter drum has not fully indexed to the flat resting spot for the fork guide, then bad things are going to happen. Acceleration load will try to force second gear back to where it came from (toward neutral). Since the transmission has the longest throw to accomplish going from 1-N-2 in one single shift actuation step, this is the most likely of the two alternatives. Also, tor this to happen, the selector drum would have to be free to rotate out of position.
5. If the shifter drum is "over indexed", then the other possibility is the thrust load will try to partially shift toward 3rd gear.
6. OK great. But what could be keeping the shifter drum from being properly indexed to the proper shift position for 2nd gear and what could allow it to rotate out of the selected shift position? I propose a couple of possibilities: a) The shift linkage and internal geometry are inadequate for the amount of throw required to fully engage a 1-N-2 upshift or 3-2 downshift. b) The detent wheel, springs, and/or selector shaft bearings are allowing the selector drum to rotate out of its intended position. c) the indexed positions on the detent wheel are not precise enough to guarantee the proper position of the selector drum for the 2nd gear shift (up or down).
7. This selector drum assembly is very complicated. The detent arm and detent wheel, along with the gear change return spring ensure that each shift indexes the selector drum to the appropriate position, and keeps it there until you grab the next gear. The detent wheel is registered to the selector drum shaft by a little alignment pin and recess in the detent wheel. This assembly is NOT intended to bear any rotational forces imposed on it from the shifter forks trying to rotate the selector drum. It is merely there to ensure the correct amount of travel on each shift attempt. Full travel of your foot on the shifter linkage is supposed to translate into full engagement of the selector drum to the next shift position (and no further).
8. If the selector drum is not fully indexing to full 2nd gear engagement, then there is a problem with the linkage geometry or the detent wheel assembly, or slop in the whole setup.
9. When the selector shaft is not fully indexed to the full shift position for 2nd gear, the detent wheel and detent arm spring bear the mechanical load of holding the selector drum in position as all the things mentioned above conspire to force second gear out of engagement. The end result is a bent detent arm, broken springs, broken detent wheel pin, and other bad stuff.
I propose the following test in the shop to see if things are still broken before wasting 8 months of your life fighting with your dealer and Triumph:
1. After the engine internals are fully assembled and the casing halves are torqued down, slide the clutch basket onto its splined shaft.
2. Attach the output shaft, differential assembly and rear wheel to the engine.
3. With a strap wrench around the clutch basket and the transmission engaged in 2nd gear, block the rear wheel from rotation. Apply 140 ft-lbs of torque in the right direction to the clutch basket while the rear wheel is held in place. If she doesn't slip out of gear, then repeat this 50 times after simulating 1-N-2 upshifts and 3-2 downshifts.
4. Proceed with bike assembly only if the transmission does not slip.
Well, that 's my story and I'm stickin' to it. It's been fun and it's taken more than 4 beers to spew all this crap out, but maybe it will help someone with a better dealer diagnose the real problem.
Thanks for reading this far.
Txclassic.