The Roloc Pad that the diamond disks attach to is a rather large, heavy piece. The typical shank used with this Roloc head is a 1/4 inch diameter.
We have so many customers that need 1/8 inch mandrels that we also offer the shanks in that size. The problem is that the torque created by the larger, heavier Roloc head can sometimes bend the shank.
Be sure to seat the shank as far into your grinder as you can and start your grinder off at a slower speed and then move to a faster speed. Having too much of the shank exposed and starting at a high speed will create too much torque for the thinner 1/8 inch shank to handle and it will bend from the force.
Just be careful when using these smaller shanks and they should perform quite well.