OR you just run 2 sets of wheels, 1 normal mtb set and 1 set of slicks. I did this and a mountain bike fitted with slicks is amazing on the road compared to mtb tyres. Higher average speed, better accelration, less fatigue and drag-free in comparison to the normal wheels. If you fit a large outer crank ring then you can really fly along nicely.
I did this for years when I was a yute and came to the opinion that road bikes suck balls as good luck swapping the wheels and going for a monster downhill in comfort/in one piece

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