M1T

Long story as to why I would like to do this, but can my 9 speed D.T. shifter work with a Dura Ace 7900 10 speed rear derailleur?

Davet

Since the overall spacing of 9 and 10-speed cassettes are the same overall width, your 9-speed downtube shifters will work but with the rear shifter in the friction mode only.

M1T

OK, was kind of afraid of that, but thanks for answering the question. Was hoping for an 'alternative cable routing' type solution/answer, but reality is what it is. Thx, Tony

rccardr 

Yes, it will work to index shift a nine speed cassette.

curbtender

So, are you keeping it 9 speed or going to a 10 speed cassette?

Leisesturm

I would imagine it is staying a 9sp. The RD is changing to 10sp. There should not be a problem. I see no reason to have to go to friction mode.

delbiker1 

The only reason I can see to use friction mode is to put a 10 speed cassette on, all ten available if friction. I found the shifting a bit fussier going into to higher gears, but less so the higher the gear. Also, slowing down the cadence helps the shift. If you ride it enough, the shift spacing becomes pretty much automatic. I am certainly not saying it is worth it to go from 9 to 10. I actually did it with original 6 speed indexed dt levers and rear derailleur. I no longer have that set up due to my preference for dual controls. I did not ride the bike enough with the dt levers.

KCT1986

You could try using "C" routing to see if it works reliably enough. Will only shift 9 of the 10 cogs though, since this is the # of positions the shifter has.



For the RD-7900, you'll need to get a different tabbed washer, either with 1 tab or 2 tabs 90* apart. Tab at green arrow is required, blue optional. Effective thickness at green arrow will determine how well it will index.

M1T

I want to keep the (new) 9 speed cassette ... and repurpose the 9.5 of 10 rear derailleur. Not the end of the world if this all doesn't happen, but that would simplify things on this particular build. Thx for the insight and replies.

KCT1986

If you're keeping it 9 speed, the RD-7900 should be fine. Normal routing at the groove. My post #8 was for 10 speed spaced cassettes.

HillRider

Except for pre-9-speed Dura Ace, nearly all Shimano road rear derailleurs from 7-speed onward are compatible with any Shimano 7-speed to 10-speed road shifter and matching cassette. So your "10-speed" Dura Ace rear derailleur will function fine with your 9-speed shifters and a 9-speed cassette. No cable routing tricks are needed.

DiabloScott

In fact my 9-speed DT shifters are matched with a Shimano mountain bike shadow derailleur (SLX) and 9-speed MTB cassette for the bigger cogs. Shifting is snappy!

M1T

It dawned on me, and please correct me if I'm not accurate, that the shifters do all the heady work (i.e., they move the cable and RD in precise increments) and the RD, maybe other than a specific tension in the springs, is just the facilitator for moving the chain on the cassette. At some point, moving back and forth between older and newer systems, the chain width, the widths between the cogs and chainrings, come into play. So I understand why it will work... I'll know shortly... not too difficult to undo if it doesn't work.

DiabloScott

Well yes, (huge over-simplification here) but if the shifter moves the cable 1mm per click, that has to correspond to the derailleur moving one gear per mm of pull. Some derailleurs need 2mm per click to move one gear. So you can't put a 1mm per gear shifter on a derailleur that needs 2mm per shift. And you can't put a cassette that uses a different spacing between gears than the shifter and derailleur want to move. 9-speed (and up) Shimano and SRAM shifters and derailleurs and cassettes are all compatible with each other. An 8-speed DA shifter is not compatible with anything except an 8-speed DA derailleur, although I think the 8-speed cassette spacings are all the same.

So back to your original question - I think your 9-speed indexed shifters will work with your 10-speed derailleur if you use a 9-speed cassette, which others have said here as well.

M1T

OK, that was exactly what I needed...

So based on a couple key details you pointed out, I found this:

Shimano 6, 7, 8 and 9 speeds - Rear shift ratio is 1.7, that is for 1 mm of cable pull/release, RD is moved left/right by 1.7 mm. Shimano calls this 2:1 ratio for marketing reasons.

All these RDs are compatible and any 6 to 9 speed RD will work perfectly with either 6, 7, 8 or 9 speed shifter. Regardless whether it’s a MTB, or road shifter, or RD. They are also compatible with Shimano 10 speed road shifters, except the Tiagra 4700 series.

Shimano Dura Ace from 1984 to 1996 period (6 to 8 speeds) - Rear shift ratio is 1.9. They are compatible only with Shimano Dura Ace shifters from the same period (that is for 6, 7 and 8 speeds).

10 speed ROAD RD - Rear shift ratio is 1.7, so it’s compatible with all the 6 to 9 speed shifters, as well as road 10 speed shifters. END

So my 9 speed shifter will indeed work with my 10 speed RD / 1mm > 1.7mm respectively. Clarity... thanks.

curbtender

Ha, the thinking part is done...

M1T

It worked just fine. The 1mm to 1.7mm movement in the shifter / RD explanation (thank you to me and to the writer who supplied that somewhere online- ha) left me with indelible confidence that it would work. And work it did.

1985 Trek 760. Not a collectible bike, but a great rider, so I took some liberties with colors and more modern components.