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Old 09-24-06, 06:44 PM   #1
.ks
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new wheel - spoke tension question

Just built my first wheel -- all went pretty well. Then I checked the spoke tension. I'm high on the dish side and fine on the non-dish side. High = 130-145 Kfg and fine = 90-111 Kfg using a Park spoke tension meter. The rim is a Mavic Open Sport 36 spoke, the spokes are triple butted DT Alpine.

Barnett's has 90-110 Kfg for all Mavic wheels. I was okay with my numbers, thinking that they will ease a bit after a few miles, then I read in Barnett's: "In the absence of the availability of such recommendations, tighten rear-wheel right-side spokes to 120130 kgf. This may still be low for some of these wheels, but using higher tensions than this without specific manufacturer recommendations puts the mechanic in the position of assuming the risk of sudden catastrophic wheel failure. Tensions lower than ideal only risk premature spoke breakage."

Sudden catastrophic wheel failure. That got my attention.

Do I back-off on the drive side, or is there still a margin before I get into the potential catastrophe zone?
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Old 09-24-06, 07:41 PM   #2
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Originally Posted by .ks
Just built my first wheel -- all went pretty well. Then I checked the spoke tension. I'm high on the dish side and fine on the non-dish side. High = 130-145 Kfg and fine = 90-111 Kfg using a Park spoke tension meter. The rim is a Mavic Open Sport 36 spoke, the spokes are triple butted DT Alpine.
I'm familiar with your problem. I built most of my wheels with DT Revolution spokes and when converting the Park deflection numbers to kgf for the central part of the spoke, 1.5 mm, the conversion chart shows that I have 160 to 165 kgf on the driveside rear spokes. This is what it takes to get enough tension in the non-driveside spokes to avoid failure at the "J" bend due to flex, and at the same time have a centered rim.
I seriously question the technique of using the Park conversion chart and the skinniest part of double butted spokes to calculate the tension at the rim. My wheels have light weight rims, MAVIC Open Pro and DT RR 1.1, and aluminum nipples. After several thousand miles I've had no ill effects from the high tension, whatever it actually is.

Al
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Old 09-24-06, 07:53 PM   #3
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Originally Posted by Al1943
I'm familiar with your problem. I built most of my wheels with DT Revolution spokes and when converting the Park deflection numbers to kgf for the central part of the spoke, 1.5 mm, the conversion chart shows that I have 160 to 165 kgf on the driveside rear spokes. This is what it takes to get enough tension in the non-driveside spokes to avoid failure at the "J" bend due to flex, and at the same time have a centered rim.
I seriously question the technique of using the Park conversion chart and the skinniest part of double butted spokes to calculate the tension at the rim. My wheels have light weight rims, MAVIC Open Pro and DT RR 1.1, and aluminum nipples. After several thousand miles I've had no ill effects from the high tension, whatever it actually is.

Al
I'm not quite sure what you're saying here. The Park tension meter measures deflection. The thickness of the spoke not only affects the deflection reading, it also affects the actual deflection amount. So Park has made a chart that determines actual deflection, normalizing the reading for spoke thickness, then converts that deflection amount into a kgf unit.

Most experienced wheelbuilders I know go to rated tension on drive side, and leave the non-drive wherever it ends up, as long as it's taught enough not to go slack. This can be mitigated by using thinner spokes on the non-drive side, since they will be stretched more to get up to tension, and will therefore be less likely to go slack in a bump hit.

Too much tension on the drive side will crack the spoke holes. It happened to one of my Open Pros before I knew better.
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Old 09-24-06, 08:20 PM   #4
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Originally Posted by waterrockets
I'm not quite sure what you're saying here. The Park tension meter measures deflection. The thickness of the spoke not only affects the deflection reading, it also affects the actual deflection amount. So Park has made a chart that determines actual deflection, normalizing the reading for spoke thickness, then converts that deflection amount into a kgf unit.
But the chart and the instruction do not tell you how to convert the deflection units to tension units (kgf) for double butted spokes. Most people assume that you should use the smallest part of the spoke which converts to a much higher tension than a thicker part of the spoke. Revolution spokes are 2.0-1.5-2.0. In other words I don't think I actually have 160+ kgf on my driveside. And I suspect that the OP doesn't either.
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Old 09-24-06, 08:53 PM   #5
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the chart and instructions require that deflection be measured in the centre of the spoke - or close to it. Past the butting anyway. The chart converts the deflection units to tension units for many different thicknesses, as well as elliptical, titanium and aluminium spokes. Sounds like you may have printed out only the first column. BTW the OPs Alpine spokes are 1.8mm in the centre.

- Joel
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Old 09-24-06, 10:01 PM   #6
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The Park tensiometer measures the deflection, and you use their chart to find the axial tension for that deflection. You use the thickness of the part of the spoke you are deflecting. A different thickness of the spokes beyond the measuring tool will not have any effect on the measurement.
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Old 09-25-06, 07:07 AM   #7
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Quote:
Originally Posted by Al1943
But the chart and the instruction do not tell you how to convert the deflection units to tension units (kgf) for double butted spokes. Most people assume that you should use the smallest part of the spoke which converts to a much higher tension than a thicker part of the spoke. Revolution spokes are 2.0-1.5-2.0. In other words I don't think I actually have 160+ kgf on my driveside. And I suspect that the OP doesn't either.
If you're measuring in the center (thin) part of the spoke, and all three contacts of the TM-1 are on that thin section, then the converted reading is correct. I suspect the tool is most accurate around 110 kgf, since that's a really common tension, but if it converts to 160kgf, you're probably really close to that measurement.

Here are the instructions. Page 4 of 6, step 2 of "Measuring Tension of a Spoke:"
"With butted spokes, position the posts so they rest on the narrowest portion of the spoke."
http://www.parktool.com/products/doc...5128_45722.pdf
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Old 09-25-06, 07:17 AM   #8
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Originally Posted by waterrockets
Most experienced wheelbuilders I know go to rated tension on drive side, and leave the non-drive wherever it ends up, as long as it's taught enough not to go slack. This can be mitigated by using thinner spokes on the non-drive side, since they will be stretched more to get up to tension, and will therefore be less likely to go slack in a bump hit.
This is why I'm such a big fan of asymmetrical rear rims! I don't know why Mavic won't make these. I suspect it's about not putting any R&D money into it since they want you to buy their high profit complete wheelsets. Next time, get a Velocity Aerohead OC for the back. It's lighter and stronger than an Open Pro and is an awesome rim.

Use the scale on the Park for a straight gauge 1.8 (Alpines have a 1.8, not 1.5 center section) since that's what the section you're measuring is. Oh, and don't exceed Mavic's spec by much. They will not warranty overtensioned rims. I'd swear they can smell the effects of high tension! LOL.
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Old 09-25-06, 07:22 AM   #9
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As already noted, you should aim for proper/correct spoke tension on the drive-side. The tension value is usually an upper limit (albeit a slightly conservative one) - too-high spoke tension can crack the rim at the eyelets. Mavic MA3 especially has problems with this, but the Open Pro is also good to be careful to stay away from too high spoke tension.

I'm with BikeWise1 on asymmetrical rear rims. Not all of them are well-executed, but in principle they solve the unequal-spoke-tension problem and some of them are well-done.
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Old 09-25-06, 08:05 AM   #10
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Originally Posted by tomacropod
the chart and instructions require that deflection be measured in the centre of the spoke - or close to it. Past the butting anyway. The chart converts the deflection units to tension units for many different thicknesses, as well as elliptical, titanium and aluminium spokes. Sounds like you may have printed out only the first column. BTW the OPs Alpine spokes are 1.8mm in the centre.

- Joel
I didn't print out anything since I have the chart and all of the printed material that came with the TM1. The spokes are not equal in thickness across the measured interval. I really question that a deflection of 22 on a Revolution spoke is equal to 157 kgf, when compared to other wheels with straight gauge spokes. If I were to set my driveside Revolutions at a more typical 110 kgf I'd be breaking spokes on the non-driveside. Been there, done that.

Al

Last edited by Al1943; 09-25-06 at 08:30 AM.
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Old 09-25-06, 08:32 AM   #11
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Originally Posted by BikeWise1
This is why I'm such a big fan of asymmetrical rear rims! I don't know why Mavic won't make these. I suspect it's about not putting any R&D money into it since they want you to buy their high profit complete wheelsets. Next time, get a Velocity Aerohead OC for the back. It's lighter and stronger than an Open Pro and is an awesome rim.
But I like my rims to have eyelets. Aeroheads don't.
Have you actually measured tension on both sides of a properly dished asymetrical Aerohead? I'd really like to see the numbers.

Al
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Old 09-25-06, 08:40 AM   #12
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Quote:
Originally Posted by Al1943
I didn't print out anything since I have the chart and all of the printed material that came with the TM1. The spokes are not equal in thickness across the measured interval. I really question that a deflection of 22 on a Revolution spoke is equal to 157 kgf, when compared to other wheels with straight gauge spokes. If I were to set my driveside Revolutions at a more typical 110 kgf I'd be breaking spokes on the non-driveside. Been there, done that.

Al
I just don't use revolutions on rear wheels

I feel that at 160kgf the alloy nipples would no longer be intact.

- Joel
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Old 09-25-06, 08:45 AM   #13
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I just don't use revolutions on rear wheels

I feel that at 160kgf the alloy nipples would no longer be intact.

- Joel
Yeah, and I'm not having any problems with aluminum alloy nipples, another reason I question the chart or the measurement technique. I think Revolutions are OK on a rear wheel but require more maintenance. I don't mind that too much but would not recommend them on the driveside for someone who doesn't do their own wheel work.

Al
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Old 09-25-06, 09:29 AM   #14
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Quote:
Originally Posted by Al1943
I didn't print out anything since I have the chart and all of the printed material that came with the TM1. The spokes are not equal in thickness across the measured interval. I really question that a deflection of 22 on a Revolution spoke is equal to 157 kgf, when compared to other wheels with straight gauge spokes. If I were to set my driveside Revolutions at a more typical 110 kgf I'd be breaking spokes on the non-driveside. Been there, done that.

Al
If the spokes aren't the same thickness across the measured interval, then you can't get an accurate conversion. Either measure across the same thickness, or just use the tensiometer for relative tension, not absolute values.

I'm building with more conventional 2.0/1.8/2.0 double-butted spokes, but I have three rear wheels (2 road, one mtb) in use under two heavier riders, and they have all held up just fine with 110kgf drive-side. The road wheels each probably have around 3000 miles of cracked Texas chipseal on them, and the spokes still all pluck the same note. Granted, these wheels are slightly overbuilt Velocity Deep-Vs runing 36h, 3x.
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