I like cats (and wheel building)

Posted by Jerry Chabot on

These are stressful times with COVID-19 concerns and all. When I get stressed, I like to think about my cat, Pumpkin. He is my wheel building assistant. I thought I would share some photos of Pumpkin for everyone, interspersed with some thoughts on wheel building and what defines a NEXT wheel, in my estimation. I posted this as a series on my Instagram account @NEXTcycling and you can visit that series and other musings by following the hashtag #builtbyJerry.

Cat Photo 1: What is not good?

It's pretty easy to get a wheel round and true. Given enough time and a good stand with dial gauges, anyone could produce a quantifiably good wheel by that metric. Spoiler alert: THAT IS NOT A GOOD METRIC.

It's analogous (from an engineering standpoint only, building wheels is easy) to the master frame builders who work with steel. Someone with hands and a spoke wrench can build a wheel. Someone with hands and a torch could join tubes into a frame and then cold set the frame (bend it) to be straight. 

What is much more difficult (one imagines) is building a frame without internal stresses by predicting how what you are doing (heating/cooling, bending, fitting) is affecting the frame such that when you are done, it doesn't need to be bent straight. It doesn't need to be forced. It just is. It actually probably only gets to be really straight in the final step when all the movement comes back around to the bullseye you aimed at in the first step. Steel is a live material, it shrinks and grows when you heat and cool it.

I don't know, I don't build frames (but if I did they'd have a samurai...) but that's my sense of the process, done well. Maybe it's a terrible analogy (but a solid Bare Naked Ladies reference).

Cat Photo 2: Round and round we go!

Ok I guess that was not the best analogy as cold setting should theoretically not damage the steel as long as it is theoretically stress relieved so that there is theoretically no internal "tension" within the structure. Theoretically. Plastic vs elastic deformation and all that, yada yada, I'm telling cat stories here not trying to explain material science. Also, theory is only correct in theory. Much trickier to execute in the real world.

The point is, I can FORCE a wheel to be round and true on the stand by pulling hard on this spoke or that spoke and bending the assembly to my will. If I'm working to a "round and true" metric, that's a great wheel. Just like Pumpkin, I can spin the wheel and grab my tail at one spot and KILL IT GRRRRRR (or over tighten that spoke in one spot). It will look great on the dial gauge, nice and round and true. However THAT IS A TERRIBLE WHEEL. 

Cat Photo 3: Nailed it!

Pumpkin's nose is perfectly centered between the fork legs here. Just like (stay with me here) the rim should be centered in a well built wheel even after years of hard riding.

Pumpkin knows the wheel needs to be round and true not just on the stand but also after a few thousand load/unload cycles that work out any "bedding in" 
or relaxing of the spokes. In other words, the internal stresses of EACH SPOKE working themselves out (refer back to Cat Photo 1) until all the forces are even and in equilibrium. Have you ever gotten a wheel and ridden it two weeks and then had to bring it in to get trued up? That is often considered to be normal practice. That's the spokes finding their balance, their equilibrium point. The spoke tension WILL even itself out eventually, because entropy. It's all entropy nowadays (or is it ball bearings?).

Wheel building, to me, is about all about managing entropy. Of course, I am a  mechanical engineer so I do tend to look at the world through a somewhat different lens than other people. It's a known issue. For people who are not mechanical engineers, I offer this from Wikipedia:

Entropy is simply a quantitative measure of what the second law of thermodynamics describes: the spreading of energy until it is evenly spread

That's a pretty good description of entropy actually. Internal energy (forces) are going to work themselves out until they are in balance, and there is no one area in a higher state of stress than another (in the mathematical sense not the cat sense).

Cat Photo 4: Destressed

Destresed  not to be confused with distressed. That's for jeans from the 90s.
Pumpkin is at rest in this photo. He has no built up stress. All his internal stress has been released and he has arrived at equilibrium and it involves purring. Destressed.

Back to entropy. Forces bound up in a closed system will release within that closed system and the system will find equilibrium. Imagine pressure in a balloon, the pressure is the same all over the balloon and if there is a thin area in the wall of the balloon the shape will deform outward there until pressure is equalized. That is actually an increase in entropy, as entropy can only ever increase in a system (if I'm simplifying). Energy wants to disperse. A spoke with a high tension will give up that energy to spokes with low tension, the energy will dissipate until all spokes have the same tension (internal energy, in a sense). This is maximum entropy state, counter intuitively, and  corresponds to the state of stable equilibrium.

What's that got to do with wheels? Simply put, every wheel chases it's tail until it's really tired and finds a nice sunny spot to curl up in and relax. Every wheel, no matter how perfect it looks on the stand, is going to relax until all spokes are at even tension, and the rim is going to move as a result. The wheel will follow the law of entropy. My job as a wheel builder is to only put wheels in the box at their maximum entropy state so that they are done moving, all the internal stress has been released. 

The hard part, of course, is ending up with a wheel that is round and true when it is in this state! That is the tricky bit.

Cat Photo 5: Relaxed 

We just established that we want high entropy wheels, or in regular person speak, wheels that are round and true when they are relaxed. Pumpkin likes to relax. He knows what's up.

How do you achieve that condition as a wheel builder? You spread the forces evenly so that the spokes are in equilibrium (refer to previous cats). You do that by adjusting the spoke tension during the build, and by forcing the spokes to stretch and bed in and straighten out so that they are fully relaxed and in axial tension only.

First, you make the wheel round and you make the wheel true. Then you make all the spoke tension on the same sides of the wheel precisely The Same. 

Side note, that is my build spec: The Same. I don't build to a particular tension figure like 120kgf so much as I build to tensions that are The Same.

That of course of puts the wheel back out of true as you are tightening some and loosening some and totally ignoring what that does to the run-out. So of course then you need to you do the round and true step again. That of course makes the spoke tensions not The Same, but hopefully to a lesser degree. So you even the tension again. Then you true again. Make The Same again. Repeat as necessary.

You also mix in some stress relieving (I prefer to call it stress equalizing) and spoke bedding and straightening and cold setting and other black magic. All of which changes the tension (bedding in a spoke makes it slightly longer, which of course relaxes the tension). That, in turn pulls the wheel out of true. Again. 

But, you do your best and you make the wheel as round and true as it let's you make it. With all the the spokes are pulling The Same and everything in equilibrium and when are no uneven stresses in the wheel that are going to work their way out later.

You may end up with everything nice and The Same on tension, but with a subsequent little hop in the wheel. Or a little side to side run-out. If you try to force out that last little hop because you are chasing that ever so tempting round and true metric. you are just putting an uneven stress back into the wheel. Stress is bad. Relaxed is good. Be relaxed. Be like Pumpkin.

A really good wheel builder will be able to achieve a result that is both round and true while simultaneously being The Same. That, dear reader, is hard to do. It is, however, very easy to feel when you ride such wheels.That feeling is what I strive to put in the box. 

Bonus cat for getting all the way to the end. Time for a nap!