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Rear Derailleurs
#1
I'm considering upgrading my Shimano Tourney 6/7 speed rear derailleur but I'm confused about them.

What makes the difference between a MTB and a road derailleur?

Derailleurs come in various speeds, but aren't they dependent on the shifter? I mean, if a derailleur is 9 speed, won't it work on a 7 speed freewheel or cassette? I read, somewhere, that it doesn't matter what speed it is because it will go where the shifter puts it.

My chainring is 44T but I read that some derailleurs won't handle that. Why not? If the chain is the correct length, what does the derailleur care about the number of chainring teeth?

I'm not having any problems with the Tourney 6/7 speed on my cruiser; I just like to change things. Is there anything to gain by putting a Deore on it?

Perhaps I'm comparing apples with oranges by comparing my Specialized Fatboy Sram X5, 10 speed derailleur with my cruiser Shimano Tourney 7 speed derailleur. I think the 10 speed is smoother changing. This is why I'm considering an upgrade on the cruiser.

Looking at some of the derailleurs and reading the answers to questions, one guy compared two Altus; one had two 13T jockey wheels, and the other had a 13T wheel at the top, and a 15T wheel at the bottom. He said the 15T is quieter. Is he correct or just BSing? I just can't see that he can tell it's quieter.
If I knew how to ride a bike properly, I'd do it every time.
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#2
Road vs. MTB is a matter of tooth capacity, both the maximum teeth in the rear and the maximum difference between smalles/smallest and largest/largest #of teeth. Starting at 10 speed, cable pull per gear (mechanical advantage) differs as well.

The #of teeth in the jockey wheels makes some difference, apparently, tuning and marketing seem to tell us Wink (I don't know...)
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#3
(05-02-2020, 01:27 PM)Joe_W Wrote:  ... Starting at 10 speed, cable pull per gear (mechanical advantage) differs as well.

The #of teeth in the jockey wheels makes some difference, apparently, tuning and marketing seem to tell us Wink (I don't know...)
I get that a 10 speed cassette would need a 10 speed shifter because of the cable pull. But the derailleur doesn't care, does it? I mean, if the derailleur is for a 9 speed, and I use it on a 7 speed freewheel, as long as the shifter is for a 7 speed, it should work.

As for the jockey wheels, I can't hear mine when I'm riding. I guess there's a reason for a 13T and/or a 15T. It's too technical for me.
If I knew how to ride a bike properly, I'd do it every time.
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#4
The derailleur cares (nowadays) :the mechanical advantage was changed. The last thing to work (with Shimano and I think SRAM) is a 10spd road shifter with anything road (up to 10spd) and MTB derailleur up to 9 speeds. The ratio of cable pulled vs. derailleur movement is different starting at 10spd MTB and 11spd road. After that nothing is compatible any more across speeds and road vs MTB. I hate that...
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#5
(05-05-2020, 11:00 AM)Joe_W Wrote:  The derailleur cares (nowadays) :the mechanical advantage was changed. The last thing to work (with Shimano and I think SRAM) is a 10spd road shifter with anything road (up to 10spd) and MTB derailleur up to 9 speeds. The ratio of cable pulled vs. derailleur movement is different starting at 10spd MTB and 11spd road. After that nothing is compatible any more across speeds and road vs MTB. I hate that...
I'm still in a fog. Here's the way I see it. Let's say the cogs on a 7 speed freewheel are 4/16th from center of cog to center of next cog, or from outside of cog to outside of next cog. The shifter has to pull the cable to move the derailleur 4/16th.

If a 10 speed freewheel has cogs 3/16th from outside of cog to outside of next cog, the shifter for the 7 speed is going to move the derailleur too far. The pull is too much. So the shifter has to match the cogs spacing. (Besides that, the shifter is only indexed for seven gears, so the derailleur won't span all ten cogs.)

If I took the derailleur from a 10 speed freewheel, and put it on a 7 speed freewheel., provided the shifter matched, I don't see why that won't work. I'm just speaking theoretically. The derailleur isn't going to 'know' how far it needs to move the chain because that's controlled by the shifter.

I don't mean to cause confusion, so I don't blame you for not continuing this thread.

Part of my original post implied quality/price of derailleurs. What will a $50 derailleur do that a $20 one won't?

Never mind. I found the answer about quality.
If I knew how to ride a bike properly, I'd do it every time.
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#6
Well, it is about how far the derailleur moves per mm of cable pulled. And that has changed, the proportionality that you assume (which was indeed the case until lately... 10 years ago Wink ) is just no longer there any more. It used to be a factor of 1.7 (cable pulled vs. derailleur distance moved) and now it is quite different.

I found this web page with the actual numbers (plus it is another explanation, that might be helpful)
http://blog.artscyclery.com/science-behind-the-magic/science-behind-the-magic-drivetrain-compatibility/

Quality wise: my sweet spot is around the 105 for road. Old stock (2 - 3 years old), so it does not break the bank. The super light stuff is not sturdy enough for me, for me as a commuter (and I am not racing any more).
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#7
(05-06-2020, 03:31 AM)Joe_W Wrote:  Well, it is about how far the derailleur moves per mm of cable pulled. And that has changed, the proportionality that you assume (which was indeed the case until lately... 10 years ago Wink ) is just no longer there any more. It used to be a factor of 1.7 (cable pulled vs. derailleur distance moved) and now it is quite different.

I found this web page with the actual numbers (plus it is another explanation, that might be helpful)
http://blog.artscyclery.com/science-behind-the-magic/science-behind-the-magic-drivetrain-compatibility/

Quality wise: my sweet spot is around the 105 for road. Old stock (2 - 3 years old), so it does not break the bank. The super light stuff is not sturdy enough for me, for me as a commuter (and I am not racing any more).
I appreciate the information, Joe, and I've bookmarked the page so I can study it further. I didn't realize how technical these things can get. What I didn't get was the pull factor. It seemed logical that if the shifter pulls 3mm the derailleur will move across 3mm rather than 5.1mm with a pull factor of 1.7. I think I got that right.

As for quality, the low end derailleurs suit me. I'm not into anything other than casual riding; four miles a day with the fat bike and four miles a day with a cruiser, with the occasional 16 - 20 mile rides.
If I knew how to ride a bike properly, I'd do it every time.
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#8
Yup, full of surprises the world is - and this one would catch one pants down. Especially since it was simple in the olden days, when we were younger... Safe riding!
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