Rab Syclon 40L Review: A Waterproof Pack for Fast and Light Adventures

Introduction

Rab has become a familiar name in lightweight backpacking and running packs, and the Syclon continues that direction. It takes the thinking behind two earlier Rab designs, the very light big rucksack and the fully waterproof long distance running pack, and combines them into one bag.

The result is a super light day or multi-day rucksack with taped seams and a waterproof face fabric. The range includes this 40 L version, a 30 L version, and women’s versions that come up a couple of litres smaller because of a shorter back length.

At 890 g, the 40 L is not the lightest pack on the market. It is still remarkably light for how fully featured it is, and that balance is what makes it worth a closer look.

Who the Rab Syclon 40L Is For

This is a pack built for fast and light adventures. It suits lightweight backpackers heading out for a single day or several, runners and ultra racers working from a full kit list who want a little of the weight transferred onto their hips, and anyone who wants one waterproof bag that can cover day walks and overnighters alike.

Because the design sits somewhere between a running pack and a light rucksack, it also works as an everyday day pack for people who value a waterproof roll top more than a traditional lid.

The women’s specific version offers a shorter back length, which is the main option in this range for smaller frames.

Waterproof Protection and the Roll-Top Design

The centrepiece is a 40 denier waterproof nylon face fabric, backed up by taped seams. Rab gives the pack an IPX4 rating, which means it is not submersible. It is built to keep rain out, not to be taken underwater.

The roll-top closure does most of the work. The straps can be clipped over the top in the manner of a dry bag, or routed down the sides to pull the load down tight when the pack is not full. That second option is genuinely useful on shorter days when you are carrying less than the full 40 litres.

Storage and Pocket Layout

External storage is generous. There is a large dump pocket across the front that will swallow a surprising amount, plus storage pockets on the sides in a durable fabric with elasticated straps. Those side pockets hold bulky items when you need them to and spring back into shape when they are empty.

The big front pockets also act as a protective shield for the lighter main fabric, and the pack uses a 100 denier spectra fibre ripstop nylon where extra durability matters. Ice axe attachments are there if you want them, longer poles can be threaded through the front, and straps along the bottom let you lash a tent or a rolled mat to the outside. That last detail is what turns a 40 L pack into a viable overnight option for lightweight camping.

There is also a separate zip pocket at the top for a water bladder, with the tube running out from it. If you prefer bottles, that pocket is a good place for a map or an extra layer instead, because it stays accessible without opening the waterproof main compartment. A small internal pocket handles a wallet and keys, the things you do not want to pull out by accident when you are digging for a jacket.

Carry Comfort and the Aerofit Back System

The Aerofit back system is light but carries a load well. The mesh back panel allows a lot of airflow, which matters most in summer when you are moving fast and running warm. The shoulder straps follow the same idea: thin, well ventilated, and wide enough to spread the load rather than cut into your shoulders.

The hip belt is one of the strongest parts of the design. It feels like a halfway point between a running pack and a light rucksack, and it carries two large zip pockets that are easy to reach while the pack is on. Emergency snacks, a phone, sunglasses or jelly babies all have a home there.

The shoulder straps borrow from the running pack world as well, with a zip pocket for a phone or a compass and a dump pocket with a small bungee at the top. That bungee is designed for a soft flask, so you can drink on the move without stopping to reach for a bottle.

Weight, Fit, and Things to Consider Before Buying

The 40 L weighs 890 g. That is the trade-off at the heart of the pack: it is not the lightest option available, but it earns the extra grams with waterproofing, taped seams and a genuinely full set of pockets.

Two practical points are worth checking before you buy. First, the IPX4 rating means the pack is rainproof rather than submersible, so it is not the right choice if you need a bag that can be submerged. Second, the standard version comes in a single back length. There is some adjustment, and the fit holds up well on taller frames, but the women’s specific version is the route to a shorter back length. Trying the pack on with weight in it is the most reliable way to confirm the fit.

Day Pack, Overnight Pack, or Ultra Race Pack

The Syclon’s versatility comes from how it handles different loads. Filled lightly, it works as a day pack for hill walks and everyday use. Loaded properly, it covers lightweight overnight trips, especially with a tent or a mat strapped to the bottom. For longer distance ultra races where the kit list is long and you want some of the weight on your hips, the hip belt and stable carry make it a sensible choice.

That range is the reason the pack is interesting. One bag covering fast day missions, light overnighters and longer events is a lot of ground for 890 g.

Buying Advice

Start with capacity and use. The 40 L is the more flexible of the two sizes because it can compress down for day use and still hold overnight kit, while the 30 L suits shorter, lighter days. If you have been looking for a waterproof rucksack and have been told that most packs are not, this is the feature to weigh up first, because the roll top changes how you pack and how much you worry about the weather.

Fit deserves the same attention as features. Load the pack with the weight you would actually carry, adjust the hip belt and shoulder straps, and check that the back length sits where it should. If the standard back length is not right for you, look at the women’s version before ruling the range out.

Conclusion

The Rab Syclon 40L brings together the light weight of a big rucksack and the weather protection of a waterproof running pack. It is not the lightest pack on the market, and the IPX4 rating means rainproof rather than submersible, but the 40 denier waterproof fabric, taped seams, roll-top closure and extensive pocket layout make it a versatile choice for fast and light days, lightweight overnight trips and longer events where kit space and weather protection both matter.