If you are riding close to someones rear, then possibly. On your own, or with a decent gap between riders then probably not.

However, in my view it's probably the weak point of any liquid cooled motor, if the rad goes you are stopped. It's always something I've considered but never done anything about - which probably answers you question... at least for the last 5 years anyway ! Who knows... in 6 months time I might change my answer to yes - you need rad protection.

The whole issue with 'protection' is you can make you bike heavy, less nimble and actually add some catch points for the scenery to grab hold of your bike. All depends on the sort of places you ride.

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Never underestimate the power of stupid people in large groups.