In-depth insights into running shoe outsoles (road & trail)

The outsole, a vital part of any running shoe, forms your crucial link with the ground during your run. Not only does it help prevent slips and falls, but it also ensures a firm grip, offering the traction necessary for a safe and effective run.

However, the importance of the outsole is often overshadowed by the advancements in midsole foams and cushioning technology. And it should not be overlooked, as the outsole plays a critical role in the success of a running shoe. In fact, no matter how much cushioning or responsiveness the midsole provides, the grip of a running shoe can potentially ruin an otherwise perfect product.

I vividly remember my father constantly repeating that I should never, ever skimp on car tires, a quality mattress, and shoes—basically, anything that keeps me separated from the floor. Let me tell you, he was damn right!

Do you always need a ultra-grippy outsole for road running?

If you're a road runner, you probably won't spot any difference between A++ and A-grade outsoles. Don't let this be the only deciding factor when picking out your shoes. In fact, most shoes offer ample grip on dry surfaces, but the situation shifts when you're running in damp conditions.

For example, let's consider the Nike Vaporfly 2. It's widely recognized as the best-selling racing shoe in the past two years and it has also achieved numerous podium finishes in the World Marathon Majors. However, its outsole falls short compared to shoes like the Adidas Adios Pro 3 or the ASICS Metaspeed Sky+. 

In dry or slightly wet conditions, this may not matter much, but it becomes a real concern when running in heavy rain, like during the Boston 2023 marathon. In that race, Adidas swept the men's podium probably, in part, due to their superior outsole performance.

So, what do we recommend? It all depends on your circumstances.

If you live—like me—in a place where it seldom rains, and the average temperature is between 15ºC and 25ºC no matter the season, you can get away with almost any outsole. On the contrary, if you tend to run on wet, ice or even mud, you definitely want a grippy shoe—like the Puma Deviate Nitro 2 with its world-class PumaGrip rubber.

Overview of running shoe brands and their outsoles

Some brands don't label their outsoles or have teamed up with other companies. Consequently, we won't feature those brands in this table.

Grip & durability: can you have both?

No, you need to make a choice.

Achieving both grip and durability in a running shoe is challenging because there is a trade-off between the two. Outsoles with excellent grip—like competition shoes—often use softer rubber that wears out faster, while shoes designed for durability tend to have harder rubber that sacrifices some grip.

It's pretty similar to the trade-off in your car or even in Formula 1 racing!

To help you understand better, let's share some data from our lab. We've tested over 200 running shoes, and here are some interesting findings in terms of hardness (HC):

As you can see, there are significant variations. For example, there is a 58% difference in hardness between the two ASICS models. This means that racing shoes tend to wear out faster, sometimes reaching their limit at around 150 or 200 miles. On the other hand, we had reports of daily trainers like the GlideRide 3 or the Endorphin Shift 2 lasting well beyond 1000 miles.

Of course, shoe manufacturers could improve durability in racing shoes by increasing the thickness of the rubber, right? For example, the Metaspeed Edge+ has a 2.0-mm thick rubber, while the GlideRide 3 has 3.4 mm. Unfortunately, that's not an option, as this would add weight to the shoe, which is a crucial factor as proven by this meta-analysis.

The mystery behind outsole wear

Excessive wear in Nike Invincible after only 120 kilometers. Source: Reddit

This is a common issue in the world of running shoes. Perhaps you and a friend bought the same pair of shoes, ran together most days, but while one person's outsole still looks brand new, the other person's outsole is already showing the foam beneath the rubber. How does this happen?

Well, there are several factors that influence outsole wear:

Absolutely normal wear in Nike Invincible after +500 km. Source: Reddit

In addition, it's completely normal to have uneven wear on the outsoles, which can range from subtle to very noticeable.

Your feet are naturally different from each other, and you may even have one leg slightly shorter than the other. These factors contribute to the asymmetrical wear pattern.

Tire companies and running outsoles: Is it all about marketing?

One of the most remarkable success stories in the partnership between two vastly different companies is the collaboration between Adidas and Continental. Over a decade ago, these two German giants joined forces, sparking a revolution in the running shoe market. Since then, Adidas has launched more than 250 different models equipped with high-performance Continental rubber.

The deal made complete sense. Each company focused on what they do best, period. The success of this collaboration has been remarkable, prompting other brands to follow suit. In fact, it worked so astonishingly well that more brands joined the bandwagon:

• Under Armour + Michelin
• Mizuno + Michelin
• Skechers + Goodyear
• Puma + Pirelli
• Vibram + Hankook

In summary: The importance of partnerships between running shoe brands and top tire manufacturers cannot be overstated. These collaborations have proven to be more than just marketing tactics. And this success paves the way for other running brands to seek out similar partnerships with renowned tire manufacturers like Bridgestone and Toyo.

Symmetry vs. asymmetry: Comparing heel outsole designs

In recent years, we have noticed a growing trend in running shoes featuring rounded heels for their outsoles. But what does this mean?

Shoe #1 (Saucony Triumph 20) has a symmetric or traditional outsole, while shoe #2 (Adidas Adizero Adios 7) showcases an asymmetrical or rounded outsole.

A 2021 study suggests that rounded outsoles can have a positive impact on biomechanics and comfort. While this may be true for some runners, in our experience, it seems to benefit neutral runners more. For individuals with pronation issues, a symmetrical pattern with a non-rounded heel is often a better choice.

Hybrid running shoes

In our in-depth guide of running shoe uppers, we highlighted hybrid shoes as an excellent option for those who prioritize durability. Now, we're going to advocate for them once again, because they just might be the perfect choice if you find yourself in the following situations and want a shoe that can handle it all:

• You mix your runs between roads and trails, but your road shoes lack the necessary grip on the trails.
• You don't typically run on wet roads, where the lugs are terrible.
• You encounter small or sharp stones.
• You occasionally run over muddy sections during your runs.
• You need a shoe that offers exceptional durability.

Hybrid shoes truly have the versatility and durability to tackle these scenarios head-on. Three examples of great hybrid trail-to-road shoes are:

• Nike Pegasus Trail 4
• Salomon Sense Ride 5
• Hoka Challenger 7

Understanding trail running outsoles

Road runners are all like, "Yeah, the outsole matters," but trail runners—they go absolutely bananas over it! That’s because when you're out there running on those mountain trails, traction becomes the holy grail.

While most trail runners focus on the stickiness of the rubber, that’s just one part of traction. Even if the outsole rubber is super sticky, it won't help you much in deep mud if the lugs are only 2mm deep and super close. On the flip side, if the lugs are deep, like around 5mm, they might not be the best for a hard-and-fast trail. 

Furthermore, the subjective perspective on grip is also of great significance, as highlighted in this study. It recognizes that both individual perception and personal experience with traction play a vital role alongside objective data.

So, the key is to find a trail shoe that fits comfortably, feels great, and suits the specific conditions of the trail you'll be running on. The terrain and conditions have a significant impact on choosing the perfect trail shoe for you.

Surface-specific lugs: choose the best for every surface

Lugs come in various forms, from big chunks of rubber to smaller patterns. They are specifically designed to enhance traction on slippery surfaces by digging into the ground. With their multi-directional design, they provide greater stability on uneven terrain.

The choice of lug type depends on the type of surface you're running on:

• For softer surfaces like grass or mud, you'll need larger lugs with more space between them. This allows the shoe to effectively shed mud and debris.
• Uneven surfaces such as sand or trails with roots require lugs with varied angles and shapes. These lugs provide multidirectional grip, helping to prevent slips and falls.
• Easier trails made of gravel or dirt are best suited for shorter and simpler lugs. These lugs offer more flexibility and responsiveness on smoother surfaces.

Salomon is a running shoe manufacturer that focuses on creating trail running shoes for various surfaces. They specifically modify their proprietary Contagrip outsole to adapt to different terrains, as you can see below.

Source: Salomon

Trail width and outsole selection

Trails are typically categorized as either single track or double track. A single track trail offers enough space for one runner to run comfortably, while a double track can accommodate two runners or more. 

When it comes to single track trails, you'll need a shoe that's agile, flexible and provides excellent grip. On the other hand, with double track trails, you can get away with less traction and more stiffness, so maybe you can try a carbon-fiber plated trail shoe, like the Hoka Tecton X of the video above.

Is it possible to replace an outsole in a running shoe?

If you're active on social media, you might have come across used running shoes with a brand-new outsole. How does that happen?

NNormal Kjerag modified with a brand-new Vibram outsole. Source: E. Roig

Some outsole manufacturers, like Vibram, sell outsoles to cobblers, who can then cut and adapt them for almost any shoe. Believe it or not, we've even seen Nike Vaporfly Next% shoes with a trail rubber sole in some races, although most of these modifications are done on trail running shoes because they tend to wear out faster.

While it's possible to ask your local cobbler to do this for you, or even use online shops that offer sole replacement services, there are 2 important things to consider:

1. Each shoe is designed with a specific outsole in mind. If you put a heavier outsole on a shoe, it can alter the weight distribution and potentially cause issues like discomfort or even injuries.
2. When you put a new sole on an old shoe, the midsole obviously keeps the same. So, we don't think it's the best idea to spend the money—usually between $40 and $80—for a shoe that already feels flat, but it can be a great option for shoes whose outsoles break prematurely after just a few hundred kilometers.

The weather factor

When running on roads, weather is by far the most important factor when it comes to the performance of an outsole.

In dry conditions, 99% of running shoe outsoles will deliver sufficient grip to have a secure ride. However, things change when the roads are wet because of rain, dew or snow—the outsole becomes the most important part of the shoe.

But how do you know which outsole is great for running on a wet surface? Well, you need to consider two things.

• Exposed midsole: if it’s wet, you want to have as much rubber as possible. Exposed foam it’s 99% of the time really slippery on roads and especially on sidewalks.
• Rubber tread: you need to use a shoe with grooves or channels in the tread pattern that help to disperse water and provide better grip on wet surfaces. The more water in the road, the more channels you want to have in the shoe.

Let's make this clearer with a visual example of 3 popular daily trainers.

The Novablast 3 has a significant amount of exposed foam, resulting in inadequate grip in damp conditions.

The Saucony Tempus, with its extra rubber and chevron pattern, grips better than the Novablast 3. It also has two main channels for shedding water.

The Pegasus 40 offers fantastic grip on both dry and wet surfaces due to its abundant rubber, lugs, and grooves. It features multiple channels that effectively evacuate the water.

Road running: 3 Top-grip outsoles

The traction performance of a road running shoe is influenced by two main factors: the real contact area and the structure of the outsole tread design, as suggested by this study.

Consequently, if you exclusively run on roads or soft trails and prioritize maximum grip, here are 3 outsoles that work wonders:

• PumaGrip: When Puma re-entered the running shoe market, they made sure to excel in the grip department, and they succeeded. PumaGrip provides an impressive level of traction, clinging to the ground like claws, while also offering decent durability.
• Continental: This rubber is widely recognized due to the successful collaboration between Adidas and the German tire manufacturer. It is highly regarded for its excellent balance between durability and grip, making it a top choice for runners. It particularly shines in wet conditions, offering exceptional traction.
• AHARPLUS: The name AHARPLUS stands for "ASICS High Abrasion Rubber PLUS." This rubber compound is specifically engineered to deliver outstanding durability and grip. While it may be slightly less grippy compared to the other two options, it excels in longevity, making it a reliable choice for runners.

Of course, there are numerous other excellent choices available in the market, and the ones mentioned are merely 3 examples of brands that provide a reliable and grippy outsole among a crowded market and based on our experience.

Are Nike outsoles so terrible in wet conditions?

Undoubtedly, there's a common belief that Nike outsoles perform poorly in rainy conditions. But, is it true?

Based on our own tests and numerous testimonials, we can conclude that while some Nike running shoes, especially the faster ones, exhibit disappointing grip in the rain, it's not fair to generalize this to all Nike shoes.

Take, for example, the Pegasus line, which is arguably one of the best-selling shoes ever. It performs well on damp roads! It may not be the absolute best, but labeling it as bad would be unfair.

To summarize, it's evident that Nike could improve their outsoles, which is probably the weakest aspect of their running shoes, considering their excellence in other areas like midsoles with ZoomX and React technology.

Source: Nike

While we shouldn't expect drastic changes in the near future, we are starting to see some positive signs. For instance, Nike is set to release the highly anticipated racing-oriented Ultrafly with a Vibram MegaGrip Litebase outsole. This move indicates that they have acknowledged the need for external assistance, which is a step in the right direction.

Interpreting outsole numbers: Insights from our lab

In our lab, we meticulously collect data on various aspects of running shoes, including specific features of the outsole. We delve into aspects such as hardness, thickness, and lug depth, shedding light on how these factors contribute to overall shoe performance.

Hardness

We use a durometer to test outsole hardness, which measures the depth of an indentation created by the device. We take 4 measurements and ignore outliers to ensure consistent results.

At the time of this writing, the average hardness for road running shoes stands at 79.8 HC. Any number below this value usually indicates a shoe designed for superior grip. Simply put, harder outsoles tend to be more durable, although they might compromise on grip.

Racing shoes usually have a hardness below 75.0 HC, except for Nike competition shoes, known for their run-of-the-mill grip. On the other hand, low-tier or rugged shoes designed for durability tend to score over 85.0 HC in this test—like the On Cloudswift 3.

Below is a list of 10 running shoes with significant outsole hardness, accompanied by their respective readings:

Thickness

Outsole thickness plays a key role in its wear-and-tear resistance. Unsurprisingly, thicker rubber often means longer-lasting durability.

We use a precise caliper to measure the thickness, and currently, the median result is 3.4 mm. Similar to the hardness aspect, it's common to find running shoes designed for performance with a thin rubber layer, like the 2.0-mm rubber on the New Balance Fuelcell Supercomp Trainer, which aims to reduce weight.

On the other hand, a durable daily trainer like the Saucony Axon 2 boasts 4.1-mm rubber, more than double the thickness of its racing counterpart.

Below is a list of popular running shoes featuring thick outsoles:

Lugs depth (trail running)

Lugs are crucial in trail running shoes as they provide the necessary traction on surfaces like grass, mud, dirt, or gravel. However, they are not suitable for road running and are therefore absent in most road shoes. Some daily trainers may feature lugs to perform adequately on light trails.

The average lug depth in our lab-tested shoes is 3.5 mm. Road-to-trail shoes typically have shorter lugs measuring less than 3 mm, such as the Salomon Ultra Glide 2 with 2.8 mm lugs. On the other hand, shoes designed for technical terrain or muddy conditions often exceed the 4 mm mark, like the Nike Wildhorse 7 with 4.2 mm lugs.

Here is a list of 10 of the most sought-after trail shoes, each with its corresponding lug depth: