Underdriving is one of the simplest but least understood performance modifications. We hope this blog lays out the facts in an easy to understand format and finally lays to rest the most common misconceptions.
Underdrive basically means to slow the speed of one pulley connnected to another pulley.
- The first method is from the drive pulley (the crank pulley or the pulley driving the other pulleys). In this method the drive pulley is made smaller, the same as the rear gears on a bicycle. As you move from the bigger gears to the smaller rear gears your cranking is slower.
- The second method is from the driven pulley (the alternator, power steering or water pump
pulleys). In this method the driven pulley is made larger, the same as the front gears on a
bicycle. As you move from the smaller to the bigger front gears your cranking is slower.
That's it folks, you are now an expert on how underdriving is and can be done!
How do these priciples apply to UR and how does the amount of underdrive designed into the part make a difference. UR does all the underdriving for its import applications from the crank pulley. So this means all our underdrive crank pulleys are smaller than the stock crank pulleys. The amount of underdrive, the smaller the crank pulley is made can have a major effect on the output of the accessories (alternator, power steering and water pump). Before we go too much deeper we need to explain that the car manufacturers tend to over spin or overdrive the accessories. This is called planned obsolesence so they can sell you parts as these items wear out. Now that you know this fact you understand why underdrive is an important part of many of our pulleys. Now back to crank pulley size. When UR develops a new application we start with a reduction based on our early years of testing. We test the prototype on the vehicle and we verify that accessory output/function is within factory specified levels. For the alternator output we check voltage with everything on in the car, even items that will never be on at the same time. This also includes simiulated volatge draw of up to a 600 watt RMS aftermarket stereo system or auxiliary lighting system. This means with UR pulleys you will have enough voltage, your car will not over heat, your a/c will still be cold and your power steering will give you all the boost you need. The percentage or amount or underdrive can vary from as little as 5% to as much as 20% depending on the amount the factory over spins the accessory.
A common mis-understood fact when it comes to voltage and electrical systems is the dimming that can occur in the dash, headlights and interior lights. Your vehicles electrical system is a 12 volt system but car manufacturers overcharge these systems to as high as 14.5 to 15 volts. This extra voltage is unnecessary for the lighting as it reduces bulb life and sometimes dramatically. As an example most people can attest to seeing a relatively new car with a tail light, turn signal or head light out. After the installation of a UR underdrive pulley it is possible you may notice a dimming at idle but realize its because your electrical system was over charged. Simply put instead of being at 14.5 to 15 volts you may now be at 13.5 to 14 volts which is perfectly fine. As we state dpreviously we specifially test each car to make sure we maintain factory acceptable outputs and function.
One side not for cars that have an aftermarket stereo system over 600 watts RMS, auxiliary lighting with a draw over 600 watts or you've added a supercharger. In these cases you need to use our stock diameter crank pulleys.
That's it folks now you are an expert on why UR underdrive pulleys are completely safe for use in your vehicle.
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