Plum SOK hardboot splitboard binding kit

Brand // Manufacturers

Plum (France)

Price

€749 – The all-in-one kit (“Two parts interface and channel”) for any two-ski splitboard. Includes: Binding plate, splitboard interface (for screw-holes or a slider-channel); Uphill-bracket (for walk mode); and heel-risers. This is what is reviewed here.

€520 – just the main binding plate itself (interface and other items can be bought separately)

€849 - The all-in-one kit, like the above but for a 3- or 4-part splitboard (“Four parts interface”).

If you need ski crampons (likely you will at some point!) you need:

Ski crampons - €89
Ski crampon channel for the Uphill bracket - €36

This all comes with a 2 year warranty.

Sizing

One-size – The binding plate fits hard-boots from with soles 270 mm to 330 mm, so depending on the boot brand/model something like Mondo 22.0 - 29.5… ish

What we liked

Loads of adjustability for great boot-hold and positioning
Light
Simple splitboard interface system - clamping the board together
Offers two walk-mode options with the one binding: (1) keep the binding plate on the board in walk-mode, using the included ‘uphill bracket’, or (2) put it in your backpack if you are using a tech toe.

What we didn’t

Heel-riser unit doesn’t allow enough rearward positioning.

The Verdict…

OUR RATING

4 out of 5 - The Heel-riser unit lowering the score.

THE SHORT READ…

The binding and splitboard interface units are excellent bits of kit - light, loads of adjustability, snug boot-fit, and can clamp the board-halves together.

The uphill bracket units are fine and give an entry-level option for touring, keeping the bindings on the board and saving you the cost of a tech toe unit (like Plum’s Pekye).

The included heel-riser unit seems the weak link in the kit - it can’t be positioned far enough back to prevent the shorter wire contacting the metal underside of the binding plate, so you’d probably want to consider getting a heel-riser from Spark or Phantom instead, to avoid damaging wear.

The adaptability of the binding - being able to use the kit reviwed here to tour with the binding on your feet, or instead using a tech toe and putting the binding in your backpack - could be great for someone looking to keep costs down initially and upgrade to tech toes later, but does mean that the bindings are not canted - so those of you with dodgy knees might find that an issue.

THE LONG READ …

We have so far tested the Plum SOK system with one week spent in nice deep snow and one week on a lot of hard-pack, icy bits and crud - covering a good range of conditions. No engineering changes have been introduced for 23/24, so all the observations here are still relevent.

We tested the following components - part of the “two parts interface and channel” kit:

The main binding plate and splitboard interface
The ‘uphill bracket’ (to attachment when using the binding for walk mode)
The climbing wires / risers
The screws are all Torx and 3 different size tools come with the kit.

Plus, the following which are separate purchases:

The uphill bracket ski crampon adapter
Ski crampons

The binding plate and splitboard interface

Board contact area- Compared to the Phantom M6 binding, the SOK binding plate looks very svelte, and it is indeed some 40g lighter. But with it’s much smaller contact area on the board top-sheet, does that mean more play is possible between the two ski-halves when riding? Technically, I think it must do, but I couldn’t detect any noticeable penalty here. The SOK binding has 4 points of contact with the board surface – feet at each corner, leaving up to 8mm of space between the board surface and underside of the binding – room for the locking-arm mechanism. However, I didn’t detect any problematic snow build-up.

Splitboard interface mounts - Very simple to put on. Each is two parts, the lower part determines your stance width & set-back and also your foot angle, through +/-30° in 3° increments. The upper part of each interface unit then determines your position across the board, with a decent 1cm or so of adjustability left or right, to help you centre the binding plate and boot across the board, to reduce heel or toe overhang etc.

Here the lower part of the lower interface unit is exposed, showing the angle choice possible.

Showing the angled undersides of the splitboard interface units

Board clamping - When setting up the interface/binding positions, by default it doesn’t clamp when you operate the binding plate locking-arms. But you can make it clamp the board by just moving one of those interface units toward the board edge a fraction more, increasing the spacing between it and the other one one by the amount of board-gap you want to close - when the bindings are then locked for riding, that gap will close. Very nice.

Binding locking arms - These look extremely flimsy and are extremely light – can they really be up to the job of locking the plate solidly onto the interface mounts? It seems so – they do work! For walk mode they also attach the toe of the binding plate to the uphill bracket.

In their locked positions, they are secured there by the little black “safety lever” shown above that you snap into position over the end of the arm. That tab is only a couple of mm thick and can be quite hard on cold glovless fingers! (or perhaps mine are way too soft).

Binding-boot adjustability – The SOK offers plenty of adjustment for adjusting to different boot sizes within the binding, and centering the boots within the binding and across the board for riding – at the front, the toe bail can sit in one of 4 discrete positions, and you also get two different length bail arms to choose from in the kit – for my large Mondo 295 boots (318mm sole length) I’m using the longer bail arms which can handle even larger boots yet. For the rear bail arm you get another two discrete positions, plus about 3cm of super-fine-tuning. That combo of different toe bail arms lengths, discrete positions and fine-tuning really allow you to get an optimum boot hold no matter what size or shape of boot (between something like Mondo 22-29.5).

I found those rear adjustment screws working loose a couple of times on my first day out on these, so do check them and tighten well. All good after that.

Canting – None. In ride mode, I did feel a bit like a Texan first time out on these! I distinctly noticed my upper thighs being forced a bit wider. While riding I pretty much forgot about it. When stood still, on the board, I did notice it again. My boots are not mega-rigid laterally and aren’t done up super tight, so that helped. This might be an issue for someone with dodgier knees and maybe stiffer boots. The reason for lack of canting is that the SOK is designed for those two uphill modes, where for the on-the-board mode you wouldn’t want canting.

Bail arms – Neither the front nor rear are sprung. On the Phantom M6 they are heavily sprung, perhaps too much so - when you want them out of the way they fight you to get back upright. Here, it’s the opposite, they stay where you put them. This works perfectly for the front bail arm, but I find myself wishing for some for the rear one – when stepping into any hardboot binding I often accidentally step on that rear bail with my heel, pushing it flat, meaning I have to either bend down to lift it back into the right position, or use the pole skirts to do the same, then try stepping in again, whereas on the Phantom it springs back up itself ready for the next attempt.

One great feature is that the SOK front bail arms are quite narrow across their width, limiting any sidewise rotation your boot toe can make within the front bail arms, around the curve of the toe level bumper, meaning less lateral movement than in say the Phantom M6, with it’s wider front bail arms.

The front bail lever is metal, with edges that are chamfered but which can still be a little hard on cold fingers, as can the teflon screw in it. Importantly, it’s boot toe-bumper is plastic and replaceable, so that’s good for boot wear and tear.

Icing up – The overall design does seem to minimise this well. I have had to clear some from two locations at times: Firstly, in the channel under the toe of the binding that the uphill bracket bar slots into; and also from around the under-sides of the two splitboard interface mounts. Plum provide a small plastic tool for clearing snow out of nooks and crannies like these. Not been any major issue so far.

Uphill/classic Bracket

A very light, simple solution that adds almost no additional parts or material to the binding plate itself. Each unit weighs just 75g including screws (including the optional crampon slot adapter. Just 54g without that) compared to the 155g of a Spark R&D Tech Toe, or the 117g of a Plum Pekye tech toe, and cheaper too.

Uphill bracket - with optional crampon slot installed.

Walk-mode - It works well, but just like any binding system where the binding plate stays on your feet during touring, it can’t prevent torsion between your boot and board as well as a tech toe system does - A tech toe unit connects directly to the boot. A binding-on-board unit like this has additional bits that reduce that effectiveness: The binding plate, the front bail arms that can twist, and the toe bail bumper-to-boot connection, all mean less rigidity than tech toe pins gives, so don’t expect to be able to hold an edge on a hard-pack/icy traverse as well as you would with a tech-toe, but it’s still better than a soft-boot binding.

I found the bars of both brackets to have some lateral play between the uprights they run between - not much, but when the binding plate is locked in, they can move about 1mm side-to-side. I doubt it affects the edge-hold much, but anything that detracts from that would be nice to eliminate.

It’s a little curious that Plum don’t provide the crampon slots in the kit as well - instead those are an optional purchase. But using crampons is so often required, it seems it would be better to just include that item. After all, Plum’s Pekye tech toe unit comes with crampon slots built-in.

Heel risers (climbing wires)

This appears to be the weak link in the kit. These risers are only intended to be used when you are using the binding plus the uphill/classic bracket together for walk mode - they are not intended for use with a tech toe. There are two riser heights: 50 mm and 65 mm, which is fine.

First, a couple of minor gripes: Getting the wires upright needs some effort - bend down and get your fingers under them. I’ve not managed it with pole tips or skirts yet; And getting them back flat again needs even more effort – even whacking them with a ski pole handle, like teeing off in golf, usually isn’t enough. Instead I have to kick them flat with my boots or get down and really giving it some with the flat of my palm.

But the real down-side here is that there’s very little forward/backward position adjustment – just 2cm range, compared to almost 7cm with the Phantom Rocket Risers (and a similar amount with the Spark Hardboot Dual Height Wires). What this means is that in walk-mode with the uphill bracket, the furthest back the riser assembly will go only just gets the tallest wire contacting the (replaceable) plastic feet under the back of the binding. The smaller wire, when upright, actually hits the metal underside of the binding plate itself. After two weeks use - there is noticeable metal-on-metal wear that can’t be good for the binding’s lifespan, I’d have thought.

To avoid the metal-on-metal wear from using the short wire, you would want to instead get a pair of Spark Hardboot Dual Height Wires, or the Phantom Rocket Risers. Which you’d need anyway if you want to use a tech toe and have boots with anything more than about 240mm sole length, to get wires that go far enough back to contact the heel of your boot.

Plum have told me they know about the issues with the current heel-riser and are going to be unveiling a new design later this year (2023).

The short risers sit against bottom of the metal binding plate itself - metal-on-metal wear…

… Whereas the tall risers do sit against the plastic feet, just - better for wear.

Metal-on-metal wear of the short wires; hardly any at front of foot from tall wires (not used so much) and main wear middle of foot from heel riser plastic body itself.

Note the forward lean the heel-riser housing gives when the wires are down:

Showing the slight forward lean when heel risers are flat.

Ski crampons

These are not included in the kit and are a separate purchase, but tested here. The crampons themselves are a simple design - and the same design as the new Phantom GT Splitboard Crampons.

A nice bit of kit, they come in two widths, 130mm and 145mm.

When using the binding and uphill bracket, note the small little gap between the top of the crampon and the underside of the binding when the heel risers are down. No big deal, you still have plenty of bite, but you will get an extra click when that gap closes each step.

However, when using the tall wire, there is much reduced bite - less than if you were using a tech toe, because boot soles typically curve downward toward the crampon, forcing it lower. Again, not really a biggie - if you find yourself in need of BOTH tall wire AND crampons, well, that’s quite a gnarly slope you are on and you might consider just getting off the board a bit sooner than any tech-toe-using companions and putting your boot crampons on pronto!

Other optional components

Pekye tech toe - As mentioned, if you want to use a tech toe unit for walking, and put the binding plate in your backpack, then Plum offer the Pekye - an exceptionally light tech toe. Note that if you use any tech toe for walking, and have a boot with sole greater than about 240mm long, you’ll need to source some climbing wires elsewhere, ones that can be positioned far enough rearwards so both wires will contact the heel of the boot - look at the Phantom Rocket Risers or Spark Hardboot Dual Height Wires.
D-board - A couple of metal plates that can fit under your riser units, or climbing bracket/tech toes, which overlap the other ski and help to reduce vertical movement between the board-halves, getting the ride-feel a bit closer to that of a solid board.

Pekye - with integrated crampon interface

D-board - overlapping plates to help reduce movement between the board halves

Weights

All the kits included components - binding plates, heel risers, uphill bracket and splitboard interface (and screws) - weigh 1526g on my scales. Of which, just the binding plates themselves make up most of that at 960g (Plum say 980g on their website), so all the other components in this interface set are very light indeed.
Compared to the Phantom M6 binding, the fairest thing is to compare each’s binding plates plus splitboard interfaces (with screws). There we see the SOK is some 70g lighter per foot:
- SOK - per foot - 597g
- M6 (large) - per foot - 667g

Overall

The SOK system offers you a very adaptable hard-boot solution – either use the baseplate itself for going uphill, or stick it in your backpack and use a tech toe instead. It feels like a great way to provide a lower-cost-of-entry starting solution (use the uphill bracket provided in the kit) which you can then upgrade later by swapping out the uphill bracket for a tech toe.

Add in excellent boot-positioning adjustability, it’s light weight, and board-clamping ability and it’s a very nice system indeed.

Marks-off are really just for the heel-riser unit - it seems odd that it doesn’t allow more rearward positioning, to avoid the metal-on-metal wear under the binding plate when using the low riser. And it also forces you to go buy heel risers elsewhere if you want to use a tech toe for touring.

I’m not sure about the lack of canting - for those with good knees, narrower stances and less rigid boots, this might not be an issue at all. I’ve a wider stance but less rigid boots and it wasn’t a big issue for me. But some of you might feel it in the knees. Yes, no canting allows for the bindings to be used for uphill in either on-board and in-pack modes, but how often do you switch back and forth between these modes, with all the change of gear between tech toe and uphill bracket that entails? Likely not very often.

In the end, I think the SOK system is excellent - except for those heel-risers. Apart from that, I’m super looking-forward to getting out on the SOK again this season, as part of a longer-term test.

We hope you find this review useful - you can check out our “Going hard” article about more hardboot gear options.

Words - Andy Beale. Photos - Plum and author’s own