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Shimano's new SH-M315 flagship cross-country mountain bike shoes aren't the lightest around but they offer a superb custom fit, excellent power transfer, and great durability.(Image credit: Jonny Irick)

Say what you will about the SH-M315 mountain bike shoes' space-age aesthetics but when it comes to the things that count – fit, comfort, durability, performance – Shimano have nailed things spot-on. Custom-molded uppers yield exceptional all-day comfort, the carbon fiber midsole is notably stiff and efficient, and the suitably armored exterior is holding up well, too.

Shimano's latest flagship cross-country shoes fit like they were shrink-wrapped around your own feet – because if you take the time to have them molded at an authorized fitter, that's exactly what they are. Heat moldable panels littered throughout the upper soften and conform to your own anatomy after sitting for a few minutes in the Shimano easy-bake oven and the corresponding vacuum literally sucks the material right up against your feet for a perfect fit.

Riders with standard-shaped feet might only get modest benefits from the procedure but for anyone with any sort of anomalies – such as Tailor's bunions – the custom process is an absolute godsend. Add in the new, mostly seam-free and notably softer upper materials on this latest-generation shoe and the result is a sock-like fit that we happily wore for up to eight hours at a time with no discomfort whatsoever.

One lone spot of fit-related contention is the included heat-moldable insole, which includes add-on metatarsal buttons and arch pads but is still somewhat lacking in support. We had better luck with aftermarket options from Specialized and Pearl Izumi. Ventilation is pretty good, though, with lots of open mesh material right around your toes and lots of perforations throughout the rest of the upper.

Pedaling efficiency is excellent with a stiff, full-length carbon midsole and a very secure fit provided by the ratcheting main strap, twin offset forefoot Velcro straps, and the well-shaped heel cup with cat's-tongue liner material. Trail riders and cyclo-cross racers might lament the midsole's almost total lack of flex but for short hike-a-bike jaunts and general standing around, the dual-density outsole offers up plenty of grip – even on glare rock – and is highly resistant to clogging with mud.

Shimano have also addressed a long-running criticism of their previous-generation cross-country shoes: the total lack of grip material behind the ball of the foot. Thankfully, the new M315 sole is almost fully coated with rubber from end-to-end so missed pedals aren't nearly as dicey a prospect as before. The nearly seamless upper and the new, stretchier materials improve the overall suppleness of the M315 relative to its predecessor and weight has improved just a touch.

As compared to some bona fide lightweights out there, the Shimanos are still veritable boat anchors at 788g per pair (size 43.5). However, that extra heft looks to bring with it excellent durability – even after three months of use on abusive Rocky Mountain terrain, our test shoes are holding up very well with only some cosmetic scuffing around the toe cap and some modest tread wear to speak of. If past experience with Shimano shoes is any indication, we expect this latest model to last for several seasons.