At Terra, we consider the On Cloudsurfer a lower stability shoe when compared to other On models. This is because it lacks the typical Speedboard found in every other model of On shoes. I needed a new road shoe since I have a goal of running a trail ultramarathon in September. I wanted a shoe that could train me for the trails even when I mostly run roads. Because it is very bouncy, the Cloudsurfer is less stable with its soft cushion and lack of Speedboard to provide any sort of rigidity. While that might sound like a bad thing, this is as close as I can get to simulating the uneven terrain of a trail while on the road. Training the ankles for rolling or wobbly motion is essential for a more secure and confident run on the trail. The Cloudsurfer utilizes a new cushioning technology created by On called CloudTec Phase; this technology gives the Cloudsurfer a very soft, plushy ride. The CloudTec Phase design works by collapsing the cushioning as you step down in the shoe and expanding the foam up in a forward rolling motion, propelling heel strikers forward. Midfoot and toe strikers benefit less from the rolling motion, feeling more of a spring than a forward roll. While neither of these functions quite mimics the trail, they do give a bouncy ride. I have now put around 70 miles of running on the shoes and strikingly my lower calf has become increasingly tight and irritated. While everyone’s foot reacts to shoes differently, I do attribute this tightness to the Cloudsurfers. Thinking critically about the design of the shoes, I am under the impression that due to the incredible softness under the heel, the shoe actually compresses more toward the heel side than desired. For me, this effect causes an over-extension of the Achilles and a prolonged tension on the lower calf. I have had to switch out to different shoes due to this strain. My hope is that this, too, simulates climbing up the mountains of a trail run. Overall, I really like the shoes, but only for runs less than 8 miles. For customers, I would recommend this shoe to a neutral runner who wants a fun bouncy shoe for 5k distances.