Specialized Trail Thermal jersey – casual ultralight coziness

Is Specialized’s new Thermal jersey the ultimate ultralight warmth booster?

Specialized Trail Thermal jersey
(Image: © Guy Kesteven)

Bike Perfect Verdict

Specialized’s Trail Thermal jersey is a super versatile, high-performance heat-keeping, sweat shifting mid layer with low key looks and high value price


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    Exceptional warmth-to-weight ratio

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    Awesome as an outer jersey or mid-layer insulator

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    Casual styling hides serious performance

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    Hidden pockets

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    Men's and women’s versions


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    Unnecessary ‘SWAT’ zip

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    Buy small for extra warmth

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It’d be easy to overlook Specialized’s Trail Thermal Jersey in their new fall clothing range but it’s one of the best mountain bike jerseys down to its value and stealthy performance. Even with that weird ‘I thought it was for a gun’ zip on the side.


Specialized has actually used quite a clever and distinctive cut for the Trail Thermal jersey including an elbow-to-elbow yoke piece across the shoulders. The truth is that Polartec Power Grid is so stretchy it’s not really needed though. That comes from the fact it’s constructed from a very thin ‘skin’ covered in a grid of tiny raised fleece squares that actually looks like a gravel tire tread. This traps a layer of warm air between you and the outside for super-effective insulation. It also wicks sweat off the skin and evaporates it into the elements extremely quickly.

From a fit point, it’s generous in the body and the sleeves are a decent length and the hem sits low on your hips. It’s doubled over for a bit of structure as well and so is the collar. The casual vibe is boosted by the double-ended marsupial/tunnel pocket across the belly which also hides a zipped valuables pocket. This is big enough to hold a phone or stuff the entire jersey into if you want to pack it down. There’s another short section of chunky YKK zip on the right hip that I initially thought might be to help access a gun in a holster (well it is an ‘murican top) but it’s apparently there to let you access the rear pocket on SWAT bibs more easily. At just over 200g it’s still light for a mid-layer.

Specialized Trail Thermal jersey

Front pocket keeps styling casual and hides a zipped security stash (Image credit: GuyKesTV)


The Polartec Power Grid is a key part of why the Trail Thermal jersey works so well. Obviously, there are loads of densities and constructions of fleece/lofted fabrics to choose from in different clothing. Especially if you look at outdoor brands rather than MTB brands rarely make proper mid-layers. Even among all the lightweight ‘3D’ fabrics the amount of warmth Power Grid can create through the way it captures warm air against your body is amazing. On high-energy rides, I’ve been hot in the Thermal on 3- 5 degree celsius temps and if you combine it with a thin shell layer like Specialized’s Trail SWAT Jacket it’ll comfortably cope with sub-zero.

The open weave also wicks sweat spectacularly well, often producing comical clouds of ‘steam’ when you stop. That keeps your skin very dry which not only means less ‘ick’ but also reduces heat loss dramatically when you start heading downhill or hit an icy blast at the summit. The jersey also dries super quick while riding so while there’s no penetration protection from rain, its heat retention makes it great for showery/misty/foggy riding.   

Specialized Trail Thermal jersey

The Polartec Power Grid fabric is a superlative sweat shifter (Image credit: GuyKesTV)

The only downside of the fabric is that wind can blow through it easily - which is where bringing a shell can help. The lack of a front zip also makes it harder to regulate temperature by opening up for an instant air con. The baggy fit means it doesn’t suck as much sweat off as it would with more comprehensive contact either so size down for maximum performance. The casual styling and hand warmer/stash pocket across the front will be a big win for a lot of people though.  

The hip zip is worse than pointless though. It’s cold and clattery when you put it on, adds cost and weight and it’s just not needed as the fabric is so light and stretchy you can pull it up easily anyway. The overall price is still very good for even a basic Power Grid top though so getting the extra pocket features is a real bonus.

Polartec® Power Grid™

The pointless SWAT zip would be better off used at the neck to adjust air con, but it's the only glitch in an otherwise excellent garm  (Image credit: GuyKesTV)


Specialized’s Trail Thermal Jersey looks like an apres ride top and works great as one too. Not least because it wicks sweat super fast and dries quickly too so you won’t be sat around wet and cold after a hard ride. 

The amount of heat it holds in for such a lightweight layer is incredible too, making it a brilliant top layer on cold but calm days or an awesome warmth booster under a lightweight shell. I’ve docked it half a mark as I’d rather have a short neck zip for temp control rather than the pointless SWAT zip on the side. The number of times I found myself keeping my hands warm in the front pocket proves how useful that is and the pricing is excellent. Just be warned that this is a seriously warm top so don’t go boiling yourself by over-layering.

Tech specs: Specialized Trail Thermal Jersey

  • Price: $150 (currently on sale at $119.95 in US) / £65 
  • Sizes: XS-XXL (XXL in men's only) 
  • Colors: Black, Harvest Gold in UK plus Slate and Oak Green in US
  • Weight: 203g 
Guy Kesteven

Guy Kesteven has been working on Bike Perfect since its launch in 2019. He started writing and testing for bike mags in 1996. Since then he’s written several million words about several thousand test bikes and a ridiculous amount of riding gear. He’s also penned a handful of bike-related books and he reviews MTBs over on YouTube.

Current rides: Cervelo ZFS-5, Specialized Chisel, custom Nicolai enduro tandem, Landescape/Swallow custom gravel tandem

Height: 180cm

Weight: 69kg