Duo

agreed, just an example of what tandems use because they need too. good catch, so to make wheels more durable perhaps add that to the list too: 145mm rear dropout.

cyccommute 

Spokes have been "spokes" for ages. The spoke you use for a 700 wheel is same spoke as you use for a 26" wheel. The only difference is the length. There is a slight advantage to the short spoke on the 26" wheel because it has less leverage to flex but I don't think it's a huge advantage. Strength-wise, you'd be better off going to a stronger spoke with a thicker head like the DT Alpine III. Those will make more of a difference on any wheel than length will.

As for the rim, there really is no advantage in a "stronger" rim...whatever that means. In my experience, "stronger" just means heavier. The wall of the rim isn't significantly thicker. If the rim is wider, it does decrease side-to-side flex a little but the spokes are already doing the heavy lifting there as well. Through building and destroying wheels, I came to the conclusion that the rim is just along for the ride and there is no need to go with a heavy rim. Whether I'm using the wheel for mountain biking, off-road touring, on-road touring or just plain riding around, I use the lightest rims I can paired with the strongest spokes I can find.

Lemond1985

I always find I fly up climbs when using 26" wheels, I suspect it's due to the lighter 26" wheels being built with spokes that are slightly shorter, saving weight. Who knew? (not really)

Happy Feet

Well, I don't know enough about wheelsets to argue specifics but I don't think you can say road 700c and mtb 26" rims are the same or that design doesnt matter between genres. If that were the case one could expect to put road 700 wheels on a 29r suspension mtb and expect them to survive technical downhill.

elcruxio

That is again simplifying things too much.

So the reason why 26" wheels are typically stronger than their 700c equivalents is because the spokes are more densely situated on the rim, they have better bracing angles and also to some extent because the spokes are shorter. But those all combined do give some significant strength increase. Though because the spokes are more densely situated on the rim one should not go over 36 spokes with 26" wheels.

As for the rim, well you're mostly wrong. Though it needs to be said that the weight of the rim is not the deciding factor but rather the shape and design combined with material choices. More weight itself doesn't do anything but extra material added to correct places will make a huge difference. A Deep V rim will build an inherently stronger wheel than a super light low profile single wall rim even when all other factors are equal.
The vertical stiffness of the rim makes a difference because that in turn has a direct relation to spoke loading in normal use.
And of course more material in the spoke bed allows for higher spoke tensions which to a point allows for a stronger wheel.

It's simple really. It is possible to build a functional wheel where the spokes are replaced with string with a conventional rim. But it is not possible to build a functional wheel where the rim is made of rubber even if the spokes were 5mm thick. It just won't work since the rim has no supportive rigidity.

To all those who aren't grasping what this comment is about a short explanation:
When the wheel spins, the lowest spokes facing the ground will experience a reduction in tension whereas all other spokes will experience an increase in tension. This is because the the hub, which is attached to the bike, pushes down towards the ground. The spokes at the top (and all other spokes as well) are supporting the hub so that it doesn't drop down. You could say the hub is suspended by the spokes.
Now if we create a scenario where the rim has zero flex to it, the decrease of tension on the spokes at the bottom will be exactly equal to the increase of tension on the other spokes around the wheel. Hence if the rim has zero flex, the decrease / increase tension cycle will be negligible especially if the wheel is built with a significant number of highly tensioned spokes. It really then comes down how much the spokes at the top are allowed to stretch (it's not going to be much)

In the real world the rim flexes though. Some flex more than others. If the rim is especially noodly there actually isn't anything to support it at the lowermost position. So the portion facing the ground will flex inwards which in turn will slacken the spokes facing the ground. This slack / tight cycle is what breaks spokes. So this is something you want to minimize in a wheel.
The ways to do this is by
1) increasing spoke count. More spokes facing the ground means there's more spokes to pick up the slack so the individual decrease in tension doesn't become insurmountable.
2) increase spoke tension. More tension means that there's less danger of the spoke going completely slack. Works great for drive side spokes, not so much for non drive side except if using a non chain geared system.
3) increase rim vertical stiffness. If the rim flexes less at the bottom it will spread the tension decrease to a wider set of spokes facing the ground. With a truly noodly rim the decrease in tension will affect maybe 2 to 3 spokes at the bottom. With a monster stiff rim the decrease will affect 6 - 7 spokes or more.

And this is why it's traditionally recommended to use high double walled box section or box/vee combo or deep vee rims when touring as those are typically vertically very stiff. My personal favorite is the DT Swiss TK540 which is a high box section / Vee hybrid rim and extremely durable. It's not particularly heavy though at 545grams per rim.

kingston 

Peter White recommends 40 or 48h wheels for touring tandems. My 26" tandem wheels are 40h.

cyccommute 

Again? Have you posted to this thread before?

You are overstating the density of the spokes on the rim. A 622mm rim has a circumference of 1954mm and a distance between spokes of 54.3mm for a 36 spoke wheel. A 559mm rim has a circumference of 1756mm and a distance between spokes of 48.8mm. The difference is 5mm or about the thickness of 4 dimes for the US or about the thickness of two €1 coins for Europe. (Two US $1 coins would work too but those are very rare here). The differences are slight. The strength differences between a 622mm wheel and a 559mm wheel are also fairly slight.

It's interesting that you say I'm wrong and then go on to say that I'm right. I've not run across anyone who has stated that you use higher tension on wheels with taller profiles. It would depend on the wheel as to whether or not you could use higher tensions. I might argue that you can't use as high a tension because more tension is concentrated in a smaller area. Again, the effect would be small. The thicker cross-section would make the rim flex less during the loading/unloading cycle which puts less stress on the spokes but I don't think it's has that large an effect. And, because you have more metal in the rim, the rim is heavier.

You are talking about the Tioga Disc Drive. But what you are leaving out is that the spokes weren't just "replaced with string". They were replaced with a plastic disc and string. The plastic disc is an integral part of the system. Without it, the wheel doesn't work since the string has no resistance to side loading. It's also curious what this website has to say about the wheel

Seems like the string isn't a good idea.

As for building a wheel with a "rim" of rubber, . Not 5mm spokes but it still has a rim made of rubber. Granted, the wheel is very different from a traditional wire spoked wheel. On the other hand, it is possible to build a fully functional, durable wheel with a much lighter rim than most people think. The rim does provide some rigidity but it's not what is carrying the load.



Pretty much what I said. Notice that it's the spokes that are doing the job of support. The rim has little to no effect on the support the spokes provide.

Two things: First there is a 4th option. Increase the strength of the spoke. What kills wheels is spoke fatigue. Damage or break a rim and the rim can be replaced as many times as you like. Damage the spokes and the wheel is compromised. If enough spokes are fatigue, the wheel is dead. Increasing the spoke strength does far more to make a durable wheel than increasing the number of spokes.

Second, while tension is important, there is a limit to how much tension you can place on an aluminum rim. At some point, you start to fatigue the metal and the rim cracks. The rim can be replaced but it's still an expense and requires a fair amount of work to accomplish. Even the "strongest" aluminum rim is only slightly thicker and "stronger" than the lightest aluminum rim. Aluminum isn't a metal that increase strength rapidly with thickness.

Finally, think about what you are trying to accomplish. As touring cyclists, we are trying to avoid broken spokes which are indicative of a wheel that may fail us in the future. The problem isn't broken rims, it's broken spokes! Fix the problem and that problem isn't a broken rim.

elcruxio

I wonder from which year that recommendation is from. If we go back 30 years it may have been a valid recommendation. It's also kinda valid for tandems with 700c wheels (but not really and I'll get to why not in a sec). But for modern 26" wheels it's really not a good idea.
The reason for this is something I got directly from velocity wheels. A rim bed has spoke holes. This is only natural because the spokes and nipples need to attach somehow to the rim. But every hole is also a reduction in material and so every hole weakens the rim. Usually this isn't an issue because there is enough spacing between the holes and thus enough material to keep the rim from splitting. However if you put 40 or god forbid 48 spoke holes on a 26" rim you are going to have a lot of holes and very little material between them.

Let's just look at the numbers.
On a 700c rim the spacing between holes is
54mm with 36 spokes
49mm with 40 spokes
41mm with 48 spokes.
Not too bad right? 40mm should be plenty

On a 26" rim the spacing between holes is
49mm with 36 spokes
44mm with 40 spokes
36mm with 48 spokes

With a roughly 10mm spoke hole diameter at the outer rim wall you're looking at 26mm of material between spoke holes if you're running 48 spokes on a 26" wheel.
Now what can happen when you have too little material between the spoke holes is that the rim simply splits in the middle like a zipper. The tire and subsequent pressure is going to push the rim walls outwards putting a strain on the rim bed. If there isn't enough stuff there to hold it together it's going to break. Higher pressure tires exacerbate the issue but with tandems the pressures need to be higher than normal anyways because of the heavier loads.

Now as to why you don't actually need more than 40 spokes even with touring tandems is because of tandem rear wheel spacing which is commonly 145mm which is 10mm more than a conventional rear hub spacing of 135mm. This extra 10mm effectively removes the need for asymmetric dish which in turn means there no longer is a weak spot at the NDS with lower spoke tensions. All the spokes in the rear wheel can be tensioned to the same tension making one bombproof wheel.

But with tandem wheels (as with all wheels) it's really more about the component quality and durability, build quality and component compatibility than spoke count. You can have 48 spokes on a 26" wheel, but if the rim is one walled steel and the spokes are some no name garbage the chances are it's going to be less durable than a 28 spoke wheel I would build and choose components for.

elcruxio

Not this one in particular but you tend to have the same mistakes in every wheel thread.

You only considered one point which was the spoke density. I also mentioned the bracing angle. Now I'll admit that the spoke density isn't the biggest factor and the effect it has is slight. However the effect the bracing angle has is not slight. Nor is the strength difference.
As a guess I'd go as far as to say that a 26" wheel is at least 10% more durable than a 700c wheel when other factors are equaled.


And again you address the point which has the least amount of effect, ie. the increased spoke tension. The much larger effect a higher profile rim has is the fact that it has more vertical wall. If you take a steel ruler it's easy to bend one way and completely impossible to the other way. Same thing in effect here. This is like basic engineering stuff. I-beams for crying out loud!

I wasn't actually. I was speaking about the fact that wheels work with tensioned spokes. So theoretically you could build a wheel successfully with string only. Though I think it might need a bit more bracing angle than is present in typical bicycle wheels. And the string would need to be kevlar or aramid or some other material which yields very little and allows for crazy high tensions.
Mind you it probably wouldn't be a good wheel but it would be possible.

Ya think?

Sigh.......
Ok... So you do realize that the spokes on a bicycle wheel are in tension right? So there isn't any compression happening. We're clear on this? Bicycle spokes do not experience compression. So why would you give an example of a wheel which has compression spokes?
Right. As to the the rim not having a large role in the whole issue. Well now you have me intrigued. I do have some steel sheet around here in the form of barrels etc. At some point I'll build a wheel out of a steel strip 25mm wide and thickness of something I'll manage to find. I wonder how thick oil barrels are....
Anyways, that'll test the issue once and for all. My guess (based on my experience with wheel building) is that because it has no vertical stiffness it will allow for the spokes to go completely slack with relatively minor loads.

But before I do that I have to say that rim vertical stiffness has a massive effect on how the spokes carry the load. If there isn't any stiffness in the rim the spokes cannot work. If there is only minor stiffness in the rim the spokes will go slack at the bottom. If there is absolute rigidity in the rim the spokes at the bottom will not go slack almost at all.

It's not though...

Well that's also true. Spoke strength does make a difference but it is still only one part of the puzzle.

And that's also true. That is why it is advisable to stay within the allowed tension limits of a rim. Or use carbon fiber which can support far higher tensions than aluminum. But in a touring rim I'd go with eyelets and towards the high end of allowed spoke tension (aluminum, with carbon I wouldn't go close to the allowed 150 or 170kgf or whatever else crazy number that is)

I get your point but it doesn't work. Rather one should aim for fixing the issue and not just the symptoms.
Want to avoid holes in tubes? Full rubber tires!
Want to avoid high blood pressure? Remove blood! Genious!
Want to avoid broken spokes? Use several different methods in unison to build the most durable yet sensible wheel possible. Component choices, compatibility, build quality and use within the limits of the build all have an effect.

Tourist in MSN

Interesting that if Velocity says that is not a good idea, you would think that they would not make a 26 inch rim with 40 or 48 holes. But they do.
Velocity Wheels - Hand Made in USA

I have learned a lot from Peter White website, and when he stands behind the wheels he builds, that suggests that he knows what he is doing.

elcruxio

That is interesting. I wonder if the person who said it still works there. Or if it was before they moved locations and the quality went down... But it does not detract from the fact that more material is more material and less is less. Where comes the critical point is unknown to me.
Though what would be even more interesting would be to know the maximum tire pressure for the cliffhangers. It's inner width is a relatively wide 25mm meaning it fits large low pressure tires which will likely not cause issues. However some manufacturers such as DT Swiss do give their rims maximum tire pressure ratings and those depend on general rim charasteristics and not really hole drillings.

Id wager though that a 32mm tire inflated to 6.5 Bar would eventually split the cliffhanger 48h version. It's not meant for that but you can't really stop a rider doing it either. And if you have a tandem loaded to 200+kg, it's going to start requiring pretty high pressures even with wider tires.

The only thing I really know about Peter White is that he makes expensive hubs. But he's also relatively old which in turn gives him experience but on the other hand can make some of his views needlessly conservative. Even the late Sheldon Brown (who I must say was almost always spot on) had some outdated views.
Personally I do not see the need for 48 spokes for 26" wheels. I don't see the need for 48 spokes even with 700c wheels. It's just too much redundancy (this coming from someone who overpacks tools and spare parts).
40 spokes for 700c is something I could perhaps somehow justify on a regular touring bike but with tandems you still have that 145mm rear hub spacing which makes the wheel a lot stronger.
Modern wheel components are just so good that 48 spokes is pretty much obsolete and has more downsides than it has upsides.

kingston 

I think you have Peter White confused with White Industries, but you are right that Peter is an old curmudgeon. I also agree with you that 40 & 48h 26" tandem wheels should have wide tires.

CraigMBA

What are you "Flat Earthers" running for pressure? 26x1.75 ultra marathons.

kingston 

This is what I normally use as a starting point
I don't remember where I got the formula, but it seems like as good a place to start as any.

djb

how much do you weigh?

CraigMBA

260 plus the gear.

djb

Given that I'm nearly half your weight, it would be better to get accurate recommendations from others similar to you.
With 40- 50lbs of stuff, i use betwwen 40 and 50psi on 2 inch tires.

kingston 

45 psi according to my formula

djb

Re that formula, I wanted to try it to compare it to my real world touring experience on various sized tires, but I do don't understand the formula.
Would you be able to explain it simply to a non math person, I'm curious to see how it compares to lots of years and monitoring pressures.
Thanks

kingston 

Multiply the total weight on the wheels in pounds by 77
Divide that number by the width of the tire in mm raised to the power of 1.6

So here's an example for a 260 pound load on 1.75" tires (1.75" is about 45mm)

260*77=20,020 (total weight times 77)
45^1.6=442 (tire width in mm to the power of 1.6)
20,020/442=45 psi

djb

thanks

so here is what that equation gave me, which my real world experience shows is much much too low

135+50 rider and load x 77=14245
50 to the power of 1.6=523
14245 / 523 = 27 psi

Ive ridden a lot with this weight of bike, rider and load, and generally run 42f and 45r, and given that I have done trips where I am riding day after day for weeks, I've had lots of time to try diff pressures, and while I might have been a bit lower than this, no way that under 30psi would have worked, so I use this as an example to explain why I dont agree with this method

I redid it adding 35lbs of bike, and putting 60lbs load instead of 50=230x77=17710/523=34psi, which is still too low and I know even this would feel wallowy and slow and unsafe in cornering on my bike (squirmmy).

re judging tire pressure, I really do think its good to try diff pressures, starting high, riding over diff surfaces, then lowering pressures by 5 or 10psi at a time and evaluating how the bike feels over the same section of road, ideally with flat asphalt, some bumps and rough stuff too.
A good trial and error way to get a feel for at what point too low a pressure starts to feel wallowy, etc., and conversely, what pressures feel too harsh for no reason.

thanks again for the explanation

Tourist in MSN

Touring with my camping gear, I probably have the same weight on the tires as you do without any gear at all.

I do not have any 1.75 width tires.

- 57mm width, 45 to 50 psi in the rear.
- 50mm width, 50 to 55 psi in the rear.
- 40mm width, 75 to 80 in the rear.
- 35mm or 37mm width, up to 85 in the rear.

Front, about 75 to 80 percent of the pressure that I have in the rear. Thus, if I had 80 in the rear, front would be about 60 to 65 psi.

The pressures above are what I pump the tires up to, so they should be considered maximum. I often will go for a week or maybe more before adding pressure, so I often run quite a bit less when you consider leakage.

There are exceptions, last spring I was on a van supported trip in W Texas where they had a very rough chip seal pavement. First day I had about 55 to 60 in my front 40mm tire and my hands took a beating, my GPS on the handlebars kept acting up too. Then next day I dropped the front pressure to between 40 and 45 psi and that was much better on my hands, the GPS was much happier too.

And off road or on gravel, I might run lower pressure for better cushioning and better grip.

ADDENDUM ADDED HOURS LATER

This is the touring forum, so I was thinking a loaded bike, but I did not clearly state that. The pressures above are what I would be using for touring with my camping gear on the bike. Riding unladen, I would be running lower pressures. I also have some bikes with skinnier tires that I did not list above because I would not be touring on those bikes.

djb

good points to bring up to give examples to other people, re how pressures will often go down gradually, so a max pressure we state always ends up a bit lower before we top up.
another good example of the rough chipseal being one of those times where lowering pressure by even just 5psi can make a big diff in bike and person being a lot less beatup.
Ive shown this photo before , but is good for folks to see to get a "worse case" example of a rough road, and how slightly lower pressures made it bearable on my 2in tires. For this day, I dont think I specifically lowered the pressures as it had been a while since I had topped up, but well maybe I did a bit, perhaps I forget. Sometimes if too harsh, I'll just remove the cap, and do a little quick "pffft pffft" to take the edge off. No need for a gauge.

kingston 

It works pretty well in the normal road bike range of weight and tire width. As you show with your example, it starts to read too low with a lot of weight and wide tires.

LeeG