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CX-Ray spokes and Wheelsmith tensiometer
 

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Nick Payne
Joined: 10 Jan 2004
Posts: 2449
Location: Canberra, Australia

1/17/13 8:39 PM

CX-Ray spokes and Wheelsmith tensiometer

I have a Wheelsmith tensiometer that's about 15 years old, and the calibration chart doesn't include readings for use with modern bladed spokes such as DT Aerolite and Sapim CX-Ray that are small enough in their large cross-section to fit a hub with normal spoke holes. Consequently, the spoke tension in the several pairs of wheels that I've built using CX-Ray spokes has always been a matter of guesswork.

My wife helped me sort this problem the other day by flipping a small branch into her front wheel while climbing, ripping four spokes out of the rim and ruining the rim in the process. Luckily she was going slowly enough that she managed to stop without coming off.

I didn't have any spare CX-Ray spokes in the correct length, but I did have some DT Revolution spokes (2/1.5/2mm), so I replaced the rim and used the Revo spokes to replace the four missing spokes. As the tensiometer does have 1.5mm spokes in its calibration chart, I was able to deduce a reasonably accurate reading for the CX-Ray spokes by comparing the deflection readings when the spokes were up to tension. I trued the wheel and tensioned the spokes until the Revo spokes were at a deflection of 40, corresponding to 92Kgf on the calibration chart, and found that the CX-Ray spokes showed a deflection of 20 at the same tension.

I then went to the other wheels I have with CX-Ray spokes, and found that most of them were under-tensioned:-( Oh well.

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dddd
Joined: 11 Jan 2004
Posts: 3280
Location: NorCal

1/18/13 1:27 AM

I guess then that you were able to equate the tension in the 4 spokes to the tension in the balance of the wheel by the fact that the rim didn't hop where the 4 spokes were?
I'm asuming that you couldn't compare the sound, given that the comparative cross-sectional area of the different spokes is unknown?

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Nick Payne
Joined: 10 Jan 2004
Posts: 2449
Location: Canberra, Australia

1/18/13 1:16 PM


quote:
I guess then that you were able to equate the tension in the 4 spokes to the tension in the balance of the wheel by the fact that the rim didn't hop where the 4 spokes were?

I replaced the rim with a new one, as the old rim was a bit out of true, especially after having to be ridden about 30km home with four consecutive spokes missing, so I operated on the assumption that the tension in all spokes was approximately equal when the spokes were up to tension and the wheel true - the tensiometer readings showed the same deflection reading on all four replacement spokes, and ditto for the half dozen of the old spokes that I measured at various points around the circumference of the rim.

quote:
I'm assuming that you couldn't compare the sound, given that the comparative cross-sectional area of the different spokes is unknown?

The CX-Ray spokes are nominally 0.9x2.2mm, but the cross section is slightly oval on the long face, so no.

ps. The DT Aerolite have almost exactly the same cross section as the CX-Ray, and so the deflection readings should be the same. The tensiometer did come with a deflection table for 0.9x3.1 blade spokes, and according to the table, the tension on those at a reading of 25 is the same as I got for the CX-Ray at a deflection of 20. So even though the absolute values in the table are individual to each tensiometer, the ratio should still be approximately the same.

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dddd
Joined: 11 Jan 2004
Posts: 3280
Location: NorCal

1/18/13 11:44 PM

Makes sense as the 3.1mm spokes are stiffer in all directions. They must also be heavier, so should ring a MUCH lower pitch for a given tension.

My Park Tensiometer chart/table lists 2.1 and 3.1mm bladed steel spokes, but by their numbers are only about two digits apart instead of 5, with both types seeing very high tensions in the upper-teen meter readings.

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