custom ud pulley

[jaykay]

I think when we talk about under drive pulley diameter limits we are considering what belt speed reduction the driven accessories can safely tolerate throughout the rpm range. You probably dont want to start running your ignition off your battery. It is also important to consider belt drive dynamics with a reduced diameter. Will the arrangement cause undue belt stress or slippage? Crank pulley size controls the belt speed for the whole system and the torque load of every accessory is reduced with a smaller crank pulley. The torque load on the engine is directly related to crank pulley radius.

A lower belt speed turns each accessory slower. One can control the speed (reduce) of each accessory individually by increasing its pulley dia. The torque load for that accessory will be reduced as well, but you will not realize the power savings that a smaller crank pulley gives unless one makes all of them bigger.

Recall that, originally the aim was to offset accessory speeds downward for constant elevated engine rpm at the track, particularly the power steering pump.

I have noted my ps pump groaning under full lock at parking speeds with the UDP....so here is another design limit cue

My guess is that that crank pulley mass is not significant when compared the massive flywheel but a quick calculation will tell you whether fancy spokes are worth the machining cost.