Spoke Gauge Sizing

[Yellow Saddle]

Cut cut cut

Of course they are designed to save weight/drag. A double butted spoke isn't any stronger than a plain guage, it just uses a thicker section where it is most stressed, thinner where it's less stressed.

The reduced weight and increased aero efficiency is a convenient side-effect. Further, spokes are not made thicker (as the incorrect term "butting" would suggest) but as swaged to make them thinner in the shank. This design mimics machine bolts that have to withstand high cyclical loads which have the effect of propagating stress cracks at the spoke thread. The mechanism it protects the thread/head is that the thinner shank does all the stretching and relaxing with each revolution of the wheel. Had the shank not been swaged, the stretching and relaxing would happen in the thread and at the elbow where the spoke is at its weakest and compromised by the manufacturing process of thread rolling and bending. It is counter-intuitive. I have previously explained why triple-butted spokes are a bad idea and also why they still exist.

I have already distinguished between strength and durability. Whilst a straight-gauge spoke is technically stronger (in tensile applications) than a butted spoke with smaller shank diameter, it is not more durable and therefore never a preferred option other than for reasons of cost. Have a look at my explanation elsewhere in this thread.

The spoke tensions can't equalise, because the rim has strength, but the tendency is for the tensions to equalise, which is why a poorly tensioned wheel goes out of true. The mechanism is that if a spoke has higher tension, the rim gets pulled over which shortens the spoke, which reduces the tension.

I agree that the spoke tension can't equalize but not for your stated reason. The reason is simple: ramp friction in the nipple prevents the nipple in a properly-tensioned spoke from going loose and conversely, no force exists that will tighten a spoke by itself.

 

[Yellow Saddle]

Hear is a paragraph lifted from DCR wheels website.

A spoke is subject to three key kinds of force. One is constant, because of the tension it is held under and the weight of the bicycle. Sapim refer to this as carry. The others are inconstant (impact and transmission); when an impact is made upon the wheel so the spoke needs to help absorb these irregularities. The spoke will only do part of this, some will be absorbed by other components; rim, frame, handlebars etc. Some is passed onto the rider. It is when a wheel receives these sudden forces that butted and aero (which are also butted) spokes become important. The dimensions of a butted spoke differ from plain gauge because they are not a continuous thickness throughout. Sapim’s butted spokes use the SCFT-system (Sapim Cold Forging Technology). This allows the spoke to be stretched without causing any damage to its molecular structure and Sapim claim this increases the strength in the central portion by at least 48%. Butted spokes can flex and absorb impacts better than plain gauge because their central butting allows them to stretch. The extra material around the elbow and nipple allow strength to be retained in the areas that commonly fail. Finally the thinner central portion allows the butted spoke to be lighter than the plain gauge.

This is very vague and possibly suffers from translation. DRC is Italian and their website has always lacked the clarity and benefit of a trained translator. Nevertheless, there are valid points hidden in the obscurity. The constant force they refer to is called static load and exists purely because of the tension in the spokes. Tension pulls against the hub and rim and even just sitting there doing nothing, the wheel is statically loaded.

In transmission or when you pedal and brake (with a hub brake) you have dynamic loads that tension some spokes more and relax others. In other words, create pulling and pushing spokes. These forces are overlayed onto the static load and adds or takes from that depending on how the spoke is oriented in the wheel. Sapim is wong to claim that the butting process adds strength. It doesn't because thinner wire is weaker than thicker wire, all other things being equal. What it does add is durability. I have explained the mechanism by which this happens. 48% is sucked out of the air because the premise is invalid. The butting (combined with stress relieving during building) improves the durability just about infinitely.

I don't like the use of "extra material" because it leads to misunderstanding. The unprocessed spoke was 2.0mm to start off with and the central shank was then drawn (swaged) to a lesser diameter. That doesn't increase the strength at the "butts" but adds durability to the spoke in its entirety.

Weight reduction is a convenient side-effect of the process.

 

[raleighnut]

Cold drawing also changes the micro crystalline structure of the steel increasing strength.

 

[Venod]

Sapim is wong to claim that the butting process adds strength

Surely if Sapim is wrong in their statement they wouldn't be putting the claims on their website as it could be reported to the trading standardss people. Butted spokes have a higher tensile strength per mm2 than plain spokes.

From Sapims Website.

"Sapim butted spokes have two major benefits: less weight and more strength! The SCFT-system (Sapim Cold Forging Technology) ’stretch‘ the spoke while retaining the linear molecular structure of the material, thereby increasing the spoke strength at the middle by at least 48%"

As an added bonus, the thin middle section of the light Laser and Race spokes also provides better shock absorption.

 

[Yellow Saddle]

Surely if Sapim is wrong in their statement they wouldn't be putting the claims on their website as it could be reported to the trading standardss people. Butted spokes have a higher tensile strength per mm2 than plain spokes.

From Sapims Website.

"Sapim butted spokes have two major benefits: less weight and more strength! The SCFT-system (Sapim Cold Forging Technology) ’stretch‘ the spoke while retaining the linear molecular structure of the material, thereby increasing the spoke strength at the middle by at least 48%"

As an added bonus, the thin middle section of the light Laser and Race spokes also provides better shock absorption.

Well, someone report it then. Butting (actually swaging) does not add overall strength but increases durability. Have a look at what I've written about durability compared to strength. Strength is a misused term when it comes to spokes. Whilst the swaging process may well increase the general strength of the wire in the swaged section, a 1.8mm piece of wire has less tensile strength than a 2mm piece of wire from the same material.

In tensile strength tests done by Jobst Brand (the Bicycle Wheel, 3rd edition, page 125) the tensile breaking point of a 2.0mm spoke was 3250 Newton and the tensile breaking point of a 1.8mm spoke only 2500 Newton. But strong enough is strong enough and none of these spokes will fail in a wheel in that mode i.e. tensile. They all break from fatigue and here the durability of a thinner swaged spoke is superior. In other words, wheels built with double-butted spokes last longer irrespective of the general strength of the spoke itself.

If I have to re-write Sapim's copy and be honest about the process it would read like this.

"SAPIM double butted spokes have three benefits: better durability, better aerodynamic and less weight. The cold forging done on the spoke creates a linear molecular structure that increases durability and allows the spoke to flex where it can do least harm to the vulnerable elbow and thread."

Note that I claim that the 48% increase in strength claimed by SAPIM when they REDUCE the diameter is absolute nonsense.

 

[Venod]

Note that I claim that the 48% increase in strength claimed by SAPIM when they REDUCE the diameter is absolute nonsense

Perhaps they mean that the spoke stretched down to 1.8mm from 2.0mm is 48% stronger than an unstretched equivalent 1.8mm spoke. I did say per mm2 when comparing strength, therefore Sapims statement colud be considered correct, if a little misleading.

 

[Yellow Saddle]

Perhaps they mean that the spoke stretched down to 1.8mm from 2.0mm is 48% stronger than an unstretched equivalent 1.8mm spoke. I did say per mm2 when comparing strength, therefore Sapims statement colud be considered correct, if a little misleading.

Yes you did clarify it and I didn't mean to contradict you by way of ignoring the rider.
However, the data tells a different story. I've posted some real figures above and a swaged spoke still breaks quicker, that's not a 48% increase in strength at all. You can punch the numbers into a calculator and see if it even represents a specific strength increase once the difference in area is factored in. My eye says no.

 

[Venod]

Not wishing to be too pedantic but looking at the graphs In Job Brandt's book it shows that a plain gauge 1.8mm spoke breaks quicker than a swagged 2.0mm ( which will be 1.8mm at the swagged section)

 

[Yellow Saddle]

Not wishing to be too pedantic but looking at the graphs In Job Brandt's book it shows that a plain gauge 1.8mm spoke breaks quicker than a swagged 2.0mm ( which will be 1.8mm at the swagged section)

Yes, but only marginally, not 48% as claimed by a competing spoke company.

The Wheelsmith graph shows the opposite of that, breaking quicker. It has to do with the way that the swaged shank is achieved. If it is just drawn, it will behave different from if it was hammered in. IIRC DT Swiss has a rotary hammer method for swaging. If you look at a DT spoke you'll note the spiral pattern on the shank whereas that is absent from a SAPIM spoke. The DT Spoke is forged in the true sense of the word whereas the SAPIM spoke is cold drawn. I suspect the result shows up the specific tensile strength of the end product. Nevertheless, the issue is moot because tensile strength does not lead to durable spokes with a high fatigue life.

If you have a copy of the book you may want to pencil in a correction on those four graphs on page 125. The top left one should be labeled DT Swaged, not straight to mirror the Wheelsmith graphs for a similar product in the bottom two graphs.

If you look at the DT graph, you'll see how DT's forge method of swaging alters the spoke's behavior at the extreme end of its strain. I'll scan the graphs and post them for the benefit of those without the book.

I also don't know if Jobst actually measured the shanks. A nominal 1.8mm will not necessary come up as 1.8 on a vernier or micrometer. I still think the answer lies in the shape of the DT graph compared to the WS one.

 

[Venod]

Yes I have the correctly labeled graphs, what you haven't said is if you agree or disagree with my reading of the graph's.

 

[Smurfy]

The washers solve no known problem in that case. Most spoke holes are drilled to 2.3mm and most spokes are 2.0mm at the elbow and therefore all "too small" for the hole by that definition. By putting a washer under the spoke head you simply increase the effective flange diameter, which is not necessary with modern hubs such as Alfine. If the washers were fitted to prevent too-small spoke heads from pulling through the hole, very strange spokes were used since heads are all the same size.

I can't be bothered to dismantle my wheel to measure the flange hole diameter. It's not about preventing spoke heads being pulled through the flange hole, it's about having a snug fit and minimising movement of the spoke in the flange hole. This website lists the flange hole diameter as 2.9mm, quite a bit larger than 2.3mm, and a rather sloppy fit for 2mm spokes.

http://www.kstoerz.com/freespoke/hub/169

These hubs are suitable for utility and hire bikes, to be used and abused, but not everyone needs or wants monster spokes. For general use, 2mm spokes and spoke washers is the best build for this hub, which is what my professional wheelbuilder did.

 

[Yellow Saddle]

Yes I have the correctly labeled graphs, what you haven't said is if you agree or disagree with my reading of the graph's.

I have replied: I said:
Yes, but only marginally, not 48% as claimed by a competing spoke company.

 

[Venod]

The Wheelsmith graph shows the opposite of that

No, it shows the same results, that a 1.8mm straight gauge spoke fails before a 2.0mm swagged (1.8mm in the reduced middle section) spoke.

 

[Venod]

I have replied: I said:
Yes, but only marginally, not 48% as claimed by a competing spoke company.

Yes now you have edited your post.

 

[Yellow Saddle]

Cut cut cut .... It's not about preventing spoke heads being pulled through the flange hole, it's about having a snug fit and minimising movement of the spoke in the flange hole.

Once the spoke is tensioned, then there is no movement of spoke inside the hole. The only time there can be movement is when the spoke is loaded so that tension is reduced to Zero. This is an unlikely and catastrophic event. In service a spoke never goes to Zero.

By your definition even a standard 2.3mm hole can therefore also not offer the "snug fit", there being 0.15mm gap all around the spoke if it is centered in the hole.

A spoke will always pull against the side of the hole and remain there, no matter how big the gap on the other side is.

As I said, the only time a spoke washer can help is if the flange is too thin, leaving too big an exposed elbow bend. However, even that can be corrected by stress relieving or forcefully flattening the elbow against the flange.