Brooks Glycerin StealthFit 21 review

We highly recommend the StealthFit variant of the Glycerin 21 for:

• Fans of the classic Glycerin who desire a more secure, locked-in upper and a lighter overall build.
• Marathoners looking for a durable, lightweight daily trainer that can handle extensive mileage with ease.
• Those seeking a versatile workhorse with a knit, sock-like upper that's perfect for winter running.

We do not recommend the Glycerin StealthFit 21 for those living in hot climates—the knit upper, while snug and comfortable, lacks sufficient ventilation and may lead to blisters and discomfort in warm weather. Instead, we suggest the Hoka Mach 6, which is far better suited for warmer conditions with its superior breathability.

Additionally, we think the Glycerin StealthFit 21 falls short in energy return compared to world-class daily trainers. If you're looking for a shoe with more bounce and an even lighter feel, we believe the ASICS Superblast is a better investment, offering greater energy return and an impressively low weight.

The upper is a crucial component of every running shoe, and we found that Brooks has elevated the Glycerin series by offering not just one, but two different uppers. This is a great advantage, as it allows you to choose between the Glycerin 21 and the Glycerin StealthFit 21 based on your personal preferences.

If you're looking for a snugger, sock-like upper, the StealthFit is the best pick. However, our tests revealed a downside—ventilation is limited, with a rating of only 2/5. Thus, we can't recommend it for hot, summer runs.

In our light test, it quickly became clear that robust ventilation isn't a strength of this model. Barely any light passed through the upper, confirming our initial findings with the smoke-pumping machine.

Curious for more details, we examined the StealthFit upper under a microscope.

We discovered small ventilation holes in the toebox; however, an additional layer of knit beneath them hampers airflow. This setup isn't necessarily negative, as it makes the shoe ideal for colder weather.

Upon manually examining the upper, we were impressed by its elasticity, which allows it to conform closely to the feet for a sock-like fit. Yet, we noticed less padding in the heel—something to consider if you prefer substantial heel cushioning.

The regular Glycerin scored 2/5 in our test, sparking our enthusiasm to evaluate the StealthFit variant to see if it could offer the much-needed enhancement.

Indeed, it does improve, albeit modestly, scoring 3/5. This outcome is encouraging, and from our perspective, it's sufficient to make us satisfied!

With the Dremel in action, we also decided to test the heel durability of this SF variant, noting that it features significantly less padding.

Our assessment resulted in another 3/5 score—a bit disappointing, though it’s not a major concern.

Shifting our focus to the outsole, we observed that it remains unchanged in the Glycerin 21. The outsole is constructed from the same durable Roadtack Rubber, which showed a hardness of 84.9 HC on our durometer test.

This rubber effectively shields the high-wear zones on both the heel and forefoot. And despite a few areas of visible foam, these do not pose any issues in terms of wear and tear based on our experience.

Given the hardness of the rubber, we anticipated impressive results in our final Dremel test. Let's dive into the outcomes.

True to our expectations, even after rigorous application of the Dremel at 10K RPM, the rubber exhibited only a slight 0.5 mm indentation. This minor mark confirms the Roadtack's resilience.

In our concluding test of the outsole, we measured a thickness of 3.2 mm. This measurement is more than sufficient to withstand significant wear and tear, as indicated by our earlier test results.

Choosing the StealthFit version brings a significant benefit—reduced weight. It trims 0.7 ounces off the regular v21, registering just 9.1 oz or 257g, which is remarkable for such a well-cushioned daily trainer.

The fact that this weight reduction is achieved solely through modifications to the upper, while maintaining or even enhancing comfort for some, is truly impressive.

At the heel, we recorded a measurement of 36.9 mm, positioning the Glycerin StealthFit 21 as a high-stack model designed for extensive use.

This feature may not suit runners who prefer a closer connection to the ground. For them, we suggest the New Balance FuelCell Rebel v4, which provides a more direct feel of the terrain.

The forefoot is also very cushioned (even by today's high standards) with a measurement of 26.4 mm from our digital callipers. This level of cushioning is more than adequate for long distances.

Brooks advertises a 10-mm heel-to-toe drop for the Glycerin StealthFit 21, and our thorough measurements confirmed it's nearly spot-on, showing a 10.5-mm offset.

This design makes the shoe particularly well-suited for heel strikers, though runners who utilise a forefoot or midfoot strike and prefer a higher offset can also enjoy them without any issues.

The insole is consistent with other models in the Glycerin line, featuring a standard thickness of 5.2 mm.

We've previously mentioned that the upper was the only change, and happily, the enhanced DNA Loft v3 found in the Glycerin 21.

This nitrogen-infused EVA foam showed a 20.6 HA reading on our Shore A durometer. This measurement indicates a ride that perfectly balances firmness and plushness, offering a middle ground that, in our view, could appeal to a broad range of runners.

After chilling the Glycerin 21 in the freezer for 20 minutes, we reassessed its softness with a durometer. Our findings indicated a significant letdown, as it increased its firmness by 36.9%. That's a bit too much, although it's a typical issue with EVA-based midsoles.

The StealthFit version shows slight variations in stability compared to the standard model, making it less supportive than its stability-oriented sibling, the Glycerin GTS 21. If support is crucial for your runs, the GTS 21 is the better choice, although it’s not available with a StealthFit upper. You’ll have to choose between one style or the other.

Nevertheless, this StealthFit edition offers a remarkably stable ride for any neutral runner. We believe that 99% of runners without stability issues should pick this one over the GTS.

We found taht the stable feel of this shoe is primarily attributed to its design dimensions, though the 3/5 rigidity we observed before also plays a significant role.

Fans of a traditional, natural ride may lean towards the standard version, which we found to be slightly more flexible (2/5) due to its upper construction.

The heel counter in the regular edition was a eyebrow-raising feature for us, as we found its super-stiff design a bit excessive for an average daily trainer.

However, the StealthFit version introduces a more thoughtful and accommodating design. From our testing, we found that this adjustment enhances comfort significantly for a daily trainer, earning it a balanced rating of 3/5 from our team.

At 116.3 mm wide, the StealthFit edition is broader than the average shoe in the daily running category. However, it doesn't reach the massive forefoot dimensions of the ASICS Novablast 4, which are similar to those found in maximalist stability shoes.

Despite its notably high sole height in the heel, our neutral running technique remained impressively stable. This stability is greatly enhanced by the Glycerin StealthFit 21's exceptionally wide design—at a robust 99.1 mm—providing a secure landing platform for heel strikers.

We discovered that the generous width not only supports a solid base but also significantly boosts the shoe’s overall support. In our view, this broad design of the heel truly sets the StealthFit 21 apart in terms of stability versus other daily trainers.

While the shoe tends toward stiffness, strategically placed grooves in the outsole enhance its longitudinal flexibility, achieving a measure of 22.4N in our definitive 90-degree bend test.

This adaptability is vital for the shoe to function as a dependable and versatile workhorse, comfortably handling multiple activities.

After chilling the Glycerin StealthFit 21 in the freezer for 20 minutes and retesting it with the force gauge, we found that it had increased it's stiffness by 17.4%.

Returning to the upper, which we previously noted as the key distinction between the StealthFit and the standard versions, our side-by-side tests of both shoes demonstrated a tighter, more secure feel in the SF version, whereas the standard was more spacious.

To confirm our observations, we measured the widest part of the upper and found that the StealthFit version is narrower by 1 mm at 98.3 mm. However, it's really the reduced vertical volume in the toebox and the snug fit of the sock-like upper that create the most noticeable differences.

The same pattern observed in our previous tests applies to the big toe area. At 77.3 mm, it's 1.6 mm narrower than the standard version, yet the sensation is significantly more tapered than this slight measurement difference suggests.

As we said before—if you're seeking a roomier upper, the standard version is the way to go, whereas the StealthFit is ideal for those who prefer a more secure, locked-in feel.

One of our major letdowns with the Glycerin 21 was its non-gusseted tongue, which seemed lacking for a £170 shoe. Fortunately, the StealthFit version addresses this by introducing a superb sock-like tongue, providing an Alphafly-esque feel that many of us love.

One of the major weight reductions in the StealthFit 21 comes from the tongue. It transitions from a cushy 11.5 mm thickness to a minimalist 2.1 mm design.

The lacing system moves away from traditional eyelets to an integrated design with the sock-like tongue, making it impossible to tie a runners' knot.

For runners who favour thin, sock-like tongues, this one's a wonder—the benefits far outweigh any trade-offs. However, runners who prefer more padding in the instep might find this version a bit challenging.

Another lovely bonus with this variation is the inclusion of a convenient finger-loop heel tab—a truly handy feature.

The insole easily comes out and can be swapped out as it's not glued to the last. It doesn't offer anything extraordinary compared to the GTS or the standard version.

We feel that it's unfortunate Brooks opted not to include reflective elements in this StealthFit edition. This really feels like a missed opportunity to enhance safety and differentiate the product further. Maybe they'll consider it for v22?