The Sierra is a 2.5 layer jacket, which sounds odd but means it has a Nylon face fabric bonded to a waterproof/breathable membrane with a printed inner surface. The latter is a sort of porous 3D layer that picks up moisture from the inside, from where it can be transported to the outside. 

Like the best mountain bike jackets out there, the Sierra also uses taped seams to stop water ingress. They’re tape-welded, including inside the arm, which is a real plus because we see quite a lot that are missing it here. Tape-welded simply means there’s a little bit of heat sealing applied to the tape, which poses a decent barrier to water.

Inside the Alpinestars Sierra is a fine mesh that draws sweat away from your body

Alpinestars Sierra Waterproof Jacket need to know

• 2.5 layer jacket with 10k waterproofing and 5k breathability
• Big collar gives great coverage, and the zip baffle is great
• Stretchy wrist cuffs, elastic pull ties at the hem, and well secured hood

All the zips are fully waterproof including the two zippered side vents, the main zip down the front and the two side pockets. There’s a pull cord to make opening them easier but there’s no little toggle on the end with the Sierra. With thick gloves in wet and muddy conditions, getting hold of this thin cord is pretty difficult.

The main zip is a branded YKK and it comes with an internal storm flap and a little zip gutter at the collar to stop the sharp end from digging in the bottom of your chin. The storm flap is a little narrow though and when closing the jacket, we occasionally catch it in the zip. Some jackets do feature a two-way zip, which can be opened from the bottom up for venting, but not the Sierra. The jacket comes with two handy side pockets for easy storage and access to a phone, wallet (if you still have one of those) and car keys.

The Sierra offers tons of protection as long as you top up the DWR, but the hem should drop lower

Performance

When buying a new waterproof jacket two features are really important, breathability and waterproofness. You’ll often see a number next to these and the higher that number the better. Alpinestars’ new Sierra jacket has 10k waterproofness and 5k breathability, which in the scheme of things isn’t that high.

But that’s not the whole story, because how effective a jacket is at keeping you dry is often more complicated than this, and due to several other features. Those figures apply only to the material it’s made from, before being shaped into a jacket. That means if brands add breast pockets, extra waterproofing at the rear hem, or simply double up the fabric in certain places this can render the numbers less helpful.

Jackets for big mountain days need well secured hoods: the Sierra’s didn’t budge

The hood on the Sierra is an over-helmet design and, while it stops a little short, there are three toggles – two on the front, one on the back – that let you get it really tight. We did some fast descents with this jacket, and it didn’t budge. The collar on this jacket is one of the tallest we’ve seen and it comes right up over your mouth, which doesn’t feel too uncomfortable and it’s pretty effective when the wind chill is testing. There’s a big baffle at the neck, but the opening is quite large, so it doesn’t impede your vision or line of sight. Our only issue is the upper toggle is integrated into the fabric and it’s therefore hard to get hold of with gloves on.

Velcro cuffs add weight and faff, but there’s no arguing with how well they keep out the wind 

To stop wind and water getting up the sleeves the jacket has angle-cut cuffs, which sit flat against your hands. There’s also a two-position Velcro closure that lets you get the cuff snug against the wrist. The jacket is loose in the arms, so it doesn’t feel restrictive, and you can easily wear a set of elbow pads underneath.

Following this theme, the shock cord on the lower hem lets you pull in everything tight but this jacket is not as long as some we’ve tested. If you like to hunker down on the bars, it can ride up and expose the rear as you lean forward. 

The face fabric on the Sierra is nice and soft, but our main issue is the DWR (durable water repellent) coating applied to the surface didn’t last more than a couple of wear and wash cycles. There was some noticeable wetting out on the sleeves and shoulders and once this starts to happen you don’t get the breathability or waterproofness. Renewing the DWR is something you have to do with all jackets but with this one, it’ll be sooner rather than later.