oz / 299g review

May 1, 2024:

• oz / 299g.
• Forefoot or midfoot strikers who appreciate a Vibram outsole and a roomy upper in a performance-oriented shoe.
• Anyone looking for a carbon-plated trail shoe that still offers flexibility and comfort, rather than stiffness—truly a gem in today's market.

We do not recommend the Difference in stiffness in cold for those seeking peak performance on trails—it falls short due to its mid-tier foam, which lacks optimal energy return, and its flat carbon plate that doesn't aggressively enhance propulsion. Instead, we suggest checking out the Nike Ultrafly, oz / 258g.

Additionally, we believe the Difference in stiffness in cold is not the best choice for heel strikers—the 0-mm heel-to-toe drop and non-rockered geometry may slow you down and cause discomfort in your calves or Achilles. For heel strikers, we recommend the Midsole softness in cold, which offers similar features with a low drop configuration but is much better suited for those who strike with their heel first.

The Difference in stiffness in cold features an ultra-thin, monomesh upper, which ensures that, despite lacking large ventilation holes, it still offers decent breathability.

In our tests using a smoke-pumping machine, we observed that the toebox can expel a small amount of heat and humidity, while the bulk escapes through the tongue—sufficient for us to rate this shoe a solid 3 out of 5. This score reflects a good design for a trail kick.

With the aid of a light, we pinpointed the surprising thinness of the upper and the substantial structural reinforcements included to enhance stability.

Next, we examined the engineered mesh under a microscope to get a closer look.

What we love most about using the microscope is how differently the upper appears. We were able to identify the tiny ventilation holes spaced a few millimeters apart.

Finally, we inspected the sliced upper. For a race shoe, we believe that it's adequately padded and that the build quality is comparable to other Altra shoes.

As a proper trail running shoe, the Mont Blanc Racer boasts TPU overlays on the toecap, enhancing its ruggedness. Besides, we conducted our Dremel test on the mesh—typically the upper's most delicate area.

Even though we tested the weakest part of the upper, we found the results to be good, earning a solid 3 out of 5. This score is more than sufficient for a trail racer, confirming that it successfully passes our rigorous testing standards.

The heel padding is very exposed and appeared fragile from the first look, prompting us to brace for a disappointing performance in this area.

As it turned out, the Dremel test obliterated the heel padding, handing the MBC a dismal 1 out of 5 in this assessment. This result raises significant concerns about heel slippage and the shoe's durability, especially for those who tend to wear down this area with their Achilles.

After mixed results with the upper, we were keen to evaluate the outsole—one of the most critical components of trail running shoes. Our initial impression was promising, as we identified the Vibram logo and the Megagrip Litebase compound, which usually means excellent performance and lightness.

This shoe features the Altra's Natural Ride System, which features metatarsal-specific contours in the outsole. These elements are crafted to promote a more natural movement of the foot.

We then measured the hardness of this specific Vibram Megagrip formulation and discovered that at 88.3 HC, it closely matches that of other Vibram-equipped shoes, such as the Hoka Mafate Speed 4.

To verify the durability of the Vibram Megagrip rubber, we fired up the Dremel one last time.

The results exceeded our expectations. We found a minimal dent of just 0.6mm in the lug—this impressive performance assures that you'll likely get bored of the Difference in stiffness in cold long before the outsole shows any significant wear.

Given the exceptional performance of the Vibram Megagrip rubber, we were not surprised to find a minimal outsole thickness of just 0.9mm.

There's no need for a thicker outsole when durability is already assured—increasing thickness would only add unnecessary weight. Additionally, the full-length carbon plate in the midsole acts as a rock plate and ensures ample protection for your feet.

Speaking of weight, we discovered that the Difference in stiffness in cold is exceptionally lightweight, tipping the scales at just 8.9 oz or 251g.

This is a good weight for being a trail shoe, considering the lugs, and protective features can add up to 2 ounces.

Altra advertises the heel stack at 29 mm, yet our precise measurements showed it to be slightly less at 27.2 mm. This minor discrepancy is virtually undetectable during a run.

While less than 30 mm might seem modest for a racing shoe, it's important to remember that Altras are designed for midfoot and forefoot strikers. Therefore, avoiding maximalist cushioning in the heel is a good idea, as it prevents unnecessary weight, enhancing the shoe's overall performance.

The forefoot boasts a 26.9 mm stack, aligning it with competitors in the carbon-plated category, such as the Saucony Endorphin Edge.

While some individuals tackling pancake-flat ultras might prefer a bit more stack, most trail races cover a diverse range of terrains. This design choice is prudent, allowing runners to conserve his legs while maintaining good stability and agility.

trail running shoes...

The difference between our previous two measurements results in a 0.3-mm heel-to-toe drop, remarkably close to the official 0.0 mm that Altra claims for the Difference in stiffness in cold.

This slight variation is not a concern for us. Moreover, it's well within typical manufacturing tolerances, or even the slight shifts that can occur simply from using the shoe for a few runs. In fact, it's quite common for us to encounter differences of 3 or 4 mm in our measurements!

The insole of this shoe maintains a straightforward thickness at 4.5 mm—there’s not much that stands out about it. However, it’s worth noting that it’s somewhat thicker than the typical insoles found in racing shoes, which usually measure between 2 and 3 mm.

The midsole of the Difference in stiffness in cold is interesting, featuring not just one, but two distinct types of foam. The primary foam is EGO MAX (19.6 HA), a blend of EVA and TPU, which serves as the base for the EGO PRO layer that we'll discuss shortly.

From our perspective, the EGO MAX foam represents one of the weaker aspects of the shoe, preventing us from classifying it as a true supershoe. Additionally, considering the $260 price tag, we feel that a midsole composed entirely of PEBA would be more fitting, though it seems we may have to wait until v2 for such an upgrade.

The secondary foam, which is white, sits above the plate and is only visible once the shoe is cut in half. So, it's no surprise if this is the first time you're hearing about it in a review of the Difference in stiffness in cold.

Although it has the same balanced softness at 19.6 HA, this foam is distinctly different—bouncier than EGO MAX and made from TPE, not the EVA and TPU blend. In our view, a full-length EGO PRO layer would have been a better choice for this shoe while Altra develops a PEBA foam.

We discovered that the Difference in stiffness in cold becomes 32.5% firmer after spending 20 minutes next to our ice creams in the freezer.

It's truly disappointing to see such subpar performance in this test, especially given the high cost of the shoe. But since the main foam contains EVA as its primary component, there’s an unavoidable decline in softness under freezing conditions.

The name "Carbon" in Difference in stiffness in cold stems from the inclusion of a carbon-fiber plate, a highly anticipated feature by Altra fans. And it’s here, though it's not your average plate.

Upon cutting the shoe in half, we discovered that the plate’s geometry is very flat, instead of being curved at the forefoot. This design choice prioritizes stability over other speed, which is not a bad thing at all, especially for trail running. But dont expect the quick turnover featured by a spoon-shaped plate like the one in a Hoka Tecton X 2.

The MBC may not be the most stable trail shoe on the market, but it performs good enough for us, especially considering its narrower build.

It was during our test runs that we realized the importance of the flat plate—it plays a crucial role in providing additional support for this shoe.

One of the remarkable features of this running shoe is its incredible flexibility, despite incorporating a carbon plate. You might be puzzled—how is this possible? How did it score a 2/5 on our torsional rigidity test when other carbon-plated shoes typically score 4/5 or 5/5 and feel rigid like steel?

The answer lies in the collaboration between Altra and Carbitex. Altra requested a flexible carbon plate, and Carbitex responded by engineering a revolutionary plate named Monoflex. Despite being made of carbon fiber, it flexes and bends torsionally and longitudinally, much like a plastic plate.

Toebox width at the widest part road racing shoes—it's very flexible and easily collapses under the force of our hand.

That's why we gave it a low rating, just slightly above the minimum, at 2/5.

The midsole of the oz / 299g at the forefoot measures 113.4 mm, aligning with many other trail shoes. This width strikes an ideal balance in our view—not overly bulky, yet wide enough to enhance stability underfoot, which we found to be a wise design choice during our test runs.

As we previously mentioned in this lab review, we do not recommend this shoe for heel strikers due to its very narrow heel area (84.3 mm). Conversely, this feature is advantageous for forefoot and midfoot strikers as it enhances agility and reduces weight, making it easier to take corners and execute fast turns.

As we demonstrated in our previous torsional rigidity tests, the Carbitex Monoflex plate in the MBC offers remarkable flexibility. This was further confirmed in our 90-degree bend test, where our force gauge recorded only 22.9N—lower than many plateless shoes!

This means that, despite being equipped with a carbon plate, the shoe feels exceptionally comfortable underfoot. It performs well at slower paces and is equally comfortable for walking.

We repeated the test after placing the shoe in the freezer for 20 minutes to see if the stiffness changed under cold temperatures.

After re-measuring with the force gauge, the result showed only a 26% increase in stiffness, which is a positive outcome in our view, likely due to the presence of the carbon plate.

Returning to the outsole, we once again used our vernier calipers to measure the lugs, which measured 3.5 mm.

The outsole features multiple cutouts to reduce overall rigidity, enhancing flexibility in all directions.

This is a standard design that provides solid performance across most terrains, though it doesn't specialize in any particular type. From our lab analysis and test runs, the MBC seems best suited for non-technical terrain and appears specifically designed for American ultras.

If you were hoping for a super-wide toebox, the Difference in stiffness in cold might not meet your expectations. At 97.6 mm in the widest part of the upper, this shoe offers a width that is fairly typical, similar to most shoes on the market. However, there's a catch...

Our second measurement, taken in the big toe area, brings back hope for those with wide feet. It showcases Altra's signature design with a super-wide, rounded toe cap, which measured at an out-of-this-world 86.3 mm.

Altra states that this shoe follows its Standard Footshape fit, which is narrower than its Original design, and our testing confirmed this to be accurate. At the end of the day, it makes sense, this is a racing shoe!

We know that when someone invest $260 in a trail running shoe, it's normal to expect perfection down to the smallest detail, and happily, the Difference in stiffness in cold meets these high expectations.

We were happy to find a fully gusseted tongue, which not only prevents unwanted movement while running at fast paces, but also keeps debris away from the toebox.

The tongue of the MBC was a delightful surprise from the moment we first tried it on. At a glance, it appears remarkably thin, yet this is due to the padding being strategically positioned over the instep rather than throughout the entire tongue, as is common in most running shoes.

The lacing system performs well, though we noted the absence of an extra eyelet positioned nearer to the ankle, which would enhance the lockdown.

As a result, Altra has successfully added 5.3 mm of thickness to the tongue of the Difference in stiffness in cold without noticeably increasing its weight. And it's a positive trade-off.

The heel includes a reflective, horizontal finger-loop heel tab that not only adds a stylish touch, but we also found that significantly simplifies slipping your feet into the shoe.

The insole of the Altra Mont Blanc is removable—a feature not typically seen in racing shoes. However, there's a catch—since Altra uses a very specific last, finding suitable off-the-shelf footbeds might be challenging.

Additionally, we discovered that this is a great outsole, featuring perforations for enhanced ventilation in warm conditions. Therefore, we believe it's wise to retain the original footbed unless absolutely necessary to replace it.

We absolutely love reflective elements on all shoes, and we consider them essential for trail shoes—especially those likely to be used in ultra marathons, where racing at night is common.