Karhu Fusion 3.5 x SAYSKY: First Run in a Distinctive Collab Running Shoe

Introduction

Every so often a collaboration lands that genuinely raises eyebrows, and the Karhu Fusion 3.5 x SAYSKY is one of them. SAYSKY, the Danish running apparel brand, has built a strong following for its clean, performance-driven designs, and this marks its first ever collaboration running shoe. The partner is Karhu, a Finnish brand founded in 1916 whose track heritage is remarkable: athletes wearing Karhu shoes, including the legendary Emil Zatopek, collected 15 gold medals at the 1952 Helsinki Olympics. Karhu even sold its three-stripe trademark to another, far better known athletic manufacturer along the way.

The Fusion 3.5 retails for £135 in the UK, weighs 309 grams in a UK 9.5, and runs a 6mm heel offset. Karhu positions it as an all-in-one shoe for neutral runners that can handle a big variety of workouts. SAYSKY detailing appears on the tongue and as a star on the heel, alongside the Karhu bear (karhu means bear in Finnish). After a first outing of nearly nine miles, here is how it performed.

Who This Shoe Is For

This is a shoe for neutral runners who value a secure, comfortable upper and a firm, ground-connected feel over the deep, bouncy cushioning that dominates much of the current road shoe market. If you like to feel the road beneath you and want one shoe that can cover steady runs and quicker sessions, the Fusion 3.5 makes a lot of sense. The construction backs that up: a full Aero Foam midsole with Karhu’s Vulcan technology worked in, intended to balance cushioning with extra support, higher energy return and a smoother ride.

The press into the midsole feels quite firm, and that impression largely carried through onto the road. If your preference is a plush, sink-in ride, this may not be the shoe you reach for first.

Main Strengths

The upper is the standout. It uses an updated Ikil knit mesh for improved breathability, with just the right level of padding in the tongue, around the ankle collar and in the heel cup. Internally, a seamless liner attaches to the side of the tongue to create a bootie-like construction, and the result feels genuinely plush wrapped around the foot. Overlays around the eyestays and toe box add durability, and a substantial heel cup gives a good stretch and hold at the back of the shoe.

Fit and lockdown were excellent from the first mile. The shoe is fairly narrow at the midfoot and toe box, and the M-lock lacing system delivered a very secure midfoot hold. Notably, the laces never worked loose during the run, which is rarer than it should be in modern running shoes. They felt just as tight at the end as at the start.

Another pleasant surprise is the weight. At 309 grams in a UK 9.5 it is by no means heavy on paper, but on foot it feels far lighter still, closer to a 250-260 gram shoe. It is a really well-balanced, nimble-feeling shoe out on the road.

Things to Consider Before Buying

Sizing is the first thing to check. The shoe runs a little small; the official guidance is to go up half a size. A UK 9.5, the tester’s usual size, felt slightly short at first, though the soft toe box meant it never caused discomfort, and the fit improved the further the run went.

The outsole raises one durability question: blown rubber covers most high wear areas, but there is none on the outer edge of the forefoot, which is a high wear zone for many runners, particularly those who load on the outside edge. Early wear patterns will be worth watching over longer mileage.

The bigger caveat is the midsole feel. The Aero Foam compound is firm and dense, and that did not transform under running weight the way some midsoles do. It delivered a firm, flat ride with a strong sense of ground connection, and the Vulcan technology felt like it was helping forward motion, but energy return felt limited compared with the softer, bouncier road shoes now common on the market. It is not uncomfortable, just firmer than much of the current field, and as a first run it is fair to allow that the compound may soften with bedding in.

Buying Advice

If you are a neutral runner who enjoys a firmer, more connected ride and values a superb upper, secure lockdown and light, balanced feel, the Fusion 3.5 x SAYSKY is well worth a look at £135. Buy your usual size plus half, and expect a shoe that rewards consistency over weeks of use rather than delivering instant cloud-soft comfort.

If maximum cushioning and bounce are your priority, this will likely feel too firm. It is a shoe that asks a little of you and gives a clean, honest running experience in return, backed by a century of running heritage from one of the sport’s most interesting brands.

Conclusion

The Karhu Fusion 3.5 x SAYSKY is a beautifully made collaboration with a class-leading upper, excellent midfoot lockdown and a surprisingly light feel, offset by a firm, flat midsole that divides opinion. First runs can be misleading, and further mileage will reveal whether the Aero Foam softens up or stays resolutely firm. Either way, this is a distinctive, well-constructed everyday trainer that deserves a place on the shortlist of any neutral runner who prefers feel over float.