MIPS liners can be found in many cycling helmets these days. The often-yellow sheaths are designed to reduce brain trauma in the event of a crash. (MIPS stands for Multi-directional Impact Protection System.) Hopefully you'll never need to test the system's safety, but you do want to be comfortable whenever you ride. Here are a couple of fit issues to consider with MIPS helmets.
1. MIPS liners can make helmets fit smaller
While some new helmets like the Giro Vanquish have been designed with MIPS integrated into the design, most helmets out there just had the liners added to existing designs without changes to the molds.
Although the MIPS liner itself is quite thin, the contact anchors and pivot points add a little width. Many brands use felt patches between the foam and the liner to allow it to slide freely. The net result is that many helmets with MIPS fit a little smaller than their non-MIPS counterparts.
For example, I was always a medium in Giro Synthe, but am a large in the Giro Synthe MIPS.
At BikeRadar, we've noticed this trend with helmets from POC, Giant/Liv, Scott, Bell and Giro.
What this means for you: If you are on the upper end of fit in a given helmet size, try before you buy.
2. MIPS liners can become the primary point of contact when the pads wear down
Every brand does it a little differently, but the MIPS liner always sits between the helmet's hard foam shell and the soft pads that contact your head.
The idea is that, in the event of a crash on your head, your cranium is allowed to pivot slightly as your helmet hits the ground, reducing the impact of your brain against the inside of your skull. Hopefully you'll never experience this! (The last time I crashed with a MIPS helmet I got knocked out - but that was because I landed square on my chin. Stupid dog!)
In day-to-day reality, pads get compacted over time from regular use. Depending on the construction of the forehead pad(s), you may well end up with the hard plastic of the MIPS liner resting on your head.
What this means for you: Keep the extra pads that come with your helmet. You'll probably want them later.