How Do Underdrive Pulleys Work?

[Robert YellowVette]

I have been doing some reading on underdrive crank pulleys, which are claimed to give you 'free' horsepower by reducing the speed of belt driven accessories, such as water pumps, alternators, A/C compressors thereby 'saving' more HP for the crank.

Some makers use a bicycle metaphor claiming a smaller '1st' gear makes more power without discussing front chain-wheel diameters versus rear sprockets...

When I ride my 12-speed bike, reducing the front chain wheel diameter and leaving the rear sprocket the same increases the crank effort so I fail to see how this reduces the power needed to turn the crank pulley and therefore the accessories...

 

[George8211]

I thought it was a pulley & belt kit that causes the crank driven accesaries to spin slower so the engine doesn't have to work as hard.


Free horse power ?? Looks like they charge for the kit ??

 

[DRTH VTR]

The underdrive pulley moves the accessory drive belt more slowly for any given engine speed. If it requires "X" amount of energy to turn the alternator 1 turn, having it turn 1000 times per unit time requires less work than 1500 turns in the same time period. The same could be argued for the other items driven by the belt.

If the engine is at wide open throttle, but the accessory drive is using less of the engine output, more is available to the rear wheels.

The other place where people talk about using a smaller pulley is on a supercharger. In this case, a given belt speed turns a smaller wheel more revolutions. More revolutions means more turns of the compressor impeller and more air packed into the engine. The amount of air that can get into and out of the combustion chamber is generally the limiting factor in power production.

 

[DRTH VTR]

When I ride my 12-speed bike, reducing the front chain wheel diameter and leaving the rear sprocket the same increases the crank effort

Think again. It is the other way around. Making the rear sprocket smaller or the front bigger increases crank effort. A small front sprocket combined with a large rear sprocket is a hill-climbing setup.

 

[Robert YellowVette]

Thanks, you are right, I was tired and not thinking clearly...