Thursday, October 7, 2010

Juliana's Wheels -- Rear Complete

You know, building wheels isn't really all that hard.  Time consuming, yes.  Made a lot easier with the right tools and an expert on hand to help, yes.  But it's not hard work or tedious work, and I'm really enjoying the process.

I finished Juliana's rear wheel in class, Monday night.  It took about two hours to get it from laced to fully tensioned and true, in a very iterative process with lots of repetition both of steps and groups of steps.  Essentially, what all of those steps achieved was to shorten the effective length of each spoke wire, tensioning it between the rim and the hub, and centering the rim between the outer locknuts of the hub's axle.  That all sounds complicated, but the mechanics are really pretty simple, as are the steps involved.

The first step of the night was to tighten the nipples down so that the last thread on each spoke end was just barely covered up by the nipple.  This basically just tee'd each spoke up with a more or less uniform starting point.  From there, we added two full turns to each nipple, to snug them down just a bit more and again, keep them more or less uniform.

Next up, Dave asked us to use the closed end of a wrench to pry the spoke ends just a bit so that they sat nice and flush up against the hub body, rather than curving out a bit.  Then came another round or two of tightening, working the spokes on each side separately (since this is a rear wheel, with an asymmetric hub and different spoke lengths per side) and then squeezing pairs of spokes together to relieve the torsional wind-up that is introduced by tightening them.  What I mean by that is that as the spoke nipples are turned to tighten the spoke, friction forces some of that twist go go into rotating the relatively flexible wire of the spoke, rather than into pulling it tighter, the result being that the spoke is then carrying an undesired torsional load in the form of that twist.  Giving pairs of adjacent spokes a good squeeze together releases the stuck parts, letting the spokes un-twist.

After a couple of rounds, the wheel was tight enough to start thinking about making it true.  Actually, it was surprising just how out of true the wheel was, given that I was trying to keep everything pretty uniform as I went.  But it was pretty far off, and that first truing took a long time to get right.  And I didn't just have to make sure the wheel wasn't wobbly at the sidewalls, I also had to make sure the wheel was circular, with the axle centered in that circle.  That's the truing stand that the wheel is in, at top, and the little pinchy thing towards the front (partially cut off at the bottom) is an adjustable guide that provides a steady reference point to true against.  As the wheel rim is spun, it's easy to see it wobble, and then the guide is adjusted so that it just barely grazes the rim at a "high" spot on one side or the other.  Tightening one or two spokes will usually take care of that high spot by pulling the rim slightly towards the left or the right (depending on whether the spokes came from the left or right side of the hub), and then the process is repeated until there are no more high spots on the rim.

I think there were a couple of rounds of tightening and truing before the next step, but honestly it all sort of became a blur in there at some point.  Either way, we eventually got to the point where we needed to make sure that the rim was centered between the outer locknuts on the axle.  In normal circumstances, an uncentered rim isn't going to ride right.  The rim won't be centered in the notch of the fork or chainstays/seatstays, for one, which means the tires might not clear and the brakes won't grab properly.  It's easy to measure this with a dishing tool, which doesn't actually take measurements or anything -- it simply adjusts to mark the distance from the rim to the outer locknut, and then you flip the wheel over and fit the tool against that side to make sure that distance is the same.  If it is, the wheel is properly dished, and if not, the spokes on one side or the other will need to be tightened uniformly, to pull the rim this way or that.  Then true again, check the dish again, adjust again, true again, check the dish again, etc...

Once the dish is right and the wheel is true, it's time to check for uniform (give or take) spoke tension across each side of the wheel.  Park makes a gauge that makes this easy.  Essentially you clamp it in turn to each spoke and the clamping action tests how much it can bend the spoke.  If the spoke bends more, it's more slack, and if it bends less, it's more tight.  The objective is to have all of the spokes sit in a range of tension so that the wheel is neither too loose and flexible, nor so tight that it becomes overstressed and prone to breakage.  It turned out I had everything a little loose, so I had to tighten everything up a bit more, then check the true, check for uniformity again, adjust, true, check and repeat.

The last step was to plop the wheel on its side on a stool with a divot worn into its seat, and apply lateral pressure to the rim, then flip it over and do it again from the other side.  As with squeezing pairs of spokes, this is intended to relieve any lateral stress on the spokes that was introduced during the build.  Then check the dish, make sure it's true, adjust, recheck and repeat.

At the end of all of that tightening, checking and truing, I ended up with a rear wheel that seems like it'll do the job.  Juli certainly seems pleased, and I'm just thrilled that I was able to do it.  In two weeks, I'll have the last class, and I'll take a crack at the front.

I have one concern, looking very closely at the rear rim:  I didn't notice it before building the wheel, but it looks like these rims have the slightest amount of stagger to the rims' spoke holes, where the holes are slightly offset alternately to the left and right.  If that's true, I got lucky with respect to lacing up the correct side of the rear wheel with the correct holes in the rim.  But since I wasn't deliberate about that mapping (I didn't notice the staggered holes before, if they are truly staggered), I may not have been so lucky with the front wheel -- have to see.  I'm not sure I'll be able to re-lace the front wheel and tension it in one class window, but if need be, I'll give it a shot.

Anyway, so far, so good.  I'm still having fun, and part of me (the part that loves toys) is thinking about wheel truing stands, spoke tension gauges and a dishing tool so that I can build my own wheels whenever I please.  Probably best to ignore that impulse right now, though.

All for now,


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