Wattsup

I'm rebuilding my front wheel, an HED rim. HED calls for a maximum spoke tension on the rear-drive side of 130kgf, but for the front disk side, only 120kgf. Why the difference? If I tension the front to 130, do I risk rim failure?

Andrew R Stewart 

I suspect that the tension guidlines are about retaining the rear non drive and the front non disk side spokes at a suitable tension, just said with a reference that is what most others use (the tighter side spoke tension being listed). The other side's tension is a result of the listed tension limit and the wheel's amount of dish. Generally rear wheels have a greater amount of tension differences (or dish) than fronts do. That and I also suspect that front wheels in general see less range of spoke tension changes during riding than rear wheels do.

Many tend to see these tension guidlines as being about how much can a rim (and/or spoke/hub flange) can take when wheel builders often see the spec. as being about how much tension the other sides have. Andy

WizardOfBoz

As a guess, the optimial weight distribution between front and rear is often said said to be 45/55 to 33/66 (F/R). The higher tension plus more weight on the rear wheel ends up giving you approximately equal compliance in both wheels?
Or Andrew's explanation. (I typed my answer before I saw Andy's).

Wattsup

Here's the actual verbiage. The other factor is the fact that the wheel will be tubeless. My rear wheel build's spoke tension was drastically reduced once the tuebless tires were installed.

For road and gravel wheels max tension is 130 kgf on the rear drive side. Rear non-drive side should have even tension.

We do not specify a non-drive side tension because that side is completely dependent on the correct drive side tension – it cannot be adjusted independently. Front rim brake spoke tension should be 110 kgf. Front disc brake brake side tension should be 120 kgf. For Fat wheels tension should be 95kgf.

urbanknight

The ideal tension is probably somewhere around 100-110. The rear DS is 130 because that's about as high as you can safely go and it will bring the NDS as high as you can to avoid them being too slack and breaking.

Iride01 

Note that is also a maximum tension. You might not even need to get to that. But it will depend on what it takes to true and keep the rim round. Along with the other things you are trying to keep the rim within.

FBinNY 

If the rim can take 130kgf, then it can, so no issue there.

However, there's no benefit to building with tensions that high. We ONLY do so on dished wheels where it's difficult to get adequate tension on the left (slack) side without going very high on the right.

Personally, I only build with DB spokes and have NEVER felt the need to exceed 110-120kgf. On fronts, I rarely exceed 90kgf.

There's a popular myth that more tension is better, but IT IS A MYTH. As long as all spokes are in the proper tension band (according to gauge), there is ZERO benefit to going higher.

In short, just because a rim has a max tension limit, there's no reason to build to it.

Wattsup

I did use 2/1.8/2 spokes. I get what you're saying about the tension, but I wonder how installing a tubeless tires affects it all. I don't have the numbers in front of me, but I remember when I did the rear wheel, the tensions dropped off dramatically after tire installation.

FBinNY 

Tire pressure presses in all directions making tires firm. But also presses on the rim pushing it inward, thereby reducing the diameter, and lowering spoke tension.

That's true, whether there's a tube or not. What's also true us that tension specs. are always based on the wheel AS BUILT, before a tire is mounted and inflated. The loss in tension is factored into the specs. and one should not think about it except out of intellectual interest.

BTW - we know that tensions are measured without tires, because otherwise they would need footnotes re. tire width and pressure.

Wattsup

Yes, I understand that. The thing is that, from what I've read, tubeless setups press even harder on the rim than tubed tires, lowering the spoke tension even further....maybe even too low. I was wanting to start with the highest tension possible.

Wattsup

I ended up with disk-side average of 114, all spokes within 10% of the average, lowest is 105. Average of the right side is 71, all within 15% of average, just barely outside of within 10%. Wheel is perfectly dished.

i find the trueing of the wheel easy. Getting nice consistent spoke tension a good deal harder. The Park Tension Meter is a little iffy, not always real repeatable. I guess the important readings are the highs, and the lows. Best to have them not far from the average.

FBinNY 

They're your wheels, and I'm certainly not going to argue with you about your choices.

However, you did come seeking advice, and, surprisingly for BF, there's a general consensus.

So, do as you will, but first consider.

How or why is more tension beneficial or necessary?

Why is there so much t as lk about tension without consideration of spoke gauge? Doesn't that matter?

If rim compression (tension loss) is important, why aren't we also considering pressure and rim width as important factor?

I pose the above because I consider much of what I hear about spoke tension to be Voodoo "science" based on a lack of understanding of basic principles.

BTW those were rhetorical questions and I'm not looking for a debate. Simply offering food for thought before you do what you're going to do anyway.

urbanknight

Too high of tension and you break a rim. Too low of tension and you break a spoke. I can tell you from personal experience that I'd much rather break a spoke than a rim. For that reason, I'm with FBinNY where I'm happy with a front wheel around 90 as it will still have the non-disc side well above 60 to keep everything in a safe zone. Rear wheels are more difficult, but even then I'd rather have NDS spokes breaking than the whole rim fail, and if that ever happens I'll make my next wheel with more spokes. So again, 110-120 DS seems to be enough to have the NDS at an acceptable tension.

Yes, truing is easy, but it's not much harder to do it with consistent tension. You just have to consider the neighbor spokes as options.

Dan Burkhart 

In discussions I have had with the folks at Velocity while negotiating warranty claims on rims,they said that low tension can also result in cracking around the nipple holes. Their research shows that the same stress cycles that break spokes also cause fatigue failure in rims.

FBinNY 

Yes, tension isn't the main factor in rim fatigue failure. It can cause immediate or early failure by excess stress at the spoke holes, but as Velocity says, fatigue failure isn't caused by tension.

What kills rims is overly stout spokes, ie. 2mm plain gauge, and/radial patterns.

With each cyclical load change, both the spoke and rim flex. But the amount each flexes depends on their relative strength. So stiffer spokes increase the cyclical flexing in any given rim. Radial patterns are especially hard on rims because they lack the "shock absorber" that over/under crosses provide.

For perspective, BITD we used very light rims and they held up fine because we built with 1.8mm spokes, usually DB for high end wheels. The problems arose when folks started using 2mm spokes, and increased tension accordingly.

Very quickly rims that had served well for years developed poor reputations, because of what I called "to much spoke for the rim".

Keep in mind that wheels are a system, and individual parts have to function as part of that system, rather than independently.

urbanknight

That makes sense, thanks. Still, cracking around the spoke holes would likely be noticed way in advance of any failure, so I'd still rather err on the low side than the high side. Sudden catastrophic rim failure is the most painful type of wheel failure in my experience. Even then, I'm talking about letting the rear NDS dip below, say 60, instead of letting the DS go above about 120. And in reference to a front wheel, ~90 is a happy middle ground that shouldn't even come close to being too high or too low.

Dan Burkhart 

Jobst Brandt once posted on a forum that he could not have written his book based on modern parts and materials due to lack of failure data. If he had hung around my shop for any length of time, I could have provided him with lots of data with all the repairs I take in.
however, these days I think a large portion of the failures I see are down to too few spokes for the job.

FBinNY 

Over the years I've seen all sorts if failures. Enough to generalize that most of all are due to failure to consider the entire system.

For example, when a rim maker designs the extrusion, he has to make an assumption about the number of spokes, their gauges, and the tension the builder will work to.

Then they drill that same extrusion for various number of spokes, use various aging or other treatments, and IME never provide build recommendations, ie. spoke gauges for best results.

I use the following analogy, probably too often, so I apologize in advance.

Engineering, like just about everything is like an impressionist painting. We're often tempted to stand too close and focus on the dots, and so end up not seeing the big picture.