liveforphysics said:
I don't know if you're insane, badly confused, or just f*cking with people, but there are no meaningful compressive loading in a spoke in a bicycle rim.
Can you tell me by what mechanism the spoke is constrained by to compress? Are you suggesting my 2 passes of electrical tape and my bicycle inner tube at 20psi (pressure I run on the back of deathbike so I get decent traction) are what the nipple pushes against to enable it to experience any compression loading?
Diameter of the head of my spoke nipple is 0.292", surface area is 0.0669in^2 times pressure in the tube of 20psi = 1.33lbs of force the tire pressure in the tube exerts on the back of the nipple. Lets say it's a roadbike wheel at 100psi with a full size mt.bike nipple in it, it's 6.65lbs of force being applied by the tube to the nipple for a best case scenario. Is this force what you believe contrains the back end of the nipple to enable a spoke be meaningfully loaded in compression???
Can you describe the mechanism in which you believe the spoke is constrained in the wheel to have meaningful compression load imparted to it?
Easy. If you had worked in a field of physics, you’d already know the answer and you wouldn’t have to ask. :lol:
Compression and Tension are vector forces that act in opposition. The Energy Balance Equation for a static object says that
- Compression + Tension = Zero.
Therefore, Compression = -Tension, or Tension = -Compression; all forces being neutral and static, Compression and Tension must balance.
A bicycle wheel under normal conditions will have spokes in tension, and the fasteners (including hooked ends) in compression. Can you describe with equation the Potential Energy of this model?
Let’s say I’m on my bike moving at great speed and I hit a Seattle pothole; forgetting about my body, the forces acting upon the front wheel goes like this:
Energy (Bike-moment before impact) = Energy (Bike-moment after impact)
Mechanically, Bike (before impact; condition: perfect) = Bike (after impact; condition: flawed). A classic non-elastic collision, yes?
With elastic impacts, such as billiard balls, the Cue ball shoots across the table, hits the bumper, and continues to move in the relative opposite angle of attack; we know some energy is absorb, but visually it appears that energy is conserved and the ball bounces with equal force onward.
In non-elastic collisions energy is absorbed and transferred into other vectors and forms. The wheel hits the pothole, and the rim compresses (there’s that word again). Let’s pick the worst case; the rim fails. Kinetic Energy is transferred at the point of contact between the part of the rim that connected with the ground (ideally the blunt point of trauma) and reflected backward (like the billiard ball) through the rest of the bike frame. At the rim and ground, the vector of force is (ideally) straight back to the axle via the spokes.
The rim: Ideally the rim wants to remain circular by virtue of the construction (cross-section rotated about an axis + metallurgical traits) and by the forces of tension which constrain the shape about the wheel axle (hub). However, in the worst case, the rim compresses and deforms. In the perfect model, compression will deform into the shape of an oval, although in reality, the momentum of the bike which is attached at the axle/hub, resists change, and moves the axle closer to the point of impact, and creating imbalanced off-center loading. Thus the spokes forward of the axle go into compression, and the spokes on the far side go into hypertension.
Now since the spokes are already in tension all the way around the wheel, the reflected kinetic energy traveling backwards has to compress against the momentum of the bike which is moving forward; this is the shock wave, the reaction, the reflected change in the immutable force that is going to break the wheel.
Let’s go back to the perfect wheel before impact: Spokes are in perfect tension, fasteners are in perfect compression, and although the forces are balanced, there is Potential Energy.
During the failure of the wheel, the rim is compressing inwards, the force of this compression, this shock wave, is moving backward through the system. The tension is removed and the spokes HAVE TO go into compression in reaction because the Potential Energy has been released, and like a spring, the spoke compresses by itself in its’ own Energy Balance Equation, and when it does, it will sing audibly in resonance as it tries to dissipate the momentary massive shock of energy. Combine this with the shock wave of impact, well… the wheel is at failure!
Where do we begin? The rim is deforming, the holes that hold the spokes are deforming, the fasteners are deforming, the spokes are reacting at several orders, and really – there’s not much to stop them from failing as well. They can become deformed, detached, or spring-loaded in which case they’re going to travel in the path of least resistance, including puncture.
But let’s spin the problem the other way around and see if we can make it work the way you are suggesting:
If the rim deforms (avoiding the word compression) then the tension in the spokes becomes less until… let’s say it becomes Zero; there’s no more tension on that side and they become sturdy rods that puncture the tube and tire, or they break or bend.
Well, first – the spokes are pulled tight like a spring to hold the rim in place. A spring that suddenly has the tension cut will react how? It’s a rod that have been elongated to create the tension, therefore when the tension is catastrophically removed, the rod shortens and wants to return to the natural unloaded shape, but instead overshoots and compresses. And it resonates when doing so, just like a FET gate has a ring to it, a harmonic.
But let’s say it didn’t compress at all, let’s say that it snapped back perfectly without bounce. That spoke has to compress to puncture the tire, it has to compress to resist the caving rim. It has to compress to resist the shock wave. And it has to compress before it bends and breaks.
That’s just the natural order of things.
Let me explain it another way:
I have a concrete beam placed across the freeway as part of a bridge span. When they fabricated the beam, they placed steel cables through it combined with rebar to provide structure and strength. Before they poured the concrete, the steel cables were placed under tremendous tension. Add the concrete, allow to cure, then release the cable tension: What happens to the beam? It bows ever so slightly upward in resistance to tension. The beam now has Potential Energy to resist collapse. We place the beam horizontally across the freeway at two fixed points, at the point-loads, the fulcrum (albeit static). We place a bunch of these in parallel then top with a reinforced concrete deck, and allow curing. A dump truck carrying a heavy load of rock and gravel crosses the span. The pre-stressed beams prevent collapse of the span whilst they are in maximum compression as the truck reaches the midpoint. A bicycle rim in normal operating conditions does not stray far from this model; we just don’t have any concrete.
Whoops! The Cascadia Plate breaks loose and we’re experiencing a 9.1 magnitude earthquake at the time the dump truck reaches the center of the span and the bridge fails. When that span hits the earth below, I guarantee it is in compression something fierce! The steel cable within the beams are acting like spokes on the wheel. If that span fails, and those beams fail, what do you think it going to happen to those cables? More importantly, do you want to be around?
Ever watch an aircraft land on a Navy Carrier? Ever seen the arresting cable snap, and what it does do people in the way? Tension = Potential Energy, and when suddenly released, compresses in reaction, and can be deadly.
What’s the next question?
~
KF
PS –
Deflection is still Compression in action.
