Our verdict

During our lab evaluation, the StealthFit edition emerged as a remarkable variant of the regular Glycerin 21, showcasing a significant weight reduction and a snug, sock-like upper. Although it compromises somewhat on breathability and cushioning, we believe these trade-offs are justified for many runners. For us, this variant excels as a featherweight daily runner, impressing us with its comfort and durability across a variety of terrains.

Pros

  • Durable outsole
  • Sock-like tongue
  • Significant weight reduction
  • Comfortable fit
  • Great for long runs
  • Improved heel counter
  • Supercritical foam

Cons

  • Needs more breathability
  • Narrower than regular Glycerin

Audience verdict

81
Good!
  • Top 28% most popular running shoes

Who should buy

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.

Brooks Glycerin StealthFit 21

Who should NOT buy

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.

Brooks Glycerin StealthFit 21 parts

Breathability

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.

Brooks Glycerin StealthFit 21 microscope

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

Brooks Glycerin StealthFit 21 micro 2

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.

Test results
Glycerin StealthFit 21 2
Average 3.8
Compared to 221 running shoes
Number of shoes
1
Breathability
5

Durability

Toebox durability

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!

Brooks Glycerin Stealthfit 21 Toebox durability
Test results
Glycerin StealthFit 21 3
Average 2.3
Compared to 155 running shoes
Number of shoes
1
Toebox durability
5

Heel padding durability

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.

Brooks Glycerin Stealthfit 21 Heel padding durability
Test results
Glycerin StealthFit 21 3
Average 3.2
Compared to 151 running shoes
Number of shoes
1
Heel padding durability
5

Outsole hardness

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.

Brooks Glycerin StealthFit 21 outsole

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.

Brooks Glycerin Stealthfit 21 Outsole hardness
Test results
Glycerin StealthFit 21 84.9 HC
Average 80.4 HC
We use an average of four tests. The photo shows one of those tests.
Compared to 272 running shoes
Number of shoes
54.9 HC
Outsole hardness
92.8 HC

Outsole durability

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.

Brooks Glycerin Stealthfit 21 Outsole durability
Test results
Glycerin StealthFit 21 0.5 mm
Average 0.9 mm
Compared to 133 running shoes
Number of shoes
0.0 mm
Outsole wear
2.0 mm

Outsole thickness

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.

Brooks Glycerin StealthFit 21 Outsole thickness
Test results
Glycerin StealthFit 21 3.2 mm
Average 3.2 mm
Compared to 291 running shoes
Number of shoes
0.0 mm
Outsole thickness
6.6 mm

Weight

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.

Brooks Glycerin Stealthfit 21 Weight
Test results
Glycerin StealthFit 21 9.07 oz (257g)
Average 9.38 oz (266g)
Compared to 292 running shoes
Number of shoes
5.26 oz (149g)
Weight
12.56 oz (356g)

Cushioning

Heel stack

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.

Brooks Glycerin StealthFit 21 Heel stack
Test results
Glycerin StealthFit 21 36.9 mm
Average 33.6 mm
Compared to 291 running shoes
Number of shoes
7.6 mm
Heel stack
42.7 mm

Forefoot stack

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

Brooks Glycerin StealthFit 21 Forefoot stack
Test results
Glycerin StealthFit 21 26.4 mm
Average 24.9 mm
Compared to 291 running shoes
Number of shoes
7.6 mm
Forefoot stack
35.8 mm

Drop

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 utilize a forefoot or midfoot strike and prefer a higher offset can also enjoy them without any issues.

Brooks Glycerin StealthFit 21 Drop
Test results
Glycerin StealthFit 21 10.5 mm
Average 8.7 mm
Compared to 291 running shoes
Number of shoes
-0.8 mm
Drop
16.1 mm

Insole thickness

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

Brooks Glycerin StealthFit 21 Insole thickness
Test results
Glycerin StealthFit 21 5.2 mm
Average 4.5 mm
Compared to 287 running shoes
Number of shoes
1.5 mm
Insole thickness
7.3 mm

Midsole softness

Note: a low durometer measurement equals a soft material, whereas a high measurement means it's firm.

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.

Brooks Glycerin Stealthfit 21 Midsole softness
Test results
Glycerin StealthFit 21 20.6 HA
Average 21.4 HA
We use an average of four tests. The photo shows one of those tests.
Compared to 219 running shoes
Number of shoes
8.5 HA
Midsole softness (soft to firm)
41.5 HA

Difference in midsole softness in cold

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.

Brooks Glycerin Stealthfit 21 Difference in midsole softness in cold
Test results
Glycerin StealthFit 21 36.9%
Average 25.6%
Compared to 218 running shoes
Number of shoes
0%
Difference in midsole softness in cold
100%

Stability

Lateral stability test

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.

Torsional rigidity

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.

Test results
Glycerin StealthFit 21 3
Average 3.2
Compared to 270 running shoes
Number of shoes
1
Torsional rigidity
5

Heel counter stiffness

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.

Test results
Glycerin StealthFit 21 3
Average 2.8
Compared to 254 running shoes
Number of shoes
1
Heel counter stiffness
5

Midsole width in the forefoot

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.

Brooks Glycerin StealthFit 21 Midsole width in the forefoot
Test results
Glycerin StealthFit 21 116.3 mm
Average 113.6 mm
Compared to 292 running shoes
Number of shoes
100.5 mm
Midsole width in the forefoot
126.5 mm

Midsole width in the heel

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.

Brooks Glycerin StealthFit 21 Midsole width in the heel
Test results
Glycerin StealthFit 21 99.1 mm
Average 90.5 mm
Compared to 292 running shoes
Number of shoes
74.9 mm
Midsole width in the heel
106.6 mm

Flexibility

Stiffness

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.

Brooks Glycerin StealthFit 21 outsole flex

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

Test results
Glycerin StealthFit 21 22.4N
Average 29.3N
We use an average of four tests. The video shows one of those tests.
Compared to 274 running shoes
Number of shoes
2.2N
Stiffness
94.4N

Difference in stiffness in cold

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%.

Test results
Glycerin StealthFit 21 17.4%
Average 36.3%
Compared to 274 running shoes
Number of shoes
0%
Difference in stiffness in cold
148%

Size and fit

Toebox width at the widest part

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.

Brooks Glycerin StealthFit 21 upper

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.

Brooks Glycerin Stealthfit 21 Toebox width at the widest part
Test results
Glycerin StealthFit 21 98.3 mm
Average 98.3 mm
Compared to 292 running shoes
Number of shoes
89.5 mm
Toebox width at the widest part
109.1 mm

Toebox width at the big toe

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.

Brooks Glycerin Stealthfit 21 Toebox width at the big toe
Test results
Glycerin StealthFit 21 77.3 mm
Average 78.0 mm
Compared to 166 running shoes
Number of shoes
60.4 mm
Toebox width at the big toe
92.5 mm

Tongue: gusset type

One of our major letdowns with the Glycerin 21 was its non-gusseted tongue, which seemed lacking for a $160 shoe. Fortunately, the StealthFit version addresses this by introducing a superb sock-like tongue, providing an Alphafly-esque feel that many of us love.

Brooks Glycerin Stealthfit 21 Tongue: gusset type
Test results
Glycerin StealthFit 21 Sock like

Comfort

Tongue padding

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.

Brooks Glycerin StealthFit 21 laces

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 favor 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.

Brooks Glycerin Stealthfit 21 Tongue padding
Test results
Glycerin StealthFit 21 2.1 mm
Average 5.6 mm
Compared to 289 running shoes
Number of shoes
0.5 mm
Tongue padding
14.2 mm

Heel tab

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

Brooks Glycerin Stealthfit 21 Heel tab
Test results
Glycerin StealthFit 21 Finger loop

Removable insole

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.

Brooks Glycerin Stealthfit 21 Removable insole
Test results
Glycerin StealthFit 21 Yes

Misc

Reflective elements

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?

Brooks Glycerin Stealthfit 21 Reflective elements
Test results
Glycerin StealthFit 21 No