Merrell MTL Sky Fire 2 First Impressions: A Stripped-Back Mountain Racer

Introduction

Trail running shoes have been getting softer and taller for years, and cushioned monsters now dominate the category. Against that backdrop, the Merrell MTL Sky Fire 2 is a bold counterpoint: a stripped-back, lightweight mountain racer built for ground feel, precision, and pace. This is the first Merrell trail running shoe we have tested in some time, and it sits at the opposite end of the spectrum from deeply cushioned options like the New Balance Fresh Foam X More Trail v3.

On looks alone, it succeeds. The design and colorways have drawn plenty of attention, and the shoe has real presence on the shelf. The question is whether it performs as well as it looks. After a first run across gravel paths, compacted trails, tarmac, slippery rooted sections, and a technical ridgeline, the picture is starting to come together.

Price, Stack Height, and Weight

The RRP is £170, which is a lot of money for such a minimal shoe. There has clearly been serious development behind it, including work with athletes in the Merrell test lab and high-tech materials such as Vibram MegaGrip on the outsole, but the price still feels high for a racing-focused build.

The geometry reflects its racing intent: a six-millimeter offset with 25mm of stack under the heel and just 19mm under the forefoot. This is not a deeply cushioned trail shoe by any measure. Where it does stand out is weight. At only 211 grams in a UK 10, it is one of the lightest trail shoes we have handled, and easily the lightest to be tested here.

Fit and Sizing: Size Up and Stay Narrow

A major warning on sizing: this shoe runs very small. A standard UK 9.5 could barely get a foot in, so a UK 10 became the working size, and even a full size up to a UK 10.5 would not have been unreasonable. It is also very narrow. The upper is all about a precise, foot-hugging fit that keeps you locked in and confident on technical ground, so runners with wide feet should think carefully.

Upper Construction and Durability Questions

The upper is an extremely breathable mesh, breathable to the point that you can see straight through it. That is great for weight and ventilation, but it raises obvious questions about how it will hold up to trail wear and tear. The original Sky Fire had durability complaints, with reports of it falling apart after 150 to 200 kilometers, so this is an area worth watching closely.

Most things are stripped back to save weight: the tongue has essentially no padding, though zonal padding around the ankle collar adds a touch of comfort. There is a substantial heel counter, TPU overlays around the lace eyelets, and structured internal layers around the midfoot and toe box for a bit of extra substance. The downsides: no toe bumper at all, with a notably soft toe box offering zero protection if you kick a rock, and a diagonal strip of elastic over the laces that looks like a bit of an afterthought, even if it does tighten the midfoot fit.

Midsole, Flex Plate, and Outsole

The midsole is a two-part dual compound using Merrell’s FloatPro foam, paired with a lightweight flex plate. That plate serves two purposes: it acts like a rock plate for protection in rocky terrain, and it adds performance, flexing and snapping back for a snappier toe-off.

Then there is the outsole: a five-millimeter aggressively lugged Vibram MegaGrip rubber design. Anyone who has run in this rubber knows how sticky it feels, delivering confident grip on rocky trails wet or dry. The wide spacing between the lugs should help it clear mud quickly, and the cutouts along the narrow midfoot help flexibility while shaving weight, since blown rubber is one of the heavier elements in a shoe. On paper, this outsole looks ready for almost anything.

How It Ran on the Trails

The first testing ground was a mix of hard-standing and compacted stony trails, soft forest ground, and a technical ridgeline. On firm, rocky sections, ground feel was abundant. The flex plate offered some protection, but rocks still pushed through the thin midsole, and it would be hard to spend long stretches on that kind of terrain. On softer ground, the shoe came alive: light, responsive, and genuinely nimble.

The ridgeline is where the Sky Fire 2 wants to be. On super technical, off-camber, rooty, rocky trails it felt dialed in, with the precise upper holding the midfoot securely and excellent lockdown. It is narrow, though, and that fit is unforgiving. It genuinely recalls an old-school precision-fit fell shoe, and it would make a strong fell running option in the UK.

There were a few wrinkles. The unpadded tongue was a worry going in, but on this run the laces caused no irritation, even if a touch of padding would help on longer efforts. The ankle collar held the heel securely with no slippage, but it gapes rather than wrapping precisely, and woodland debris collected around the collar and worked its way inside the shoe. After the run, a surprising amount of grit and twigs had gathered inside.

One honest note: pushing hard on the ridgeline straight out of the box, without a proper warm-up, led to some lower-leg tightness and a shortened run. A reminder that a low, minimal racing shoe asks more of your calves than a cushioned trainer.

Who Should Buy the MTL Sky Fire 2

This is a performance-driven, specialist shoe, and arguably not that versatile. If you want a soft, bouncy, cushioned trail ride, this is not it. Even for runners who do not want extreme stack heights, an extra couple of millimeters of midsole would improve comfort on firmer trails and tarmac without losing the connection that defines this shoe.

But if you are a trail purist who wants a super lightweight, stripped-back feel, maximum ground feel, and a precise fit for fast running on technical, soft ground, the Sky Fire 2 will likely be exactly what you are looking for. The Vibram MegaGrip outsole, in particular, looks set to be a real strength once fully tested in mud.

Conclusion

The Merrell MTL Sky Fire 2 is a striking, featherweight mountain racer that delivers exceptional ground feel, snappy performance from its flex plate, and serious grip credentials. Its compromises are equally clear: a very small and narrow fit, minimal protection, thin cushioning on firm terrain, a debris-prone collar, and a premium price for a minimal build. For fast, technical running on soft ground, it has a lot to offer. For everything else, choose with care.