Originally posted by Barnaby
Hi Pokey-Thanks for the reply. As to the Sierra Guide thing, I think they commissioned the book from the author Raymond Bridge. He is a seasoned cycle tourist and the author of 10 outdoor-related books. The book is a bit seasoned too, his references are to 5 and Ultra 6 speed set-ups, but his section on the drive train is in depth-35 pages.
His recommendation is:
46/42-14,17,20,24,28 for the five-speed
He likes this because the shifts are (eliminating the extreme two) as follows low to high:
6.8,4.5,4.9,5.4,4.6,15.6 (which can be spit if you want to use that last gear into 7.9 and 7.7 inches.
-the other jumps are almost perfectly spaced
-the high gear is about right for touring
-the 40 inch low is about right for a conditioned rider in the mountains.
-the first two shifts do not require a double, so no hesitation on the hills
-After that every second is a double, so easy to remember.
He sees these reasons for not going to a 3-rings:
1-the crank axle is longer
2-the chain-line is moved out, so that unless the frame is beefed up, there is more whip.
3-" The tension on the right side...between the crank axle and the rear whell axle is given increased leverage by the longer dimension."
4-The angle of the chain is greater from either chain wheel
5-The extreme combination gears are worse than in the 2 chain wheel set-up.
6-The largest chain wheel is likely to wear quickly.
7-You must use a longer and heavier chain
8-You must use a large capacity rear derailleur which is less precise in shifting, and heavier.
On the other topic though if I order new cups will I see an improvement in swaping a 125 mm free axle unit weighing 305 grams with a 110 mm sealed unit weighing 233 grams ( cups included), in terms of stiffness and Q-factor?
I think the stated reasons for 2 chainrings vs 3 is mostly applesause.It's an OLD book and things have changed.There are now triple axels that are shorter than many of the old doubles.Shimano splined spindles are also stiffer. Frames are stronger.Don't use the extreme gears.The largest chainwheel wears least quickly.You co not need a longer heavier chain.You don't even have to use a long cage rear derailer. IT"S MOSTLY HOOEY!.....I think many would take exception to his 46/42 crankset and 28 tooth big cog for loaded touring in the mountains.I could not say how much improvement you will see since I don't know how much of a real problem you have especially with BB spindle flex. I use the shortest BB spindle I can get away with .There could be an improvement in shortening the spindle by 15mm,but your crankset and frame could be the bigger problem.