Salomon S Lab X-Series vs. Salomon S Lab Sense 4 Ultra – 911 vs. GT3

In a recent review of the Salomon S lab X-Series “road” shoe I hinted that, based on limited trail miles, it seemed that the X-Series might prove to more of a trail shoe than expected. Well, now, after a little less than 200 miles (300 km), I can affirmatively attest to the trail-worthiness of the X-Series. In fact I will go so far as to say that this shoe is hitting an optimum mix of characteristics for all but the most gnarliest of trails- even though Salomon classes it as a “road” shoe with some trail capability.

So, I have tested the X-Series against Salomon’s top-of-the-line trail shoe, the S Lab Sense 4 Ultra. This comparison test has been conducted on all of the same trails, conditions, and terrain. The primary trails are mountainous buffed to mid-technical, the terrain has included steep (>60% grade) trail, off trail talus, slickrock, bog, stream, and river crossings, and the conditions have included snowfields, clay mud, ice, and sand. A good variety that includes just about everything that one might encounter in the Rockies with the exception of the “moondust” that typically develops later in the summer on some trails. The following is a summary of the differences, similarities, and superiorities between these two shoes.

In the end the two shoes compared in a way that a 911 might be compared to a GT3– both models are very high performance options, but one is much more “edgy” and tuned to a specific purpose whereas the other is a more widely usable and comfortable design with numerous advantages.

Note: 23 April 2015- I have posted an update on the performance of the S Lab X-Series at about 700 km (435 miles) of use here.


The S Lab X-Series shoe was announced last summer in Europe and the US and came as a bit of a surprise since Salomon have not ever played in the “road” shoe market but have, rather, focused on the trail shoe market. When this “road” shoe was introduced many cast a wary eye given Salomon’s lack of experience in this area. However, usually when Salomon does something they do it right and “different” than the competitors. And this holds true for the X-Series.

Salomon X series overall

Salomon S Lab X-Series

The S Lab Sense 4 Ultra is an evolutionary derivative of the  original Sense line brought out in 2012. As can be gleaned from the name of the shoe, Salomon are on the 4th generation of the Sense. In 2012 the shoe introduced super light weight, high traction, incredible proprioception, ultra-light ProFeel carbon fiber rock plate, some unique materials, and, most importantly, Endofit construction to the trail running world. The EndoFit construction has been universally acknowledged as a superior approach to fit and feel. The progression of the models since have fixed performance and durability issues primarily with added traction (more and deeper lugs) and new, more durable, materials and constructions. Such is the same with the latest iteration in the Sense 4. This shoe is the flagship of the Salomon trail running offerings but the model has become a bit “long in the tooth” since there have been no real advancements beyond those first introduced in 2012. It is expected that Salomon will disrupt the trail running market sometime soon with an all-new model, perhaps with a dose of the carbon technology that they have been applying to the cross country ski boot and mountaineering boot markets.


Salomon S Lab Sense 4 Ultra



The X-Series and Sense 4 both employ many of the same construction elements and materials- EndoFit, ProFeel film, SensiFit, and ContraGrip outsole. The differences lie in some of the materials in the upper, the amount of dual-density EVA incorporated, the design and attachment of the outsole wear layer, and the geometry of the shoe itself.

The X-Series offers a wider forefoot (about 3 mm on my size 7.5US, 40 2/3 EU) and a wider, squarer toebox. The heel is also wider by about 5 mm on these size 7.5US.

Salomon X series comparison forefoot

Toebox and forefoot area of S Lab Sense 4 Ultra (left) and S Lab X-Series (right) showing the wider, squarer toebox and a 3 mm wider forefoot dimension in the X-Series (size 7.5 US (40 2/3 EU)). The heel of the X-Series is also wider- by about 5 mm.

Perhaps a more significant difference is the midsole stack where the X-Series has a 19 mm heel/11 mm forefoot and the Sense 4 has 13 mm heel/9 mm forefoot stack. Both models use “dual-density” EVA where a cushier EVA can be placed where needed (I assume that the lower density, softer, EVA is in the forefoot and heel). The overall stack height (including the outsole) for the X-Series is 23 mm heel/15 mm forefoot giving an 8 mm drop. The Sense 4 adheres to the “low drop” Sense design ethos with a total stack height of 19 mm heel/ 15 mm forefoot and a 4 mm drop.

The outsole design is quite different as expected since the X-Series is intended for roads and “city trails” (whatever that means). The Sense 4 continues with the minimalist chevron array that has proven to provide outstanding performance in everything but ice and, to some extent, water crossings on submerged rocks. The X-Series outsole has a much larger surface area of contact but still with significant, deep lugs in strategic areas. The X-Series also has a segmented outsole wear layer that is glued on whereas the Sense 4 outsole wear layer is one piece. Salomon had durability issues in the original Sense model with such segmented wear layers coming off due to failure of the glue and the associated very long exposed glue-line. Hopefully they have since figured out a reliable and durable way to glue these segments.

Salomon X series outsole

Salomon S lab Sense 4 outsole

S Lab X-Series outsole (top) and Sense 4 outsole (bottom) showing large difference in traction design between the two models.

The X-Series also has a deep cutout extending from the middle of the outsole rearward into the heel area. It is not clear what this design feature is for but it may provide additional longitudinal stiffness through the middle of the outsole. Upon examination the X-Series is much stiffer longitudinally in this area than the Sense 4, although the forefoot stiffness is similar between the two models.

The X-series employs a significant “rocker” geometry, similar to that in the Hoka line of shoes. The Sense 4 retains the low, flat profile of past Sense models.

Salomon X series compared to Sense 4

Salomon S Lab X-Series (upper) and Salomon S Lab Sense 4 Ultra (lower) showing the substantial “rocker” geometry incorporated in the X-Series when compared to the Sense 4 Ultra.


The heel cups are similar but the ankle cups are different. The X-series has a symmetric ankle cup whereas the Sense 4 has an asymmetric set-up. I have felt no difference between the two while running, so it is not clear what the higher inner ankle cup does on the Sense 4 other than perhaps protecting a bit more in rocky conditions and from opposite side heel scuffing.

Salomon S Lab X-series back comparison

Rear view of the Salomon S Lab Sense 4 Ultra (left) and the Salomon S Lab X-Series (right) showing similar heel support structure but very different ankle cup configurations.


Salomon X Series ankle cup comparison

Ankle cup comparison showing Salomon S lab Sense 4 Ultra (top) and Salomon S Lab X-Series bottom. The Sense 4 has an asymmetrical cup whereas the X-Series is symmetric. The Sense 4 also has much more rear roll-over of the padding.


The upper materials are quite different where the Sense 4 utilizes the same (or similar) 3D fine mesh fabric throughout the upper with the heel wrapped in a tough solid polymer as seen in previous Sense models. The X-series uses primarily three different materials in the upper- a “barely there” mesh on the outside panel for ventilation, lightweight Lycra through the toe and forefoot region, and a beefier nylon fabric around the heel and throughout the inside are area up to the forefoot.

Salomon X Series side veiw

Salomon X series viewed from above

View from above of the Salomon S Lab X-Series showing a toebox and forefoot that is wider than that in the Sense 4 Ultra.



The s lab sense 4 uses the same (or similar) 3D mesh material as found in previous Sense models for the entire upper with the exception of the hard solid polymer around the heel.



View from above of the Salomon S Lab Sense 4 Ultra showing the slim and fast profile that is a fundamental part of the Sense line.

The polymer overlays look similar but they are actually very different. They both have the signature “zig-zag” side features but the X-Series toe area is much more pliable than the Sense 4. The Sense 4 overlay is much more structural and therefore the Sense 4 provides much more protection for toes than the X-Series. Both models have similar toe bumpers however as this is the most likely place that one will stub a toe.


The two models have very similar weights. The S Lab X-Series shoe is quoted as weighing in at 218 gms (7.7 oz) for a size 9 (US). My size 7.5 (US) (40 2/3 (EU)) tipped the scales at 215 gms (a little less than 7.6 oz). That is lighter than the quoted 238 gm for a size 9 (US) in the Sense 4 and the actual measurement of my Sense 4 Ultras in size 7.5 (US) at 227 gms.


The X-Series is $160 US and the Sense 4 Ultra is $170 US. Both are expensive shoes but if they last as long as other S Lab shoes I have used, the “pennies per mile” metric should make these shoes a reasonable offering from a price perspective.


RED! Salomon have simplified the graphics and the all-red X-Series are quite distinctive. No other colors are available in either model.

Running Performance Comparison

As outlined above I have tested these shoes on the same trails, conditions, and terrain. They have both seen about 200 miles (300 km) of use on buffed to mid-technical trails and terrain varying from steep (>60% grade) trail, off trail talus, slickrock, bog, stream, and river crossings. The conditions during testing have included snowfields, clay mud, ice, and sand. This is a variety that includes just about everything that one might encounter in the Rockies with the exception of the “moondust” that typically develops later in the summer on some trails.

General Running Characteristics

When comparing these two shoes the primary difference one will note at the outset is the cushioning and the “rocker” geometry of the X-Series. This makes the X-Series stand apart from the Sense 4 not only on the road but very much so on the trail. Proprioception is better in the Sense 4 but the X-Series has very good trail feel albeit with a bit of dampening.

The cushier ride of the X-Series is welcome, particularly to those used to the direct, somewhat hard ride of the Sense. While the Sense is likely to comparatively increase stride efficiency this comes at a cost of comfort and durability as it concerns ones feet. The X-Series provides a nice compromise between the “hardcore” trail feel of the Sense and the disconnected trail feel of a maximal shoe like a Hoka Stinson.

The X-Series also has a slightly larger total area outsole and much larger surface area of contact. One will feel this immediately as well and, just as such larger area outsoles do with the Hoka line, there is a perceptual “smoothing” of rocky trails in comparison to the Sense. Although I have yet to be on highly technical, fast running trails yet, I have not noticed a significant decrease in control between the Sense and the X-Series. The X-Series feels just as nimble as the Sense on typical buffed to mid-technical trails.

The larger, squarer toebox of the X-series is noticeable and in certain situations one can feel a bit more movement in the forefoot, particularly on steep downhills. I have run a couple of 3 mile ( 5 km) continuous steep (>15%) downhills in both models and found similar pace and feel except in the steepest (>50% grade) of areas where the X-Series begins to break loose where the Sense does not. The EndoFit is obviously doing a great job of helping one to maintain control in such challenging conditions and the ProFeel rockplate is doing an outstanding job protecting against sharp rocks.

In a couple of long (30+ km) runs on the same course the X-Series proved to be a very comfortable option when compared to the Sense 4. My feet were ready for some relief at about 32 km in the Sense 4, whereas, in the X-Series, I was looking forward to continuing on. That’s a big difference and one that will play a major role in a 50-100 km race.

Performance in Water, on Snow, and on Ice

One concern I had initially was that the Lycra upper of the X-Series would not drain after a water crossing as well as the 3D mesh of the Sense does. Not so, and, in fact, the X-Series is draining better in my experience.

More importantly, the X-Series performs much better on wet and/or slippery rocks. I found very good traction in stream crossings with the X-Series where the Sense would be somewhat unstable, particularly if any algae was present. The X-Series crosses rock-strewn streams with confidence and stability. This is most likely due to the simple physics of increased surface area for the ContraGrip when compared to the Sense- a lowered net shear stress for the X-Series allowing for significantly increased traction. I will update on performance in this condition once the summer algae thickens up on the submerged rocks.

A similar experience is found on snow and ice- the X-Series just plain outperforms the Sense on snowfields and ice. First, the increased traction helps establish control. Second, glissading is much smoother and controllable with X-Series as once static friction is overcome and gliding begins the larger surface area and less aggressive lugs allow for a “ski feel” and the associated control. Being a cross country skier helps but I was able to traverse a 200 m snowfield in one controllable glissade whereas in the Sense this same snowfield required three starts and stops due to “dig-ins” and lack of control. The same has been my experience on ice- the X-Series is much more stable on ice. Of course the Sense is particularly bad on ice but, with the X-Series, I was able to safely cross numerous “luge runs” of glare ice  that routinely  develop in the woods as winter fades and spring arrives. I would not even try to do these crossings with the Sense.

One outstanding question is how hot will the Lycra be; the material is thin and drains well but Lycra is not a particularly cool fabric in hot weather. Durability of the Lycra in abrasive conditions is another concern- time will tell.

Off Trail and Performance in Mud

I have taken the X-Series across some significant talus fields at running pace both up and down and have found very good performance, and, in some cases, better performance than the Sense which is already an outstanding performer on talus. Once again the source of the superior performance of the X-Series on talus is the increased dry traction when needing to make a big move on the talus field during descents. The larger surface area provides just that little bit more of a direction-changing force needed when heading directly for a large boulder or drop-off.

When crossing bogs I feel no difference between the models and the X-Series performs admirably right through the best muck and bramble that Idaho can offer. In pure mud, I was concerned that the big cutout in the outsole would just serve as a reservoir for mud and a large area for attachment of additional mud, particularly in clay-based mud. This did not happen and I find that the X-Series actually accumulates less mud than the Sense; not sure why. But as far as traction in pure mud on trail, the Sense 4 is superior; the widely spaced and aggressive lugs on the Sense 4 do a good job of piercing through the mud and hanging on to whatever grip is available whereas the X-Series tends to float a bit until one engages the outer, deep lugs. In practice this means that you will slide around a lot more in the X-series on mud so if you are regularly running in mud, the X-series would not be a good choice. My experience here in the Northern Rockies is that on the rare occasions when we do have significant sections of mud nothing seems to work very well (read the accounts, for example, of the 2014 Bear 100).

Concluding Comments

What I have found with the S Lab X-Series “road” shoe from Salomon is quite unexpected from what I thought the shoe would be. I expected a road shoe that could be used on buffed trails. The X-Series turns out to be an excellent, very nimble trail performer in a broad range of conditions even rivaling the outstanding performance of the Sense 4 in certain cases. This all comes with a nice dose of cushioning. Although I look forward to seeing how durable the outsole and upper is in long term use on muntainous trail conditions, it seems that Salomon may have hit an optimal mix of performance and comfort with the X-Series.

The S Lab Sense 4 Ultra continues with the outstanding, high performance, and “edgy” character of the Sense line giving unequaled proprioception and control on high speed trail downhills and all around performance for mountain running. The ride is “direct” and hard but efficient. It is also marginally comfortable at distances greater than about 50 km.

Bottom Line

Just as the 911 offers a nice mix of performance and comfort when compared to the “edgy’, stark, but super fast and controllable GT3, the S Lab X-Series does the same when compared to the S Lab Sense 4 Ultra. The Sense 4, like the GT3, has capabilities second to none in certain conditions, but this performance comes at a cost- long distance comfort. The X-Series offers a little less performance (and “egdy-ness”) in some cases and better performance in others, but a lot more comfort for the longer races and runs.

I’m going with the X-Series for my trail running going forward this season. And I am not the only one- Max King ran the LA Marthon in 2:17:30 in the X-Series, Justin Houck just won the 2015 Gorge Waterfalls 100k Montrail Cup Race, and Ellie Greenwood ran the Vancouver Marathon in 2:47 in the X-Series. Quite the versatile shoe!

I’ll keep you updated.

Note 12 July 2015

It seems that Kilian used the S Lab X-Series for some portion(s) of his 2015 record-breaking performance. Here he is changing shoes at Telluride aid station before climbing 1400 m (4500 feet) up Bear Creek on road, then trail, then snow fields over Oscar’s Pass and then a 950 m (3100 foot) decent into Chapman. Looks like he is changing into the X-Series (with an all black wear layer) from the 2016 Sense (or some new prototype). This is a testament to the versatility of the X-Series, a supposedly “hybrid” road/trail shoe!

Kilian Hardrock w_2016 shoes

Kilian changing shoes at the 2015 Hardrock 100 Telluride aid station before a difficult 1400 m (4500 foot) climb and 950 m (3100 foot) decent. Looks like he is changing into the 2016 X-Series from the 2016 Sense. Photo credit:



9 thoughts on “Salomon S Lab X-Series vs. Salomon S Lab Sense 4 Ultra – 911 vs. GT3

    • Hi Erik,

      I think you will be surprised with the on trail performance. I am adding a bit more to the review as it concerns mud performance compared to the Sense 4. I need to make sure that it is clear that the X-Series is not nearly the mud shoe that the Sense 4 is as far as traction, but the X-series does handle mud quite well even with the less aggressive outsole.

  1. I have a pair coming my way real soon.
    I have been quite satisfied with my S Lab Sense 4 Ultra, very durable, comfortable and a good fit.

    • Hi Laurent,

      Yes, the Sense 4 Ultra is a great shoe… I am surprised how much the slightly larger platform and extra cushion on the X-Series affects comfort on long runs. X-Series still holding up well after about 350 miles at this point. I expect that you will like the X-Series, at least on the buffed trails, but perhaps on other, more technical ones as well.

  2. Wondering if the X-series could be the right fit for the runnable and not-too-technical trails of the Lavaredo Ultra Trail. Or, as suggested, a good choice on my drop bag for a specific course section.

    • Hi Corrado,

      I have not run the trials in the Cortina area, so I can’t give a reliable recommendation. But if the trails are runnable and not too technical I would expect the X-Series to be a superior choice. I continue to be surprised at how well the X-Series performs in truly technical terrain. The well developed proprioception and endofit are primary reasons why.

    • Hi Simon,

      You could be right, however the last on Kilian’s shoes looks a lot like the X-Series and the ankle cups appear to be symmetric as they are on the X-Series. Ellie Greenwood has been running in an X-Series with an all black wear layer, although I have no idea what the topology of this layer is. The shoes Kilian is switching to in the picture are likely a new prototype with the monochrome look with what appears to be incorporation of X-Series geometry. I guess we will find out in August when the 2016 line is announced. Thanks for the pic from Frosty.

      • On the side it says prototype. So it may be a brand new shoe! All the Salomon athletes (Frosty, Kilian, and Iker) at Hardrock we’re wearing it so it must be good!

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