Monday, May 03, 2021

More tar pits than cake

 Maybe it starts with a rusted bolt, or a stripped-out socket head cap screw. Maybe a bike that was too grimy for quick diagnosis at check-in turns out to have multiple problems once the veil of crude oil and sand is wiped away. Or a suspiciously clean bike is corroded into a single block of inseparable metals because its owner has diligently hosed it off after every ride. The basic tuneup becomes a trap, capturing you for hours as you try to outwit or overpower the sticky quagmire.

A minor version would be this bike with a rear disc brake rubbing. The caliper pistons needed to be reset. To do that I needed to take the pads out. The little bolt that holds the pads in place was basically welded in there. The socket head rounded out almost instantly under the first attempts to dislodge it.

Delicate work with the drill gained me a purchase for a screw extractor.

Screw not sold separately, of course, but I found one from a different brand that fit well enough.

We can't possibly charge enough to cover the costs of some of these repairs we make to preserve the usefulness of a bike that is otherwise quite recoverable. And further time is often lost as we play phone tag to reach a customer and then try to explain what weird thing is wrong, and get authorization to fix it. In the worst cases, the tar pit opens after we've expended time and resources to nearly complete a repair before getting sucked into that "one last thing" to finish it up and clear the ticket.

Or maybe it's a simple, albeit somewhat mysterious issue that draws you in a few minutes at a time, like the mountain bike that came in during the early afternoon last Saturday. The rider reported that the chain would derail two cogs when he tried to backpedal in low gear. The bike is a Diamondback full suspension model with a 1X11 SRAM drive train.

It was never a good idea to backpedal extensively on a derailleur-gear drive train, but it has become increasingly true as more and more cogs are added, especially trying to serve a ridiculously wide range from a single chainring in front. However, mountain bikers in particular need to be able to pedal back at times, to reset their foot position for technical sections, so they should be able to get at least a full crank rotation before the inherent flaws of the system flare up. The rider said that he had formerly been able to backpedal enough, but that since he "tagged a tree with the rear derailleur" it was no longer allowing him to operate unimpeded.

To the eye, nothing looked terribly deviated, despite the tree contact. He left the bike for us to consider in due course, when it came up in the queue, but El Queso Grande thought that I might be able to kick it through very quickly, since its ailment looked to be so minor. I agreed that it would probably respond to some obvious treatment.

The derailleur hanger did turn out to be very slightly deviated. That should do it! Actually, since the derailleur is below the cassette, it would have little effect on whether the chain feeds smoothly onto the top of the cassette when the rider pedals in reverse. But maybe, just maybe, since it was the only apparent variable...

Nope. But you knew that already, didn't you?

There was a chance that the derailleur itself was bent. We've seen it before. Especially with the long, long cages needed to handle the chain wrap and cog size of super-wide gear ranges, and the intolerance of systems cramming 11 and 12 cogs into very little more than the space initially carved out for eight, tiny deviations can lead to intractable shifting and chain feed issues. But again, the derailleur is below the problem, not above it. With alignment and adjustment again dialed in, the chain still dropped.

In the process of investigation, I had to remove the rear wheel. The through axle did not want to come out. EQG theorized that the through-axle itself was bent, but it didn't seem to be. My theory is that the dropouts aren't parallel. Who knows if they ever were. The bike looked sharp at first glance, all bright orange with the brand name also in bright orange, very subtle. It could also be a way of hiding your identity so it wouldn't be blatantly associated with your crappy product. A lot of things have to be exactly right on a rear suspension swingarm and a through-axle wheel mount. It's another version of the press-in BB with misaligned bearing seats. It ain't never going to be right. Y'all had one chance to make it so, and you blew it. At least with a BB you have some chance of finding a Wheels Manufacturing thread-together unit that will put the bearings in proper alignment relative to each other, sidestepping the error in frame manufacturing. Misaligned dropouts are a tougher nut since the through-axle format does not lend itself readily to the old style of alignment tools, and materials like carbon fiber or aluminum either can't or shouldn't be tweaked.

Tell me again how much better our lives are with this temperamental bullshit?

Of course I had to have the wheel in and out of those dropouts a dozen times or more as I tried cassette spacers and different cog sets to try to coax the chain line into a more cooperative orientation. I also removed the bottom bracket -- an outboard thread-in model -- to put a skinnier spacer behind the drive side to pull the crank in a tiny tad. Can't go too far, or it will act up at the high-gear end of the range. But sometimes just a little more than a millimeter can be just enough to get by.

Not this time. Nor did any cassette yield a result worth what we would have to charge for the parts and labor. I had noticed that the original cassette had a slight bend in that 42-tooth low gear, but replacing the cassette with a brand new one with a well-made, beefier 42 produced no improvement. I put the cheesy bent one back on because it made no difference.

Bent chain links can cause no end of disruption, but this chain was neither bent nor excessively worn. Hardly worn at all, in fact. Was lack of lube making the chain less laterally flexible? You couldn't prove it by me. I juiced it right up and it still hopped off.

At the end of three hours we had disassembled and reassembled the drive train multiple times and come up with absolutely nothing billable.

The wonderful world of Internet forums had no definitive guidance. There were blessed experts who had sure-fire solutions to this problem, in the same thread with the vast majority of people who said either that you should just never backpedal because the chain always derails, or suggested trying each and every thing we tried -- all of which have worked on other bikes.

There was no obvious sign that the whole bike had been bashed out of alignment by the tree encounter. And it's true that wide-range, 1X drive trains are very prone to this problem. It's another reason that I no longer feel any sense of accomplishment when one works, only a sense of foreboding, wondering when it will stop doing so, and why.

A cake tuneup, by contrast, is one on which everything goes so smoothly that the bike is on and off the stand within half an hour. These tend to happen on older bikes that have been coming to us for years. If they were thoroughly assembled, almost nothing -- and sometimes nothing at all -- will have gone out of adjustment in a year. The same is true if a tuneup was once done to full specifications. It helps if the bike is ridden with some sensitivity to its vulnerabilities, but it can still be ridden a lot. We regularly have alumni come in who have worn the chain to the point of replacement and perhaps cooked a shift cable or two, but whose hub and headset bearings are exactly where they need to be. Locknuts are called locknuts for a reason. Or a bike might need only a genuinely simple, specific repair that works as it should. But on the whole, we deal with more tar pits than cake.

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