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-   -   Theory: PSI & spoke life (https://www.bikeforums.net/general-cycling-discussion/1145305-theory-psi-spoke-life.html)

Troul 05-27-18 09:41 PM

Theory: PSI & spoke life
 
700 x 32 with tubes 32 spokes double walled

Been running the psi within 5 to 10 psi under the higher tolerances by the oem [max rear 75 max front 95] of the tire & have had no wheel or spoke issues.
before doing so, I've typically ran 20 psi under the max & a spoke coming loose or breaking seemed to happen every now & then.

is it coincidence? Probably jinx myself now...

Road Fan 05-28-18 05:34 AM

I think your spokes are not tight enough.

big chainring 05-28-18 06:22 AM

In 45 years of cycling and 10's of thousands of miles I have only broken maybe 6 spokes. The wheels on my bike now are from 1971. All original spokes. Maybe they make wheels differently now. But spoke breakage is or should be a rare happening.

Kovkov 05-28-18 06:40 AM


In 45 years of cycling and 10's of thousands of miles I have only broken maybe 6 spokes. The wheels on my bike now are from 1971. All original spokes. Maybe they make wheels differently now. But spoke breakage is or should be a rare happening.
This. I broke 2 spokes so far. One on a wheel from 1956 and one on a wheel from 1963. Both broke in 2017.

cyccommute 05-28-18 07:36 AM


Originally Posted by big chainring (Post 20363784)
In 45 years of cycling and 10's of thousands of miles I have only broken maybe 6 spokes. The wheels on my bike now are from 1971. All original spokes. Maybe they make wheels differently now. But spoke breakage is or should be a rare happening.


Originally Posted by Kovkov (Post 20363807)
This. I broke 2 spokes so far. One on a wheel from 1956 and one on a wheel from 1963. Both broke in 2017.

Why is it that when broken spokes are mentioned, all kinds of people tell us that spokes "should ever break"? People break spokes all the time and it's not just from damage. Spokes do fatigue...which is how we know about concepts like "spoke fatigue"...and occasionally break. If you've never broken a spoke, just consider yourself lucky.

And, yes, I know all the tricks of wheel building. I know how to form spokes to the hub, how to stress relieve the wheel, how to tension properly, how to choose good spokes, etc. I've been using these since the mid-80s. But I also know that spokes sometimes fail. Again, if they never failed we wouldn't know anything about spoke failure.


Originally Posted by Troul (Post 20363484)
700 x 32 with tubes 32 spokes double walled

Been running the psi within 5 to 10 psi under the higher tolerances by the oem [max rear 75 max front 95] of the tire & have had no wheel or spoke issues.
before doing so, I've typically ran 20 psi under the max & a spoke coming loose or breaking seemed to happen every now & then.

is it coincidence? Probably jinx myself now...

I would say that it's a coincidence. Tire pressure causes the spoke tension to decrease as the pressure increases.

GerryinHouston 05-28-18 09:26 AM

Spokes break due to bad spoke tensioning, not tire inflation pressure. If you radically under-inflate your tires and the rim makes hard contact, you will get snake bite punctures and might get a broken or bent rim. It is highly unlikely that your spokes will break...

blue192 05-28-18 09:28 AM

I work on a theory that there is a relation in spoke longevity and body mass... aka fat man breaks more spokes. xD

cyccommute 05-28-18 09:52 AM


Originally Posted by GerryinHouston (Post 20364038)
Spokes break due to bad spoke tensioning, not tire inflation pressure. If you radically under-inflate your tires and the rim makes hard contact, you will get snake bite punctures and might get a broken or bent rim. It is highly unlikely that your spokes will break...

You are missing the point about tire inflation. I agree that lower pressure doesn't cause spoke tension problems. Higher pressures cause deformation of rim which have an effect on the spoke tension that may no have been accounted for when building. I would also say that impacts of the rim on the ground are going to cause momentary cycling of an individual spoke's tension which could lead to fatigue issues if done often enough.

I would disagree about spoke breakage being due to bad spoke tensioning only. Tension is part of it but so is using spokes that are too small a diameter for the load that they are being used for. A spoke with a 1.5mm elbow isn't going to have the longevity of one with a 2.0mm elbow or a 2.3mm elbow. Thicker elbows have a significant impact on the amount of stress the spoke can take before it breaks.


Originally Posted by blue192 (Post 20364045)
I work on a theory that there is a relation in spoke longevity and body mass... aka fat man breaks more spokes. xD

Wrong and insulting.

ThermionicScott 05-28-18 11:55 AM


Originally Posted by cyccommute (Post 20364096)
Wrong and insulting.

+1. Cue @FBinNY's "the ballerina and the linebacker" story. :thumb:

Kontact 05-28-18 12:51 PM

Changing tire pressure does different things to the wheel.

Decreased pressures soften bumps transmitted to the spokes. (Good for spokes.)

Increased pressures decreases spoke tension uniformly. (Could be good or bad depending on start tension.)

Increased pressure makes the rim more rigid. (Definitely good for spokes as spoke wear and breakage comes from tension changes as the wheel rolls across the contact patch.)


Of the three, increased pressures making the rim flex less strikes me as having the greatest effect on wheel life since road bumps aren't going to affect momentary tension variations to the extent that the contact patch does. So if you want to run lower pressures, it is best done with a larger volume tire to keep the same sort of net force on the rim.

Reynolds 05-28-18 01:07 PM


Originally Posted by cyccommute (Post 20364096)


Wrong and insulting.

Let's rephrase it:
More weight on the rear wheel breaks more spokes (all other factors being equal).
You agree?

UniChris 05-28-18 01:15 PM


Originally Posted by cyccommute (Post 20364096)
I would disagree about spoke breakage being due to bad spoke tensioning only.

You obviously have orders of magnitude more experience with this, but I thought a loose-ish spoke was likely to go slack at the bottom of the wheel, therefore be more likely to fatigue and break? (And if higher tire pressure de-tensions the wheel overall, then ones that were borderline could now be loose-ish ?)

Of course one that's loose enough to never be meaningfully contributing probably wouldn't have that problem.

ThermionicScott 05-28-18 01:16 PM


Originally Posted by Reynolds (Post 20364444)
Let's rephrase it:
More weight on the rear wheel breaks more spokes (all other factors being equal).
You agree?

Still depends on when and how the weight is applied. Is the rider lifting out of the saddle for rough terrain, or sitting on the saddle like a sack of potatoes regardless? ;)

Kontact 05-28-18 01:22 PM


Originally Posted by UniChris (Post 20364452)
You obviously have orders of magnitude more experience with this, but I thought a loose-ish spoke was likely to go slack at the bottom of the wheel, therefore be more likely to fatigue and break? (And if higher tire pressure de-tensions the wheel overall, then ones that were borderline could now be loose-ish ?)

Of course one that's loose enough to never be meaningfully contributing probably wouldn't have that problem.

Spoke tension increases, decreases then increases again as they roll pass the contact patch. But the tension never goes "slack" to zero.


Originally Posted by Reynolds
More weight on the rear wheel breaks more spokes (all other factors being equal).

Hard to say since the rear wheel is also the drive wheel, the dished wheel and the wheel that receives the greatest lateral loads.

UniChris 05-28-18 01:25 PM


Originally Posted by Kontact (Post 20364461)
Spoke tension increases, decreases then increases again as they roll pass the contact patch. But the tension never goes "slack" to zero.

That would depend on how loose it is to begin with.

Anyway, to my understanding, the problem is that spokes that become untensioned enough at the bottom of the wheel that they move relative to their shape when tensioned, are susceptible to fatigue cracking.

Thinks like forming the bend probably help, but short of something like a straight pull spoke I doubt you can get it formed perfectly enough that there is no movement at all as the tension changes.

Kontact 05-28-18 02:27 PM


Originally Posted by UniChris (Post 20364463)
That would depend on how loose it is to begin with.

Anyway, to my understanding, the problem is that spokes that become untensioned enough at the bottom of the wheel that they move relative to their shape when tensioned, are susceptible to fatigue cracking.

Thinks like forming the bend probably help, but short of something like a straight pull spoke I doubt you can get it formed perfectly enough that there is no movement at all as the tension changes.

What you're describing isn't a bicycle wheel - it is a bunch of spokes attached to a rim. If you want to talk about how something works, you can't start with the assumption that it is wrong or broken and then extrapolate from there.

Spokes fatigue from variations in spoke tension within their normal tension range. If the elbows aren't seated properly the fatigue load gets concentrated there and the elbows break. If there are too few spokes the tension they carry increases as does the amplitude of tension they see as the wheel rolls, wearing the spokes faster.

In a properly spec'd and built wheel, the spokes will never break from fatigue. The rim will fail first, followed by the aluminum flanges. Wheels that break spokes have something essential wrong with them in either design or execution, because steel spokes are much more fatigue resistant that the aluminum rim and hub.

Bill in VA 05-28-18 03:31 PM


Originally Posted by ThermionicScott (Post 20364454)
Still depends on when and how the weight is applied. Is the rider lifting out of the saddle for rough terrain, or sitting on the saddle like a sack of potatoes regardless? ;)

I believe this is the big issue,

Not specifically lifting, but the whole way a person rides.

I subscriber to the theory that all things are breakable, so what needs to be done to minimize breakage?. To me this trumps the science of why the spoke broke.

I am a road rider, so I avoid holes (if possible). I do not jump curbs, even though I have in the past when clipped in. I lift for any visible bump, including the NIMBY speed humps even if the newer gradual type, and even my driverway, but more for comfort than anything else. I also rode the same bike and wheels for 40 years with long periods of inactivity and resulting weight gains and loss. The wheels were hand built with strong rims, quality HF hubs and 36 quality DB spokes. Over the years I never had to true those rims, and all the tires were 27" x 1" (25mm) @80-90 PSI or 1 1/8 (28mm) @ 75PSI with wire beads as the rims were not hooked. I also used that bike to ride the C&O towpath and long stretchs of dirt roads in Canada and never had an issue. If the path/roads got too rough, I would dismount and walk over the rough terrain. I have also never had a pinch flat on any bike, even my old Columbia cruiser as a kid.

In short my goal was to finish the ride riding, and not walking home due to a breakage I could have avoided. I also think that over many of those years, if you broke you walked back. No cell phones for rescue for many of those years. Even so, I did and still do with my new bike, always carry 3 spare spokes, 1 front, and 1 for each side of the rear wheel. I carry them in a piece of clear vinyl tube inserted into the seat post. I would use them if the tire had flatted and it was grievously out of true, otherwise it could wait for down time (especially the cassette side of the rear)..

I still follow my rules for the current bike with 700c x 28-35mm tires. I run the 28s @90PSI front and 100PSI rear and the 32s at 70/80 F/R. I have 2 sets of wheels, the factory set and a hand built set optimized for the wider tires. Both have been problem free. It has worked for me..

But after all that, I do admit that i do find the science facinating.

Kontact 05-28-18 03:56 PM


Originally Posted by Bill in VA (Post 20364631)
I believe this is the big issue,

Not specifically lifting, but the whole way a person rides.

I subscriber to the theory that all things are breakable, so what needs to be done to minimize breakage?. To me this trumps the science of why the spoke broke.

I am a road rider, so I avoid holes (if possible). I do not jump curbs, even though I have in the past when clipped in. I lift for any visible bump, including the NIMBY speed humps even if the newer gradual type, and even my driverway, but more for comfort than anything else. I also rode the same bike and wheels for 40 years with long periods of inactivity and resulting weight gains and loss. The wheels were hand built with strong rims, quality HF hubs and 36 quality DB spokes. Over the years I never had to true those rims, and all the tires were 27" x 1" (25mm) @80-90 PSI or 1 1/8 (28mm) @ 75PSI with wire beads as the rims were not hooked. I also used that bike to ride the C&O towpath and long stretchs of dirt roads in Canada and never had an issue. If the path/roads got too rough, I would dismount and walk over the rough terrain. I have also never had a pinch flat on any bike, even my old Columbia cruiser as a kid.

In short my goal was to finish the ride riding, and not walking home due to a breakage I could have avoided. I also think that over many of those years, if you broke you walked back. No cell phones for rescue for many of those years. Even so, I did and still do with my new bike, always carry 3 spare spokes, 1 front, and 1 for each side of the rear wheel. I carry them in a piece of clear vinyl tube inserted into the seat post. I would use them if the tire had flatted and it was grievously out of true, otherwise it could wait for down time (especially the cassette side of the rear)..

I still follow my rules for the current bike with 700c x 28-35mm tires. I run the 28s @90PSI front and 100PSI rear and the 32s at 70/80 F/R. I have 2 sets of wheels, the factory set and a hand built set optimized for the wider tires. Both have been problem free. It has worked for me..

But after all that, I do admit that i do find the science facinating.

While these are all good ideas to avoid beating up your bike, impacts are unlikely to specifically break spokes, if that's what we're talking about. Impacts will brake rims and axles. There isn't a good way for an impact to concentrate force on just a few spokes to overstress them to break them. For impacts to damage spokes over time, you'd have to keep hitting bumps with the same section of the rim.

cyccommute 05-29-18 08:19 AM


Originally Posted by Reynolds (Post 20364444)
Let's rephrase it:
More weight on the rear wheel breaks more spokes (all other factors being equal).
You agree?

Yes, except all other factors are seldom equal. There are too many factors to be taken into consideration to make a blanket statement. Weight is a factor but so is spoke count, spoke gauge, tire inflation, riding habits, etc.

cyccommute 05-29-18 08:21 AM


Originally Posted by UniChris (Post 20364452)
You obviously have orders of magnitude more experience with this, but I thought a loose-ish spoke was likely to go slack at the bottom of the wheel, therefore be more likely to fatigue and break? (And if higher tire pressure de-tensions the wheel overall, then ones that were borderline could now be loose-ish ?)

Of course one that's loose enough to never be meaningfully contributing probably wouldn't have that problem.

It's the "only" part of the statement that I disagree with. Again, there are lots of other factors to consider to say that only one thing is responsible for spoke breakage.

cyccommute 05-29-18 08:31 AM


Originally Posted by Kontact (Post 20364560)
Spokes fatigue from variations in spoke tension within their normal tension range. If the elbows aren't seated properly the fatigue load gets concentrated there and the elbows break. If there are too few spokes the tension they carry increases as does the amplitude of tension they see as the wheel rolls, wearing the spokes faster.

In a properly spec'd and built wheel, the spokes will never break from fatigue. The rim will fail first, followed by the aluminum flanges. Wheels that break spokes have something essential wrong with them in either design or execution, because steel spokes are much more fatigue resistant that the aluminum rim and hub.

Your two statements are at odds with one another. I agree that spokes fatigue from variations in spoke tension. Their fatigue life isn't infinite. I also agree that fewer spokes put more load on each spoke and result in increased fatigue.

The fact that we know about spoke fatigue, however, negates your second statement. "Never" is a long time. Spokes can certainly outlast rims but a worn out rim isn't the end of a wheel. It usually is because finding an identical rim or even a rim with an identical ERD usually isn't worth the hassle and expense. But the spokes will eventually start to fail with enough miles on them no matter how well built the wheel is.

I would also disagree that the hub flange goes before the spokes. Hub flange failure is rare even when the hub is reused multiple times in wheel building.

Spoonrobot 05-29-18 08:41 AM

Like clockwork I've broken a spoke on every road wheelset (3 so far) I've ridden far enough, between 6,000-8,000 miles. This is 700cx28, 650bx42 and 700cx23.

I'm also of the camp that sometimes spokes just break.


For impacts to damage spokes over time, you'd have to keep hitting bumps with the same section of the rim.
This happens more often on mountain bikes but it does happen on road bikes too, hit the same section of rough road or potholes often enough and one section of rim is always going to take harder/more impacts than the rest. I recently broke a spoke on a mountain bike at the elbow and looking at the remaining section of elbow the amount of fretting was impressive. Wheels were tensioned close to the rim maximum and had not loosened during riding. Once I started paying attention I realized it's possible to hear the impacts that will cause the spokes to loose significant/all their tension.

I think "spokes should never break on a well-built wheelset" is mostly a party line spread about both by this forum and general word of mouth marketing by independent wheelbuilders trying to build cachet.

Troul 05-29-18 09:36 AM

glad my thinking isn't isolated & it's a rather thought bout subject.

Kontact 05-29-18 10:28 AM


Originally Posted by cyccommute (Post 20365620)
Your two statements are at odds with one another. I agree that spokes fatigue from variations in spoke tension. Their fatigue life isn't infinite. I also agree that fewer spokes put more load on each spoke and result in increased fatigue.

The fact that we know about spoke fatigue, however, negates your second statement. "Never" is a long time. Spokes can certainly outlast rims but a worn out rim isn't the end of a wheel. It usually is because finding an identical rim or even a rim with an identical ERD usually isn't worth the hassle and expense. But the spokes will eventually start to fail with enough miles on them no matter how well built the wheel is.

I would also disagree that the hub flange goes before the spokes. Hub flange failure is rare even when the hub is reused multiple times in wheel building.

They aren't at odds with each other - it is very clear that "the wheel" is what is built with the original, new components, not used parts. "Never" is only in the context of starting with a new wheel.

Hub flange failure is rare. Spoke elbow failure would also be rare - if all wheels had their spoke elbows properly bedded. But as noted in the Wheel Fanatyk article about Alpine III spokes you previously posted, lower cost wheels don't get that sort of hand work so the elbows are compromised. Like many things mechanical, you can get away with doing things not-quite-right for awhile, but if it isn't right there will be eventual consequences - like unbedded elbow breakage. You can put a 2.3mm bandaid on the problem, or bed your spoke elbows. Savvy pro wheelbuilders choose the latter.

Reynolds 05-29-18 11:52 AM

Spokes don't last forever and I don't know why they break, but I can tell you that with my usual setup and ride style - 36 spoke wheels built by myself, ACI double butted spokes, Shimano or Campagnolo hubs, Mavic or Weinmann rims, 70kg rider weight, avoid potholes, lift off the saddle at bumps - I've never broke a spoke in more than 50000kms.


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