Fox Union BOA Flat shoe
Test Locations: Washington and Oregon
Test Duration: 2 months
Reviewer: 6’, 160 lb / 183 cm, 72.6 kg
Size Tested: US 11
Blister’s Measured Weight per shoe (US 11): 444 g
Upper: Rubberized TPU
Closure: Dual BOA
Sizes Offered: EU size 37 – 47 (US Men’s 5 – 13, Women’s 6.5 – 14.5)
Pedals Used: Chromag Dagga
MSRP: $224.95 USD
Intro
Fox is a relative newcomer in the shoe game, but we were super impressed with the original Union when we reviewed it last year, and Fox has continued to expand their lineup since. The Union BOA Clipless shoe was introduced in 2023 as Fox’s top-end offering for clipless riders, and now they’ve taken the same construction and double BOA closure to create the new Union BOA Flat. So we were curious to see how Fox’s top dog in their flat pedal lineup compared to some longer-standing competitors.
Features and Construction
Aside from a slightly lower stack height at the sole, the Union BOA Flat is nearly indistinguishable from the Union BOA Clipless. The upper is constructed of a single piece of rubberized TPU material with perforated venting over the toes and on the side alongside the arch. A nylon shank adds stiffness to the fairly low-profile EVA midsole, and the toe area gets a rubberized cap for a bit of extra protection from rock strikes and such.
The heel cup is fairly deeply molded to help with heel hold, and the insoles feature a replaceable velcro insert that allows riders to choose between two different heights of arch support.
The big differentiator over the regular Union Flat is the addition of the BOA closure. Where some shoes go for one BOA dial, Fox has implemented a dual BOA closure, with the lower dial managing the forefoot and midfoot, while the upper dial handles a narrower area just below the top of the tongue. The dials are BOA’s Li2 dials, which are fairly low profile and allow easy micro-adjustments.
Perhaps the most important part of any flat pedal shoe is the rubber, and Fox has stuck with their Trailtac rubber found across their range of shoes. Fox claims that it “balances grip and durability”, which was worth investigating on the trail as we’ve found other shoes claiming good durability to miss the mark in the grip department.
Fit
Sizing-wise, the Union BOA Flat is bang-on in my typical US 11 size. I’ve mentioned in other shoe reviews that I have rather narrow feet, and I’ve been impressed with the ability of the Union BOA Flat to accommodate them. They aren’t a particularly narrow shoe — Five Ten’s Trailcross XT fits a bit wider, while I find the Ride Concepts Tallac and Hellion Elite to fit a bit narrower — but the BOA system on the Union offers a large adjustment range, opening wide enough to put on with ease before then cinching down evenly around my foot. I also have high arches, and the option to run the higher arch insert with the adjustable insole adds a welcome degree of support beyond most stock insoles.
While they did soften up within a couple of rides, the upper material of the Union BOA feels quite stiff when new. This led to a bit of initial pinching in some areas near my ankle, but it wasn’t much of an issue on the bike — more while walking around, and it went away entirely after a few rides.
On the Bike
As someone who made the somewhat unusual switch from clipless-only to splitting time between clips and flats, I generally prefer a slightly stiffer flat pedal shoe to make the switch back to stiffer clipless shoes feel a bit more natural. I found the Union BOA Flat to hit a nice Goldilocks zone of feeling stiff and supportive through the arch and heel while offering decent flexibility in the toe. The Union BOA’s midsole contours nicely around the pedal without being so soft as to induce foot fatigue on longer rides. I would put them relatively on par with my preferred Ride Concepts Tallac, and substantially stiffer than the Five Ten Trailcross XT for example.
I find that BOA shoes require a slightly different tightening approach than laces. With most lace shoes, I crank them down pretty tightly, given that the laces allow for some stretch, but that same philosophy with the less elastic BOA cables will make my feet numb in no time. The dual BOA arrangement on the Union BOA Flat gave a much better fit than most shoes with a single BOA dial, where I could snug up the forefoot section for a precise fit where it counts, then leave the upper BOA adjuster a bit looser to allow better blood flow.
For being slightly stiffer than average, the Union BOA Flat also has an impressively low stack height, particularly compared to the Ride Concepts Tallac. They don’t feel quite as well cushioned as the Tallac, but they offer a more integrated connection with the pedal and bring a bit more “feel” as well, whereas the Tallac can feel a bit muted by comparison.
Five Ten reigned supreme for years with their ultra-sticky Stealth rubber, but several other companies have started to crack the code and catch up as of late. While I still think Five Ten wins for the stickiest rubber, I’ve found myself appreciating the efforts of some other companies like Ride Concepts, Shimano, and Specialized for offering marginally less grip, but a greater ability to adjust position when a foot inevitably gets knocked out of place during a rough descent. Fox’s Trailtac rubber definitely falls into that category of being quite close to the level of grip on offer from Five Ten’s Stealth S1 rubber, while offering slightly better ability to reposition. I would put them on par with the stickiest options from Ride Concepts, or maybe even slightly more grippy, which is a strong compliment in my book as I’ve generally really enjoyed the balanced feel of the Max Grip rubber used on the Hellion Elite and Tallac. Where shoes like the Tallac and Five Ten Trailcross have a slightly deeper lug pattern on the soles, the Union BOA Flat sticks with a slightly lower profile design that prioritizes a more consistent feel on the pedals.
As you can likely tell at this point, I’ve been extremely impressed with the performance of the Union BOA Flat, but I have one small gripe that would potentially be a big one in other climates: the rubberized finish of the Union BOA Flat’s upper shrugs of mud and water with ease, but that same weather resistance means it does not breathe well. Hotter rides in these shoes can get fairly swampy, and while the perforated vents certainly do something, they aren’t quite big enough to keep up on higher output rides or in particularly toasty conditions. I found them to be fine for my purposes, but riders in particularly warm locales might be better off skipping the BOA option in favor of the Union Canvas, which should allow more airflow.
Off the Bike
The initially stiff upper of the Union BOA Flat made for some discomfort when walking during the first couple of rides, but that dissipated entirely once they had a chance to break in.
I mentioned the lower-profile sole of the Union BOA Flat, and while there is adequate spacing between the lugs to grip in most conditions, hike-a-bikes in muddy conditions can be a bit dicey once the small channels clog up with dirt. Shoes like the Five Ten Trailcross XT or Ride Concepts Tallac feature larger lugs toward the toe and heel, which can bite into soft ground more effectively than Fox’s sole design. As with most flat pedal shoes, their grip on rock is excellent.
Durability
The Fox Union BOA Flat is expensive, but fortunately, it seems that the build quality goes some way toward justifying the initial cost. As my daily driver shoe during the test period, I subjected the Union BOA Flat to routine dunkings in gritty mud puddles, scrambling around through shale and rock, and routine hose-downs without any problematic signs of wear. Sure, they have a couple of battle scars, but nothing has managed to penetrate the burly material on the upper or cause any functional damage, and the lack of sewn seams means that the shoes are less vulnerable to damage.
The sole is perhaps the most impressive bit — despite all that grip and being mashed into the fearsome pins of my Chromag Dagga pedals, it’s still in great shape. While I don’t doubt that the Daggas will eventually chew away at the soles, all indications point toward the Trailtac rubber having a longer lifespan than average.
Bottom Line
Fox had their work cut out for them with a late entry to the mountain bike footwear space, and fielding a solid flat pedal offering is a particularly daunting task. The Union BOA Flat is inarguably expensive, but its premium features, adaptable fit, and admirable grip go a long way toward justifying the asking price. Add in the promising durability, and the Union BOA Flat has eclipsed my prior favorite shoes as a go-to for most rides — unless the weather is truly sweltering.