Originally Posted By: tom slick
Originally Posted By: Lorenzo
I'm convinced that in most cases when someone reports how much better their new oil feels, they are simply experiencing the effect of replacing old worn out stuff with fresh. Same thing for tires, brake pads, etc.
my bike's shifting gets "notchy" within 50 miles of off road riding. keep in mind that is varied high rpm and constant shifting. it must be something with the way the shift drum and forks interact.
I don't know if it is stick-slip. ways on a cnc machine are much closer tolerance and finer surface finish than anything in a MC engine.
Having done a shift-fork and transmission gear replacement (a complete transmission tear-down) on my '86 FJ1200 I don't think there's a great deal of friction at the shift forks/gear/shifter barrel. It's not usually steel on steel either. Most japanese bikes have shift forks made of aluminum. Incidentally, the surface of the shift forks, the channels in the gears in which they ride, the channel in the shift barrel and the center bore in the puck-shaped gears (that ride on the gear arbor) are very finely machined.
In any case, if you look at the part of a transmission most subject to wear, and the parts most likely to "grab" and create notchy shifts, it is going to be the dogs and windows in the sides (flat-side) of the puck-shaped transmission gears. These sections of the gears are not machined at all, but are typically rough castings. It is this connection that must be disengaged in order to change gears. The less the transmission is loaded up with torque, the easier it will be for the dog to slide out of the window in the adjacent gear. We are talking about usually 3-4 points of engagement per gear. You see significant wear in these places, and only see shift-fork wear (or actual bending) when the dogs/windows are rounded from missed shifts, causing the gears to want to push themselves away from one another under load. The only thing resisting this malfunction is the shift fork, which doesn't have enough mechanical advantage to do so.
This is why some high performance bikes like the hyabusa have undercut dogs and windows. This is another way of saying that an angle is cut or cast into the contact surfaces of dogs and windows so that when on gear is applying twisting force to its adjacent gear through the dogs in the side of the gear, the angle cut into the dogs will tend to squeeze the adjacent gears together into the proper position rather than spreading themselves apart in the case of rounded dogs/windows resulting in a false neutral under maximum power).
I'm not sure what I have added to the discussion other than to add some detail to the discussion around the architecture of a typical cassette style motorcycle transmission.
