FBinNY 

I disagree. Rim stress (spoke tension) limits aren't related to total load. They're about local stress at individual spoke holes which are pretty much independent of each other. OTOH the longitudinal (circular) compression limit of rims is staggering, and there's virtually no possibility of exceeding it.

So spoke count isn't a factor, except that with fewer holes, the span between them is greater, meaning more rim flex, and a need for more spoke elongation (tension) to compensate.

Alan K

Strictly speaking, that is untrue. There is no such thing as no possibility of exceeding the limit because under high tension, one or some spokes/nipples will fail. Once the rim is under uneven tension,… well, you know the rest!

FBinNY 

Context and sequence matter.

First of all the quoted statement was about rim compression vs. spoke hole load, and I stand by it. In fact, I might have been conservative in saying "virtually", because I doubt that it's at all possible to compressive overload a rim via spokes. The rim would fail at a spoke hole first.

As I said, sequence matters. If a rim were to taco after a few highly tensioned spokes failed, that wouldn't be considered a rim failure within the context at hand.

Alan K

Context does indeed matter. We all know that without even tension on spokes, rim in itself has virtually no strength.

The discussion in this thread is about how much tension is appropriate on spokes.
In my opinion, it not the best idea to go to the maximum tension at which a rim is rated.
I don’t know how cold it gets in your area but in my place, some days it gets 20 below 0F. Thermal expansion of metal, being a physical property, does not forgive anyone. And thermal expansion most decidedly works in the opposite direction equally well. I have seen new wheels laced by novice enthusiasts in the summer at or close to the maximum tension that end up with broken nipples in the winter in their garage.

The key to a strong wheel is even and appropriate tension on all spokes… I would say the more spokes the better but this will bring out strong reactions from those who have never had any issues 3 spokes. 😉

Kontact

So you've never seen someone potato chip a rim with too much tension? Where have you been hiding?

Yes, total tension is very much a thing.

https://spokecalc.io/spoke-tension-t...tive-guide.php

FBinNY 

I don't know, maybe I hang out with a better class of wheel builder.

Seriously, I've been building wheels for over 50 years, and, yes, I've seen folks potato chip (I never liked the "taco" analogy) wheels more than a few times. Mostly newbs who get carried away when "stress relieving" and once or twice while in service. However, I've seen that at all tensions so, don't consider it related to excess tension.

FWIW- I learned wheel building in an era of light spokes and rims, and have NEVER understood why folks see the need for crazy high spoke tension. To me, their logic sounds comparable to parking cement trucks along a bridge span to make it stronger, So, as one who worries more about staying above minimum tension, I'm not as exposed to the issues of excess tension. When I see consequences of high tension it's generally immediate spoke hole failure, or premature stress cracking between the holes, and not the compressive failure you describe.

Consider-- the rim is laterally braced by the spokes and at a low potential point analogous to resting in the notch at the top of an "M". The rim can only move over the side and "chip" if a lateral force is applied. Increasing tension will not create that side force, so nothing will change, especially on high spoke wheels where the span between spokes is shorter. So I stand by my opinion that excess tension will lead to hole failure long before compressive failure is possible. (unless spoke tensions are so uneven that the bracing symmetry is lost).

Kontact

Here's the thing: I agree that high tensions aren't necessary. But what I'm saying is that the rim need a certain amount of tension to stay rigid, and not a lot more than that. When you put the same tension on 20 spokes as you put on 40 spokes, you have doubled the tension on the rim. Not only is there no reason for that, but it can exceed the rim's structure.

You learned to build wheels with very light rims and thin spokes, but 36 of them. I'll bet the tensions you used were nowhere near as high as what a 20 spoke rim requires. Those were uncommon even in the '90s with Rovals being so uncommon I have yet to see a pair of them.

This guy seems to share my view:
https://wheelfanatyk.com/blogs/blog/too-few-spokes


If you don't think total wheel tension is an important limit - fine. But you already said you don't see the need for excessive tension, and if you have more spokes you only need minimum tensions for them to do their job.

FBinNY 

You and I agree on the basic thinking about tension vs. spoke count, and if you read my earlier posts here, you'll see that.

So no problem with the idea that fewer spokes generally want more tension, and more spokes want less, but our reasoning is different. We also disagree on the potential consequences of excess cumulative tension, but that's purely academic since we'd never run into the issue anyway.

The "problem" is that we're in the minority these days. I can't count how many times I've heard, "These will never hold up. Can't you make them tighter?" from referred clients.

BNB

thanks for your post. good points. so hard to know, so many ideas out there. one thing for sure is I will be checking this wheel after every ride for a while and then periodically from that point with the period being frequent! for sure the original tension on that wheel was way too high.

BNB

thanks for the input. yes, it's a tandem bike. 36 spokes. I agree with you about pitch. Pitch might work for someone doing this often (though I'm skeptical) but would absolutely not work for me. Would be interesting to see an experiment on pitch (human ear) vs tool (non human).

BNB

In the event this is relevant: I got around to measuring the tension on the front wheel, 36 spokes, and it was the same as the original rear wheel, 36 spokes, from the builder; all spokes read with Park Tool TM-1 at 20 and 21 (approx 140-155 kgf from the posted graph in this thread - if I managed to read it right - exceeding the spec of the rim of 120kgf). The front wheel has not failed (yet). The rear wheel failed at the rim holes at the same tension with far more weight in the rear of the tandem. It might well be that front wheel will fail too over time so I will tension those spokes lower.

Test ride went well today with "new" rear wheel.

mpetry912 

based on all the above I believe I'd shoot for a reading of 14-17 on your tensiometer

the comments about not shooting for the max tension are worth noting. Especially on a tandem rear.

/markp

SJX426 

@FBinNY - I get the max recommended tension for a rim. Do you have a rule of thumb for adequate spoke tension based on spoke count? Something like 90% of max for a 20 spoke, 85% for a 24, etc.
I suspect you may take rim cross section into account that may influence due to spoke spacing.

FBinNY 

I don't have a simple rule that will satisfy you.

It's primarily a function of gauge, since I want enough elongation to absorb rim flex without ever hitting zero tension. I start by looking at ALL factors and choosing rims and spokes accordingly. Generally I prefer lighter rims, but will use stiffer rims when reducing spoke count. That usually also means thicker spokes to keep total steel within range. In turn, that means higher tension.

So, it's a systems approach with every thing considered in context of the whole.

By example, we know that dished wheels call for greater tension on one side. So there's dilemma of maintaining minimum tension on one side without exceeding max. on the other. I compensate by using different gauges, giving me much more latitude in tension.

mpetry912 

I'm with FBinNY on this one, there is no absolute rule on the right spoke tension for any rim, spoke count, spoke guage combination.

This is an area where experience and good judgement come into play

If you think you are going to get a finished wheel that is true, round, and properly dished with all spokes in a 3% range

then you have enough experience to not have to ask this question.

Again I would look for a spoke tension between 14 and 17 on your tensiometer and shoot for uniformity rather than max tension

especially on a wheel being built for use on a tandem

/markp

FBinNY 

Some thought to consider when trying to understand how much tension is desirable. I'm not looking for an answer, just trying to help folks think about how spokes work.

1- we know that wheels go out of true, so what is that process, and what does it imply?

2- for those who've broken rear wheel spokes, which side sees more breaks? What does that imply about setting tension goals?

As I said, please don't answer, let folks stew on it a while.

Kontact

The problem with these questions is that they suggest tension is always related to breakage or going out of true, rather than to something like elbow seating, nipple lubrication, etc.

FBinNY 

Not at all, it's tabula rasa.

I'm inviting people to THINK rather than.simply relying on what someone tells (or has told) them.

I don't know why you assume they'll ignore factors other than tension.

In any case, I'm a believer in the Socratic Method. I believe that the insights folks might glean through thinking and analyzing lead to greater understanding vs. simply absorbing what they're told.

But, I'm happy to see what happens when folks compare what they believe with what they observe.

jsallen

Yes. And it isn't only example: it's physics. The spoking on each side must pull the rim toward its side equally, and the lower the angle, the tighter the spokes must be. Spoke gauge in proportion to tension on the two sides of a dished wheel results in approximately proportional reduction in tension due to all loads -- radial (weight), lateral and torque. And so also, the same change in length, and that the bottom of the wheel does not flex sideways under radial load. If spoke gauge is proportional, the musical pitch of the spokes on both sides of the wheel will be the same. These issues are addressed at https://sheldonbrown.com/spoke-pitch.html..

jsallen

I wonder how this is measured. It may vary from one manufacturer to another Iis it the maximum loads which the rim can withstand, or the recommended tension when the wheel is no under load, and if so, based on what assumptions? It is well-known that weight load decreases tension. however lateral and torque loads increase it on one side of the wheel -- though at the same place weight load is decreasing it. A mathematical analysis of this is possible, but I don't know of any. In The Bicycle Wheel, Jobst Brandt only examines each independently, and the graph he shows of right-and left-side spoke tension under lateral load in a dished wheel is strangely incorrect because it shows spokes going into compression. Much better computational tools are available now, and an analysis using them would be interesting indeed.

FBinNY 

It's actually very simple.

ANY spec must conform to something comparable to the "Four corners rule". Meaning that it can and will be measured under known and logically applied conditions.

So the max tension specs for a rim is exactly that ---- the maximum spoke hole load when and as built. Any other consideration would depend on assumptions and would therefore be meaningless.

jsallen

Any other consideration could be based on an understanding of how much spoke tension can increase in normal use. This would require the analysis that I am not aware has been done. Wheels are subject to stresses din use which they do not experience on the truing stand!

FBinNY 

Not relevant to the spec.

I agree that rim makers have to consider all kinds of load factors, but that's THEIR job. When they publish a spec it's just what it says it is, no more, no less.

In this case, it's a Maximum tension limit. Not a suggested ideal tension, payload limit, suggested spoke gauge, etc.

There are also other implied considerations that would fall under "suitability for purpose", ie. a 700c rim marketed as a road rim may not be suitable for MTB, while one sold for that purpose would be expected to be robust enough for that.

There's also the builder's responsibility for making smart choices factoring purpose, right weight, and objectives, ie. road racing, touring, utility, etc.

jsallen

Sheldon Brown once built a wheel with 72 spokes for a heavy friend. It worked. Aside from that, the way to avoid spoke-hole damage is to use thinner spokes, not to use thick spokes at less than their optimum tension. The thinner spoke will retain better control of the rim due to their being able to shorten more while remaining under tension.

Kontact

In use, spoke tension does not go up, it goes down as the wheel touches the ground and the rim momentarily flattens out.