davidad

If they undergo a loss of strain then they get shorter. The high tension lengthens the spokes.

79pmooney

I won't speak for Jobst Brandt but the answer is simple. Look at the spoke coming vertical from the rim. You don't move until you have rotated the hub enough to move the spoke head forward of the nipple. At this point it acts like any non-radial spoke because it is no longer radial. It is also simple math to show the tension rise in the (at rest, radial) spoke to transmit that forward force from the hub to the rim will be quite high. (I wouldn't bet against a velodrome monster simply ripping the hub out the wheel doing an aggressive kierin start.)

cyccommute 

But the shortening of the spoke isn’t due to compression. Brandt is assuming that compression and tension are the same force with opposite signs, an assumption that is in error.

Gresp15C

The way a physicist might think about it, is to consider a balance of forces. All of the forces on an object, including inertia, add up to zero. For the object to accelerate, or to move against friction, the forces pointing in one direction must exceed those pointing in the opposite direction.

This corresponds to a slight increase of the tension on the "pulling" spokes, and a slight decrease of the tension on the remaining spokes. It's simplistic but I think it's accurate enough.

downtube42

Wheel engineers are smart, but spokes are stupid. Really really stupid. They don't know, for example, that they are weak in compression and in fact don't have any support to prevent them from sliding right through the rim. So they try, they give it their all. The spokes below the hub do everything in their power to resist that hub and all the weight on that hub from smashing down to the ground. But they fail miserably with hardly a fight, In fact, before the hub moves downward enough to even overcome the tension in the spokes below the hub, the spokes above the hub stop that hub cold. BOOM. Silly spokes who dream of working in compression. Now the wheel rotates 180 degrees and the tide has turned. Those wannabe compression spokes are now the tension champs, taking their turn at keeping things off the ground. And so it goes, round and round.

Likewise for lateral strength.

Bike Gremlin

I've measured spoke tensions when a bike is stationary, with a rider weight on the saddle, and without a rider.
When the rider is seated, the spokes at the very bottom lose some tension, the 1, or 2 spokes right next to the contact patch (at both sides) gain just a little tension.

When a torque is applied (front brake locked to keep the bike in place, while a pedal is pushed by the rider's whole weight), I have measured trailing spokes gain tension.

Both of these measurements align with the way Jobst describes the load transfers in his book. At least the way I understood it.

The reason for my experiment was Jobst's claim that the rear wheel left-hand side spokes don't take almost any driving torque. My assumption was that modern hubs might be a bit stiffer, and just might allow the left-hand side spokes to take the driving torque.

Bottom spokes are loaded in compression, but the way for a spoked wheel to take such load is by having all the spokes in tension, and then the compressive load is taken by a decrease in tension, because a spoke can't take compressive force without any tension - it would buckle.

cjenrick

have you ever sat on a bare rim? they do not deform much, especially something like an Araya 7X.

add all the tensioned spokes to the rim and now you have a super strong system that surely does not deform much,

Kimmo

I'd say bladed spokes are pretty nifty. You can't get a better spoke than a CX-Ray.

And I reckon straight-pull spokes are even better, nothwithstanding that I'm only counting bladed straight-pulls, because round ones are beyond stupid... Bye silly failure-prone elbows, hello sleek AF flangeless hubs (you'd hope... dammit, Shimano).

As materials and methods have improved, stronger rims allow for fewer spokes, which are under higher tension and are thus less prone to fatigue. Sleeker and stronger, yes please.

As for radial... seems to work. Looks a bit nicer, uses less metal. Whether the hub needs to be beefier is probably the other way around when it comes to straight pull.

Kimmo

So what were your results?

Bike Gremlin

Trailing spokes on the left-hand side had gained some tension - similarly to the right-hand-side spokes.

I hadn't measured the leading spoke tensions - the focus was on whether the left-hand side spokes transfer any torque, not whether the leading spokes on either side get slacker (which is to be expected).

cyccommute 

You are looking at this in the wrong way...just as Brandt and others have. The bottom spokes can’t be compressed because there is no mechanism for that compression. Nothing can push up on the spoke. The end of the spoke at the rim isn’t connected to the rim in a way that allows for compression. Look at my picture again




The spoke is, in essence, a rope that is being pulled on. You can’t push it. There are even fiber emergency spokes that work on the principle of pulling on that rope.

The spoke is undergoing less tension during loading but “less tension” isn’t more compression. That’s why the wheel isn’t “standing on the spokes” because there is nothing fo the spoke to stand on nor anything to push up on.

Bike Gremlin

Saying the same thing, using different words, aren't we?

davidad

What's it due to then, magic?

cyccommute 

No. Because there is no compression of the spokes in a tensioned bicycle wheel. There is reduction of tension but there is no compression. The rim is compressed but it doesn’t have a connection to the spoke that can be compressed.

cyccommute 

The spoke undergoes a decrease in tension which isn’t the same thing as compression...not even close. The rim is deformed through compression but not the spoke.

Andrew R Stewart 

Interesting points that I disagree with and fully do. Unlaced/tensioned rims most certainly do deform under someone sitting on it. less for deep rims with thick sides, less for shallow ones, have you ever mounter a sew up tire on a bare "classic" rim (to stretch the tire)?

I've read that a tension spokes wheel has one of the higher strength (or is it stiffness) to weight ratios of common structures.

Who here has read AR Sharp's book? Why do we need to relearn proven lessons? (That last line is not meant to be answered) Andy

79pmooney

Andy, I raced those 250 gm Super Champion Medaille d'Or rims (front only). You could sit on one of those unlaced once! (Like you can sit on an egg once. You end up on the floor with both.) A year after my last race I was doing a town line sprint when a newbie put his QR in my front wheel. Took out 9 consecutive spokes. Good news was that was a Weinmann Concave. It still ran through the brakes and fork. I rode it to a standstill from 30 mph. (Did take the paint off the inside of the left fork blade on my brand new Mooney but keeping me up and all those behind me where they belonged - I'll take it!) Ben

Edit: regarding the strength of laced wheels - that super light Medaille d'Or. I dented it on RR tracks late in a race and had to use a puller to return to round but that same wheel hit a big frost heave in Vermont (Smuggler's Notch) when I was going 50+ and got thrown 2 feet in the air. Stayed perfect. Oh, that wheel was light! Not just the rim. 15-17-15 spokes. 250 gm Clement Seta. Yes, 36 spokes but still, not a heavy wheel!

Andrew R Stewart 

Ben- We have similar stories. Medaille de oro, Mavic Champ de Mondes, ArayaTitan Ace, Ambrosia Montreals. All under 300g and with 36 stupid light spokes. Clement cottons for me due to rain here. Dropping water bottle at 40mph and skidding on it. Building close to 100 wheels a winter for a few years because machine wheels didn't exist back then. Helping Weyless with their hub testing wheels. Building the first Mavis SSC blue rims in the area. Denting Rigida 1320s and going back to sew ups because they held up better. Moving past Fiamme Reds and yellows because the angled brake tracks sucked. Hating Weinmann concaves (both the 124 and 129s) because the seams sucked. Robergel, Torringtons and chromed Union spokes being replaced by stainless as that became less brittle. Oh then there's the Ukai foam filled tubulars that replaced the wood filled Euro rims.

I have to admit that current rims are far better made, stronger, stiffer but so many lack eyelets and with the nasty dishing need so badly balanced spoke tensioning. Too bad wheel building is such an infrequent job these days or many wrenches would have a better feel for the dynamics a wheel sees. Andy

Bike Gremlin

To quote Brandt:

"With tension, wires can support compression loads up to the point where they become slack.
The same loads that increase compression in wooden spokes, reduce tension in wires.

...

As in a wooden-spoked wheel, the bottom spokes of a wire wheel become shorter under load, but instead of gaining in compression, they lose tension.
With the same load, the net change in force is the same for both wheels.
The algebraic sum of negative and positive forces (compression and tension) is the same."

English is not my native, but to me this sounds like the same thing you said, and the same thing I said, only using different words.

cyccommute 

My problem with that quote is that there is no compressive load on the wire spoke. There’s no need to even discuss compressive loads on wire spoked wheels. I’ll agree that the spoke is elongated through tension and it shortens with loading of the rim because of a reduction in tension. The rim is compressed but the spokes don’t experience compression of any kind. Brandt is looking at compression and tension as they are simply forces with the opposite sign. Again, that is not the case.

Consider what happens if you loosen the nipple on a tensioned spoke. Tension decreases as the nipple is loosened. Is the spoke undergoing compression? Obviously not. When the rim deforms under compression only the rim experiences compression. The spoke experiences a decrease in tension. Of course the other spokes pick up that decrease by increasing in tension.


Yes, but... A wooden spoked wheel (or car wheel) is compressed by the weight of the vehicle. The rim is compressed...I prefer to say deformed...by the weight of the vehicle. But that’s where the comparison ends. The wire spoke of a wheel doesn’t undergo the same compressive forces as the wooden spoke does. In a double wall bicycle rim, the wire spoke is hanging in the space between the walls of the rim. There is nothing pushing up on the spoke at all. This is why the load hangs from the rim at the top of the wheel instead of standing on the spokes. There’s nothing but air to stand on.


The highlighted sentence is what I see as his biggest mistake. He is treating compression and tension as an algebraic sum as if when compression goes up, tension decreases. That is not the case. Think of the wooden spoked wheel. I agree that the bottom spokes of the wooden spoked wheel get shorter under load because they are being compressed. However, when the load is removed and the wooden spokes lengthen, they can’t be said to be lengthening due to an increase in tension because the wooden spokes are never in tension. They are only being decompressed. In other words, compression becomes zero but tension is also zero.

The opposite happens in a tensioned wire wheel. The spokes are lengthen through tension and when the rim deforms under load, the spokes shorten but that is because tension is decreasing. The wire spoke itself is never under a load other than tension. Tension goes back up as the rim is unloaded but compression on the spoke itself is zero.

In short, compression and tension aren’t added to find the force on the spokes of either a wooded spoke or wire spoke. The force on each is either compression or tension but not both. That holds for any discussion of compression and tension. An object can’t be pulled and pushed from the same point at the same time so compression and tension aren’t algebraical connected.

Your English is excellent and far better than my ___(insert any language here). My objection is to the use of the term compression (except when talking about the rim) with regard to a wire spoke. To compress something you have to have a force that is pushing on the object. There is nothing to push on when it comes to a wire spoke.

Bike Gremlin

I'm not a mechanical engineer, but I understand the book as explaining the loads that a wheel takes, and explaining how spokes help keep it rolling - dealing with those loads.
Wheels do take compressive loads, and that is explained and discussed.

It is possible I'm missing some fine language, engineering, or even logical thing (one not excluding the others), but the way Jobst explains it in The Bicycle Wheel aligns with my practical experience and the little theoretical knowledge I have.
It works for explaining any problem causes, as well as any solutions.
It works for visualising the problem and the solution.

It seems as if we're going in circles.
Don't think I can explain this better than Jobst. To me the explanation makes sense, it holds water based on all the other knowledge and experience I've had.

The way I understand the problem, and Jobst's explanation:
There's compression on the rim, that is taken by the spokes (or the wheel buckles). Spokes take it by decreasing their tension.

From what I could measure, there really is a loss in total tension (aggregate tension of all the spokes). The ones at the ground contact point lose tension. The ones next to the contact point gain just a little bit of tension. The others have no measurable tension change. Total net-tension change is negative - some tension is lost.

I also measured this using a super stiff rim with very few spokes. Same results.

Yes, the spokes aren't (directly) pushed. I agree. Just: I didn't get from reading The Bicycle Wheel that it is explained as spokes being (directly) pushed.