I bought a cheap treadmill and want to turn it into an incline trainer (25-30 degree incline). I have made the necessary mechanical modifications to get the incline but now I need a way to slow the belt down. At this incline the treadmill moves faster than I want with no electrical power required (gravity powered now). I would rather not resort to friction braking it. I was hoping a load (light bulb) applied to the motor circuit with a dimmer switch would allow me to slow/control the speed. This somewhat works but I need FAR more slowing down. Ideas?

 

Use a high wattage, low value resistor, or switch a few selected values in.
What rpm (roughly) is the motor rotating at at the slope required?
Max.

 

Rough estimate I would say 60-70 RPM at the motor.

 

Check the motor voltage for rated rpm and pro rate the voltage for 60-70 rpm and this will tell you the generated voltage, will give you an idea of the minimum resistance you could use without motor damage.
Max.

 

7,099 RPMs
2.5 HP @130 VDC
So about 13 volts? Not sure how to factor that into a resistor value??

It's this motor- https://www.amazon.com/United-Technologies-Automotive-Fitness-Lifestyler/dp/B07FB6S384

 

You would not need much in the way of resistor value, just a few Ohms, one means of experimenting on the value is to obtain a replacement open coil element, and use a small section at a time.
A local appliance parts store would have one.
You may be able to just short the output and monitor for any motor over heating.
Max.