Do older derailleurs care how many speeds
 

I am hoping to put a 6 speed 105 derailleur on an 8 speed Shimano drivetrain. Sora 8 speed STI and 8 speed cassette.

I know that with higher number of gears, this probably won't work. I think maybe on the older ones, derailleurs were pretty much the same, just the shifters and cassettes changed.

What say ye?

If you have put it on and see does it work

In friction shifting, it doesn't matter much, as long as the derailleur can sweep the full width of the cluster. With indexed shifting, you have to be sure that the amount of cable pull each shift requires matches the travel of the derailleur, or you'll never get proper indexing.

Shimano has changed the cable pull specs between models and over the years. I'm not sure that a 6-speed 105 derailleur uses the same cable pull as an 8-speed STI lever.

I have an older downtube shifting bike in 105 as a gift for my wife. Once I hand it over, I assume she'll want STI.

So I'm just thinking ahead about what that may take.

Cassette, new wheel with cassette body hub, respace rear triangle, and shifters. Not sure if I'd need a new derailleur.

Maybe she'll love 6 speed downtube shifting. I do.

Shimano 6 through 9 speed derailleurs, and 10 speed road derailleurs, all have the same 1.7:1 actuation ratio, so if you match the shifter and the cassette speeds you should be OK. Science Behind the Magic | Drivetrain Compatibility ? Art's SLO Cyclery https://bike.bikegremlin.com/1278/bi...compatibility/

Shimano has never, to my knowledge, stated that their past ders are forward compatible. This is much of what i have found when working with indexed systems. Andy

If it's a 105 RD-1050 which was an index derailleur it should be fine. Note that there is a difference between a "6 speed derailleur" and a "derailleur from a 6 speed" .

VeloBase.com - Component: Shimano RD-1050, 105

The rear derailleur probably doesn’t matter, but the front derailleur can be tricky. This is a 600 FD6401 I happen to have right in front of me:

https://cimg1.ibsrv.net/gimg/bikefor...52a8d54a7c.jpg
I unscrewed the cable clamp-down screw so you can see that there are two grooves you can set your shift cable in. The one closer to the pivot is for STI , and the one further away for down tube shifters. So clearly Shimano had changed the pull ratio right around this time. Your 1050 predates STI’s so I don’t know what pull ratio it has.
The 6400 series also predates STI. When they introduced STIs to the 6400 group, Shimano has to make a new FD6401 front derailleur. The cage , if I understand correctly, is narrower and has different contours.
TLDR: get the correct FD to go along with whatever STI you are running.

Nothing ventured nothing gained. Especially if you already have all the components. My bet is that it will work.

My guess is that it will shift and seem to be working on the stand, but on the road it'll be finicky and frustrating. There will always be at least one gear that is noisy.

Is your wife going to ride it if she's always having problems with the gears? Or will she just give up? I know what's happened when I've tried this.

That was sort of my experience. I used a Suntour V-GT for about a year for indexed 7-speed shifting. It would miss a shift every so often, but the missed shift was unpredictable. Sometimes the miss would be an upshift, sometimes a downshift. I missed shifts on every cog, but most shifts worked essentially perfectly. The one consistent dysfunction was that the 2nd and 3rd smallest cogs were noisy on the road.

All great ideas.

She's not much of a cyclist but a pretty serious runner. She's got a 650c Motobecane that works but that size is getting rarer and for some reason, is always a pita to fix flats.

Anyway, I set her up with the bike trailer while the car was in the shop and I was at work. Since then, I've found her bike in the driveway nearly every day after work. So she's getting to like it, or kids napping, or something.

This corresponded with me stumbling over a Centurion Ironman in her size. The full 6 speed restore will be done this afternoon. It's prettier, cooler, probably smoother, somehow lighter, and has the ubiquitous 700c tires. I think it's a keeper.

Maybe she'll love the bike as it is, I do. I'm just think aloud.

I'd probably buy her new parts if she wanted to convert, not pillage the Moto, as that would render it un-sellable. So it's not as simple as putting it together and seeing if it works.

I'll include a new front and rear derailleur in my mental budget if she wants to go this route.

I didn’t follow exactly what front and rear derailleur you now have. However since you are going to swap out both derailleurs, now is the time to figure out what cassette would be best for your wife.

If she needs a wider range of gearing for hills, then get a derailleur that will support a larger max cog. I set my wife up with a 12-34 cassette so she would have enough range.

My wife used downtube index shifters on an 80’s Univega for years but that doesn’t translate into rider strength. STI brifters just made it easier, and safer, for her as the years have gone on.

John

I have an old custom running ST-7400 Sti brake/shifters with a FD-7400, RD-7401, 11-28 Shimano cassette and it is quite. You should have no worries.

If you already have it then put it on and find out. Shouldn't take that long to get the only answer that matters and then put it back the way is was if it doesn't work.

This thread is essentially closed at this point. I think I've got my answer.

I was clear in my follow-up posts that I do not have the parts. I'm making a list of what I might need if my wife wants STI.

I've got it running now and working so nicely with 6 speed downtube shifting that I hope we're good, at least until she catches the bug.

8 speed STI will work perfectly, so long as the shifters aren't Dura-Ace, since 6-8 speed Dura-Ace rear derailleurs had a unique actuation ratio that only DA levers work with. That caveat aside, the 6-speed 105 will index just as well with an 8 or 9 speed STI shifter as it did with the 6 speed downtube levers it came with.

Front derailleurs can be trickier, so just get one that came from an STI group. The 9-speed 105 is a great choice here... dirt cheap, pretty, shifts great, and about as reliable as a shovel. Actually, that's true of 105 parts in general, and has been since 105 made the "group bike" a thing back in 1987.

--Shannon

If it's reasonably flat where you live, and you've chosen the optimal set of gears, six speeds may be fine.

Its not about the speeds, never was.

If you switch to STI, 8 speeds is your minimum. Sure you could set your stops to use only 6 (5) of the clicks but what fun is that?

I never asked about gearing or climbing or how to maintain a cadence up a friggin hill. I find it very difficult not to get angry at responses to things I didn't ask. Probably why I quit teaching 15 years ago.

Brand new 7 speed sti
https://bike.shimano.com/en-EU/produ...rney-a070.html

I'll bet your students were pretty happy when you quit, too.

By the way, this is your statement to which I was responding:
"'I've got it running now and working so nicely with 6 speed downtube shifting that I hope we're good, at least until she catches the bug."
My response:
"If it's reasonably flat where you live, and you've chosen the optimal set of gears, six speeds may be fine."

I think your anger is misplaced, albeit still a bit sad.

Agree. Rosefarts you owe him an apology