Originally Posted by
mstateglfr
Instead of going from 42/18 and only dropping to the 28t ring in preperation for a hill, why not drop to the 28t ring AND change the rear cog down to 14 and you will maintain speed at 110rpm.
There, hypothetical problem solved with a couple simple shifts instead of just one simple shift.
The shifts you propose aren't all that simple. Shift down the front first and you are still in a high RPM situation until you make the two shifts on the rear. Shift up on the back and you're now in too high a gear
and the front derailer might not make the shift because the torque is too high.
Trying to shift on the front and rear at the same time, much less shifting twice on the rear is fraught with possible problems as well. Too many moving parts doing too many opposite things is asking for the chain to drop off the front...which really kills momentum...or asking for rear derailer to wrap around the axle which really, really kills momentum.
Originally Posted by
mstateglfr
Reality time- I would most likely accept i won't go 18 up a hill and shift later than what you find acceptable. This would prevent the absurd 'need' to spin at 140rpm and would also prevent me having to spin at 110rpm.
Again the problem is torque. Delay too long with the large chainwheel difference and there is a good possibility that the chain won't drop in the front. This has always been a problem in mountain biking. If you delay the shift too long, the spring on the front derailer just isn't strong enough to knock the chain off and you grind to a halt.
Even if you do happen to make the shift, the very large difference in the gears has to be dealt with.