# Park Tension Tool conversion chart and Sapim CX-Rays question



## tinball (Sep 24, 2014)

Well I'm elbows deep in my first wheel build. I got the 18:9 Pacenti wheelset that Mike T posted about in another thread except with Sapim CX-Ray spokes. I have to say thanks to Mike for all the tips on your site they are helping ALOT. 

Anyway, at this point, I'm in the process of bringing up the tension on the rear wheel. I know some like the tone method but I've always been shall we say musically challenged and don't really trust myself with that method until I get some idea of what to listen for. So I'm using the Park Tools TM-1 tension meter. 

The issue I'm running into is conflicting information on what size spoke to use for the CX-Rays. On Parks website, they list them as 2.1m x 1mm bladed steel. On Sapim's website they are listed as 2.2x0.9. There are 2 different conversion columns that match these sizes and the differences are 10-15Kgf per reading increment. 

In another thread, it was listed as the recommended tension for the Pacenti SL23's is 125Kgf. I'd like to get as close as possible to that.

I'd like to get as stiff a wheel as possible without risking damage to the rim. If I go to low I'm afraid I'm going to get a flexy wheel but am concerned about risking damage to the rim by going too high. 

Or am I just over thinking this?


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## thosj (Mar 24, 2010)

I've built two of these 18-9's. Pacenti SL-23 27 hole rims, one BHS hub, one Camp hub skipping every other hole NDS, one with CX-Rays and one with round Sapim Race. I use the Park chart for .9 x 2.2 spokes for the CX-Rays. I built 'em 14/15 on that chart, around 125kg. The real beauty here is the NDS spokes are 90/95% the DS spokes!! On the Park chart with Park tensiometer, you get 15 on the DS you get 14 on the NDS. Way nice.

I'm a big guy, 220, and so far, so good!! 300 miles on each one, they're staying straight and true and tight. Minor tuning after the first 3 or 4 rides, and I mean MINOR, and now very good.


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## Mike T. (Feb 3, 2004)

tinball said:


> I'd like to get as stiff a wheel as possible without risking damage to the rim. If I go to low I'm afraid I'm going to get a flexy wheel but am concerned about risking damage to the rim by going too high.
> 
> Or am I just over thinking this?


Yes you are because tension, above the level needed to prevent spokes actually going slack during use, does not contribute to wheel stiffness. I know you won't believe me and I'm not an engineer who can trot out the formula to prove this but there is one over at Mountain Bike Review.com - *Meltingfeather* - and he's posted the proof and given the explanation. He designs pre-stressed structures (cable tensioned cement bridge beams) for a living and a bike wheel is a rotational version of his beam. If I unleash him on you (and I've had him post here before) you'll hear more than you ever wished for on the subject. I wish I had kept the link to his teachings.

Spoke tension decides how much the tension swings are in a rotating wheel during the load/unload cycle of the spokes. Low tension = big swings = faster fatigue. High tension = small swings = slower fatigue.

Just don't shoot the messenger or I'll unleash him on ya.


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## dcgriz (Feb 13, 2011)

Why do we need to trot out formulas to prove what has been proven already over and over again. Can't we just form a circle, hold hands and say " Thou shall not increase tension to increase lateral stiffness ever again"?

Jokes aside....to the OP two bits of advice:
-higher tension increases durability which is a good thing to have. 
-the Park tensionmeter is not the instrument to use if you are after absolute tension values unless you calibrate it for the specific spoke and tension you are after.


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## tinball (Sep 24, 2014)

Thanks everyone. I'm having a blast - except for when I lost the nipple washer in the rim! PIA to get it lined up with the valve hole and out. 

Mike, haha I believe you. Please don't sic him on me! My father and uncles are all engineers....  I guess, I just want to make these wheels the best I absolutely can because... why not? I'm trying not to obsess over every little detail but it can be difficult. I want to be proud of these hoops when I roll out on them for the first time in a group.

dcrigz, I want as durable a wheel as I can make it. And as consistent a tension on each side as possible. I do plan on seeing if my LBS has a tension meter I can calibrate it to next week - no harm in having it as accurate as possible even if I'm going to have to use it for consistency rather than absolute accuracy. 

Theosj, I'll use the same one as you then for reference. I have the wheel in dish but I want to get a bit more tension and I have a very slight hop and out of true about .5mm at this point. I'm sitting about 13.5 right now avg on DS and 11.5 NDS avg. If the tension meter were accurate (i'm not saying it is) that would be about 113Kgf DS and 92Kgf NDS. 

Wheel seems SOLID however. If I put it on the floor on with DS down on a block to stress/flex it, it does not seem to flex at all.


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## Kerry Irons (Feb 25, 2002)

dcgriz said:


> -higher tension increases durability which is a good thing to have.


Except when the tension is so high it causes the rim to crack. I would be interested to see if anyone has ever studied the range between "high enough so your spokes won't go slack and therefore get chewed up and result in spoke breakage" and "too high and you get premature rim failure."


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## nsfbr (May 23, 2014)

Mike T. said:


> Yes you are because tension, above the level needed to prevent spokes actually going slack during use, does not contribute to wheel stiffness. I know you won't believe me and I'm not an engineer who can trot out the formula to prove this but there is one over at Mountain Bike Review.com - *Meltingfeather* - and he's posted the proof and given the explanation.


I am an engineer and I think the explanation is actually quite simple. Unless you yield the steel that the spoke is made of the tensile force per unit elongation is the same no matter what the tension. That means the wheel will experience the same rebound force for any spoke tension as long as none of the spokes gets to zero tension. Put another way, if you hang something on a spring so that it is partially extended and then put a 10 pound weight on it, it will go an amount x. If you then put another 10 pound weight on it (so you've now added 20 pounds plus the original amount) it will go down another x amount. Thus, weigh scales have linear scales. (The reason for the first weight to get it started is that most springs need to have their coils all not touching before they are in the constant spring constant region.

So, think of your spokes as springs, they really are as they are using the same elastic modulus that a regular spring does (or a torsion spring, etc.) When the wheel is resisting the force that the weight of the bike and rider makes through the axle by pushing against the ground, the spokes at the bottom are shortened a bit, and the ones at the top are stretched. The force that the bottom ones exert on the hub is reduced by the amount of deflection multiplied by the "spring constant" of the spoke. The ones at the top have their force increased by the same amount (assuming a rigid rim). Note that the tension doesn't enter the equation at all. (The equation is the regular old spring equation F = -kx, where k is the spring constant and x is the displacement. The negative sign means the force is always restorative.) 

The only way to change this is if the bottom spokes become slack, because spokes are essentially noodles - the can't "push". 

You want a stiffer wheel, there seem to be three ways: Stiffer rim, higher spring constant spokes (not necessarily thicker ones), or more of them.

The tl;dr version of this is simply the equation: F = -kx, dF/dx = -k, 

Or, the derivative (slope) of the force is constant.


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## dcgriz (Feb 13, 2011)

Kerry Irons said:


> Except when the tension is so high it causes the rim to crack. I would be interested to see if anyone has ever studied the range between "high enough so your spokes won't go slack and therefore get chewed up and result in spoke breakage" and "too high and you get premature rim failure."


How high is high enough to cause the rim to crack? Good question but the answer must have quite a range based on the rim profile, extrusion thickness, type of extrusion, and a dozen other reasons.

From my empirical experience on the subject I have found box section rims to be more susceptible to cracking than the pointy variety. Pacenti PL23 and Velocity Synergy exhibited the beginning of bulging at around 110kgf and this is where I would stop. Putting the tire on further reduces the tension by 5-8 kgf so the rim sees a net tension of around 100kgf. The answer for these rims is "use a lot of spokes".

From the pointy rims, I have worked mostly with Archetype, HED and Deep V. My final tension before the tire is installed is at 130-135kgf. These are true values because the meter is specifically calibrated for the actual spoke and the tension it sees. Never ever had an issue with the rim cracking or even beginning to bulge at these tensions even after extended use. Are these tensions at the upper limit of what the rim could withstand before cracking? I do not know but my gut feeling and deduction is that for rims of that profile and overall weight the answer is "no". Now, if these rims were to be modified to be taller and wider but also loose 30 pr 40 grams in the process then the answer may be "maybe".


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## Bridgestone (Sep 6, 2007)

dcgriz said:


> These are true values because the meter is specifically calibrated for the actual spoke and the tension it sees.


Can you post pictures of your calibration fixture please. I want to build something with the help of my neighbor who is a fabricator. I was thinking of using weights from a weight set.


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## coachboyd (Jan 13, 2008)

Bridgestone said:


> Can you post pictures of your calibration fixture please. I want to build something with the help of my neighbor who is a fabricator. I was thinking of using weights from a weight set.


This is the one that we built.
The main parts needed are an S-beam load cell and a digital display read out. We have a CX Ray spoke being held between a fixed object and the load cell. This spoke has (as you can probably tell) 120.2KgF on it. 










For the Park tools TM1 we use this fixture to make it so that at 120KgF the TM1 reads exactly at 15. We use the TM1 to bring the wheels up to tension, get it fairly consistent and in the ball park of being at the right tension. Then the final round of dialing in the tension to an exact number and getting it even the whole way around is done with a dial indicator tension meter. I got a few from Ice Toolz and have been pretty happy with them.


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## dcgriz (Feb 13, 2011)

Bridgestone said:


> Can you post pictures of your calibration fixture please. I want to build something with the help of my neighbor who is a fabricator. I was thinking of using weights from a weight set.


I had posted a thread about the Park Tensionmeter calibration some time ago

link: Park TM-1 tension-meter accuracy post#24 and #25

My TM-1 was reading 19kgf too light- 130kgf vs. 111kgf actual load. 
Weights from a weight set would be just fine. Or your weight plus some weights from the weight set.


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## JohnF2 (Sep 21, 2014)

The set I’ve been riding now for several weeks also used the 2:1/27 spoke rim/hub, and used cx-rays. I tensioned to 14 DS (Park tension gauge – assumed to correspond to around 110kgf), and NDS came out close to 13. Tension felt sufficiently taught on both sides (based on general experience with spoke tensions), so opted not to tension further. 
My concern with 1:1 laced wheels has been with getting all of the NDS spoke tensions high enough (above 50kgf), but that isn’t an issue with the 2:1 wheel. 
On the road, the wheelset feels rock solid (front 20spoke and rear 27), and have been running 90/85 rear/front pressure max) – very pleased with their feel. The only other attribute to test is longevity – that will take time. Hopefully spokes won’t break, rims won’t crack, and hub flanges won’t fail.


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## dcgriz (Feb 13, 2011)

I typically model the wheel on paper and calculate the theoretical NDS tension based on hub offsets, lacing patterns and DS tension before the build begins. If after setting the DS tension at 130kgf I don't calculate the NDS to be more than 55 kgf I look for another hub.


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