Update:After eight months of testing, one of the rings has loosened,that is why I do not recommend you to do this mod! For the upper tensioner the size of ball bearings are:internal diameter-6mm/external diameter-22mm/width-7mm.You will also need one washer(for M6 bolt which you will put from the outside) and ring with dimensions:internal diameter-22mm/external diameter-28mm/width-2mm.The internal diameter of the ring must be very precisely made in order for the ring to become tight on the bearing and not to move or the belt will slide over the position sensor of the wheel axis and will destroy the wheel base.Best find a professional turner with a lathe to make these rings for you. For the lower tensioner the size of ball bearings are:internal diameter-6mm/external diameter-19mm/width-6mm.You will also need three washers(for M6 bolt,you will put one from the outside and two from the inside) and ring with dimensions:internal diameter-19mm/external diameter-25mm/width-2mm.Again the internal diameter of the ring must be very precisely made in order for the ring to become tight on the bearing and not to move. Why did I do this mod?Because while I was playing with the wheel I felt a few things.Small fine details from the initial moment when the rear of the car starts to slide were missing.I also noticed there was a slight lag from the moment in which the car starts to oversteer until the moment I felt it in the steering wheel.Another thing I felt was that when there is a change of direction of rotation a dead zone in the force feedback appears for a fraction of a second.The reason for these problems is that there is not enough tension in the belt.The original belt tensioners have 18mm external diameter while those I put have 22 and 19mm external diameter.This will add little more tension in the belt and fix these problems without adding extra friction to the single belt drive mechanism.I tried to put ball bearings with 22mm external diameter on the lower belt tensioner but the tension is too high and this only adds unnecessary friction.After this mod I can feel these small details from the initial moment when the rear of the car starts to slide,now there is zero lag from the moment in which the car starts to oversteer until the moment I felt it in the steering wheel and no more dead zone in the force feedback when there is a change of the direction of rotation of the steering wheel.Also the rattle when I go over curbs/bumps is little less violent now and I do not use FEI any more(before I used FEI set to 5).Now this single belt drive mechanism is extremely impressive.This mod cost me 7€. If you decide to do this mod it is for your own risk. This mod will void your wheel base warranty!