why do trials bicycles have holes in the rims?

[petrol mobility]

I was thinking of putting trials rims on my mountain bike as i thought the wide rims would look pretty sick. but i was wondering if the holes in some rims takes much strength out of them and what purpose thhe holes have.

 

[Bob A]

Sorry they wont fit.

 

[two wheeler =D]

the holes are for saving weight, no point on putting it on an MTB, it also weakens the rim a fair bit