The central support for the shift mechanism is a splined
spindle that is bolted at the rear to the adapter plate.
The adapter plate is, in turn, rigidly attached to the handlebars.
Note: for the purposes of this analysis, we'll ignore the braking function of the lever. Everything is taken from the viewpoint of a rider seated on the bike.
|The shift mechanism is mounted on the spindle. Some parts of the mechanism are splined and engage with the spindle but the ratchet drum is free to rotate on the spindle. The gear shift cable is attached to the ratchet drum and is pulled as the drum rotates.|
|The ratchet drum is tensioned by a spring in it's base.|
|Two pawls in a cage (ratchet pawl cage) inside the ratchet drum engage with teeth on the inside of the ratchet drum and control the position of the drum, and hence the gear cable, to correspond with the required derailleur position.|
The outer lever is mounted to the end of the spindle by
inner and outer cone bearings.
The outer lever is free to rotate around the shift mechnism.
Notice the spring loaded ball that provides a detent
stop for the outer lever in it's home position
and stops it rattling around.
And the screw that projects through the lever just outside
the bearing race and provides a limit stop for the
Both operating against the spindle end plate.
The inner lever is mounted on a pivot in the outer lever. The upper end of the inner lever moves through an arc below the shift mechanism.
Shifting to a larger sprocket or chainring:
Pressing the outer lever causes the outer ratchet pawl, located in the lever, to engage with the outside of the ratchet drum which is rotated thereby pulling the gear cable. Pictured is a right-hand lever which rotates clockwise about the spindle in operation.
When the outer lever is released it returns to it's home position under control of the return spring inside the front cap.
Shifting to a smaller sprocket or chainring:
The mechnism is under tension from springs in the ratchet drum and, via the gear cable, the derailleur. Operating the inner lever moves the outer pawl (in the outer lever) out of engagement, so the ratchet drum can rotate back, and rotates the release plate. In the right-hand lever, the release plate move the outer pawl, in the left-hand lever, the inner lever move the pawl directly. The release plate engages with the inner pawls and moves them into the correct position to release the shift mechanism by one position only. The right-hand lever release plate has two cam tracks which engage with pegs on the pawls. The left-hand lever release plate has two projections on it's underside which engage with the pawls.
Releasing the inner lever re-engages the pawls to hold the ratchet drum in it's new postion ready for the next shift.
The initial movement of the inner lever causes the ratchet drum to rotate slightly in the wrong direction so as to unload all the pawls and allow them to move freely. It then prevents further movement in the wrong direction as it moves further and operates the downshift described above. Wear in the mechanism affecting this aspect of operation is the most common cause of failure - see below.
A used Dura Ace ST-7700-C right hand shifter
was found on eBay, described as follows:
As the inner lever is operated
it rotates clockwise about it's pivot.
The long tang engages with the release plate and
rotates it anti-clockwise.
At the same time the outer lever rotates
clockwise about the spindle for a short distance until it is stopped
by the step on the inner lever
(just above and to the left of the long tang in the pic)
coming into contact with the top plate of the ratchet pawl cage.
That step on the inner lever then sweeps along the curved edge of the
top plate, preventing the outer lever moving further,
while it completes the mechnism release operation described above.
However, in this case the leading edge of the pawl
cage top plate has been worn away by use and, instead of moving
under the plate, the lever digs into it and will
not move any further.
Some precision is required for the inner lever to allow just the right amount of movement in the outer lever before moving below the pawl cage to block it. It looks like a design flaw for such a critical region to suffer abnormal wear.
At this point we'll return to the original owner's claim that this lever was just two years old and had covered 3500 miles. One is forced to conclude that either some eBay sellers are economical with the truth or Shimano has a serious quality control problem and the plate has not been properly hardened. Perhaps both.
Whatever the truth, the expectation is that Dura-Ace components will be robust enough to withstand the demands of pro cyclists and STI levers should outlast at least a few chains and tyres. Unfortunately, while the upper part of the inner lever is made of very hard steel, Shimano appear to have made the pawl cage out of Brie. Initially the inner lever would have required just a little extra pressure, then more force until finally it simply wouldn't move. To produce the damage seen here would have require considerable force to be used over some time. It certainly would have been obvious that there was a problem. But I have seen inner levers snapped off when the rider has refused to take no for an answer. Moral: at the first sign of trouble send your levers back to Shimano. Remember, there's a three year warranty on Dura-Ace components.
Pushing the outer lever outwards rotates it anti-clockwise
a fraction with respect to the mechanism,
bringing the inner lever lower and missing the step
so that it can move
past the leading edge of the pawl cage top plate as it should,
whereupon the mechanism operates perfectly.
Although I have no doubt there's a bin full of these cages somewhere in the Shimano factory, they're not available to us peasants. Therefore, the best option would be to canabalise a low mileage crash damaged shifter for a good replacement. It's difficult to imagine any crash that could inflict serious damage to this part of the shift mechanism. Failing that, build the plate up by welding and then file/grind back to shape. But find a good weldor.
Some folks have asked 'can't I just file the burrs off the cage?'. Sadly not. The profile is very important to syncronise the motion of the inner and outer levers, as described above. You have to add metal first. A TIG with a stainless steel filler rod would be ideal because stainless work hardens.