Well, I think all you need is the right tool 
The internal lockring of the KS3 should be tightened with a torque of 50 - 60 Nm,
guess two nails are not able to achieve that torque. You need a hook key Ø 95 mm.
Once again, at this chance; the belt tensioning method shown in the Strida manual (without using a hook key) is definitely wrong!