Road Bike, Cycling Forums banner
1 - 20 of 23 Posts

·
Registered
Joined
·
69 Posts
Discussion Starter · #1 ·
If a cable is fixed at both ends, i.e. at the bolt on the rear derailleur and at the shifter, and free to move between these fixed points to achieve movement of the derailleur by pulling/releasing the cable, how can the adjustment barrel possibly move the derailleur?

The cable passes through the adjustment barrel screw, just as it passes through the cable housing and bottom bracket guide, and must slide freely.

The adjustment barrel screw pushes on the cap to the cable housing but the cable itself still passed through both and must slide freely.

Someone please demistify the illogical adjustment barrel, solve the conundrum, and restore sesibility to the cycling universe.
 

·
Registered
Joined
·
22 Posts
Think of adjusting the length of housing not the cable. When using a barrell adjuster you are essentially changing the housing length and leaving less or more bare cable exposed. Hope that helps.
tdxloki said:
If a cable is fixed at both ends, i.e. at the bolt on the rear derailleur and at the shifter, and free to move between these fixed points to achieve movement of the derailleur by pulling/releasing the cable, how can the adjustment barrel possibly move the derailleur?

The cable passes through the adjustment barrel screw, just as it passes through the cable housing and bottom bracket guide, and must slide freely.

The adjustment barrel screw pushes on the cap to the cable housing but the cable itself still passed through both and must slide freely.

Someone please demistify the illogical adjustment barrel, solve the conundrum, and restore sesibility to the cycling universe.
 

·
BikerFox Wannabe
Joined
·
696 Posts
Barrel adjusters

tdxloki said:
If a cable is fixed at both ends, i.e. at the bolt on the rear derailleur and at the shifter, and free to move between these fixed points to achieve movement of the derailleur by pulling/releasing the cable, how can the adjustment barrel possibly move the derailleur?

The cable passes through the adjustment barrel screw, just as it passes through the cable housing and bottom bracket guide, and must slide freely.

The adjustment barrel screw pushes on the cap to the cable housing but the cable itself still passed through both and must slide freely.

Someone please demistify the illogical adjustment barrel, solve the conundrum, and restore sesibility to the cycling universe.
Barrel adjusters effectively lengthen or shorten the cable housing length thus creating tension or slack in the cable respectively. Very logical. The barrel adjuster itself does not move the derailleur, but if, for instance, you unscrew it and create more cable housing length you will effectively tighten or tension the cable and that can pull on the derailleur.
 

·
Registered
Joined
·
29 Posts
very timely

OK so this is really timely.

I'm just putting together a Cannondale with these things. Where is the best place to positon these? Close to the brase on's, Close to the lever, or close to the derailuer?

Thanks.
 

·
Registered
Joined
·
69 Posts
Discussion Starter · #5 ·
zero85ZEN said:
Barrel adjusters effectively lengthen or shorten the cable housing length thus creating tension or slack in the cable respectively. Very logical. The barrel adjuster itself does not move the derailleur, but if, for instance, you unscrew it and create more cable housing length you will effectively tighten or tension the cable and that can pull on the derailleur.
Ok, we are getting close but I still believe in magic.

The derailleur must move when the barrel is turned - at least the jockeys move toward or away from the wheel. Maybe it also rotates forward and backward as the cable is shortened or lengthened at the point where it attaches to the frame.

For someone as thick as me, I can understand your explanation about 'effectively lengthen or shorten the cable housing' but the derailleur must move - no?

The cable itself is the same length and still slides through all cable housing and adjusting barrels.

To make it even more mystical, consider the adjustment barrel on the downtube. Does the shifter/brake lever also move somehow? Following the cable housing theory, something moves, shifter or derailleur.

What do you say?
 

·
BikerFox Wannabe
Joined
·
696 Posts
Nothing moves...

tdxloki said:
Ok, we are getting close but I still believe in magic.

The derailleur must move when the barrel is turned - at least the jockeys move toward or away from the wheel. Maybe it also rotates forward and backward as the cable is shortened or lengthened at the point where it attaches to the frame.

For someone as thick as me, I can understand your explanation about 'effectively lengthen or shorten the cable housing' but the derailleur must move - no?

The cable itself is the same length and still slides through all cable housing and adjusting barrels.

To make it even more mystical, consider the adjustment barrel on the downtube. Does the shifter/brake lever also move somehow? Following the cable housing theory, something moves, shifter or derailleur.

What do you say?
...if the cable is already so slack that screwing out the adjuster doesn't take up enough slack. Stop thinking of things "moving" when the barrel adjuster is turned and realize that things move when the cable pulls or releases them. (And the barrel adjusters effect cable tension.) Barrel adjusters are a simple five minute "lesson" at any bike shop that is customer friendly. Ask your LBS for a demonstration.
 

·
BikerFox Wannabe
Joined
·
696 Posts
Don't use 'em

Bluesref said:
OK so this is really timely.

I'm just putting together a Cannondale with these things. Where is the best place to positon these? Close to the brase on's, Close to the lever, or close to the derailuer?

Thanks.
On my bikes (Campy equiped) I don't use the "cable insert" type adjusters. There's enough adjustment in the barrel adjuster on the RD and I just set the FD correct initially (by pre-strecthing the cable, etc) and don't need one. I think they're ugly and not needed and add weight. (On frames with downtube adapters they are at the frame boss adapter by default.)
 

·
Registered
Joined
·
491 Posts
over thinking

I think your over thinking the problem. Barrel adjusters are simple, they simply add or subtract tension from the cable. If you start turning the barrel adjuster all the way out you increase cable tension, and this increase in cable tension cause the der to move ever so slightly in one direction, which is how use can use barrel adjusters to adjust shifting on derailuers, in allows fine tunning and is not made to take up tons of cable slack. Hope this helps
 

·
Registered
Joined
·
69 Posts
Discussion Starter · #9 ·
The ends are fixed points.

Thanks for the replies everyone.

I comprehend the fact that adjustment barrels work and I have no problems with the rear derailleur shifting on my bikes.

However, I dare to be a pain and overthink this vexing issue.

The ends are fixed points and cable tension increases as those fixed points are pushed further from one another.

See if you all agree with this (sorry no diagrams):

Consider the rear derailleur attached to a frame, no chain, no cables. The bolt that attaches the derailleur to the frame is a pivot point - it can rotate forward and backward. Now attach a cable. Turning the rear barrel lenghtens the cable by pushing the derailleur forward which is in turn translated through the other joints of the derailleur to effect the jockey position. The shifter never moves, only the rear derailleur, so when the downtube barrel is turned it has the same effect again by moving the derailleur.

Cool.
 

·
Fini les ecrase-"manets"!
Joined
·
9,416 Posts
tdxloki said:
See if you all agree with this (sorry no diagrams):

Consider the rear derailleur attached to a frame, no chain, no cables. The bolt that attaches the derailleur to the frame is a pivot point - it can rotate forward and backward. Now attach a cable. Turning the rear barrel lenghtens the cable by pushing the derailleur forward which is in turn translated through the other joints of the derailleur to effect the jockey position. The shifter never moves, only the rear derailleur, so when the downtube barrel is turned it has the same effect again by moving the derailleur.

Cool.
Umm, what? I think it's good that you've got an explanation you can live with, but I don't really get it.

The way I look at it is that the cable housing is like a movable third point the cable goes over.

Imagine holding a cable by the ends (one end in each hand). A friend puts a finger under the middle and lifts up. This increases cable tension--because the cable has to go farther to get over your friend's finger (straight line is the shortest distance), you either have to let out more cable or move your hands toward each other. And your friend can move his finger back and forth, but as long as he keeps it the same height, the tension remains the same (discounting friction, of course).

Now, thread a housing over this imaginary cable. Now your friend is pushing the ends of the housing toward each other (which is what the barrel adjusters do). Because the housing isn't compressible, the middle of the housing will bend and rise into a hump. Though it's not exactly the same as a finger creating a third point in the cable, the cable now has to go over the hump created by the housing. Cable tension will rise because going over the hump is farther than the straight line you started with. And you can slide the housing back and forth on the cable, just as in the first example.
 

·
Registered
Joined
·
21,635 Posts
My head hurts!

tdxloki said:
However, I dare to be a pain and overthink this vexing issue.
Man, you can say that again. We're all sorry that you can't understand this simple concept, and we just hope that you don't have any other "misunderstandings" that you want to share - like why does the contact patch get smaller when you raise tire pressure or some such. All I can say is: Wow! :)
 

·
Banned
Joined
·
2,767 Posts
Well, now that we know the answer to the barrel adjuster dilemma, maybe someone can help me with this:

I'm running Campy Record 10, and I think my RD (short cage) has a busted jockey wheel. See, one spins clockwise, but the other spins counterclockwise. Has anyone seen this before? If I put a reverse threaded screw in the one that spins counterclockwise, can I get it to spin clockwise? What if I ride on the equator: will the jockey wheels spin at all?
Does this happen with Shimano RDs? Or is this a quirk of Eyetalian componentry? Help, please. Nothing I've done so far seems to fix the problem.
 

·
eminence grease
Joined
·
18,538 Posts
You're looking at it from the wrong vantage point. If you go around to the other side of the bike, the wheel motion is reversed. From there, you will be able to see the wheels moving in their proper directions.
 

·
Registered
Joined
·
234 Posts
OK,anybody ever get this one....

On more than one occasion I have tried to explain to my riding buddies that you can't eliminate tire rub at the brake shoes by selectively tightening one side of the quick release or the other, as though doing so spaces the tire in one direction or the other. I cannot convince them it doesn't matter which side of the quick release you turn, the result is the same. All I can imagine is that in twidling the QR, you might raise one side or the other in the dropout, and thus affect the position of the wheel.
 

·
Banned
Joined
·
2,767 Posts
terry b said:
You're looking at it from the wrong vantage point. If you go around to the other side of the bike, the wheel motion is reversed. From there, you will be able to see the wheels moving in their proper directions.
Ok. I tried that. It worked for one jockey wheel, but now the other one is spinning counterclockwise. Maybe if I use a mirror so I can see both sides at the same time......
 

·
Banned
Joined
·
2,767 Posts
elvisVerde said:
On more than one occasion I have tried to explain to my riding buddies that you can't eliminate tire rub at the brake shoes by selectively tightening one side of the quick release or the other, as though doing so spaces the tire in one direction or the other. I cannot convince them it doesn't matter which side of the quick release you turn, the result is the same. All I can imagine is that in twidling the QR, you might raise one side or the other in the dropout, and thus affect the position of the wheel.

No can do. If your friends found out that was the case, then they'd go back to worrying what would happen to 'em if the Earth stopped spinning. Don't want that.
 

·
eminence grease
Joined
·
18,538 Posts
alienator said:
Ok. I tried that. It worked for one jockey wheel, but now the other one is spinning counterclockwise. Maybe if I use a mirror so I can see both sides at the same time......
You might try one of those special composite mirrors, the one made up of the two mirrors mounted to form a right angle. I know they provide the correct orientation of you face when you look in them, I'm sure they will work for the jockey wheels.

Barring that, have you thought about lying on your back and looking up at the derailleur? I wonder which way they turn in that situation?
 

·
Banned
Joined
·
2,767 Posts
terry b said:
You might try one of those special composite mirrors, the one made up of the two mirrors mounted to form a right angle. I know they provide the correct orientation of you face when you look in them, I'm sure they will work for the jockey wheels.

Barring that, have you thought about lying on your back and looking up at the derailleur? I wonder which way they turn in that situation?

Ok. I tried that, laying on my back. I couldn't see the jockey wheels, so I guess it's fixed. Thanks.
 

·
eminence grease
Joined
·
18,538 Posts
Glad to help.

Now I have a question - how come when I spin my wheel under the neon lights in my Bike Haus the spokes go backwards while the wheel spins forward? Shouldn't that break the wheel?
 
1 - 20 of 23 Posts
Top