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Old 12-31-13, 11:52 AM   #1
crewdoglm 
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Dura-Ace 7900 rear hub numbers

Contemplating a build using a 7900 straight-pull 20H hub. I have so far deduced that Shimano made the 7900 rear hub as standard drilled flange and also as a straight-pull 20H on Dura-Ace wheelsets only e.g., C35. The perplexing thing is that I see no part numbers which distinguish the two hubs. There is tech data from Shimano giving the 7900 dimensions but I'm guessing this is for the standard flange model which came as part of a group set. Any insight on this? Does anyone know the hub data for the straight-pull hub?

Addendum: I know it can be measured. The the exact spoke circle would, I think, have to be computed using the polygon formed by the five spoke anchors. This is a polygon inscribed in a circle with the veritices on the circle. We're solving for the circle perimeter only knowing the length of the polygon sides. I can't find or recall the procedure for that. I think this what was called "squaring the circle" (impossible). Any math types feel free to weigh in there.

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Old 12-31-13, 12:30 PM   #2
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I suspect that all tech info relating to the shell and spoking would be for the standard drilled hubs, since Shimano didn't (doesn't) offer the straight pull version for sale separately. The rest of the data, such as service and internals may be common to both.
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Old 12-31-13, 12:51 PM   #3
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I suspect that all tech info relating to the shell and spoking would be for the standard drilled hubs, since Shimano didn't (doesn't) offer the straight pull version for sale separately. The rest of the data, such as service and internals may be common to both.
I'm pretty sure that's correct. Those straight pull hubs were only sold as part of complete prebuilt wheel sets.

I have a pair of Shimano WH-R560 prebuilt wheel (~105 level) that uses a specific radial laced front hub and a specifically designed rear hub laced radially on the drive side and 2X on the non-drive side and both hubs have the nipples at the hub flanges. I have never seen anything like them sold as individual hubs. Their internals and freehub body are otherwise completely conventional Shimano.
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Old 12-31-13, 03:48 PM   #4
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Measuring j-bend hubs is easy but I have no idea how to measure a straight pull hub.

This is taking the long way around, but you could look at the tech docs and see what spoke length Shimano used. Then figure out the ERD of the rim they used. This might get you in the right direction???
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Old 12-31-13, 04:23 PM   #5
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Measuring j-bend hubs is easy but I have no idea how to measure a straight pull hub.

This is taking the long way around, but you could look at the tech docs and see what spoke length Shimano used. Then figure out the ERD of the rim they used. This might get you in the right direction???
The tech docs I found do not give a spoke length but you're right; you could technically solve for the flange diameter if you re-wrote the spoke math having the ERD and spoke length as known numbers. That would be more work than we need I think. This is going to come down to playing with the calipers and the hub until we're close. I don't have expert verification of this, but I think the computed length of a J-bend spoke will be fine for straight pull. (That "J" is not included in the overall length.) This going to be an 88MM carbon aero TT wheel with old-school 3MM blades tied & soldered - if I can get SP's REALLY short. Obviously we've got a way to go on it and it's just being done for the aesthetic value. (Although, a tied/soldered wheel has better power transfer IMHO.) Thanks for the thought.
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Old 12-31-13, 04:35 PM   #6
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I don't have expert verification of this, but I think the computed length of a J-bend spoke will be fine for straight pull. (That "J" is not included in the overall length.)
J-bend lengths in classic hubs don't match direct pull lengths in "star" hubs. Among the other things is the adjustment for the thickness of the star point where the spoke sits.

However I believe that MrRabbit here on BF has a conversion, or a calculator for these. Otherwise I've always done it with a sketch and a calculation of the straight line distance to which I add 1mm for every 10mm of center-to-flange measurement.
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Old 12-31-13, 06:01 PM   #7
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That is good info. Thanks. If the right center-to-flange is 18, understand you would add 1.8MM to the number given for a J-bend?

As to my initial question, thanks to Dr. Peterson of Drexel University for straightening me out. (This is not a case of "squaring the circle.") Happy to now know: there is a constant relationship of "regular" polygons to a circumscribed circle around said polygon. The constants are set forth in the CRC Standard Mathematical Tables. Measuring one side of our pentagon-shaped hub yields a drive side diameter of 48.14 MM and a left side diameter of 28.3. I would be fascinated to learn how this compares Shimano's unpublished data. The difficulty is in positioning the calipers to measure precisely between the spoke heads.

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Old 12-31-13, 06:17 PM   #8
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That is good info. Thanks. If the right center-to-flange is 18, understand you would add 1.8MM to the number given for a J-bend?

.
No you misunderstood me. The 1:10 adjustment I referenced was the compensation for the center to flange distance. It's the difference between the flat distance from flange to rim vs. the "dish" distance.

In a perfectly tangent spoke pattern the spoke length would be very close to 1/2 the ERD plus the CTF adjustment.

Looking at your hub, calculate/measure how far from the halfway plane the spokes end at, subtract from half the ERD, add for CTF and you'll be within 1mm. I do this using a careful sketch of the hub profile, then indicate the spoke routing and measure.
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Old 12-31-13, 07:12 PM   #9
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Got it. Your technique obviates the need for a spoke circle diameter number. And that number is often absent with the "star" hub. Still straight pull wheels are being built all the time which invites the question of a standardized technique. I guess I'm thinking there must be a correction for j-bend lengths out there using an effective spoke circle D. DT Swiss publishes this number for their straight pull hub which has a less pronounced star shape.

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Old 12-31-13, 07:19 PM   #10
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Got it. Your technique obviates the need for a spoke circle diameter number. And that number is often absent with the "star" hub. Still straight pull wheels are being built all the time which invites the question of a standardized technique. I guess I'm thinking there must be a corrected j-bend lengths out there using an effective spoke circle D.
Yes, my methods dat back to before there were cheap scientific calculators, or the internet. To do a spoke calculation required a decent amount of time using trig function charts, and pencil and paper. Or you could use direct measurements and rule of thumb adjustments to get an answer without complicated math.

BTW-mas I said earlier, I believe MrRabbit has a spoke calculator that handles star hubs (or a formula for adjusting standard measurements. If he doesn't post here soon, you might PM him.

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Old 12-31-13, 07:42 PM   #11
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Roger that. Sounds like you've got some years on me but FWIW I appreciate that perspective truly. I teach math/charts & graphs stuff in aviation and see I excessive reliance on technology. The problem is that bad conceptual knowledge prevents errors from being caught. (The machine isn't wrong but it won't tell you when you're f--ing up either.)
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Old 12-31-13, 09:20 PM   #12
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Roger that. Sounds like you've got some years on me but FWIW I appreciate that perspective truly. I teach math/charts & graphs stuff in aviation and see I excessive reliance on technology. The problem is that bad conceptual knowledge prevents errors from being caught. (The machine isn't wrong but it won't tell you when you're f--ing up either.)
Over reliance on technology is now getting some recognition after a few airplane crashes, but it's not a new problem. Engineers trained through the sixties were just starting to use computers, but were trained using slide rules, and hand calculation. The standard response when asked for a complicate calculation was "do you want a rough answer now, or a precise answer later". The limitations on computer power and speed made people rough up answers in their heads so they could decide whether something was possible, before they bothered working out the details.

I built wheels for over 20 years and never knew the spoke tension. There were only three tensions following the Goldilocks rule -- too loose, too tight and just right. I still use my judgement, and tactile feedback while building, then use a tension meter only as a sort of reality check. While the tension meter gives me a number, it's not that useful since I don't have a predetermined target number, only a range.
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Old 12-31-13, 10:14 PM   #13
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Impressive. I am happy as hell to get it true and tight any way I can and the tension meter plays heavily into that. LAUGHING AT MYSELF HERE. Those aviation crashes you mention are ultimately driven by the dollar. When every flight must produce revenue, the company wants to minimize the number of human beings in thing receiving a salary. Worse, they're never going to admit that the lack of a real systems expert (flight engineer or the loadmaster) is what killed everybody. The FAA could start requiring it... Happy New Year to you.

Do you tie and solder?
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Old 12-31-13, 10:40 PM   #14
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Those aviation crashes you mention are ultimately driven by the dollar. When every flight must produce revenue, the company wants to minimize the number of human beings in thing receiving a salary. Worse, they're never going to admit that the lack of a real systems expert (flight engineer or the loadmaster) is what killed everybody. The FAA could start requiring it... Happy New Year to you.

Do you tie and solder?
I very rarely tie and solder. Only for sprint track wheels for podium quality riders. Otherwise it kills serviceability with zero real benefit (to say nothing of the extra work). In my life I think I've tied and soldered 2 or 3 pairs of wheels.

As for airplanes, I don't think an additional crewman would make a difference. The problem is that the avionics are so good, and so reliable that many pilots have zero experience actually in control. So, in those rare instances where the systems fail you have a pilot that despite 10 years flying, and never dealt with a crisis. Moreover that have no feel for the controls and are incapable of seat of the pants flying.

I learned this when I was certified as a scuba diver, where the process involved many hours of classroom work, learning nonsens like decompression theory, and how to plan dives using tables. That was followed by pool and open water work drilling emergency procedures such as such as controlled out of air ascents. At the end of the course the instructor commented that, sadly, all the drills were probably wasted effort since the equipment was so reliability that years might go by before we needed to manage a crisis, and by then we'd have forgotten all the drills.
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Old 01-01-14, 09:53 AM   #15
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True the equipment is very reliable, but failures are essentially random so though years may go by between failures on average, it's possible for only a day to go by before one. You can choose to remain current with emergency procedures either by reviewing and practicing procedures on your own, or by taking recurrency classes. Failures are really rare in commercial passenger airline operations too, but pilots are required to undergo recurrency training twice a year anyway.
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Old 01-01-14, 10:18 AM   #16
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True the equipment is very reliable, but failures are essentially random so though years may go by between failures on average, it's possible for only a day to go by before one. You can choose to remain current with emergency procedures either by reviewing and practicing procedures on your own, or by taking recurrency classes. Failures are really rare in commercial passenger airline operations too, but pilots are required to undergo recurrency training twice a year anyway.
While practicing emergency procedures once in a while certainly helps (most open water divers don't) good luck and good equipment breeds confidence, reliance and complacency. But more important, reliance on technology removes the intimacy that manual operation brings. It isn't about knowing what to do in an emergency, it's about recognizing an emergency early and responding by instinct or reflex.

If you look at the diving world, most open water divers are pretty complacent, however cave divers who are very aware of the unforgiving nature of the environment do multiple predive checks, including various emergency drills before every dive.

I'm not anti technology at all, but it's important to recognize that relying on technology means a loss of the day to day experience that creates the instincts we may need in an emergency. Somehow we need to find a way to use modern tools without letting old skills atrophy.
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Old 01-01-14, 12:22 PM   #17
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Working with straight pull hubs and spokes from a calculation standpoint is pretty simple actually:

1. Just like the end points of the spokes define a virtual circle with a virtual diameter we call "ERD"...

2. Straight pull spoke seats define a virtual flange diameter that we typically reference as flange diameter - center-to-center.

With twin-thread setups it's even more fun - because you have to establish the minimum insertion required by the manufacturer - THEN - define the virtual flange diameter. It'll always be smaller then the physical flange - just like with standard hubs.

Calculating the spokes as usual is a piece of cake.

I share the beef of others who wish manufacturers of boutique wheels would publish their hub dimensions with references and their rim dimensions with references - not everyone trusts the repair skills of the LBS that sold 'em the boutique wheel.

=8-)

As an aside, one of my favorites is the bmx hub that looks like a straight pull hub but requires traditional j-bend spokes. I don't remember whether it's Atomlab or someone else...

You stick the j-bend spoke in - and then turn the spoke 90 degrees - just like that you have a straight pull j-bend spoke.

So the "effective flange diameter" - otherwise hereby know as "EFD" (smacks self in face) is roughly 2mm or 3mm below the surface of the the hub shell.

Yeah...I know...

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...and enjoy your knew reference.

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Old 01-06-14, 01:18 PM   #18
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Working with straight pull hubs and spokes from a calculation standpoint is pretty simple actually:

1. Just like the end points of the spokes define a virtual circle with a virtual diameter we call "ERD"...

2. Straight pull spoke seats define a virtual flange diameter that we typically reference as flange diameter - center-to-center.

With twin-thread setups it's even more fun - because you have to establish the minimum insertion required by the manufacturer - THEN - define the virtual flange diameter. It'll always be smaller then the physical flange - just like with standard hubs.

Calculating the spokes as usual is a piece of cake.

I share the beef of others who wish manufacturers of boutique wheels would publish their hub dimensions with references and their rim dimensions with references - not everyone trusts the repair skills of the LBS that sold 'em the boutique wheel.

=8-)

As an aside, one of my favorites is the bmx hub that looks like a straight pull hub but requires traditional j-bend spokes. I don't remember whether it's Atomlab or someone else...

You stick the j-bend spoke in - and then turn the spoke 90 degrees - just like that you have a straight pull j-bend spoke.

So the "effective flange diameter" - otherwise hereby know as "EFD" (smacks self in face) is roughly 2mm or 3mm below the surface of the the hub shell.

Yeah...I know...

Hippy New Beer!


...and enjoy your knew reference.

=8-)
Thanks. You are confirming my hypothesis: the flange's spoke-circle diameter is still the thing we need here. It's just trickier to get it with a pentagonal hub and the manufacturer making it classified for God's sake. Anyway I played with the math and discovered that in the case of the 7900 factory-wheel hub, we have spoke holes making the points of a pentagon with all sides & angles equal i.e., a "regular" pentagon. There is a mathematically constant relationship between said pentagon and a circle around it where the vertices are on the circle - high school geometry which I simply forgot. Turns out, for any regular pentagon the length of one side x .83065 will give you the radius of it's circumscribed circle. I am getting 42.5 for left and 48.0 for right. I would love to know what Shimano's numbers are. Planning on calling Shimano USA in CA and just see if they'll give up the numbers.

So when you do a length computation on a funky straight-pull hub, having found the EFFECTIVE flange diameter, is there any OTHER correction made to the length indicated for a J-bend spoke? Guessing not Thanks!
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