Good morning KenChoo,
You are correct the S5101 is a "cored" motor. The smoothness that you notice is most likely from the Dual Ball Bearings that they have.
I hope you find this information helpful. Should you have any further questions or concerns, please do not hesitate to contact us at 217-398-8970 for Support, 217-398-0007 for Service or via fax at 217-398-7721 or by email at
[email protected].
Sincerely,
Krysta
Lead Futaba Customer Service and Programming Technician
Also you may want to check our Website at (
http://www.futaba-rc.com)