The Castelli Insider Jersey is highly breathable and, although intended for indoor training, can also function as an ultra-lightweight jersey for hot summer rides. If it weren't for the silicone gripper in the hem, which is disproportionately tight, this would otherwise be perfectly fitting jersey that performs well in its intended scenario.
Two rear pockets
Doubles as a summer jersey
Disproportionately tight around the waist
By Stefan Abram
The Castelli Insider Jersey is the lightest jersey the Italian brand has ever made, and it certainly feels that way – there really is nothing to it. Designed for indoor training when modesty requires a top, it should also make a pretty handy hot-weather jersey come a summer heatwave.
Castelli Insider Jersey: construction
The fabric is a semi-open mesh which, while not entirely see-through, doesn’t leave that much to the imagination. That said, there is enough opacity that it does do its job of keeping you decent when you can’t go topless.
With two pockets on the rear, not only are phones and chargers easier to transport to your turbo setup but taking the jersey out on road rides is a genuine possibility. That said, the jersey is not SPF rated, so suncream will be a necessity for outings in the summer sun.
At the hem, the silicone gripper is wide and effective at locking the jersey in place, but it does feel noticeably tight. The cut of the jersey is supposed to be figure-hugging and this is achieved across the shoulders and chest without stepping over into feeling restrictive – in juxtaposition to the hem.
The sleeves are shorter than the current fashion of just above the elbow, coming halfway down the bicep, and the collar is a little lower than usual on a short-sleeve jersey.
Coming from not using a jersey at all for indoor rides, although this jersey is incredibly thin, there is still a discernible difference when the fan is first turned on. You do feel slightly more isolated from the wind.
But after a few minutes on the turbo, you soon get used to the jersey. The difference is comparable to a 1°C variation in room temperature: noticeable if a sudden change; imperceptible once you’ve gotten used to it. Even in frenetic Zwift races I didn’t overheat, and the jersey didn’t cause any distraction from the efforts.
Although the hem does feel disproportionality tight when putting on the jersey, it wasn’t actually noticeable once riding along. Although when you do unzip the jersey fully, it does open with a ping.
The pockets performed quite well, managing to hold a phone, Allen keys and even a laptop charger. There was a fair degree of sagging when stuffed to the brim, but nothing more than you would expect from such a lightweight jersey. Having somewhere to stash the surprising quantity of paraphernalia that indoor riding can require proved to be a genuinely useful function of this jersey on multiple occasions.
Given that I was testing the jersey in the later part of autumn, I didn’t assess its performance on any rides outdoors. But from my experience so far, I am fully confident that it would work excellently as an ultralightweight jersey for super-hot conditions.
At £75, the Castelli Insider Jersey compares favourably to the Le Col X Wahoo Indoor Training Jersey which is £120 and has the downside of arm grippers that are prone to getting sweaty.
But if all you’re after is something to cover you up when on the turbo, the Endura Fishnet would provide this function for only £29.99. However, in being a base layer, it is less versatile than the Castelli Insider Jersey as it won’t double up as a jersey of the peak of summer.
Starting off riding mountain bikes on the South Downs way, he soon made the switch the road cycling. Now, he’s come full circle and is back out on the trails, although the flat bars have been swapped for the curly ones of a gravel bike.
Always looking for the next challenge, he’s Everested in under 12 hours and ridden the South Downs Double in sub 20. Although dabbling in racing off-road, on-road and virtually, to date his only significant achievement has been winning the National Single-Speed Cross-Country Mountain Bike Championships in 2019.
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