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RD Question

Old 12-02-20, 05:14 PM
  #1  
Sjtaylor
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RD Question

Iím rebuilding an 80s bike. The RD that came off shows wear. I have an identical nicer one in my stash with a sheared stop pin. I might try to remove the broken off stud and rob the stop pin from the worn one. I also have a Shimano 600 RD that could be the replacement. Any thoughts?

Would the derailleur work without that pin?

Thanks.


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Old 12-02-20, 07:14 PM
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The stop pin is needed to keep tension on the spring inside the A knuckle. Without it, the arm doesn’t return to keep tension on the chain. Look at your two derailers above. The arm has to be behind the parallelogram or it just won’t work.

Where the stop pin comes from probably doesn’t make any difference. If it fits in the threads and is long enough, it will work.
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Old 12-03-20, 01:56 AM
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The stop pin (campy calls it a "grub screw) keeps the cage from unwinding when the wheel is removed. As long as the wheel and chain are mounted, the pin has no function; the chain keeps the tension.

But it's a kludge to use a rear derailleur without its stop pin. I'd use the best derailleur you have. In this case, it's probably the shimano 600.
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Old 12-03-20, 09:32 AM
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What's worn on the good DR?
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Old 12-03-20, 09:39 AM
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Originally Posted by SurferRosa View Post
The stop pin (campy calls it a "grub screw) keeps the cage from unwinding when the wheel is removed. As long as the wheel and chain are mounted, the pin has no function; the chain keeps the tension.

But it's a kludge to use a rear derailleur without its stop pin. I'd use the best derailleur you have. In this case, it's probably the shimano 600.
Thatís incorrect. The spring pulls the chain backwards against the crank. Without the stop screw, the spring will rotate the arm forward and make the chain slack. The chain wonít keep tension on the derailer. Look at Sjtaylorís picture. Without the stop screw the derailer spins toward the front of the bike.
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Old 12-03-20, 12:15 PM
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Originally Posted by cyccommute View Post
Thatís incorrect. The spring pulls the chain backwards against the crank. Without the stop screw, the spring will rotate the arm forward and make the chain slack. The chain wonít keep tension on the derailer. Look at Sjtaylorís picture. Without the stop screw the derailer spins toward the front of the bike.
Here's a pic of one of my derailleurs. I've circled the grub screw (stop pin). It has zero function when mounted with chain and wheel/cogset. Without chain, cogset and grub screw, the cage would unwind all the way. Without chain but with the grub screw in place, the cage unwinds only to the grub screw, which makes mounting and dismounting the wheel far easier. That's its purpose.



I ve only been overhauling derailleurs for a dozen years...
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Old 12-03-20, 12:46 PM
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Furthermore, if the grub screw is what is preventing your cage from unwinding when mounted on the bike (with chain and cogset in place), then your chain is too long and will be slack!
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Old 12-03-20, 12:51 PM
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Originally Posted by SurferRosa View Post
Here's a pic of one of my derailleurs. I've circled the grub screw (stop pin). It has zero function when mounted with chain and wheel/cogset. Without chain, cogset and grub screw, the cage would unwind all the way. Without chain but with the grub screw in place, the cage unwinds only to the grub screw, which makes mounting and dismounting the wheel far easier. That's its purpose.
On further reflection, I was mistaken on the direction of the spring, however, the stop screw serves much more purpose than you are giving it credit. While you might be able to thread a chain through an old Campy derailer with a weak spring, that canít be done on any derailer made since the Suntour Vx. You simple canít hold the spring tight and thread the chain through.

Running a derailer without the stop screw would be inviting disaster on shifts to the lowest ring on a triple as well as wrapping the derailer around the axle if the chain should fall off or making a tangle of the chain if you should ever need to remove the wheel. It would be even worse with modern derailers that have much stronger springs than Sjtaylor is using.

I donít care how many years youíve been working on derailers (40+ for me), running a derailer without the stop screw is just inviting disaster and very bad advice.
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Old 12-03-20, 01:14 PM
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Originally Posted by cyccommute View Post
On further reflection, I was mistaken.
We know. But you simply made the subject more confusing for the op.

While you might be able to thread a chain through an old Campy derailer with a weak spring, that can’t be done on any derailer made since the Suntour Vx.
They're no weaker than Suntour, although Suntour usually gives you more options for spring tension. Only one time did I run a derailleur without its grub screw; it was with a Suntour ARX.

You simple can’t hold the spring tight and thread the chain through.
If removing a wheel to fix a flat, for example, the chain will already be laced through the cage. All that will be needed is to pull back the cage when mounting the wheel.

running a derailer without the stop screw is just inviting disaster and very bad advice.
But not as bad as not knowing how it works in the first place.

And I didn't advise it. You can read post 3 again if you're still confused. I​​​​​ did advise using the Shimano 600.
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Old 12-03-20, 05:07 PM
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Originally Posted by SurferRosa View Post
We know. But you simply made the subject more confusing for the op.
So you never make a mistake? Like saying that running a derailer without a stop screw isnít going to be an issue?

They're no weaker than Suntour, although Suntour usually gives you more options for spring tension. Only one time did I run a derailleur without its grub screw; it was with a Suntour ARX.
Suntour is the next generation from the Campy and their springs are stronger than the Campy. The derailers in Sjtaylorís post are a few generations along from the Suntour Vx and are even stronger. As mountain biking got going, they needed stronger springs to keep chain slap down.


If removing a wheel to fix a flat, for example, the chain will already be laced through the cage. All that will be needed is to pull back the cage when mounting the wheel.
Does Sjtaylor say that the chain is laced through the cage? From what I can see the derailer is off the bike so, without the stop screw, the derailer would have to be wound around and held while lacing the chain through. Kind of makes your point moot.

But not as bad as not knowing how it works in the first place.
Again, never made a mistake?

And I didn't advise it. You can read post 3 again if you're still confused. I​​​​​ did advise using the Shimano 600.
But you did advise running the derailer without the stop screw. You said ďAs long as the wheel and chain are mounted, the pin has no function; the chain keeps the tension.Ē The wheel and chain arenít always mounted. Drop a chain in a shift and the whole thing is going to be ripped off. You might be able to get away with riding if the stop screw fell off but anyone who is doing that is flirting with disaster.
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Old 12-04-20, 08:11 AM
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Originally Posted by cyccommute View Post
But you did advise running the derailer without the stop screw. You said ďAs long as the wheel and chain are mounted, the pin has no function; the chain keeps the tension.Ē The wheel and chain arenít always mounted. Drop a chain in a shift and the whole thing is going to be ripped off. You might be able to get away with riding if the stop screw fell off but anyone who is doing that is flirting with disaster.
Interesting discussion. My initial mulling on this tells me that your prediction of "disaster" is overstated, but having no experience with losing a pin, I really don't know. Dropping a chain with out the pin may allow the cage to rotate a bit further than it should, but shouldn't the chain tension prevent it from going too far? If it does rotate too far, would the cage or chain then jam on something? Mental visualization is not as effective as a hands-on study, which my curiosity will demand that I do next time I have a bike on the stand. I don't plan to do a road test though. My curiosity has limits.

How does one of these pins shear anyway? I can't tell from the pic, but it looks like the pin hole elongated to the point that the pin couldn't be fixed in the threads. This could happen if the pin wasn't properly tightened.

All in all, the advice to not use it without the pin is absolutely right.
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Old 12-04-20, 01:21 PM
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SurferRosa is absolutely correct. a stop screw is not needed when the freewheel and chain is in place.
It is only a nice feature for wheel changes. I remember Huret derailleurs without a stop screw. when
reinstalling the wheel you had to rotate the derailleur and then install the chain.
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Old 12-04-20, 01:30 PM
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I'm still wondering what was wrong with the DR that was called worn, but presumably had the pin or stop that is being hotly debated.
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