Thanks Peter for your detailed response. I would like to be prudent in how I spend my money to
determine next steps. It seem like first replacing used chain with new and if that does not work to
try bb replacement.
Should a good shop or frame builder, I have spoken to Mikkelsen here in the SF Bay area, be able to
do the measurements to determine if the frame is within Shimano specs?
Your response about "does the derailer know which frame it is on" makes sense and does suggest bs
And as it relates to the difference between ghost shifting and derailment, the tech at Shimano said
the Trek problem was that somehow the chain would be derailed onto the bb rather than ghost
shifting--going up and down on its own from middle to small chain ring.
Still curious what frame your da triple was on???
Thanks for engaging in this dialogue.
(Peter Headland) wrote in message
> > the drive side crank at the pedal end is quite a bit further from the stay than the one on the
> > non-drive side.
> That's normal for the D-A triple (and various other triples on the market).
> > So you think xtr bb not only evens this out
> I don't think, I know. I have this setup fitted on my bike. I measured before and after.
> > but also will allow me to move drive side in to improve chain line and shifting???
> See above.
> > This frame is a TST ti frame with a
> > 1.250 seat stay.
> But how long are the chainstays (measure centre of the crank to centre of the axle)?
> > I have now spoken to a tech at Shimano twice asking about updated middle chain ring. He said the
> > only frames that these do anything for are Treks
> Gosh, I wonder how the chainring knows what brand of frame it is fitted to? If he had said that
> this fix only works for a certain range of frame geometries, and maybe only for fairly stiff
> frames, I would think that reasonable, but this "Trek only" story smells like BS.
> > and those are the only ones covered under warranty.
> Yes, well Trek has enough "clout" to demand a solution. If your frame is within Shimano's
> technical specifications for this drivetrain (chainstay length, chainstay to seat tube angle, seat
> tube diameter), then Shimano should stand by their product and make it work (or refund your
> money). If your frame is outside their parameters, all bets are off.
> > He said the new ring fixed a derailment problem and would not address the ghost shifting
> > problem.
> I must be thick - I don't understand the difference between "derailment" and "ghost shifting".
> What you described is exactly what I experienced - the chain shifted down to the small ring (call
> that "derailment", if you want), and then either shifted back up or rubbed horribly (because the
> FD was still positioned for the middle ring).
> > What brand of frame was your DA triple on?
> Trek 5500. But see above - any frame with similar geometry should benefit from the fix, IMO.
> > I will definitely check out your idea re: xtr bb. Would any shimano bb do the same thing -- or
> > something special about xtr?
> Only XTR 116.5 will do. The other MTB BBs have a different spline. All the road triple BBs have
> the same offset as the one you already have.
> > It seems like moving the middle and small ring in a bit would do the trick.
> I estimate that, to eliminate the problem without chainging the middle ring, you would have to
> move the chainline in about 8mm. Even if your frame had that much clearance (which it doesn't),
> that would mess up the chainline from the small and middle rings to the small sprockets.
> > What do you think about a spacer under the middle ring bolts to bring both the middle and small
> > cog inward?
> A small spacer (0.5mm - 1mm) won't help. Even a small spacer may spoil shifting and/or cause
> chainsuck. Any significant extra space where you describe would give the risk of dropping the
> chain in between the big and middle rings (and you *really* don't want that to happen). Plus
> you'll run out of thread on the chainring bolts and end up with a severely weakened crankset.
> Don't go there!
> > Seems the front d has the necessary range to cover a couple of extra mm of inward movement onto
> > middle and small ring.
> It does. These FDs are designed to work on a variety of seat tube diameters, so they are quite
> tolerant of lateral displacement relative to the rings.