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Screw that moves rear D on hanger

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Screw that moves rear D on hanger

Old 06-27-21, 10:44 PM
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SkinGriz
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Screw that moves rear D on hanger

Not the barrel screw.
Not the H or L limit screws.
But the one that can rotate the whole rear D a little.

What is it for?
Thank you.
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Old 06-27-21, 11:20 PM
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B-Tension screw? It adjusts the distance between the cog teeth & chain/jockey wheel. Needed for precise shifting.
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Old 06-28-21, 05:55 AM
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Originally Posted by Crankycrank View Post
B-Tension screw? It adjusts the distance between the cog teeth & chain/jockey wheel. Needed for precise shifting.
Based on my experience of having the B-tension screw all the way out (i.e., almost unscrewed) in a 105 medium cage rear derailleur to shift an Ultegra 12-25 cassette, the only function of the B-tension screw is to set the minimum distance between the guide pulley and the biggest cog on the cassette. Even if that distance is too large, it does not seem to affect shifting. So it is much less critical than the other 3 settings mentioned by SkinGriz .
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Old 06-28-21, 06:39 AM
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Originally Posted by SoSmellyAir View Post
Based on my experience of having the B-tension screw all the way out (i.e., almost unscrewed) in a 105 medium cage rear derailleur to shift an Ultegra 12-25 cassette, the only function of the B-tension screw is to set the minimum distance between the guide pulley and the biggest cog on the cassette. Even if that distance is too large, it does not seem to affect shifting. So it is much less critical than the other 3 settings mentioned by SkinGriz .
It is true that the B-tension screw's function is to set the vertical position of that guide pulley. Different derailleur designs have slightly different ways by which is accomplished (it adjusts spring tension on traditional derailleurs or acts as a physical forward movement limit in many modern derailleurs). The influence of this screw will likely be minimized with a 12-25 or similar cassette, because there's not a lot of size difference between the largest and smallest sprockets. When the cassette's range increases, and the larger sprockets really grow in size, the B-tension screw has much more influence over shift quality (at least in my experience). If it's too far away from the largest sprocket, then it's usually way too far awayfrom the middle sprockets, which creates slow and vague shift performance.
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Old 06-28-21, 06:59 AM
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I have found, empirically speaking, that about 3 chain link pins between the large cog & the upper jockey wheel is about the correct b-screw setting in big/big combination. Too much less, & the chain is simply shoved over with out smoothly guiding the chain onto the next cog resulting in chunky shifts; Too far away & the angle between the upper jockey wheel & the cog isn't great enough to derail the chain to consistantly obtain a shift.

It is there to accomodate all the possible variations of possible drivetrain & chainstay length combinations. Chains come in integer 1 inch length increments. Chainstays & various gear combinations, not so much.
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Old 06-28-21, 07:27 AM
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Originally Posted by base2 View Post
I have found, empirically speaking, that about 3 chain link pins between the large cog & the upper jockey wheel is about the correct b-screw setting in big/big combination.
That sounds pretty far. Shimano's Dealer Manual (for road rear derailleurs) says:
Mount the chain on the smallest chainring and the largest sprocket, and turn the crank arm backward.
Turn the B-tension adjust bolt to adjust the guide pulley to be as close to the sprocket as possible but not so close that it touches. Next, set the chain to the smallest sprocket and repeat the above to make sure that the pulley does not touch the sprocket.
And most YouTube videos I have seen recommend a single digit mm distance.
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Old 06-28-21, 07:47 AM
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Originally Posted by SoSmellyAir View Post
And most YouTube videos I have seen recommend a single digit mm distance.
Totally agree. Having set up countless RDs, I'll be even more specific, 5-7mm seems to be the right distance for most RDs. It gets the upper jockey wheel as close as possible to the cassette, without the top of RD cage fouling the cassette as it is shifted into the higher gears.
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Old 06-28-21, 08:27 AM
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Originally Posted by hokiefyd View Post
It is true that the B-tension screw's function is to set the vertical position of that guide pulley. Different derailleur designs have slightly different ways by which is accomplished (it adjusts spring tension on traditional derailleurs or acts as a physical forward movement limit in many modern derailleurs). The influence of this screw will likely be minimized with a 12-25 or similar cassette, because there's not a lot of size difference between the largest and smallest sprockets. When the cassette's range increases, and the larger sprockets really grow in size, the B-tension screw has much more influence over shift quality (at least in my experience). If it's too far away from the largest sprocket, then it's usually way too far awayfrom the middle sprockets, which creates slow and vague shift performance.
This observation is fair. The 12-25 cassette is designed well within the geometry of a short cage derailleur. Since my bike came with a medium cage derailleur, I have to have the B-tension screw almost unscrewed to minimize the distance between the cassette and the guide pulley.
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Old 06-28-21, 09:04 AM
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On a bike with a horizontal dropout, the B-tension screw allows you to change the axle position in the dropout while maintaining the proper B-gap. Without it, you're limited to placing the axle in the position that accomplishes this.
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Old 06-28-21, 09:08 AM
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Well, I'm not one to argue with Sheldon Brown.

The instruction sheet that came with my first dérailleur, an old Benelux Mark 7, recommended adjusting the angle stop screw so that there were 1 1/2 links (three half-links) of straight chain from the jockey pulley to the largest rear sprocket when the bicycle is in its lowest gear. In my experience, this distance (or just a little more) has given the best results with many different types of dérailleurs. This distance is measured along the center line of the chain from where the chain straightens out after being curved around the jockey pulley to where it is again curved as it goes around the large rear sprocket.
FWIW: About 3 chain link pins usually results in about 6mm of space between the upper jockey wheel & the biggest cog, but also accounts for the incline of the derailleur from vertical as viewed fron the side of the bicycle, (i.e. "clocking of the derailleur.) It is easily viewed with out the trouble of actual measurement & virtually guarentees proper "as-installed" function in any given system. Lots of bikes have clocked derailleurs of some degree or another. Only on conventional diamond framed bikes is the hanger tab usually at about the 7 o'clock position.

The oft-quoted 5-7mm vertical distance spec is a proxy for an actual 1.5 inches distance from jockey wheel to cog wheel contact points, which is admittedly much more difficult to measure with a measuring tool unless you consider that each pin in a chain is exactly 0.5 inches apart. From the derailleurs perspective the more relavent distance is that from contact point to contact point to obtain the proper angle to derail the chain as intended.

Last edited by base2; 06-28-21 at 09:44 AM.
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Old 06-28-21, 10:33 AM
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Old 06-28-21, 11:31 AM
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Originally Posted by base2 View Post
Well, I'm not one to argue with Sheldon Brown.



FWIW: About 3 chain link pins usually results in about 6mm of space between the upper jockey wheel & the biggest cog, but also accounts for the incline of the derailleur from vertical as viewed fron the side of the bicycle, (i.e. "clocking of the derailleur.) It is easily viewed with out the trouble of actual measurement & virtually guarentees proper "as-installed" function in any given system. Lots of bikes have clocked derailleurs of some degree or another. Only on conventional diamond framed bikes is the hanger tab usually at about the 7 o'clock position.

The oft-quoted 5-7mm vertical distance spec is a proxy for an actual 1.5 inches distance from jockey wheel to cog wheel contact points, which is admittedly much more difficult to measure with a measuring tool unless you consider that each pin in a chain is exactly 0.5 inches apart. From the derailleurs perspective the more relavent distance is that from contact point to contact point to obtain the proper angle to derail the chain as intended.
I see! The 1.5" refers to the length of the chain as it forms a tangent from the largest cog to the guide pulley, whereas the 5 mm to 7 mm refers to the gap in space between the largest cog and the guide pulley.
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Old 06-28-21, 03:09 PM
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Very informative.

Thank you.
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