Reviewing inov-8’s new Terraultra G 260, with its Graphene-enhanced rubber sole, got me thinking. If you’re a climber, walker, mountaineer or runner, you’re pretty much dependent on the rubber on the soles of your shoes or boots to keep things sunnyside up or – in the worst case scenario – for stopping you cratering. But mostly we just take rubber compounds and technology for granted. So what’s going on? What makes one brand’s sole unit stickier than another’s and who makes the stickiest rubber soles?
The big picture? Rubber soles are made from a mix of different rubbery compounds, sometimes natural, soft, latex rubber which does grow on, or maybe in trees, but more frequently these days synthetic rubbers, which like pretty everything else in the modern world, are based on petroleum chemicals.
On top of the various rubbers, you’ll also get fillers, colorants and so on. By adjusting the proportions and types of the various rubber compounds in the mix, companies like Vibram can tweak the grip, durability and other properties of the sole.
It’s grip alchemy if you like. Rubber engineers can micro-design compounds to optimise grip or durability or performance at different temperatures by changing the ingredients.
It’s Not Just About The Rubber
Before I get deeper into different rubber compounds, and trust me, this is a journey of discovery for me as well as you, it’s worth remembering that it’s not just about the rubber, the configuration of the sole can make a big difference too. Some basics: on rock, the more rubber in contact with the surface of the rock generally, the better. That’s why rock boots are slick, like dry-condition racing tyres and technical approach shoes often feature smooth ‘climbing zone’ areas mostly under your big toe. The flip-side is that on softer surfaces, smooth rubber can be near lethal – just ask anyone who’s topped out on a rock climb which finishes on a steep, wet, grassy slope – shudder.
No, what you want as things get soft is big, deep, lugs with enough clearance to shed the gloop. All-round walking boots with classic soles and sharp heels mostly do okay on this sort of thing, but the ultimate is a more studded / spiky, fell running-type shoe with a pattern that gives grip in all directions. And on snow? The same’s broadly true, but classic, stiff-soled boots with serrated edge lugs are ideal for kicking side-on into nevé like a saw. In general, the more pronounced the tread blocks, the more grip you’ll get on softer surfaces, but beware tread that runs mostly in one direction like a caterpillar track, it may slip more easily on off camber slopes.
To cut a long story short, different lug shapes and configurations work in different situations, for mountaineers and walkers though, it’s mostly about all-round balance.
Rock Boot (Low) Rebound Rubber
Rock-boots are the undisputed kings of sticky, but they don’t necessarily work as you think they might. The first sticky-soled rock shoes were apparently based on the very soft rubber used for aircraft tyres, which needed to be able to grip hard, even when not warmed up. Where things get more subtle is that grip isn’t just down to softness. What’s happening at a micro level when you stand on a climbing foothold is that the rubber moulds microscopically to the contours of the rock surface under your weight.
That’s why, incidentally, if your foot slips off a hold when you first shift weight onto it, weighting the foot for a couple of seconds and then standing up once the rubber has had a chance to conform to the rock can make the difference between gripping and slipping.
It’s not quite that simple – if the rubber’s very soft, it’ll be more likely to roll off an edge- but it’s mostly how it works. All the big climbing boot brands do a decent job these days; Five.Ten’s Stealth rubber variants are well regarded – they had a version at one point that stuck to glass – but there are plenty of other options, including Boreal Zenith and the various Vibram XS Edge and XS Grip compounds used by Scarpa.
Are all sticky rubbers the same? Nope, Five.Ten for example, reserves its super-sticky Mi6 – the Mi stands for Mission Impossible – rubber for just one, top-end shoe designed to be used on the hardest routes, with ‘normal’ rock boots getting a harder compound C4 that’s less likely to deform on edges.
Sticky Approach Shoes
This is where things get interesting. With rock boots, sheer grip is what matters, but with approach shoes and scrambling boots, other criteria come into play. Durability is one factor. The general rule – even if Graphene looks set to change it – is that the softer and grippier the sole unit, the faster it’ll wear out.
Some brands don’t care. Five.Ten uses the same mix of rubber compounds as it does for its rock boots, albeit with different sole patterns aimed to give a little more grip on soft surfaces. Others seem happy to adopt climbing-style looks, but with a compound that’s far less sticky than you might think. The Haglöfs Roc Icon, for example, is a good-looking boot that turns out to be more of a sheep in wolf’s clothing – do sheep even wear clothes? – thanks to a less than super sticky Vibram sole even though it has a ‘Climbing Zone’ area.
By contrast, the Scarpa Crux, with is Vibram Megagrip compound sole – think halfway to sticky rubber – proved impressively grippy on everything rocky on the recent Lowe Alpine media launch in the Austrian Tirol, but has a reputation for wearing out relatively quickly with regular use.
How do you know? The time-honoured thumb-nail test will give you a very rough idea, but any of the Five.Ten compounds used by the brand itself along with its owners, adidas in their Terrex range is reliably grippy as is Vibram Megagrip.
This video – https://www.youtube.com/watch?v=-e6cZ2qmTIQ – gives an idea of how Vibram tests its sticky-ish compound. The brand also has a compound called Idrogrip which is optimised to work on wet rock and aimed mostly at watersports athletes and is closely related to their climbing compounds. Scarpa uses it for its Gecko approach shoe.
How do rubber sole makers develop and assess compounds? Not only do brands like Vibram, the market leader in the outdoors, use wear-based tests like wet ramps and different trial surfaces, they’ll also run lab tests on prototype compounds to determine abrasion resistance, hardness, density, tear strength, cushioning and lamination strength.
In other words, it’s not just intuition or guesswork, there’s proper lab testing before lab life-type testing and finally outdoor field testing. Phew…
For walkers and all-round mountaineers, it’s all about using a rubber compound – and tread – that gives an optimal balance between grip on different types of surface and in different conditions. And of course, they also need to factor in durability. There’s no point in a walking boot with a sole unit that only lasts 200 miles even f it grips like an over-excited terrier to a stick. Most all-round walking and mountain boots – and trail-running shoes for that matter – are aimed somewhere in the middle with a balance between grip and durability, but when conditions get demanding, there can be big differences.
Wet rock is particular problem with some soles. For example, I find Salomon’s Contragrip rubber compounds generally fine in the dry, but often quite slippery on wet, smooth rock. Likewise some Berghaus soles. Standard issue Vibram tends to be a good compromise, stickier compounds generally stick better, but it’s very hard to generalise.
Again, stickier rubber – in very general terms – seems to work best in the wet, but at the expense of higher wear rates. But even then, it’s not quite that simple. Running shoe specialist Asics – which own Haglöfs – has developed an own-brand rubber compound called WET GRIP Rubber, which doesn’t just use softer rubber based on automotive winter tyre technology, the soles also incorporate particles of ‘chaff’, in this case, rice husks. on the basis that they can cut through the film of water which lubricates the contact between the rubber and the ground and increase the contact area of the sole.
Haglöfs used it briefly in an approach shoe called the Rocker, but it’s hard to disentangle the rubber from the chaff as it were.
When The Going Gets Cold
Proper cold is, believe or not, another issue for sole makers. For perspective, winter spec car tyres are designed to works at temperatures of 7˚C and below, retaining their softness thanks to a careful mix of natural and synthetic rubbers, while ‘normal’ summer rubber compounds become rigid and far less grippy. The same basically happens with footwear, but there are special compounds available which are designed to work at lower temperatures. Swedish cold condition specialist Icebug, for example, uses a winter-specific sole unit produced by Michelin.
Meanwhile, Vibram – yes them again – has developed a technology called Arctic Grip, which is designed specifically to work in winter conditions and specifically on wet ice, frozen snow and in sub-zero ground conditions. The technology is rubber based with a filler, designed to work in sub-zero conditions and uses broad lugs to maximise contact between the rubber and the ground. It’s claimed to be around three times grippier on wet ice in particular, than other Vibram soles.
It’s not however a technology that’s taken off in the outdoors market. Saucony uses it for a winter trail-running shoe and Merrell has a couple of winter hiking boots. So realistically, ice time is crampon time still. Or studded sole time.
Mostly though, it helps to be aware that your rubber sole may simply not have the same level of grip on rock in sub-zero temperatures as it does in milder conditions.
Back To The Future
Technologies like super sticky rock boot rubber and that Vibram Arctic Grip sole show that careful development and shrewd use of different rubber compounds and additives can create impressive levels of grip, but that in turn generally means softer rubber and increased wear levels. Which is why Graphene Enhanced rubber compound just launched by inov-8 is so interesting. By adding durability without impacting on grip levels it opens up the potential to have, say, stickier approach shoes or mountaineering boots, which don’t wear out in double quick time.
Or alternatively, even stickier soles, that last as long as the current offerings. Potentially it’s a proper game changer.
In general terms, softer rubber is grippier rubber, but Graphene technology means you may soon have all that grip, but without the accelerated wear. And that could mean not just stickier scrambling shoes, but also humbler hiking boots that simply feel more connected and less likely to slip on wet rock in particular.