Mosin - please re-read my post.
I did not say that elasticity causes the belt creep.
I said it is an assumption and that if the belt doesn't stretch then there is no creep.
Mark Kelly's calculation of belt creep is :
Creep = T/r x A/E
Where T is torque transmitted, r is pulley radius, A is belt cross sectional area and E is the elastic modulus of the belt material for small strains. Thus a torque of 1mNm on a 10mm diameter pulley using a belt of 10 mm2 and made of rubber with a modulus of 50MPa will display 0.4% creep. If the torque reduces by half so does the creep so the speed change on load for a 0.5mNm load variation is around 0.2%. The 0.5mNm load variation is pretty typical of the stylus drag changes found on turntables.
If the belt has no elasticity, then the Elastic Modulus (E in the above calculation ) is ∞ ( infinity ), and Belt Creep will be effectovely 0 ( zero ) exactly as I said.
To give some examples of tensile strength :
Rubber 15mpa
Human Hair 380mpa
Silk 1000mpa
Aramid fiber 2557mpa
These are single fibres only.
Here are the calculations using Mark's example, and assuming the cross sectional area is arbitrarily 1/10th the size of the rubber belt.
T R A E Creep
1 5 10 50 0.04000
1 5 1 380 0.00053
1 5 1 1000 0.00020
1 5 1 2257 0.00009
On the subject of thread drive TT's, they do have to be designed properly, as in the Final Audio. The Final Audio uses an AC synchronous motor with precisely controlled regenerated sine/cosine waves for the motor and variable voltage regulation to optimise the torque applied to the moving high mass/high inertia platter. In addition to this the pulley profile must be designed for a thread rather than a belt. If I recall correctly the pulley should present a concave hemisphere to the thread.