slowster wrote: ↑16 Jan 2023, 10:06pm
If you replace the 10 speed cassette with an 8 speed one:
a. You might have issues with the chain, depending upon what speed chain you choose. I suspect that the extra width of an 8 speed chain might not suit a 10 speed rear derailleur, although you might be able to use washers to increase the space between the jockey arm plates to address that.
An 8 speed chain will not in my experience work well with a 10 speed front derailleur - the extra width means the chain will be very prone to rubbing on the inside of the cage in many gears, and you would probably find yourself continually making fine adjustments with the DT lever to stop the rubbing after making a rear shift.
My guess is that a 10 speed chain would be too narrow for an 8 speed cassette, and I suspect that might also apply to a 9 speed chain.
b. If you are considering that 11-34 8 speed cassette because you currently have an 11-34 10 speed cassette, I think you are likely to be disappointed. The big benefit in my opinion of the 10 speed cassette is the 2 tooth gaps in the middle of the cassette. The 8 speed cassette has 3 tooth gaps starting from the 15t sprocket. For many people, including me, those 3 tooth gaps are too large for comfort.
I suspect Bice's problems with 10 speed friction shifting are to a significant degree down to:
i. A ladies' frame which is probably more flexible than your Galaxy, i.e. 'pull-back' will be much worse,
ii. Use of a rear derailleur with a much smaller cable pull ratio than your mech.
iii. A Suntour Powershift lever with a small barrel of a size suitable for 6 speed, not 10 speed.
Your 10 speed XT derailleur requires 3.4mm of cable pulled to move the arm the distance required between two sprockets on a 10 speed cassette. In contrast a 10 speed road rear derailleur requires only 2.3mm, a 9 speed derailleur only 2.5mm and an 8 speed mech 2.8mm.
In other words, a 10 speed MTB rear derailleur will be much more tolerant of the set up being less than perfect and of any cable movement. It just needs a shifter that has the capacity to pull 3.4mm x 9 = 30.6mm. The Dia Compe friction shifter I linked to has that and a longer lever as well to suit.
That all said, I doubt that you have supplied all relevant/correct information. If you have 10 speed down tube levers and a 10 speed MTB rear mech, your levers should not pull enough cable to change from smallest to largest sprocket, whereas you state that the lever pulls enough cable to move from smallest to largest in 7 clicks.
I described the Diamant ladies' frame build and the 10-speed v 8-speed here:
viewtopic.php?p=1705713#p1705713
(I did use a 10-speed and then an 8-speed chain.)
I have found no problem with shifting an 8-speed chain with a 10-speed Ultegra FD-6503 front derailer, but because of a braze-on on this bike, the shifter is set far higher than I would normally do: Peetee reported that it would work fine and has same sort of set-up, and so it does. Perhaps setting it higher may help accommodate an 8-speed chain, but it just hasn't been an issue.
On the rear, for both the 10-speed experiment and the current 8-speed success, I am using an Acera X which came off a 1990s 7-speed MTB. (The bike is in the attic for the winter and I can't read the part number.)
As I explained, the Suntour ratcheted downtube shifters had no issue shifting up and down a 10-speed block, and are fine with 8-speed; they shift the front derailer fine as well.
I toured with this set up over 300 miles in the summer with no issues: I was with my club, all on light road bikes through Hants, Dorset, Wilts, so I was shifting a lot in order to keep up and I was carrying 14kgs in the panniers (far more than the others).
I have no idea what would be the cable pull on the Acera to shift from one gear to another: the joy about friction downtube shifters is that you do not need to worry about any of that. Just be sure the levers pull enough cable to get you all the way from the smallest up to the biggest cog on the block.
I, too, would expect the Diamant ladies frame to flex a bit, so there may be 'pull-back', although it feels solid compared with an ultra lightweight steel road bike.
Quite a few people who have used friction downtube shifters on 10-speed have not got on with them, as they do require precision.