Concentricity

So the big news for today is that frames are arriving to us on Monday.  All told, the timing came in pretty much within a whisker of exactly what we'd been told they would, way back when.  I'd always counted on frame lead time being one of these super high standard deviation deals, where you had about a 1% chance of them arriving within a few days of plan, a huge chance of them arriving REALLY late, and a small chance of them arriving meaningfully early.  Our agent, who we haven't mentioned lately, deserves a golf clap from Mike and me for recommending our manufacturer to us as one with a track record of dependable delivery.  That was a big part of our whole decision tree, and it's paid.  We'll be QC'ing those puppies all day and night Monday - fire up the espresso machine - and turning them around with astonishing rapidity.

We're finally caught up with our wheel builds and are working on current orders.  That was A LOT of wheels to get built.  Early in the process of getting a large number of carbon wheels built, I realized that I had adopted a pretty significant change in the way I was approaching the builds.

Most people can figure out whether a wheel is true or not.  You set your brake calipers to "tighter than you'd ever ride with them" and if they bind on the wheel, it's not that true.  A widely used standard for true is +/- .5mm, which means that if you roll the wheel through a gap that is 1mm wider than the rim, it will pass without hitting.  There are small variances in rim width on the wheel, so you have to set the gauge to the widest part of the wheel.  We're talking about small segments of a millimeter here, but when your feeler gauges on the stand are only hitting the rim when they are hitting both sides of the rim simultaneously, you realize that your tolerances are getting there.  

Figuring out whether the wheel is round or not is more subtle when you don't have a truing stand.  It's very hard to talk about absolute tolerances since the rim itself might not be 100% perfectly absolutely "British scientists use it to calibrate their instruments" round, and there may be hard spots in the rim (I'll get into this in the next installment of "How It's Made").  Chasing these things down comes at the expense of having very very balanced spoke tension, which is super important.  Also, the perceptibility of slight out of roundness is basically nil, because tires aren't THAT round, and they are pliable.  If it were possible to have a perfectly round tire and you pumped it up to 200 psi, you might feel something in a rim that had a +/- 1mm tolerance.   In the real world, you never would, and the brake adjustment wouldn't bear that out either - I've adjusted a lot of brakes and it's freaking impossible to get both pads in EXACTLY PRECISELY the same spot on both sides.  

But while truing a carbon rim is the same as an aluminum rim, getting the roundness right is different.  With aluminum rims, it's possible to move the rim from circular - extra spoke tension will pull a high spot in.  So there's some fudge factor there.  You're basically adjusting the circle of the rim in a "hub-o-centric" universe (thanks, Galileo!).  Carbon rims are WAY too stiff to deal with this, so you can crank up the spoke tension until the cows come home (and the cows generally come home just after you've demolished the rim with too much spoke tension) and the rim ain't moving.  This is part and parcel of why full figured riders (but not necessarily the extremely powerful among us) are best off on deeper sectioned wheels - they don't "squish" under load, which means that the spokes don't load and unload as much, which means the spokes last longer.  But that discussion of "which wheels are best for which ride" discussion is best saved for another day. 

In any case, when you are trying to make carbon rims roll round, instead of manipulating the rim to be the correct distance from the hub, you are actually placing the hub in the center of the circle defined by the outer edge of the rim.  If you have a high spot in the rim's rolling roundness, you need to move the hub slightly toward that high spot.  It is almost always the case that a high spot is 180 degrees away from a low spot.  So you move the hub slightly away from the low spot and slightly toward the high spot and that does it.  And then another part of the wheel gets out of whack so you chase these deals around in your best imitation of "whack-a-mole" and eventually you get to the point where you can't see daylight between the guide and the rim, but you don't hear the rim scraping on the guide.  And then you destress the spokes and go through it all all over again. 

It's a labor of love.