Added auxiliary normal speeds and forces for two wheels, trying to test contact forces in three directions.
Updated the longitudinal unit vector of front wheel with cross product of E.2 and g_3, but actually the old one (E.2 cross A.3) is also right after I checked it.
Introducted the biModel as an attribute, and apply doit() function for eliminating the zeros of derivatives.
Added commonts on the ^ and & which are cross and dot; Denormalized the lateral_v since it's already been done.
Fixed the indent of substitution.
Manual derivation of bicycle model.