The solref and solimp parameters can be adjusted to obtain different restitution effects but they are not in one-to-one correspondence with the notion of restitution, and it is generally not possible to guarantee a fixed coefficient of restitution. You can make contacts more bouncy by increasing the second parameter in solref; it corresponds to a damping coefficient: 1 = critical damping, less than 1 = under-damped, more than 1 = over-damped.
Hi, I tried to tune the values of solref and solimp, but the bounce is very unstable. And I couldn't understand why... Here is the model.