40-spoke rear hub?

[Crumbling Nick]

Son's commuter has been suffering rear spoke failures. The wheel has 36 double-butted spokes, On-one double-fixed hub, DRC ST19 rim. Rider weight is about 110 kg, plus up to 10 kg baggage.

I built the bike & wheels during 2009, for a 1 mile journey to the station, though about 2.5 years ago that became a less frequent 16 mile round trip. Estimated mileage to date is around 8 000. For comparison my 36 rear spokes have lasted about 45 000 miles. Original build on the Goldtec hub was by LBS. I transplanted a new rim about 20 000 miles ago. I weigh 72kg.

The first breakages were after hitting a pothole downhill. I repaired & retrued/tensioned the wheel. It seemed sound once the failed spokes had been replaced, i.e. tension still fairly even & suitably high. All 3 failures were at the hub end & looked uncomfortably like fatigue. I don't have much detail of subsequent breakages, other than that there have been no obvious incidents that could be blamed.

It's most likely that the failures are due to fatigue. That means either that the wheel has been badly built, the spokes(unlikely)/hub have a quality problem, or the specification is not good enough.

First thought is a new 40-spoke wheel. Does anyone make a double-fixed hub for 40 spokes? I assume that a rim would be less problematic.

Has anyone any other ideas, including suggestions about improved building technique? (Mods, these might be better split off to the Knowledge).

[LittleWheelsandBig]

Just rebuild the wheel with DT or Sapim DB spokes, with spoke washers if the hub flanges are superthin (shouldn't be).

40 hole rims and fixed hubs are speciality items. Even track tandems are happy running 36 hole hubs.

[Jonah]

http://www.philwood.com/products/hubspgs/touringhubs.php

[Crumbling Nick]

http://www.philwood.com/products/hubspgs/touringhubs.php

Thanks for the link, even though it led to freehubs. I could see no sign of any 40 hole fixed hubs on the site, apart from the offer "other spoke hole count options are availabe upon inquiry". I presume that they are used to delivering on that promise, but price & schedule may be unpredictable.

Slightly more disturbing was the advice " Some cog manufacturers are making cogs that are not ideal for use with Phil Wood hubs due to their thickness (which is generally a result of plating). For this reason, we do not recommend using Surly or Euro-Asia cogs with our track hubs. The additional thickness can result in stripping of the lockring and/or hub threads because there is not enough thread engagement". That sort of message does not inspire the sort of trust that would encourage me pay their prices to a non-European supplier.

[Crumbling Nick]

Just rebuild the wheel with DT or Sapim DB spokes, with spoke washers if the hub flanges are superthin (shouldn't be).

40 hole rims and fixed hubs are speciality items. Even track tandems are happy running 36 hole hubs.

Hub flanges are not superthin; scantlings seem to be the same as the current System EX (& aliases) rear hubs.. I've no experiece of spoke washers, though can grasp the theory well enough

I agree with our son's view that durability of current build is simply not fit for purpose. Repeating the same mistake would not look very clever on my part.

I'm not sure about the relevance of track racing tandems, which are not expected to be durable & are used on smooth surfaces, to a country lane commuting bike with luggage. Can you explain, please? I was aware that 40 spoke rear wheels were no longer the standard after the 1970s, which is why I asked the original question.

[LittleWheelsandBig]

Track tandems cope with up to 1000 watts through the rear wheel and lots of sideways loading.

You've not actually mentioned what type of spokes were used first time round. DT produced a batch of spokes with excessive elbow length several years back. They tended to fatigue quickly. I'm quite sure that many wheelbuilders (including me) could reuse the hub and possibly the rim and produce a reliable wheel for you.

[Jonah]

http://www.philwood.com/products/hubspgs/touringhubs.php

Thanks for the link, even though it led to freehubs. I could see no sign of any 40 hole fixed hubs on the site, apart from the offer "other spoke hole count options are availabe upon inquiry". I presume that they are used to delivering on that promise, but price & schedule may be unpredictable.

Slightly more disturbing was the advice " Some cog manufacturers are making cogs that are not ideal for use with Phil Wood hubs due to their thickness (which is generally a result of plating). For this reason, we do not recommend using Surly or Euro-Asia cogs with our track hubs. The additional thickness can result in stripping of the lockring and/or hub threads because there is not enough thread engagement". That sort of message does not inspire the sort of trust that would encourage me pay their prices to a non-European supplier.
[/quo

Sorry, I wasn't looking closely enough; I just saw 40-hole and assumed they were fixed. 

I second your advice on the Phil hub.  My double-fixed Phil has now been reduced to a single-fixed after I used an EAI cog and stripped the thread.  To add insult to injury, I later stood on the shard of thread in bare feet, requiring me to remove it from my bleeding heel.

I must say I was seduced by the shiny EAI cog but it has now cost me dearly.

[Crumbling Nick]

http://www.philwood.com/products/hubspgs/touringhubs.php

Thanks for the link, even though it led to freehubs. I could see no sign of any 40 hole fixed hubs on the site, apart from the offer "other spoke hole count options are availabe upon inquiry". I presume that they are used to delivering on that promise, but price & schedule may be unpredictable.

Slightly more disturbing was the advice " Some cog manufacturers are making cogs that are not ideal for use with Phil Wood hubs due to their thickness (which is generally a result of plating). For this reason, we do not recommend using Surly or Euro-Asia cogs with our track hubs. The additional thickness can result in stripping of the lockring and/or hub threads because there is not enough thread engagement". That sort of message does not inspire the sort of trust that would encourage me pay their prices to a non-European supplier.
[/quo

Sorry, I wasn't looking closely enough; I just saw 40-hole and assumed they were fixed. 

I second your advice on the Phil hub.  My double-fixed Phil has now been reduced to a single-fixed after I used an EAI cog and stripped the thread.  To add insult to injury, I later stood on the shard of thread in bare feet, requiring me to remove it from my bleeding heel.

I must say I was seduced by the shiny EAI cog but it has now cost me dearly.

I share your pain; it could easily have been I.

Today's search for what happened to HubJub (now resolved) produced, by chance a thread initiated by Sergeant Pluck that gives chapter & verse to the problem you experienced. It's annoyingly wise after the event stuff from your POV, but I'm starting to think that yacf is not the forum it was for this type of information.

[Crumbling Nick]

Track tandems cope with up to 1000 watts through the rear wheel and lots of sideways loading.

You've not actually mentioned what type of spokes were used first time round. DT produced a batch of spokes with excessive elbow length several years back. They tended to fatigue quickly. I'm quite sure that many wheelbuilders (including me) could reuse the hub and possibly the rim and produce a reliable wheel for you.

Thanks for the response.

The spokes were Sapim, since that's what LBS supplies. I've only used DT spokes on one wheel so far.

The track tandem part was only surprising in one aspect...

Power output of 1 kW at a speed of 72 km/h means a tyre force of 50 Newtons. Rider weight (bike+riders=200kg) is around 1 000 Newtons on each wheel. 5% extra force isn't that important.

Meanwhile in my version of the real world, a gentle half metre dip of 15mm in the road is enough to double the load on the wheels. It's a ripple, not a pothole, but it's road, not track.

The suprise was the 1 000 watts figure. I would have expected somethng more like 3 kW. 15% is not quite trivial, so I spent some time working through Jobst Brandt's book (F.E. analysis, complete with some typos) before concluding that braking & power forces have a tiny bit less effect on spoke tension than weight.

This isn't getting any closer to solving the problem. A sequence of bad experiences with son's Galaxy (still unresolved) has left me with some distrust of the advice (as opposed to the craftmanship) of professional wheelbuilders.

[LittleWheelsandBig]

Don't forget to include the effects of lateral loading from sprinters accelerating out of the saddle. You've assumed that that all forces are in plane.

You suspect that the spokes have failed from fatigue. Tandems are following the solo bike trend towards stiffer rims and fewer spokes. 36 spokes are demonstrably holding up for both road and track teams. Most wheel builders use the same spoke tension for 28, 32 or 36 spokes if the rim is able to stand the resultant compression. If the spoke fatigue problem is the result of an insuffiently supported spoke elbow (as seems likely), how will 4 more spokes fix it?

[blackburnrod]

Rebuild using 13/14 gauge single butted spokes.

[Crumbling Nick]

Long pause for thought, for which I thank you...

Don't forget to include the effects of lateral loading from sprinters accelerating out of the saddle. You've assumed that all forces are in plane.

Agreed. But it's not possible to quantify lateral loads AFAIK. As a fixed wheel & tandem rider I am aware that riding out of the saddle is a very complex dynamic balancing act. As a result of my riding experience I am inclined to share Brandt's opinion that lateral forces are not critical. I suspect that the (secondary, in IC engine jargon) out of balance effect of legs at a cadence above 180rpm, with a seated rider, may be bigger. I'll listen if some empirical evidence turns up to show otherwise.

You suspect that the spokes have failed from fatigue. Tandems are following the solo bike trend towards stiffer rims and fewer spokes. 36 spokes are demonstrably holding up for both road and track teams. Most wheel builders use the same spoke tension for 28, 32 or 36 spokes if the rim is able to stand the resultant compression. If the spoke fatigue problem is the result of an insufficiently supported spoke elbow (as seems likely), how will 4 more spokes fix it?

I don't think I understand what you're saying that about insufficiently supported spoke elbow. I'm keeping an open mind.

Meanwhile some basics. Fatigue failure is fatigue failure. It's caused by cycles of repetitive stress that are lower than the stresses that would cause immediate failure. Preventing failure in the egineering world is usually based on reducing stresses, though sometimes specifying better materials or manufacturing techniques (ever heard of nitrided crankshafts for car engines?) may work.  Undergraduate text books explain it better than I can.

The wheel in question has had about 5 million load cycles (in this case rotations).   S-N data for steels shows a fatigue limit (the stress at which fatigue failure will never occur) is reached after 50 million cycles and this typical fatigue limit is only about 5% lower than the stress for failure at 5 million cycles.

So, increasing spoke count by 11%, which, at my level of naivety in stress analysis should reduce spoke stresses by a tad more, may well be enough to reduce stresses below the fatigue limit.

Oops - minor linguistic problem: if a 40 spoke hub & rim had been readily available, it might have solved the problem. I think my original question has already been answered, within the limitations of this forum.

Are you still with me? I'm not very good at explaining things to people I don't know, especially without the feedback that comes from face to face contact. S-N curves are commonplace stuff for Mechanical Engineers (which I am not), but there is more to come.

The problem with the fatigue failure stuff is that it doesn't tackle the complexities of the real life load cycles that bikes experience on roads. That's the point at which I resorted to empirical evidence. Son & I both have 2 bikes. 2 have symmetrical rear wheels, 2 have dished rear wheels. I'll fill in details if you think it will help to provide a solution to the original problem.

[rogerzilla]

Rebuild using 13/14 gauge single butted spokes.

They just concentrate the stress in the wrong place.  The clever thing about double butted spokes is that they concentrate the stress in an area where spokes very rarely break, and give a bit of extra elasticity too so that a properly tensioned spoke can't ever go slack in use.

On an undished wheel you shouldn't need 40 spokes, even if you're camping.

The cause is simple - the LBS did a substandard wheelbuild.  Rebuild it with DT Competition or Sapim Race spokes and get them tight.

[Biggsy]

There are triple butted (or even bladed) spokes for those who want a bit more insurance.  DT Alpine III are 2.3/1.8/2.0 mm.

[Crumbling Nick]

Roger, you may have had a problem reading my OP due to a special character. I have edited it out (it works fine on my Andriod phone!) so that you can read it. You may wish to reconsider your reply.

[rogerzilla]

Ah...it was the wheel on *your* bike that was built by the LBS, I see.

If they're breaking at the hub end you need more tension and a savage stress relieving session.  An old crank used as a lever at each spoke crossing is best.  The objective is to set the bends against the hub (and straighten out the inside spokes) so that there can be no movement in use.

[Crumbling Nick]

Rebuild using 13/14 gauge single butted spokes.

They just concentrate the stress in the wrong place.

Can you explain or provide a source for that assertion. I'm planning to use those spokes on the drive side of son's Galaxy, which is getting a bit OT. 

The clever thing about double butted spokes is that they concentrate the stress in an area where spokes very rarely break, and give a bit of extra elasticity too so that a properly tensioned spoke can't ever go slack in use.

On an undished wheel you shouldn't need 40 spokes, even if you're camping.

That was pretty much my starting point when I built the wheel. I don't need 40 spokes and TBH might get away with a 32 spoke rear wheel, but the rider in question is 50% heavier than I. I'll let you use a slide rule to work out the quantitative implication.

[snip]

If they're breaking at the hub end you need more tension and a savage stress relieving session.  An old crank used as a lever at each spoke crossing is best.  The objective is to set the bends against the hub (and straighten out the inside spokes) so that there can be no movement in use.

I've been building with higher tension than LBS (whom I respect and work well with) who almost certainly use a spoke tensiometer, since most of their builds use Mavic rims, which have a reputation for cracking around the spoke holes if tensioned above Mavic's limit. However I don't have a tensiometer, so can't quantify quickly.

There is a more subtle issue of higher mean stress reducing fatigue life, but I can't locate the source of that (possibly Chris Juden), so will have to treat it as speculative for now.

The spokes were stress relieved. I use Brandt's technique, rather than Sheldon Brown's, which you describe. I have big hands & strong fingers...

You missed out Brandt's advice on correcting the spoke line at the hub (& for that matter the nipple end, though for that build, Polyax nipples worked without my assistance), which I routinely follow.

[LittleWheelsandBig]

Does he weigh more than a tandem? Low spoke count wheels have proved quite reliable for tandems.
http://www.rolfprima.com/products-Tandem.php

[Crumbling Nick]

Does he weigh more than a tandem? Low spoke count wheels have proved quite reliable for tandems.
http://www.rolfprima.com/products-Tandem.php

Short answer has to be no, certainly not as much as a pair of strong track riders.

But there seems to be a big gap between what you find works & what CTC's Chris Juden had found not to work, to his embarrassment, on touring bikes. That latter is closer to my bitter experience than yours.

Any ideas on resolving what is looking increasingly like trench warfare (other than Biggsy's safe solution to the immediate problem, which I had already started to explore, though the equivalent Sapim spokes aren't available in the UK)?

[LittleWheelsandBig]

The conventional approach doesn't seem to be working for you, so the unconventional approach you are pursuing might be a good option.

That said, there is no sensible reason why that wheel is causing problems, so I'd still just rebuild it with very similar spokes and only try something else if the second attempt failed prematurely.

[Crumbling Nick]

I tried that approach with son's Galaxy - obviously a different case since it's a dished wheel. This was before I started building wheels.

Second builder, whose reputation was enough to generate a waiting list several months long, produced a wheel which started breaking spokes after a couple of thousand miles, so no better than the first build.

That's what prompted my first reply to you. Frankly we still seem to be living in completely different worlds, and I don't understand why our experiences are so inexplicably different.

[jsabine]

Rebuild using 13/14 gauge single butted spokes.

They just concentrate the stress in the wrong place.

Can you explain or provide a source for that assertion.

I thought there was commentary to that effect in Brandt's book, but couldn't find it on a cursory look just now. I seem to recall discussion - with acerbic input from JB - in newsgroups, probably r.b.tech.

That said, there is no sensible reason why that wheel is causing problems, so I'd still just rebuild it with very similar spokes and only try something else if the second attempt failed prematurely.

I'd probably do the same.

FWIW, though I'm a good bit lighter than your son, I spent quite a while carting 30kg or so of tools round on a bike, a good bit of which was using a somewhat inappropriate 28-spoke fixed hub built onto an Open Pro. No spoke breakages, though I did manage to tear one of the hub flanges off after a few months - I second the idea that a normal number of DB spokes (DT Competitions in this case) is perfectly adequate. I'd expect a rebuild with new spokes to leave you frustrated at this problem having to be written off as just one of those things.

[Biggsy]

(I'm using metric sizes because I forget what old-fashioned gauge is what, and quoting the actual diameters makes them easier to visualise).

Rebuild using 13/14 gauge single butted spokes.

They just concentrate the stress in the wrong place.

Can you explain or provide a source for that assertion. I'm planning to use those spokes on the drive side of son's Galaxy, which is getting a bit OT. 

Roger basically explained that with his next sentence:

The clever thing about double butted spokes is that they concentrate the stress in an area [the middles] where spokes very rarely break,

Sapim Strong do have thicker head ends as an attempt to make them harder to fatigue, but they don't have thin middle sections for the stress to concentrate to.  DT Alpine III provide the best of BOTH double butted and Sapim Strong spokes, and they're lighter as well.  They're 2.3 mm at the head end (like Sapim Strong), 1.8 in the middle (like DT Competition and Sapim Race), 2.0 at the rim end (like all these spokes).  Sapim Strong (2.3 head end, 2.0 rest) only make logical sense for (daft?) people who don't believe in double/triple butted spokes but want something beefier than 2.0 mm plain gauge.

So, I reckon the only sensible choices are: DT Alpine III if you don't mind the expense; or DT Competition or Sapim Race double butted for the lower cost and greater availability; or perhaps bladed spokes like Sapim CX-Ray for the ultimate fatigue resistance (and low weight and good aerodynamics, but high expense).  DT spokes (particularly Alpine III) had a problem with overly long elbows at one time, but I think that was corrected some years ago.

A compromise (or even an advantage) is to have thinner spokes on the rear non-drive side.  (Theory explained on SheldonBrown.com).

I don't do tandem, but I do do wheels with plenty of redundancy built in (because I've got no mug to come and rescue me if I get stuck out in the sticks; and I carry very heavy shopping on one bike).  My rear wheels have 36 spokes; 3-cross DT Alpine III drive side; 2-cross Sapim Laser or DT Revolution non-drive side.  I have the non-drive side like this mainly to save weight, but I'm guessing you're not bothered about a few extra grams on a tandem.  However, with 40 spokes, I would have thought ordinary double butted would be fine if the build was really good.  (You don't necessarily get a really good build from a professional).

[Crumbling Nick]

I have in the workshop the failing wheels from both of son's bikes. I shall replace broken spokes, but it's evidently only a short-term bodge.

The hub wear patterns both show clear evidence of fretting/hammering, which can only be caused by spokes unloading to such an extent that they are wearing oval holes in the hub, presumably when they tighten up again. This is familiar enough;  last time was an undertensioned build (or 2) on my bike nearly 40 years ago.

I hypothesise that the hub damage has occurred due to the very high loads on bumpy country lanes, and that the damage has resulted in spoke failure due to inadequately supported spokes at the hub. Thanks to Little Wheels and Big for the germ of that idea.

The big surprise was a completely separate ovalisation of the spoke holes on the fixed wheel hub. I don't think it's just the one that failed most recently that shows this. The erosion is radially inwards, so nowhere near along the line of the spoke. This is much deeper on the spoke side & I find it difficult to explain. Part is easy : it can only have happened when the spokes were unloaded in a very big way. The wear pattern is what I'd expect from a spoke under compression, but it's in the wrong direction. Sorry I can't provide photos. Time is limited & photographic technology is fragile.

Has anyone else seen anything like that?

I've thrown out all my ideas for rebuilding & am literally starting again from the beginning. The hubs can only be re-used if re-spoked in a different direction, but nothing short of many more spokes or different sorts of spoke looks likely to produce durable wheels. Thicker ends at the hub now look unlikely to work. I feel as though I'm running out of options, even for the symmetrical wheel.
 

[uphillbothways]

40h hubs are very much a rarity these days, because even tandem users are generally well served by 36 spoke wheels. Rim stiffness is a key factor in wheel strength - a really stiff rim like a Velocity Deep V will handle a great deal of abuse even with a relatively low spoke count, because the spoke loads are more evenly distributed.

If you're after the strongest possible wheels, I'd suggest looking into bits made for cycle polo. 48h hubs and rims designed for this purpose are available at quite reasonable prices. Velocity make a polo-specific rim called the Chukker that builds up into a phenomenally strong wheel. Brick Lane Bikes will sort you out, call 0207 033 9053.