Road Cycling - Shfiting Question

Currently I have a Shimano 105 setup with a triple crankset and a 9 cog rear cassette. I notice that if I am in the largest cog on the crank and the largest cog on the rear cassette, which I believe is the gear of least resistance for that cog on the crank I hear a clicking noise as if the chain is slightly touching the front derailer. I aksed my LBS about this and they said I should not ride in that gear and shift down to the middle cog on the crank. Thier reasoning was that even though it's a 27 speed bike there are only about 20 speeds and 7 duplicates.... (estimate). The LBS recommended that I only use the first 5 or 6 gears on the rear cassette per cog on the front crank? Does this sound right to you? Would a better Shimano set do better? For expample, Dura Ace.... Anyway any thoughts on this would be appreciated.

Scott


Hmm, if I could spell shifting..... Sorry

You should not be riding in the big/big or small/small combinations. As the cogs got wider to accomodate more gears the chain got pulled to angles that create front derailleur rub. The LBS is correct and upgrading to a different groupset would not fix the problem.

No. It is generally accepted that cross gearing (big-big, small-small) is bad for the drivetrain. The issue is the amount of chain deflection. On modern roadbikes with even tighter chainstays this becomes more of an issue. A triple only exagerates the situation. Also, if you have to bail out of that front big chainring to the middle or inner ring, while attempting to power up a hill, you might get a nasty surprise. The amount of deflection and tension on a chain will be sufficiently high that when you drop the derailleur, the chain will go flying laterally and you'll end up throwing the chain... oftentimes with some rather uncomfortable side-effects. This will be true for any group. I run Dura-Ace and I get rubbing in cross-geared combos. It's more a matter of chainstay length, deflection and teeth count not quality of the group.

I see a lot of people mashing away on a 3x9 system with the front on 3 and the back on 2nd or 3rd and a horrible rubbing noise, I can usually hear them blocks away.
If you look at how the chain is in relation to the orientation of your frame, it looks like this:
Frame Chain 3x1
|------------|
|-----------/
|----------/
|---------/
|--------|
Or Vice versa: 1x9
|--------|
|-------- \
|----------\
|-----------\
|------------|
When it SHOULD look like this 3x5-9.
|-------------|
|-------------|
|------------/
|------------|
|------------|
What happens is first and foremost, your chain's running crooked, VERY VERY crooked and the chain's not only rubbing against the front der. but also the rear der. cogs and your cassette cog teeth too which would wear down your drivetrain pretty quick.

Good points. I've noticed the same thing and try not to go big/big, small/small very much, if at all.

I have had that nasty surprise a few times and always wondered why... Well now I know! Thank you all very much... esp. svloid who offered the visual... I wonder if he can do a woman with ascii art? ;)

Scott