Why? For five somewhat overlapping reasons: fast acceleration, nimble steering, eager climbing, smooth bump absorption, and, well, look at it.

Quick acceleration
Prior to riding the Wabi, my main frame of reference was my Surly Straggler. Compared with the Straggler, the Wabi is a rocket. Part of the speed disparity is due to weight. My size 61 Wabi weighs 21.6 pounds with a saddle bag, while the Straggler (size 58) tipped the scales at 28 lbs, even as a single speed (but including its rack, its dynamo front hub, and aluminium fenders).

Apart from the weight, I attribute its brisk acceleration to its wheelset and its stiff rear triangle. Somehow, the wheels, which came stock on a sub-$1,000 complete bike, are a glaring exception to Keith Bontrager's aphorism: "strong, light, cheap: pick two." The acceleration also comes from the frame stiffness. When I pedal hard on the Wabi, I sense that most of the energy goes towards spinning the rear wheel. Not true with the Straggler; some gets used to bend the tubes like macaroni. Admittedly, the Straggler has no pretense about racing, as its name implies, so the Wabi-Straggler acceleration comparison is perhaps unfair. Still, after having been introduced to steel by Surly, as many are, I was pleased to learn from the Wabi that steel does not have to mean slow acceleration.

In hopes of keeping the all-silver-everything aesthetic, I tried a Soma HWY One, which fit into the stock 26 mm stem clamp and certainly looked the part. Comfort was an improvement over the stock bar, but I still didn’t love it. ​

Especially with comparably priced 11- and 12-speed options available
I couldn't justify a new 11-speed or Eagle drivetrain when my 10-speed shifter (X7) and derailleur (X9, long cage) were in perfect working order. On the other hand, the XG-1099 retails for $320 for a 10-speed cassette; if that kind of coin was on the table, why not just upgrade the whole drivetrain? Budget aside, I chose the 10-speed because the 11-speed and Eagle versions of the X-DOME require the XD driver, which would have made single-speed conversion complicated at the time of purchase (that problem has since been solved). The initial driver of this cassette search was the pursuit of simplicity, after all.

Plus, I assumed that I could expand the range of the 10-speed, given the array of aftermarket options to increase the range of popular cassettes (e.g., by Wolf Tooth, OneUp, or First Components). The search proved more difficult than expected. 

The only company I found who would add teeth to the XG-1099 cassette was Ari, an Italian maker of bike components.

So I ordered a 39-tooth in red, by emailing info@aribike.it. Email correspondence was pleasant and prompt.

Installation and weight
Removing the aluminum outer ring from the steel set of cogs was nerve-wracking, given the value of this precious metal, but not that difficult. I used a flat-head screwdriver to pry it off slowly. Ari has instructions here. 

​Installation was also simple, following the instructional video on Ari's website. Cog first, then spacer, then the rest of the cassette.

Cost of cassette, including Ari Cog

• The Ari cog cost €70 EUR plus €26 EUR shipping ($111.59 USD total, June 2017 conversion rate).
• I found the XG-1099 used on ebay for $135 (retail is $320). Buying used was the move, because the steel body is reportedly durable, and the faster-wearing aluminum outer ring was to be replaced anyway.

My total cost: $247 

Durability and performance
That's admittedly a pretty expensive cassette, but it checks my criteria:

• easy to take on and off
• no need for the XD driver
• enough granny (39 teeth)

As for how the cog works, I've really liked it after 3 months. The aluminum outer ring has stayed flush with the steel cassette body, with no sign of movement. The teeth have not noticeably worn. The bright red paint has slowly given way to its metal roots, but I'm alright with the more subdued look.

If the Straggler is a slow road bike and an under-biked moutain bike, is it great at anything? Yes. It's great at being a bike that rides mostly pavement but enthusiastically seeks dirt. The multi-tool field is crowded these days, but for the money, it's hard to beat a Surly. To complete my multi-tool, I needed a tire that would roll fast on pavement but wouldn't be intimated by the occasional singletrack.
​
And on my quest to find the perfect tire for the Straggler, I had one major constraint.  It had to fit under Handsome 45-mm fenders.  (As an aside, I don't understand why the gravel-bike scene hasn't embraced fenders. Well, some have, but it's not a unified scene. There's a reason my purple paint still glitters strong after three years of year-round commuting.)

Handsome says their 45-mm fenders will fit 38s. This width seems about right for the job, regardless of fender restriction. Much fatter, and it'd be slow on the road; much skinnier, and it'd be sketchy on dirt. After considering the Maxxis Rambler 38, Schwalbe G-One 38, and the Panaracer Gravelking 38, I decided on the Hutchinson Override 38. While the other three would have presumably done the job well, I went with the Hutchinson because it looked like the roadiest of the four, and, if I'm being honest with myself, that's where the bike will get the most use.

True to Handsome's word, the fatties fit fine under the 45-mm fenders after a little wrenching with the struts.

As advertised, I have been impressed with how fast these tires roll on pavement--not much slower than the Panaracer 32s and A LOT faster than the Rock N Road 43s. I attribute the speed to the low-key center knobs and the respectable 418-gram weight. They're comfortable, too, pairing nicely with the vibration-dampening Natch frame.

And they have held their own on local ITP singletrack, with enough side knob and cushion to hold a dirt corner, but not so much so that the pavement ride to the trail is a slog.