dasding

here's a bicycle wheel physics question for all you wheel smiths out there...I've just built a set of wheels for the frist time and I was talking about the structure of the wheel with a friend--we came up with a question that we couldn't figure out the answer to:

can a wheel withstand the same stresses applied to the top of the wheel as those that are normally put on it via the hub? in other words, if you lean your weight onto the top of the rim while it is standing up vertical, does this put the same stress on the wheel than if you put the wheel on the bike and stood your weight up on it (i.e. the static load is on the hub now)? Granted, putting your weight down on the top of the rim isn't something you'd normally do...

urbanknight

I don't think so. Since the rim "hangs from" or "pulls on" the spokes, putting your weight on the rim will cause the rim to take the entire load in compression, so the spokes and hub are getting very little stress.

But I'll let some engineers weigh in not.

rydaddy

I will take the yes position. The wheel could withstand the same amount of load applied at the top as compared to applying the load at the hub. The load path changes, phycics do not. This assumes uniform spoke tension around the wheel. My definition of wheel strength is the load at which the lower spokes go slack. In this case, the top and bottom spokes will go slack at the same time. Any additional load applied will cause the wheel to become unstable and possibly buckle. I'm sure I will see plenty of disagreement, as usual.

Al1943

This is one of the hottest discussions or arguments in the biking world. Jobst Brandt, Stanford PHD, engineer and author of the book "The Bicycle Wheel" says that a wheel stands on the bottom spokes, meaning that the bottom spokes are compressed, though I think this means less tensioned. Tension measurements seem to support this position.
Leaning on the top of the wheel will compress some spokes, I suppose on the top and on the bottom but possibly not the same amount. When in the riding position the compression must be from the hub to the bottom of the wheel. Leaning on the top would compress the wheel from the top and from the bottom.
I hope I'm not falling for another troll.

dasding

I didn't mean to re-ignate any old fires here...also I am familiar with Brandt's view that the wheel actually stands on the bottom spokes by lessing tension, but not actually putting them into compression. the question i had was more about how the load on the top gets distributed, and whether it really just is the same load expressed in a different way. i was thinking that if you take an un-spoked hula hoop and push down on the top, it tends to bulge at the sides, becoming oval-shaped. but if you imagine spokes on this hoop, then the sides that want to bulge away at 90 degrees to the downward force are actually pulling on spokes that are connected to the hub in the center. obviously the load isn't simply getting put entirely into the rim because these spokes are exerting a force. but i'm not sure if it simply the same as if you put the same force onto the hub. sorry if this has just become a physics discussion...

ratdog

no, pushing down on the top of the wheel will distribute the force outward along the rim & and lessen the tension on the spokes whereas pushing down on the hub will pull the spoke down and distribute the force more evenly along the rim but pull the forces towards the center of the circle.

davidad

The load will go from top to bottom spokes through the hub. The stress will be less per spoke because of this.

FBinNY 

The theory that the hub hangs from the top of the rim is problematic because it leads to flawed thinking as shown in the first response. It's better to think of the spokes as trying to keep the rim round, while loads at the hub and rim are trying to distort it. In that vein, when you consider a rim load at the top, it's sort of a mirror image of the normal loading of the rim on the ground.

Though the situation is different because there's symmetry in the top and bottom loads vs, the asymmetry of a normal situation, the basic physics remain the same.

AEO

wouldn't the spokes to the sides keep the wheel in tension as the rim ovalizes?

It's not so different from broadsiding a car, where your fork gets bent instead of your wheel collapsing.

Andy_K 

It seems to me that pushing down on the top of the rim is significantly different in the it applies force in the direction of the hub to both the top and bottom of the rim. This is the way I've been stress relieving spokes, and I typically hear the spokes "ping" when the ones I've been working on are near the bottom.

While we're on the topic of wheel trivia, it seems to me that Brandt's "standing" position is typical of an engineer. The actual force distribution involved is certainly complicated enough that I can't work it out in my head. At this point, an engineer measures something and says, "There's your answer." It strikes me as being a lot like the discussion over whether or not centrifical force actually exists.

rydaddy

The pings you hear are the twisted spokes unwinding, not the sound of stress relief (which makes no noise).

Andy_K 

Yes, I realize that, but aren't the spokes unwinding because the tension on them has been reduced?

rydaddy

Yes they are. I guess I am confused by you method of stress relieving. You need to increase the tension on the spokes in order to effectively stress relieve them.

Al1943

A agree. I think pushing down on the top of the wheel is very different from loading the wheel in the riding position.

dasding

woah...I wasn't expecting to have all of my anecdotal wisdom about wheels over-turned in this thread! what's this about stress relief not existing? I did find it interesting that nobody agrees about the proper way to stress-relieve a wheel (squeeze parallel spokes, push down on rim at 9 and 3 while wheel is on ground, use a crank to lever spokes a la sheldon brown) but I didn't know such a thing didn't exist. what's the thinking behind this?

dasding

FBinNY, what do you mean by the normal loading of the rim on the ground--I don't think I follow what you mean by the "asymmetry" of this situation. I think I was agreeing with your "trying to keep the rim round" view, however, when I gave my example of hula hoop, but this was just me imagining off the top of my head, and not actually applying any engineering knowledge.

Al1943

Maybe but not necessarily. When riding loose spokes can ping when there is no unwinding involved. I know this from an experience I had with some bladed spoke wheels.

rydaddy

I didn't mean that stress relieving did not exist, I meant an affirming sound while stress relieving does not exist. There are several ways to stress relieve, and it's the builder who decides.

I tend think the ping while riding is the nipple of the loose spoke losing contact with the rim (going slack) and popping back into contact as the wheel spins. That, or the spoke crossings are rubbing and making noises as the wheel flexes. A friend of mine had an annoying tick and it turned out to be one under-tensioned spoke.

cal_gundert05

What if you put a wheel in a truing stand and sat on the top of the wheel?

(That) + the typical action of placing your weight on the hub as the wheel stands on the ground = sitting on the top of the wheel while it stands on the ground.

So...

(math is more my thing, I suck at physics)

garage sale GT

Do you have any evidence? Did you ever put index marks on to see if there's any turning?

Andy_K 

Maybe this is a sign that I don't know what I'm doing. I thought the whole point of stress relieving was to get the spokes to unwind so you could fix the resulting loss of tension.

urbanknight

I took the question as if the wheel were not attached to anything. Did I read it incorrectly?

Captain Blight

Okay... so you push down on the top of the wheel, the sides want to bulge out, and that places the spokes radiating to the sides in greater tension. I *think* the hub still 'hangs,' just from different points on the wheel.

Complicating this is that unless we're talking about radially-laced wheels, the tensions in questions are vectors at at some tangent angle. ( probably didn't phrase that right, I'm terrinble at maths).

rydaddy

Experience and it's mentioned in every book I have read on the subject. I have noticed the more careful I am about not letting spokes wind up in the first place, the less pings I hear. In fact, when I build with bladed spokes there are no pings at all. Thus, my conclusion that the ping you hear is spokes unwinding.

Two different things. Stress relieving temporarily increases the tension in the spokes. This is meant to erase the "spring" from the bend in the spoke. It helps for long term durability of the spokes by increasing the fatigue life. Unwinding the spokes is a good practice otherwise they'll pop and ping when you first ride the wheel and it'll be right back in the truing stand.

Tunnelrat81

This is sometimes lost in the discussion between people who have read actual wheel building books. For those whose wheel building book is BF, this terminology gets all confused.

Untwisting the spokes is what builders do by turning the nipple further and relaxing back a bit during the tensioning process...or after the fact by placing the wheel on one of it's flanges (or on it's axle ONLY with cup/cone hubs) and pressing all around the wheel at 3 and 9 oclock until all the lower spokes have been sufficiently relaxed to allow them to relax to an untwisted condition. Flipping and repeating on the opposite side.



The term "stress relieving" may SOUND like what you're doing when you untwist the spokes, but it's entirely different as rydaddy mentions here. Stress relieving involves LOADING the spokes so that their elbows are pulled to a highly loaded state beyond the normal forces experienced while riding. As the theory goes, once then relaxed, the spoke elbow will be existing in a "stronger" state than before it was forced to support the great load. This is what's shown by the "grabbing parallel spokes and squeezing HARD" method...and also on Sheldon's site where he advised using a crankarm to twist the spoke against each other at the cross to accomplish the same "over loading."

The nice thing about stress relieving is that the practice has a benefit even beyond what happens in the spoke elbow, but it also creates a high enough force to theoretically "seat" both the spoke head AND the spoke's nipple into the aluminum (flange and rim) a bit. Just like the spoke un-twisting, this process very slightly changes the spoke tension, resulting in a loss of true. This is something that will happen eventually during riding (only the seating process, not the actual elbow stress relieving) but like everything else, you'd like these tiny spoke tension changes to happen during the building process so that you never have to bring the wheel back in to true it.

A wheel built using these techniques will have the best possible chance of longevity, assuming that we're arriving at well balanced tension as well.

-Jeremy