Racing is about exploring
your limits. Why lower them when you don't have to by using semi-slicks.
I believe semi-slicks also contribute more to trail damage than full
knobbies because of reduced traction. Semi-slick mtb tires are not suitable
for 99+% of riders because they offer reduced traction when it is needed
most: Under braking and descending.
Rolling fast is not worth
it if you can not stay on the trail. Last year in a race I passed 4
or 5 riders as they touched their brakes, locked the wheels and skidded
off the side of the trail. Not only did they lose time as they pulled
themselves back on the trail, the riders left long skidmarks and a
broken-down trail edge. Using full knobbies I was able to carry more
speed, use my brakes effectively and stay on the trail.
