cyccommute 

As a large, aggressive ride who regularly ride loaded touring bikes and loaded touring bikes, riding on the lightest rims available hasn’t been a problem for many years. I do build wheels from “strength” but discovered long ago that the rim isn’t a place where a lot of strength is needed. Build with strong spokes but the rim doesn’t really do much other than go along for the ride. This kind of problem is rather rare. It happens but it isn’t any reason to run out and buy super heavy rims to solve the problem.

Lombard

I have to disagree here. The rim is clearly the weakest link in a wheel. Over tension your spokes and what will fail - the rim, not the spokes. Use too few spokes for the load and what will fail - the rim, not the spokes. While I agree there isn't a reason to buy super heavy rims, using the lightest rim available isn't wise either.

L134 

On self supported tour last summer with 13 experienced people so 26 wheels. Two rim failures that required hitching to the next town. I don't think anyone broke a spoke. I have to admit I was surprised.

cyccommute 

If you use too few spokes for the load, the spokes fail by breaking. That’s why heavily loaded bikes have more spokes. Rims can crack, of course, but spokes are far more likely to break if the wheel is overloaded.

Lombard

Really? I have to question this. And even if so, it is a lot easier to replace a few spokes than to re-build a whole wheel with a new rim.

Granted too little tensionwill more likely break spokes but too much tension is more likely to crack spoke holes.

cyccommute 

Yes, really. Why do tandems and touring bikes and bikes that carry heavy loads have more spokes than lightly loaded race bikes? 36 hole wheels are common on single touring bikes and 40 hole wheels are common on tandems if not 48 spoke wheels. More spokes means that each spoke experiences less load. Yes, there are people out there riding tandems with low spoke count wheels but it is not a common practice. Here’s what Velocity says about spoke count

This is common advice and has been for decades. Here’s what Sheldon Brown said many years before he died (pre 2008)

I’ve experience that extra weight in low spoke count wheels myself. I replace 20 spoke Vuelta wheels with 32 spoke wheels I built with White Industries hubs, triple butted spokes, and Velocity A23 rims. I lost 2 lb of weight in the process. I was shocked at the difference.
I’m not saying that replacing individual spokes is difficult but it’s relatively trivial to replace a rim as long as the rim has an ERD close enough to the original rim’s ERD. The spokes can outlast several rims. However, if more than a few (3 or 4) spokes break, the whole wheel should be rebuilt…otherwise you are just going to keep chasing broken spokes. Rim failure, even the kind that headasunder has experienced, does not signal wheel death. Multiple spoke failure, on the other hand, does. Even a single spoke failure is cause for careful monitoring of the wheel health.

By the way, loose tension on spokes is just as likely to result in rims cracking as over-tensioning is. Tension that is too high puts a lot of strain on the aluminum in the rim as the wheel goes through the tensioning/detensioning cycle that all wheels experience. The spoke tries to pull through the aluminum. Too little tension allows the rim to flex more in that cycle which also causes the aluminum to crack.

grumpus

Is that still a problem? I know some 1990s Mavic rims had a reputation for cracking between the spoke holes but there can't be many of those left.

Lombard

Precisely my point! And less load on each spoke is less load on each spoke hole!

As for why low spoke wheels became a thing, certainly weight wasn't the reason nor was cost. Running fewer spokes means you need a heavier rim (as stated in your Sheldon Brown article) to compensate for the increased load on each spoke hole. Obviously the need for a heavier rim is not to prevent spokes from breaking.

The real reason for going to low spoke counts was that so many riders (buyers) find them sexy. Read again - BUYERS. They are giving the masses what they want.

I myself build my rim brake wheels 24F/32R and my disc brake wheels 32F/R. And I'm only 175# soaking wet. A tandem is carrying an extra person's weight - two me me is 350#. At least 40 spokes seems like a bare minimum for a tandem.

Further reason I don't like low spoke count wheels ia that if I break a spoke on a 24 or more spoke wheel, I can adjust other spokes to make it true enough to get home. Less than 24 spokes and I'll be walking or calling for a ride.

sweeks

Here are some more images of cracked rims. These are on Kinetix 20" (406) rims, all in service on the rear wheel of a folding bike. All of these rims (total of 3) exhibited "thumping" during brake application, and they were associated with cracks in the brake tracks. I assumed that they started out as cracks from brake-worn rims which then propagated to the center of the rim, but one rim did not have excessive brake wear. After discussing these problems with Bill Mould we concluded it was the reverse process. The rims, which were narrow (17mm inside, IIRC) with quite wide tires running t about 60 PSI, split down the middle due to tensile stress on the rims, and then in some cases spread to the brake track. I think the rim's expansion caused cyclic fatigue of the brake track during braking, resulting in the cracks there.
The problem went away when I started using wider rims (Sun-Ringle ENVY).

FBinNY 

Arguing whether spokes or rims are the weakest link is pointless. Wheels are a system, wherein each element works with others to produce the desired result.

The goal is to achieve something like Holme's "One Horse Shay" which lasts because no single element fails prematurely.

For my part, I'm a happy camper if I dent an old wheel on a pothole, only to notice the brake track is worn, and there are stress cracks here and there.

cyccommute 

Note what is missing from Velocity’s recommendations about what to use for heavy riders/loads…rim suggestions. Kind of curious coming from a company that makes rims but not spokes. They don’t say “use heavier rims”. Instead they say “use more spokes”. Yes, fewer spokes put more stress on the rim but, again, rim failure through cracking or pull through is less common than spoke breakage. The kind of failure that headasunder and, now, sweeks detail, are not related to spoke tension nor spoke count.

headasunder

Some useful advice and observations mixed in with the usual digressions, in the end I took fbinny's about not over thinking it and stuck with 2 cross,
spokes(pillar triple butted) and rim are in the hood first time building with dt Swiss squorx nipples and accompanying phr washers so should be interesting.

CliffordK

One thing about the failure under the tape. It seems relatively independent of the spokes, with greater relation to the tire and tire pressure.

So, 32, 36, or 40 hole may not make a huge difference. In fact, with 40h, one has more nipple holes and less pace between nipples.

One difference in the spoke design might be how much weight bearing stress the wheel experiences. And, fewer crosses and more spokes may well create less flex in the wheel.

Lombard

If course they say use more spokes. Do you really think they will admit that their rims are too delicate?

Don't know if you remember the infamous paired spoke design of some wheels of the mid-2000's, notably the Bontrager Race wheels. Those were known for spoke hole cracks. Mine started cracking at the spoke holes at 4K miles. I discovered multiple cracks around spoke holes when..........I broke a spoke. Who knows how long those cracks were there.

Granted this has nothing to do with the OP's issue. I've never seen a rim crack that way.

grumpus

Is that obvious? A heavier rim is stiffer, so better for spreading the load between spokes and thus reducing the cyclic tension variation that kills spokes.
They are giving the masses what they /think*/ they want, apparently.


* I'm using this in a fairly broad sense.