Originally Posted by silencery
Try moving the fan further from the motor. Depending on the strength of your motor's rotor, it could be interfering with the magnets on the fan causing the fluctuation.
You can also test this quickly by turning on the car (make sure the fan is running) and rotating the poles of the motor by turning your drive wheels. If the fan speed fluctuates, you know the motor's magnet is too strong.
Thank you for the tip and you are right the fan interferes with the magnet.
But even at a distance of 10mm, which is a lot in a touring car, the fan struggles.
And this is with a X12 17.5 Stock spec.
I made the test with a 40mm R1 works and a Yeah racing fan also.
The results are the Yeah racing is running strong even direct at the motor, but the 40mm R1 struggles and runs only with about the half speed.